498 Wireless World, November 1975 Wireless World Teletext decoder 1 The background

by Philip Darrington

The first of a series of articles on the Teletext television information display system and the construction of a decoder for use with domestic television receivers.

In succeeding articles of this series we transmitted on lines 16 and ns of each ments were being carried out in 1970. A shall describe the design and construc- frame3. It apparently did not, at the method of data transmission not using tion of the Wireless World Teletext time, occur to ORTF, who originally the vertical blanking intervals was Decoder. But, before we do that, it used the system, that the technique described by P. T. King of Hazeltine seems only sensible to make sure that could be developed to allow the trans- Research, Inc., at the SMPTE Technical readers are up to date with current mission of visible information, but the Conference in 1973. In this system, activity in the field and have an idea of system was used for switching remote known as an "add-on" type, a low-level other systems which have been transmitters, identification for remote subcarrier was inserted on the vision proposed. We will also describe the supervision purposes, etc. signal at an odd multiple of half the line basic operation of Teletext and men- Other systems of transmission aimed frequency, a method which is well tion the techniques used to decode the at "still" display, with or without sound known in colour television circles as a transmissions for display on a domestic and in colour or monochrome depend method of avoiding interference television receiver. on the exclusive use of a television or between two signals in a common In the early years of "wireless", the sound channel. For instance, the NHK waveform. The added signal is shifted in vision of millions of people being "A" system4 is able to carry over 50 phase from line to line and the visible entertained by transmissions was just channels of colour still pictures, with effects are reduced. The actual fre- not credible. Television was thought to sound, in the bandwidth of one normal quency is chosen to be between 2 and be sheer science fiction and wireless television channel. The pictures are 3MHz in the NTSC standard, itself was for the transmission of vital transmitted at the rate of one still per because radiated energy is at information. Even if it had been frame, together with a code to enable a a minimum in that region in a possible to communicate phonically, magnetic disc memory in the receiving typical picture. The data signal is that was not the idea at all — ships were equipment to record a selected frame, biphase modulated at 21 kilobits per the real users of radio and if amateurs which is then continuously replayed. second and at the receiver is synchron- could derive pleasure from listening to Sound is multiplexed with video infor- ously detected, the data decoded and marine morse traffic or Eiffel Tower mation and transmitted in the frame used to operate a character generator, time signals, so much the better. intervals. which can, for example, produce sub- Then, of course, radio telephony was The NHK "B"4 system falls into the titles in languages using the characters developed and the entertainment multiplexed television channel cate- available. possibilities were recognized. It is now gory, using uncoded signals. Three These two techniques — the use of an extraordinary and rather depressing unused lines per field are used to previously blank lines in the vertical fact that out Of the few thousand transmit three lines of a still picture, blanking interval and the provision of a megahertz available for radio trans- which is magnetically recorded at the character generator at the receiving end mission, nearly 600MHz are taken up receiver. Sound is transmitted by the — paved the way for Teletext. A system by broadcasting stations, most of use of additional sound carriers at low of this kind is able to carry more which put out entertainment of one level, spaced further in the spectrum information, more flexibly, by leaving sort or another. Sound is reasonably from the vision channel than the the receiver to do rather more of the restrained in its use of bandwidth, but original sound carrier. work than an analogue or uncoded television occupies 566MHz — a stag- In 1971, W. D. Houghton of RCA digital system. If the transmitted signal gering figure, particularly as the described the Homefax5 system in is made to carry instructions to the television signal contains so much which additional information was mul- receiver on what to do, the receiver can redundancy. Anything designed to tiplexed with a broadcast television then hold much of the information in make better use of this signal is to be waveform to produce "hard copy" from store, releasing it when commanded to welcomed and Teletext is one way of an electro-optical printer, using a do so. doing that, in that a previously unused cathode-ray tube to produce the "print- Peter Hutt of the IBA described an part of the waveform is used to ing" on Electrofax paper. The informa- embodiment of this principle in a paper transmit more information. tion, uncoded, was carried in an unused read to the IPC, 19727. The system was Many ways of using television vertical interval line and transmitted in named SLICE and was primarily transmissions to better effect have the normal way — each of these lines intended for the labelling of pro- been proposed'. 2. Since the early sixties, producing part of one line of a row of grammes with a source identification. test signals inserted in the field blanking print on the receiver paper. The information was carried on lines 16 interval have been in common use, and Transmission tests of this system had and 329 in the form of 112 binary digits have been supplemented by coded data been carried out in 1967, and improve- per line and, at the receiver, was read

Wireless World, November 1975 499 into a memory where a complete message was assembled. SLICE was the fore-runner of ORACLE and CEEFAX. (Hutt mentioned the possibility of a domestic information service in his paper.) ORACLE p1 Thu 14 Aug 1# 44 $4

Teletext ORR if ORACLE was developed by the Inde- pendent Broadcasting Authority in the E C light of its experience with SLICE and N E W IIIEr 200 ASCU — .101 the BBC announced their own system, 201 mess 22 a PUBLIC ADVICE —550 which was very similar and bore the 23 • HELP, 75C name CEEFAX. (ORACLE is an '4EWSr LASk .25 POLICE 5 . ,, 72C acronym — Optional Reception of An- WEATHER 30 SERuICES 70C nouncements by Coded Line Electron- PROGRAHMES .40 ics — CEEFAX is See Facts as pro- "RAVEL 50 nounced by an adenoidal dyslexic.) EMTERTA/HMEHTS 60 SHOPPING 115 There is little profit in trying to decide LE:SURE . 16 HONE ADVICE 350 which organization hit upon the idea RECORDS * BOOKS 19 JUNIOR ORACLE $50 first, or which was on the air first, as the *H6 :*u REGIONS 80 ENGINEERING two were quickly and sensibly to agree 450 on a common standard of transmission, in conjunction with BREMA8. Both names will probably continue to be used for each organization's broadcasts, but the generic term "Teletext" is now common, and will be used henceforth. Teletext has been broadcast on the present standard since September 1974 Fig. 1. The main index page (p. 100) of ting blank lines, a practice which is — the beginning of a two-year experi- ORACLE for August 14, 1975, produced already followed by the IBA and which mental period. The system uses a by the W.W. decoder on a Thorn colour the BBC intend to change to shortly multiplex-with-vision type of transmis- receiver. (they already omit blank lines at the sion and uses lines 17 and 18/330 and bottom of a page). 331 to carry coded information. There is To select a Teletext page, one first no accompanying Teletext sound. switches the signal path through the decoder, selects a page by means of Before proceeding further, a look at thumbwheel switches or a set of push- the facilities offered by a Teletext group of, perhaps, four such pages buttons and waits for the selected page transmission and a résumé of some of which change at about one minute to be assembled. the specification will be a help when intervals. They will usually be on a related topic, such as sport, and are considering the equipment needed in Teletext signal the receiver. Briefly, the Teletext page identified by the letters A B C or D, the consists of 23 rows of 40 characters relevant letter being in a different The method of transmission of the each, plus a page header, which is only colour so that the viewer knows which Teletext data is of interest at this point. partly displayed. As seen in Fig. 1, point in the set of pages has been As has been said, the data signals are the header gives the name of the reached. This type is really the same as carried by lines 17 and 18 and the service (Ceefax or Oracle), the page the first variety, but pages are changed corresponding lines in the alternate number, the date and the time. The time automatically at minute intervals. The field, 330 and 331. The choice of lines is display is continually changing and is third kind of page can be selected by the influenced by the need to avoid the always visible. time-code on the header, so that it can early part of the vertical blanking Visible characters are either capitals be received, placed in store and read out interval and the few lines near the start or lower-case letters assembled on a 7 x at a convenient time. of the video signal (lines 23 and 336). If 5 dot matrix and in one of six colours or The data to be displayed can be early lines were used, it is possible that white: diagrams or low-resolution "pic- presented as a complete page or, in the data would appear on the field flyback tures" can be assembled by the use of a case of news flashes or subtitles, can be on some receivers and if lines later than block of cells on a 3 x 2 pattern, each of inserted in a blank rectangle on the 18/331 carried the information, which can be "on" or "off", in colour. A screen, leaving the rest of the picture receivers with incorrect picture height variety of other symbols (commas, visible. The full page can be superim- adjustment or with a downward-shifted brackets, (6), £, etc.) can also be shown, posed on the picture, but in our picture might show the data as an and the set of characters is known as experience this is a good way to ruin extremely "busy" pattern of dots at the the ISO-7 code, which is a version of the one's eyesight. top of the screen — this sometimes ASCII code with some of the "National A typical magazine has the capacity occurs even with the present line usage" characters substituted. Char- of 100 pages. Four lines are transmitted allocations. A further reason for avoid- acters can be made to flash on and off, per television frame and there are 24 ing lines 19/332, 20/333 is that they are though our own feeling is that this will lines per Teletext page. A full magazine currently used for insertion test signals, be used rather less when the service is would therefore take 600 frames or 24 which are often visible but, being static, finally in use than it is now, if it seconds to cycle. The BBC and IBA are not obtrusive. infuriates other people as it does the appear to differ on what they consider A form of binary code is used for the writer. acceptable access times; the BBC sug- data, known as non-return-to-zero, The Teletext editing teams can use gest a typical time of 12 seconds (for an which possesses the advantage over a the pages in three different ways, the unspecified number of pages) while a complemented-element code of a norm being single pages which appear recent check on Oracle gave an access reduced bandwidth requirement. As its when selected by the viewer at any time time of 28 seconds for well over 100 name suggests, the resulting waveform and which are up-dated perhaps once or pages. The theoretical time can be is not a continuous train of pulses, but twice a day. A second type is one of a reduced considerably by not transmit- rather a series of voltage levels. A 500 Wireless World, November 1975 typical sequence is shown in Fig. 2(b), serving odd parity, are not rejected and the beginning of the line are not where it is seen that a "1" level does not will be written. concerned with the displayed message. automatically return to zero at the end Address data bits, on the other hand, Three groups of bits are transmitted, of of the pulse and if the succeeding level is are more heavily protected by being which one is two bytes (16 bits) also 1, the voltage merely stays up — transmitted in a type of code which will designed to lock the receiver's clock in similarly with a succession of "0". One detect 2, 4 or 6 errors in the byte and will frequency and phase with the trans- result of this is seen in Fig. 2(c) where a detect and correct a single error. This mission, in a similar manner to the way train of the form 0 1 0 1 0 1 etc. turns out type of code was described by R. W. in which the colour burst dictates to be a square wave at half the Hamming in a classic paper9 in 1950, and phase in a colour receiver. The 16 bits repetition frequency of the clokc. As is known as the Hamming code. It takes are-termed the clock run-in and consist transmitted the pulses are "raised-co- the form of four parity bits in positions of a train of 1 0 1 0 Is. It is a penalty of sine" in form and the square wave 1, 3, 5 and 7 of the eight-bit byte (the the n.r.z. code used that this run-in is becomes a half-clock-frequency sinu- addresses are in a four-bit code). Three necessary, as it entails a code which is soid. of the parity checks are associated with not self-clocking. In other words, The diagram in Fig. 3 shows one line groups of three of the four message bits unless the data are of the 101010 of the complete picture frame. A and the overall parity check covers all variety, which conveys no information, complete line occupies 64µs, of which message and parity bits. Failure of the the transitions do not occur at every about 1212s are taken up with the overall parity indicates an error and the interval, and the data pulses them- synchronizing pulse, back porch with position of the error is identified by the selves could not be used as a clock colour burst and front porch. The first checks on the groups of three bits. If the source, if it were not for the choice of 40 of the 360 bits are concerned with overall parity appears true, but the odd parity for the protection code, clock synchronization (in a manner individual checks show that a correc- ensuring at least one transition per similar to that of the colour burst), a tion is required, there will have been a character, which can be used to refresh series of bits which constitute a "start" double error and the byte will be the clock generator. The 16-bit burst of sequence and 16 bits for control and row rejected as unusable. 3.5MHz is also detected and used to address information. The rest of the line Figs. 3 and 4 indicate that the bits at identify the transmission as Teletext contains 320 bits for display and control. and not some other data system such as Figure 4 shows the layout of informa- SLICE (IBA), ICE (BBC) or insertion tion in the line. test signals. The second group of 8 bits forms the Coding (a) framing code, which is always the same All transmission paths are subject to and is used to indicate the start of the errors, and television has its share. 0 0 1 1 0 1 1 1 first 8-bit word. The order of bits used is Noise can be a problem, but the type of 11100100 — an arrangement which is distortion caused by multipath propa- designed to avoid errors. A common way to detect the framing code is to gation is perhaps the deadliest source of (b) error. Both BBC and IBA have done pass all data through an 8-bit shift work on this in the U.K., Sweden and register and to examine the parallel Germany, and have come to the con- outputs of the register. There will be clusion that Teletext transmissions are only one step, when the register fairly robust, but require a good perfor- contains all eight bits of the framing mance in the receiver. This relative (c) code, when the above order of bits is invulnerability to attack is assisted by present at the outputs of the shift the application of code protection — Fig. 2. The non-return-to-zero method register and gives a positive additional bits of information of transmitting binary information is at comparison with the "permanent" transmitted with the data. (b) with the attendant clock at (a). A word used for detection. The framing Code protection is applied in two succession of 101010 etc. gives a code is shown in progress through the levels. Data bits which are intended for half-clock frequency waveform and, if shift register in Fig. 6, which indicates addressing and control are heavily the pulse shape is a raised cosine, as that, even in the presence of bits from protected, while those used to produce used in Teletext, the result is a sinusoid the clock run-in (c.r.i.) and succeeding the displayed characters are protected at 3.5 MHz. information, a maximum of 5 bits "look rather less. The transmitting authorities like" the framing code. By means of consider that the occasional error in, or more or less extensive circuitry, the Fig. 3. A line of rejection of, a character is not serious as Teletext data. Sync. framing code detector can be made to it will quite probably be corrected on pulses are the 30% pulses and the recognize the code in the presence of the succeeding transmission of the colour burst is on the back porch. one error. page, but that an error in an address would lead to complete nonsense and • 64µs must be avoided in the presence of the 100°/a average amount of noise and ghosting. 52µs The character code is basically 7-bit white ISO-7 and is protected quite simply by 800/0 ) 5 C

means of a parity bit, giving odd parity. ts

This means that if the total of "ls" in the 6 bi (1 ) basic 7 bits is odd, the parity bit is "0". ) ts ts An even total of "1"s in the 7 bits EL 8 bi 6 bi EV 1 dictates a "1" parity bit to maintain an ( data (320 bits) ( ddresses de

odd overall total. On examination at the AL L & a n-in

co

receiver, any byte (the name for a ls ng SIGN k ru tro

"word" of 8 bits) with an even number mi loc on c fra of "1"s is seen to contain one error and c ( c is rejected. Previously-correct words in 300/0 a display will not, therefore, be over- written by an incorrect one and the effect is a reduction in errors in succeeding pages. Double errors, pre- 0/0 hne 17 18 330 or 331

Wireless World, November 1975 501

sync addressing ISO-7 coded characters for display 0 sr:4 c cil ml e CRICRIFC CFIAG IsPI S Ispl I I e spl t I ImIeIsPIfloIrlsplal1[11 gl 01 01 dl sP .PI

ISO-7 coded characters

control and I clock framing clock row-address N 0 w sp 0 sp 0 m e run-in run-in code group

1 0 1. 0 1 0 1 0 1 CIMICIIIII:1110101111i1C101i1CICILICIOCII2CIGICIGICIGICIEJIZIGICICIIIIIII0E101111111111111:11111111r Il Hamming-code groups 150-7 coded characters 'P' = protection bit = message bit

Row addressing. The 16 bits following The detected vision signal is taken from Fig. 4. A row of data It is seen that the framing code are concerned with the television receiver, having been each group of 8 bits (1 byte) has a the identification of the following data. disconnected from the video amplifier, separate function. Clock run-in and Each of the 24 rows of characters must and taken to the decoder, where clock Control and Row address each have be identified and this is done by regeneration, framing-code detection, two groups. numbering each with a row number parity checking, serial-to-parallel con- from 0-23 and a magazine number (1 to version and page selection are carried 8). The bottom line of Fig. 4 shows the out. The selected and organized infor- output of the counter is a pulse at mation is then passed to a store (either a layout of bits in the 16-bit group and one-eighth the rate of the clock and is multitude of shift registers or a ran- also indicates the fact that these are used to identify correctly-framed words dom-access read/write memory) until protected by being Hamming-coded. of 8-bit words in the shift register. In the selected page is assembled. When As, at the moment, we are not other words, framing-code detection this has been done, the store addresses concerned with protection, the bits indicates a reference point and the the character generator in the output marked "P" can be ignored, and the counter produces a pulse every eighth processor (a read-only memory con- groups become 4-bit words. In fact, this clock period which transfers the group taining the ISO-7 characters) which is an artificial distinction, because they of eight bits currently in the shift indirectly drives the guns of the display form one word of three bits and one of register, through latches, to the data five. The first group of three message tube. lines. The output if the latch only bits (marked "M") identifies the changes when the counter indicates magazine, and the remaining five Input processor. This preliminary that the eight bits being presented to it contain the row address in a pure canter through the decoder is, of course, are, in fact, a word. Complete char- binary code (00001 is Row 1, 10111 is grossly over-simplified and a closer look acters are therefore presented on the Row 23). The least significant bit is at the sections is necessary, the first of eight, parallel data lines in serial form, transmitted first. these being the video processor which reducing the 6.9375MHz rate to 867kHz. Data. The rest of the line is filled with operates on the video signal to derive The parallel data presented to the data for display, the previous bits not, data and clock pulses. As has been said, latches are also examined by the of course, being visible on the screen. the n.r.z. code in which data are Hamming-code checker which, as was Fig. 5 shows the complete set (in use transmitted, is not a self-clocking code seen, can correct one error and detect 2, until Sept. 1, 1975) of available and the decoder must contain its own 4 or 6 errors in address and time characters and blocks for graphic clock. The clock run-in contained in the information. Following this block, an displays, together with the address Teletext signal can operate as either a 8-bit latch finds and holds the 3-bit code (bits 1-7). Bit 8 is the parity bit and locking sequence for an oscillator which magazine address and 5-bit row plays no part in the display. A slightly is continuously running, but which address, referred to in Fig. 4, after a modified table will be used exclusively drifts out of phase with the Teletext two-character holding time, inserted to after September, 1976, and the system is clock between lines, or can be used to allow the two four-bit words (or one now in transition between the two. excite a passive oscillatory circuit three-bit word and a five-bit word) to be which then rings for at least the assembled. Decoding duration of the Teletext line, being Parity checks on character words are The circuitry for decoding Teletext automatically in phase and frequency performed by the parity block, operat- transmissions is extra to that in an synchronization with the signal when ing from the main data line, its output ordinary receiver and is mainly digital the LC circuit is properly tuned. Each determining the acceptability or other- in nature. When the services are estab- transition of the data "refreshes" the wise of a word. lished and commercial receivers are tuned circuit. The rest of the input processor is sold (several firms already have The diagrammatic input processor of concerned with the selection of a page models), the decoders will be built in, Fig. 7 shows that the data is first passed by the viewer, who will be provided but before that happens, many viewers through a serial-in, parallel-out store — with three thumbwheel switches or a will want to convert their existing the shift register, which is clocked at keyboard with ten number keys and receivers, an add-on unit being the data rate. The register is 8 bits long and keys for several other functions. A page obvious solution, with video signals into can contain one word, complete with its having been "dialled in", the row and out of the decoder. U.h.f. input and parity bit (4 parity bits in the case of address latch block examines the row output will probably not be used, the Hamming-protected words). The 8 bits, address words until row zero is detected cost of a colour modulator, for instance, in parallel, are examined by the fram- — the page header — when the selector being prohibitive. ing-code detector for coincidence with will examine the following address The extra circuitry needed can be the 11100100 "start" sequence which, information to compare it with the considered in three groups; data acquis- when detected, resets a required page keyed in by the viewer. ition, storage and output processing. "divide-by-eight" counter, to zero. The On detection of the required page code, 502 Wireless World, November 19'

b7 0 1 0 1 0 1 1 1 Bits In --.. 0 0 0 0 1 0 1 1 b 5 --.. 0 1 0 1 0 1 0 1 C91r4s. 4 b3 b2 b1 0 1 2 3 4 5 6 7 b4 4 4 4 Rovq, ©I I pc!. SP 1 EE () ', re @©1 @1 p I p 0 P 0 0 0 0 0 I 1.1.1 Graphics i = L 0 0 0 1 1 Red 1 nse I A A C) Q Q ® a 111 q Graphics 1 ® 0 0 1 " 1 Soo 8 (D R1 n.r1= 0 2 Green min 2 ,I lainw B R :-. Graphic 0 0 1 1 3 lis E II 3 , S , zs rim r c c so, c — s T imm Graphics 0 1 0 0 4 Blue $ It'll 4 1 D I D T T ® d % t iiiiiii 1 I 0 1 0 1 5 I GraphifAsagenta °/c1 5 U E E' U U T e u L Graphics F 011 0 6 , Cyan & I el-1 6 e F ® V V l f 14 v ' l: Graphics 0 1 1 1 7 White ' i r 7 2 G G ® W W C-) g,11: w C 1 0 0 0 8 ( 8 H 1 H 1 X X ® hIDI x'

Alpha" 1 0 0 1 g Red ) 7iii 9 I I ® Y Y ® i I AI y Alpha" 1 0 1 0 10 Green ) I L : irA J i J C) Z Z" , 1 z j Alpha,' 1 0 1 1 11 Yellow + I Ls ; 7 K I K 1 ( /I $ k 2 l' 3 Alpha" ® Flash 01 1 1 0 0 12 Blue v r L L \ , \ ® I I M, a Alpha" ow 1 1 0 1 13 Steady Magenta - !, = P M M ° I C')I , II mIli 1 1 we we _, End Box Alpha" 1 1 1 0 14 Cyan • iimm > ow N N G I. t I1A I n i HI i ® ® i 111115 Start Box Alpha" White / woe '') ow 0 0 ci 1 .. DEL

control characters (columns 1 and 2) bi b2 to be displayed as spaces b 3 b4

Notes b5 b7 CD This character code (position 0/3) is reserved for internal use by broadcasters UK version of national use character in the IS0•7 code Graphics display In the graphics mode when bit 6 = 0 the corresponding alphanumencs mode rectangle showing character should be displayed character in- character the allocation of bit All character rows start in the 'Steady', 'Alphanumeric White' and 'unboxed. alphanumerics in graphics numbers to the condition, without control characters mode mode individual cells

Fig. 5. Characters possible in a Teletext array of semiconductor devices, often manufacture to perform a variety of display. Graphics are not bistable circuits, set to "1" or "0" by functions, but the one in a Teletext in the ROM, being produced directly by input signals and which can be interro- decoder is a character generator, the data bits. gated non-destructively when required arranged to contain the characters by examining any desired location in shown in Fig. 5. Bits 1-7 in the config- the memory. No "order" is entailed: urations shown on the left, control the data can be lodged at any part of the selection of display at the output. Bits memory. 1-4 determine which row of character the data which follows is written into Data are stored in 7-bit code as bits should be read out, while bits 5-7 the store, assuming that the parity received from the 7-bit latch driving the indicate the column. For example, if the checker is in agreement. data line in Fig. 7. seven bits from the store were 0011010, The outputs of the input processor bits 7, 6 and 5 (001) indicate that one of are taken to the store and are (a) the Display. The display of characters the undisplayed control characters in 7-bit data (b) the "write" or "reject" depends on the use of a large-scale column 1 should be generated and bits 4, command to the store (c) the row integrated circuit — another form of 3, 2 and 1 show that ROW 10 — alpha- address and (d) the character address. store called a read-only memory. This is numerics, green — is required. The use again an array of memory cells, but this of this invisible control character means Store. Storage can take many forms, but time the pattern of bits read out of a that the visible, succeeding character the most convenient way to store the given address in the memory is not shall be a green alphanumeric one, as data while the page is assembled is the under the control of the user. The form opposed to the graphics alternative in random-access memory, which is an of the data is decided at the time of columns 2, 3, 6 and 7. The next group of

Vireless World, November 1975 503

Movement of data stream Nn Data in shift register corresponding bits could be 1101001, in which case the hits result would be a green, lower-case "i" 0 0 0 o 0 011 0 1 0 1 0 1 0 1 0 1 on the screen. A control character is not 0000011 01 01 01 0 101 0 4 displayed, so that whenever the mode is 000011 0 10 10 10 101 01 4 changed — from alphanumerics to 0 0 011 0 1 0 1 0 1 0 1 0 1 0 1 0 4 0 011 0 1 0 1 0 1 0 1 0 1 0 1 0 1 4 graphics or from picture to insert or Clock run-in 011 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 4 from white to red — a space must moving through 11 0 1 0 1 0 1 0 1 0 1 0 1 0 1 01E1. 4 appear. shift register 0 1 0 1 0 1 0 1 0 1 0 1 0 1 01 Ifiiiiii There is one exception to this, and the Framing code 1 0 1 0 1 0 1 0 1 0 1 0 1 01C:Eli: 3 moving space can be avoided when an alphanu- into 0 1 0 1 0 1 0 1 0 1 0 1 0111M:4A 3 shift meric character is needed close to register 1 0 1 0 1 0 1 0 1 0 1 01 tiiiiiiiiiiiEWO: 4 graphics, with no control character 0 1 0 1 0 1 0 1 0 1 01M::i1:;iwow!1 space. Although the graphics char- 1 o 1 0 1 0 1 0 1 o 1 Ctim:0:.:0;:iU0.*: 3 Framrtg code acters are shown in Fig. 5, it is not shift register 0 1 0 1 0 1 0 1 0 ivimmoi:i:li::0::vi 8 meant to imply that they are contained 1 0 1 0 1 0 1 0111::1 :1iiii0iMilliiiIitil x 4+X in the ROM. Only the alphanumeric 0 1 0 1 0 1 011UCI:V.:i:MI:]:0:Aiix X 2+2X characters are held in the memory, the 1 0 1 0 1 011iii:w4i OiiiMiliiii)Ox x x 3 +3X graphics being generated directly by the 0 1 0 1 01[1::::::1:::::1::4 a:44%0 x x x 2 +4X 7-bit code as received, as shown in Fig. 1 0 1 0 [tiiilMitiOi:MOilx Details x 1 +5X 5. The area occupied by a character and 0 1 0 1 ItiliiiCiall:iiiiiViVil x i',,,T=ex 0+6X 1 01tEl::ilmiwito alx x data x 0+7X its surrounding space can be separated Ett:Mmtmvxm xxxxxxxx 0+8X into six "cells" which each have a bit of Test word for comparison :4:1::4::11:ie:Mr.ti data allocated to them. All codes with data in shift register intended for display as graphics have a 1 as bit 6. However, if, while in the 6-counter produces an output at each Fig. 6. The framing code in progress graphics mode, a code occurs with a 0 as character. These two outputs are used through the shift register, and in bit 6, then those characters in columns 4 to serialize the outputs of the ROM, and position on the 10th row down. At no and 5 of Fig. 5 will be displayed — a a 40-counter with a 6-line output is time other than at coincidence are limited selection of alphanumerics, applied to the store, instructing it to more than 5 bits in the register in termed "blast-through" alphanumerics. provide successive character codes to correspondence with the reference Fig. 8 shows the output processor, the ROM which generates the pattern word 11100100. complete with character generator and for each new character. timing circuits, synchronized by line Line sync is also applied to a and field sync pulses. Characters are "divide-by-10" counter, which ensures for inserts and a "flashing" instruction. built up from dots at the 7MHz clock that the store addresses the ROM for 10 Graphics are produced, under command frequency and a dot clock is therefore lines in each character, giving 7 scan- from the controls decoder, by the required. The number of elements or ning lines for the character and 3 graphics generator on the principle dots in the width of a character is six (5 scanning lines space between char- previously mentioned and illustrated in in the character and a space) and a acters. A 24-counter addresses the 24 Fig. 5. rows of data in the store. Finally, the interpolator can be used A further block in the display cir- to obtain a slightly improved appear- cuitry is the controls decoder which ance to characters which include a recognizes the control characters in diagonal (Y, Z, K, etc.). The principle is Fig. 7. The input processor block columns 0 and 1 of Fig. 6, to produce to take advantage of the fact that diagram. RGB drives, blanking to provide a box interlacing effectively provides 14 lines

D. line sync sync ) f ield sync 0 separator ► odd/even p. data

data video 8- bit shift register parity check processor latch

clock reject/write \ e

Hamming framing-code 8-bit protection latch detector check 4 row address

IV start to , 2-character ram hold store

clock character :8 +4;",' \8\ ► address

row address

selector clqck + 8 3 magazine number means

etc thumbwhee switches or push-buttons

504 Wireless World, November 197}

in a character, not 7, and to "fill in" the steps produced by the 7 stored lines. An graphics odd/even field command can be derived and used to synchronize the interpola- tor, which is also used to make the odd /even flashing signal effective.

That is the principle of the decoder, character very briefly. In a short article, it is not generator interpolator ► • possible to cover all aspects and it was (Rom) not the intention. Forthcoming articles Flashing will describe the circuit in detail and t t A B graph/ provide complete information on the alpha construction of a decoder that will differ from this general picture in several respects. Many savings in cost have controls G been found possible by circuit changes decoder which have also made possible a unit which is much smaller than envisaged. lloblanking box (To be continued) character References address to 1. Gassman, G.-G. "Twelve Sound Channels ram character 6 during the Vertical Sync Interval of the row address (B) Television Signal", ITT Chicago Spring Con- ference, June 1970. 0 ) .1 4-0 2. Ball, J. D & Wells, D. R. "Encoded field sync captioning of television for the deaf". International Television Symposium, Mon- start 1' treux, 1973. line field line 3. Guillermin, J , Dumont, J & Guinet, Y. sync sync sync "Transmission of remote control and identi- fication signals within audio and video Fig. 8. A typical output processor. Fig. 9 An example of the use of channels in radio and television broadcast- graphics. ing", I.E.E. Conference Publication No. 25, November, 1966. 4 Numaguchi, Yasutaka. "Still Picture Broadcasting System", NHK Laboratories Note No. 174, January 1974. 5. Houghton, W. D. "Homefax — a consumer information system", RCA Engineer, Vol. 16, No. 5, Feb /Mar., 1971 6. King, Patrick T. "A Novel Television Add-On Data Communication System", Journal of the S.M.P T E., Vol. 83, January 1974. 7. Hutt, P R. "A system of Data Transmission in the Vertical Interval of the Television Signal. IBC 1972 Conference Publication No. 88, pp. 131-140. 8."Specification of standards for information transmission by digitally-coded signals in the field-blanking interval of 625-line television systems". Published jointly by BBC, IBA and BREMA. October 1974. 9. Hamming, R. W. 'Error Detecting and Error Correcting Codes," Bell System Tech- nical Journal, Vol. XXVI, 2 April, 1950.

Fig. 10. Extracts from an Oracle index oo oP OP 00 0040*** r r 0 os 0 of a rr 40O 4.10 or a. OP 0 off Of Of' Of, 00 0 Of 0000 • page, showing the normal appearance .00$00410 W Of oo of Or Of 00 AO r1111 rr 0 . at (top) and the effect of character a. 4100 a. Mr 40 of or of 4 0 0f oo 0 0 of of ft0 40f. I0 0 0 0 * rounding (bottom). This is an IBA photograph taken from the screen of a studio monitor. On a domestic receiver, the effect is not as pronounced.

400 SHOPP I N • 1 15 500 HOME CE 350 400 SHOPPING . 115 500 HOME ADVICE • 350 Wireless World, December 1975 563 Wireless World Teletext decoder

2—The decoder system

by J. F. Daniels*

This article describes the facilities offered by the Wireless World decoder and also covers, in general terms, the methods of installation in a commercial colour receiver. The problems likely to be encountered with such a project are also discussed.

When contemplating the design of a left to the individual constructor. Operation project as complex as this one, there are Details of how the unit may be con- The decoder can be built into a box many factors which have to be consi- nected into various types of commercial measuring about 8.5 x 10.5 x 2in, which dered. For instance, to build a single colour receiver will, however, be fully is a convenient size to rest on top of a Teletext decoder with cost and size covered and this should leave only normal domestic TV receiver. The virtually no object and expensive test problems of a mechanical nature to the power supply is not included in this box equipment available is comparatively individual. for a number of reasons, some electrical, easy, but this is of little interest to the The cost of the decoder will be in the but mainly to keep down the size and home constructor. What is needed is region of £85, and although this may heat dissipation in the decoder unit. something which can be built relatively seem a great deal of money to pay, Space can usually be found in the cheaply, can be mounted in a small, people who have seen the resulting cabinet of most domestic TV receivers attractive cabinet, and can be installed display of pages on the TV screen agree to take the decoder power supply. and made to work with only the that the service is well worth while and The front panel of the decoder carries minimum of adjustments, preferably has great potential for the future. The two sets of thumbwheel switches, and requiring only a cheap multimeter. Wireless World decoder will be capable various other function switches. In the This design will fulfil these require- of utilising most of the features cur- latest version, the function switches ments. This does assume, however, that rently offered by the system, including take the form of a row of pushbuttons as the unit is constructed without any display in six colours and white, al- shown in Fig. 1. The bank of three wiring errors and with no faulty com- phanumeric characters, graphic char- thumbwheel switches are for magazine ponents — in a unit using around 85 i.cs acters, and flashing display. Two circuit and page number selection, the one on and their interconnexions, there is some options will be described; one which the left being for magazine number; the room for error! includes both upper and lower case other two for page number tens and Not to be too discouraging at such an characters, and another, slightly units. The bank of four thumbwheel early stage, however, it should be simpler circuit, with upper case char- switches are for the selection of timed pointed out that printed circuit boards acters only — a worthwhile option for pages. which may only be transmitted will be made available, from normal cost conscious contructors. The circuit for a one-minute period during each sources, which should eliminate most does not include any form of interpola- day, and therefore require selection by wiring error problems. Further, digital tion (character rounding) because it means of time code and storing, for i.cs tend to be very reliable, in my was thought that the extra cost of about viewing later. The switches can be set to experience anyway, as long as they are £15-20 was not justified in a discrete- any given time during a 24-hour period, not obtained from one of the sources of component decoder of this type. and in this mode of operation a page will unmarked, untested devices. The use of Before going on to describe some only be written into the store at the time such i.cs in this project must be strongly problems, which can be encountered shown on the thumbwheel switches. It discouraged, as even if they appear when dealing with commercial TV should be pointed out here that at the satisfactory on a d.c. test, they may well receivers, it is necessary to describe in time of writing, no pages are being be out of tolerance on delay time or more detail the performance of the transmitted in this manner, although fan-out, which could have disastrous Teletext decoder. the operation of the circuitry can easily effects in some parts of the circuit, be checked, because all pages carry time where correct delay time through i.cs is coding information. However, a cost an important factor. Fig. I. Suggested front panel layout of saving of the order of £6 could be made For the constructor who has access to the Wireless World Teletext decoder. by omitting this facility. an oscilloscope, waveform diagrams will be given at various points in the circuit to help those wishing fully to understand the circuit operation. It is not intended, in this series of articles, to give full constructional details, and the choice of suitable box, CLEAR T V TELE SUB NEWS TIME TEXT TITLE FLASH SELECT and method of mounting p.c.bs etc. is

'Designer of the Teletext decoder 564 Wireless World, December 1975

The row of pushbutton switches mitted, the displayed subtitles will interlocked with the other buttons so mainly controls the form of display on automatically change as they are trans- that time-selected pages can be written the TV screen. The four in the cents e are mitted. This may be a very useful into the store while watching a TV all interlocked, latching pushbuttons, facility for the future, as subtitles take programme — possibly for later reading the one of the left is an individually- up very little transmission time in the during the commercials! The time operating, momentary-action type and Teletext waveform, consisting of only a selection facility is not operative when the right-hand one is individually few rows of information. However, at subtitle or newsflash buttons are latching. The "TV" button merely the present time they are only trans- selected. selects the picture on the screen in the mitted in test form. No facility is provided for superim- normal manner, although the decoder The operation of the "newsflash" posing the complete Teletext display on will still be operative and can store button is somewhat similar to the the picture, as in the author's opinion pages in the usual way, ready for instant subtitle button, but with an added this gives a meaningless display which viewing when the "Teletext" button is facility. After selecting the newsflash makes both the picture and the Teletext pushed. The latter merely replaces the page number on the thumbwheel display difficult to interpret. picture with the video output of the switches, the current newsflash — These, then, are the basic facilities decoder and, in this mode, all the which may have first been transmitted offered by the Wireless World decoder. normal features of Teletext display are some time ago — is displayed in a box in Without doubt, as the Teletext system available. the TV picture in the normal manner. If, progresses, more facilities will be The page header contains a contin- however, the clear button is then offered by the service, and it should not uously changing time indication in the pushed, the picture returns to normal, be difficult to add extra facilities to the top right-hand corner, but a fixed page and no data are then displayed until a decoder as required. number display — the number of the new newsflash is transmitted, whereu- page selected. When a different page is pon this is displayed in the usual way in Installation required on the display, the momen- its box. If the current newsflash is There is really only one satisfactory tary-action, left-hand button marked required to be seen again, after pushing way to connect the decoder to a "clear" is pushed. This clears all the the clear button, the newsflash button is domestic colour receiver if all the information from the display except for simply released and reselected. facilities described earlier are required, the page header row, which then starts The next button, marked "time" and this is shown in Fig. 2. It can be seen "rotating" i.e., reading out all the page brings the time-select thumbwheel from the diagram that there are only headers as they are transmitted until switches into operation, when the four points of connexion into the set: a the new page number selected is selected page will only be written into feed of composite video from the output reached, whereupon the new page is the decoder store during the one-min- of the receiver if. strip, and feeds of red, read cut into the screen. ute period displayed on the thumbwheel green and blue (or possible R-Y, G-Y The next button is marked "subtitle" switches. This page will then be held in and B-Y) to and from the inputs to the and is used to select the "insert" mode the store until either the clear button is receiver video amplifiers. It is possible of operation. When this button is depressed or a different mode of opera- that a fifth connexion, from the set's selected, the TV picture is displayed on tion is selected. This button is not flywheel oscillator, will be required if the screen until the subtitle page, the the set is in use in a low signal area and number of which has been selected on displays a noisy picture, as this can be the thumbwheel switches, is detected, used to remove horizontal jitter on the when the subtitle message will be read Fig. 2. Suggested method of connexion Teletext display caused by the noise on out in a box inserted in the picture. If a into a domestic colour receiver, using the video signal. However, this possibili- new subtitle, or indeed a continuous an interface board containing three ty will be considered later during the stream of different subtitles is trans- simple electronic video switches. circuit description. The interface board is a small video switch unit, mounted inside the receiver, fairly close to the video amplifiers, and serves to switch elec- tronically between the picture and the Teletext display, when commanded by either the function switches, or by COLOUR TV RECEIVER "hole-cutting information from the H tuner to c r t decoder. The design of this unit will R G B vary slightly, depending on the type of LLE receiver used, some sets having, R, G t and B feeds to the video amplifiers and others using colour difference signals p R (R-Y, G-Y and B-Y). If the facility of colour video putting newsflashes and subtitles in decoder r. G amps f strap boxes is not required, then this unit B could probably be replaced by a three pole change-over relay, controlled solely by the function switches. This, then, is the only practical way in which a decoder can be installed into an video input existing TV set, if a coloured display is required and this is the only method electronic video that will be described in detail in this change-over switches series of articles. However, for those who rent a colour set, there is another, outputs somewhat less attractive possibility, shown in Fig. 3. Here, a separate tuner TELETEXT and i.f. strip are used to provide video DECODER INTERFACE UNIT C>," BOARD for the Teletext decoder. The R, G and B outputs of the decoder are then ma-

Svvi ch control trixed together, and fed to a u.h.f. 565 Wireless World, December 1975 modulator. This in turn feeds the aerial 0000 socket of the receiver, which is tuned in to the modulator on an unused channel. This will, of course, only give a mono- aerial chrome display, but would at least socket have different shades of grey to repre- sent different transmitted colours. tuner It is not practical to modulate the decoder display into PAL colour form, partly because of the high cost of a colour coder, but mainly because the R results woule be unsatisfactory due to TELETEXT DECODER video u h f the fact that the bandwidth of the PAL video system would be insufficient to cope f strip input UNIT input modulator B with the Teletext display waveform. - ve mixed --i\A/\/--• sync Data signal 'Before starting a description of the decoder block diagram, there are two more important points to be made to Fig. 3. Alternative arrangements for unit has been kept down by splitting up prospective constructors. Firstly, there rented television sets. This has the the boards in this way. is the question of obtaining a suitably disadvantage that only a black and The large boards measure 91/2 x 51/2in„ undistorted data signal from the TV white display will be obtained. and are arranged to mount one above receiver. the other, spaced about '/:in apart. The Distortion of the data waveform can analogue board measures 51/2 x 3in and be caused in a number of ways; poor is spaced 1/2in above the digital boards. bandwidth or non-linear phase response former in the mains supply to the TV This gives an overall size for the in the receiver i.f. strip; reflections receiver, and I would strongly decoder electronics of about 91/2 x 51/2 x (ghosting) on the picture, caused either recommend this course of action for 11/2in. The digital boards, which each by external multipath interference or anyone who does not regularly work hold about 40 i.c.s, are double sided, but' aerial mismatching; co-channel inter- with live equipment. If, however. the for cheapness do not have plated- ference; and finally noise. All these can constructor feels absolutely confident through holes. The "plating through" cause errors to be made in the data that he can carry out the installation process is carried out by the construc- display and, in extreme cases, prevent without electrocuting himself, then tor, using tinned copper wire soldered operation of the decoder at all. there are two important points to note. on both sides of the board. Generally speaking, however, The first is to ensure that the receiver This simplified block diagram in Fig. 4 satisfactory results can be expected chassis is connected to the neutral side shows the main functions contained in from the majority of colour sets dis- of the mains and not the live — this the decoder, only the main data paths playing a ghost-free picture. Noise on should be a simple matter of connecting being shown for simplicity. The heart of the picture, unless of sufficient ampli- the plug the correct way round but it the circuit is contained in the clock and tude to be objectionable, is unlikely to must be checked with a multimeter. The line divider blocks, and there are many be a problem, as the decoder employs second is to make sure that the decoder waveforms from these sources which circuits capable of detection and cor- cabinet (if made of metal) or any are distributed to the rest of the circuit rection of errors caused by noise spikes. metallic parts on it such as switches, blocks. This initial description is only Secondly, the performance of the etc. are not connected to the decoder intended as a guide to circuit operation, decoder in the presence of interference electrical earth. so that an overall picture can be in various forms is determined almost A three core mains lead must be used, obtained, before starting a detailed solely by the performance of the front with the earth connexion taken to the description of each circuit block. end, i.e., the circuitry which separates decoder cabinet, if this is made of metal. The function of the analogue board is the data from the video waveform, and Probably the best solution, though, is to to take the composite wide-band video converts it into t.t.l.-compatible form. It use a wooden cabinet and ensure that signal from the receiver i.f. strip, and is proposed to describe first a fairly the thumbwheel switches and pushbut- produce from it t.t.l.-compatible mixed simple data separator, which is extre- ton switches are suitably insulated from syncs, data, and clock waveforms. The mely easy to set up and which will be their electrical contacts. The earth single clock line includes the outputs of adequate under good reception condi- connexion should only be made after two clock generators, one derived from tions. This will enable the rest of the the decoder has been tested and set up, the incoming data, and another free- digital circuitry to be tested and set up. as it could create a hazard while running oscillator used during the In a later article a more complex form of actually working on the decoder. Of display time. Switching between the data separator will be described which course, after testing is finished, when oscillators is achieved by using a will give an improved performance the earth is connected, protection is waveform from the line divider circuits, under adverse signal conditions alth- ensured against the decoder box which switches from the display ough it will be rather more difficult to becoming live due to faulty insulation. oscillator to the "data locked" oscillator set up initially. during part of the field blanking interval Construction (between lines 10 and 20). The free-run- Safety Prototype decoders were constructed ning oscillator has a preset frequency The most important problem of all is on 12 x 7in pieces of ordinary Vero- adjustment which controls the width of one of safety. If the decoder is to be board Olin matrix sheets. There is no the Teletext display, and is also trig- installed in the manner to be described reason why this method of construction gered by a line blanking wavefrom to rather than by using a u.h.f. modulator, should not be used, apart from the fact ensure that it starts up in the same as mentioned earlier, then a direct that it is very laborious, and wiring phase at the start of each television line. connexion must be made to the receiver errors can easily be made. Clock and data waveforms from the chassis, which could under some cir- For those who have less time to spare, analogue board are fed to the serial-to- cumstances be live. printed circuits will be available in the parallel converter, which in turn feeds There is only one way to prevent the form of two large p.c.bs for the digital the data latches and the framing-code decoder itself from becoming live, and circuitry, and a smaller p.c.b. for the detector. The output of the framing that is to use a mains isolating trans- analogue circuits. The overall size of the code detector is used to reset the clock

566 Wireless World, December 1975 dividers, and a =8 clock waveform is in dress output of the clock dividers, to the The data store consists of seven turn used to operate the data latches. code convertor circuit. ("Column ad- 1024-bit random-access memories, ad- It should be explained at this point dress" refers to the 40 vertical character dressed in parallel — one for each of the that the clock and line dividers perform columns and "row address" to the 24 seven bits of data. The other input to the the dual role of data aquisition and data horizontal character rows.) store is the read/write input. This input display dividers, and this constitutes The divider circuits are both arranged is normally in the read condition, when quite a saving in circuit components. so that the data on these eleven wires is data already in the store is read out onto Bits 1-7 from the data latches are fed correct during both data aquisition and the screen, but changes to the write straight to the inputs of the data store, data display, and this obviates the condition during Teletext data lines 17 while all eight bits are fed to the parity necessity of complicated switching in and 18, when instructed to do so by the checker and Hamming-code corrector. the address inputs to the store, The code read/write control logic. The output of the Hamming corrector convertor is required for the following The seven-bit output of the data store consists of bits 2, 4, 6 and 8, suitably reason. the 1024-bit random-access is fed in parallel to three circuit blocks, corrected in the case of a single error, memories are arranged in a 32 x 32 as shown. Alphanumeric characters and and also an output which indicates an matrix which can, of course, be graphic characters are generated for even number of errors. If an even error addressed in any of its store positions by each of the 960 display positions on the is detected during a row address group, a 10-bit address input. Our display screen. The control codes decoder then the even error output of the matrix, though, is arranged in a 40 x 24 ,decides which will actually be dis- Hamming corrector is used to inhibit pattern as previously described, and this played, what colour it should be, and any data from being written into the requires an 11-bit (6 + 5) code to whether or not it ought to be flashing or store on this row. address each individual position. How- boxed. It does this by suitable switching Bits 2, 4, 6 and 8 from the Hamming ever, there are -many unused positions in the output control unit, which also corrector are fed to the row and page which can be addressed by the 11-bit blanks control characters. recognition circuitry, and also to the code and by a suitable rearrangement of This, then, is a necessarily brief line divider circuits. The line divider the addresses, the 11-bit code can be introduction to the Wireless World. circuits count line syncs during the reduced to 10 bits, without actually Teletext decoder. In the following display period, but when data lines are losing any of the 40 x 24 matrix articles, detailed descriptions of each of detected during field blanking, the positions. A simple calculation showing the circuit blocks will be given, with counters are preset to the correct row that 40 multiplied by 24 comes to less waveform diagrams and explanations number, indicated by the Hamming- than 1024 indicates this possibility. where these are relevant. Finally, corrected bits. The five-bit row-address circuits will be given for various types of output of the line dividers is fed, Fig. 4. Teletext decoder simplified block "interface" board. together with the six-bit column-ad- schematic. (To be continued)

line blanking

video input sync mixed syncs frame synoc separator separati n reset data display hOriz L data Shift w Separator reset line divider 4 2,4,6,8 (during display circuits 4 period ) counter lines preset column 10-20 inputs data address waveform clock clock divider 11 bit to 10 bi code circuits 6 converter —411-7/6 row display display address width 'clock

switch reset COntrOlA clock w latch A waveform lines 10-20 framing wave orm code detector line blanking waveform A data control serial o data data store parallel data 7x1024 bit codes 8 latches decoder converter rams

le read/write w Switch data input control

!lamming parity code cheCker alphanum R corrector character generation G 2,4,6,8 W write allow Output control B Switching row zero flashing detection and blanking magazine read/write 1 0 0 and page control detection logic graphic shapes time code generation 9 2 8 detection

"cut-hole" information Wireless World, January 1976 37 Wireless World Teletext decoder

3 — Line and clock dividers by J. F. Daniels

Last month we looked briefly at the displayed as characters on the screen. discriminate between data acquisition overall operation of the decoder by Other outputs are also obtained from and data display: i e., data is only considering a simplified block schema- this part of the circuit, namely separat- "looked for" during the periods just tic diagram. I also mentioned that the' ed field syncs, line address information mentioned. If all 625 lines' were exa- heart of the circuit lay in the design and for the character generater read-only mined for data, false framing codes operation of the clock and line divider memory, the vertical component of the would be detected at random during the circuits. This month we shall look in display blanking waveform and a field video information, and cause lines of greater detail at the operation of these blanking waveform which goes to "1" "rubbish" to be read into the store. It two circuit blocks, as these together during the period from lines 11 to 21 and would of course be possible to look for provide most of the waveforms used 324 to 334. This latter waveform is used data only on lines 17-18 and 330-331, but elsewhere in the decoder circuitry. We extensively elsewhere in the decoder to it is possible that broadcasters may shall begin by looking at the operation change the positions, or increase the of the line dividers. number, of data lines transmitted, and The main function of this part of the Fig. 1. The line divider circuitry. IC3 is a so the above method was adopted to circuit is to provide row-address infor- 74121 monostable, IC5 and /C14 are 7490 avoid later modification in this respect. mation for the store during the opera- decimal counters, IC„ is a 7474 dual It may appear, from a quick glance at tion of writing data into the store and D-type edge-triggered flip-flop and IC20 the circuits, that the i.cs are numbered when reading it; i.e., when the data is is a 741771 74197 binary counter. somewhat erratically, but this is

data allow neg field strobe from WFM syncs IC47 pin 3 IC9 pin A data allow WFM rrom IC9 pin 6

strobe from IC47 pin 2 6 IC4 IC15( IC7 12 neg mixed syncs IC 7 51 110 13 12 3 IC13 IC15

11 IC4. 12 5 13 I IC6 ) L_ sz:56 inputs from c, C2 Hamming corrector 10n TT x1On bit Nos. IC6

0 0 6 0 0 IC12 CLK PR Rot Rog Rgii Rot R02 9 A A 10 IC5 ICia C/CLR IC 11 IC20 ioad0— 11 OCLR IB 0 B IC15 C6 CLK2 0A QA QB QC °A QB Qc in D Q Q GB QC QD 0 0 0 0 0 0 0

11 9 3 IC13 10 R3 IC6 470 IC4 IC12

10

IC13

y y V Imes 11-21 display WFM field row address for blanking WFM row address outputs character generation for data store

38 Wireless World, January 1976

line numbers

622 623 624 625 2 3 4 5 6 7

neg (a) mixed syncs

0 output (b) IC 3

(c) field sync pulses from IC 4 pin 13

because they are designated according Fig. 2. Waveforms from Fig. 1, showing are used for the display of alphanumeric to their position on the p.c. boards — the detection of broad pulses. characters, and gates (6,10) and (4,10) IC1.40 being on the uppermost board and serve to inhibit character generation during three of the ten lines. These IC41_50 on the lower circuit. During circuit descriptions, multigate i.cs will gates are not required when the upper be referred to by their IC number, and removes spikes which are caused by the and lower case version is built, as the output pin number. For example (7, 12.) delay through IC3. The method by blanking of unused character lines is indicates the inverter gate for field sync which the broad-pulse separation is achieved in a later part of the decoder. pulses in Fig. 1. achieved is more clearly shown by Fig. The operation of the rest of the circuit 2. waveforms (a), (b) and (c). is best understood by referring to Fig.3, The broad pulses are used to reset the which shows some of the more impor- Line dividers first part of the line-divider chain, which tant waveforms through the circuit. The Negative-going mixed syncs from the consists of two i.cs forming a divide-by-8 counter IC14 is fed with one analogue board trigger monostable 3, divide-by-80 circuit. The first i.c., a pulse every 10 lines from IC5 and its which is adjustable by means of a preset divide-by-10 counter, provides line- outputs QB and Qc are gated in (13, 8) to potentiometer. Elswhere in the circuit address information for the character provide a start pulse for the display the variable width of the output pulse is generater r.o.m. and sets the height of period at line 51 (also 364). C4 removes used to shift the display horizontally, each "character box" at 10 lines per any spikes caused by timing differences but for this part of the circuit the output field. between the QB and Qc outputs of the pulse width is not critical, so long as the In the "upper case only" version of counter, and C5, R1 and R2 shorten the pulse is wider than the input sync pulse. the decoder, only seven of these lines pulse into a narrow negative spike to set By gating together the input and output the vertical display-blanking flip-flop, waveforms of this i.c. in a NOR gate formed from gates (15, 3) and (15, 6). (4,13), the broad pulses during the field The positive transition at the output of blanking interval are separated from gate (15, 3) is coupled via C6 and R3 into the mixed sync waveform. Capacitor C, Fig. 3. Waveforms in the circuit of Fig. 1. gates (12, 8) and (6, 4), which has the

character display rows

0 2 3 4 5 6 7 8 9 110 11 12 13 14 15 16 17 18 19 20 21 22 23

field pulses

OD IC5 ILIULJULI OA IC14

OB IC14

QC IC14 L 1C13 pin 8

display period start pulse IC 15 pin 3 display field blanking

reset pulse

0 output IC11

CA IC 20 5 bit OB IC 20 row address QC IC 20

OD i c. 20

1C 13 pin 3 display period finish pulse

IC12 pin 6 lines 11-21 WFM 111 Wireless World, January 1976 39

Hamming coded groups 40 display characters

150-7 clock clock framing magazine 5 bit row coded character run-in run-in code number address number r_ PMPMPMPMPMPM APM P - protection M - message part y bit

(a)

32 display Hamming groups characters A ,r clock clock framing mag row page no page no minutes minutes hours hours: spare spare '''' tt run-in run-in code no address no units tens units tens units tens I control control tt P c ear page newsflash I I subtitle bit bit bit

( b)

effect of setting the outputs of the 5-bit Fig. 4. Layouts of typical rows of data the magazine number, enable any counter, formed from ICs 11 and 20, to bits. A display row (row 5 of magazine combination of page numbers between zero. From this point onwards, the 5-bit 1) is shown at (a) and at (b) is a 100 and 899 to be selected. counter counts through the character- page-header row. The next four groups contain infor- display rows until the counter reaches mation indicating the time of day. The 24 (the end of the page display). This first group contains four bits of "min- utes units" information and the second, point is detected by gate (13, 3) and its "1" during the equalizing and broad output is used to reset the vertical three bits for the minutes tens. The pulses. This is of little consequence, spare bit in this second group is used as display-blanking flip-flop. however, as no video is present on these During lines containing data the 5-bit a "clear page" control, which goes to lines so that no false framing codes "1" whenever the following page con- counter is preset to the correct row could be detected here. number, which is available in binary tains new information and the old page Before continuing with a description must be erased from the store. The next form at the output of the Hamming of the clock divider circuits, it would be corrector during the control and row group contains a 4-bit "hours units" as well to consider in greater detail how message and the fourth group contains address groups, at the start of each data each of the data rows is made up. line. a 2-bit "hours tens" indication. There The time at which this data is entered are two unused bits in this group, the , Data row format into the presettable counter is controlled first being used to indicate a newsflash by gates (4, 4) and (13, 6). The least Display row. The data bits are trans- page, and the second a subtitle page. significant bit of the row address arrives mitted in groups of eight, and each line first, and a strobe pulse from the clock of data contains 45 of these groups, or Clock dividers divider circuits (to be described later in bytes. The first five groups have the There are two more groups of Ham- this article) set ICH at the appropriate same function on all rows and a typical ming-protected data before the page time. The next data group to arrive row is shown in Fig. 4(a). The following header display starts, and these will contains the remaining four bits of row 40 groups consist of 1S0-7 coded eventually contain more control infor- address information and this is written characters, the eighth bit in each group mation. into IC20, again at a time determined by being used as an odd parity bit. The The clock-divider circuit block pro- a strobe pulse from the clock divider magazine number and row address vides a large number of pulses and circuits. The other inputs to gates (4, 4) information are coded in a different waveforms for use elsewhere in the and (13, 6) are waveforms which only manner from the display data, being decoder, as follows: a display line- allow the strobe pulses to set the more heavily protected in a form of blanking waveform which goes to "1" counters during lines which have been Hamming coded. Only four useful bits, during the 40-character-wide display confirmed as containing valid informa- of data can be extracted from each of period; the "data allow" waveform tion. These waveforms are used in a these eight-bit groups, but a single error which goes "1" on detection of a number of places in the decoder and we in the group can be corrected and an framing code and returns to zero 42 shall call them data allow (DA), for a even number of errors i.e., 2, 4 or 6 bits bytes later; 6-bit column-address for waveform going to "1" when a framing in error, can be detected. addressing the store, which is used code is detected and returning to "0" at when writing data into the store, and the end of that line and 15A for its Page header row. The page-header row when reading from display; strobe pulses which discriminate between inverse. (row zero) contains eight more of these A 3-input NOR gate (12, 6), provides Hamming protected groups following the Hamming-coded words in normal the field" blanking waveforms which the row address number, and only 32 rows and in the page header row and goes to "1" during lines 11 — 21 (324 — groups of 1S0-7 coded data for display, finally, groups of pulses used elsewhere 334), by gating together the QA and Qc as shown in Fig. 4(b). The two groups for the formation of the vertical ele- outputs of IC14 together with the immediately following the row address ments of the alphanumeric characters. vertical display-blanking waveform. It number contain the page number units Figure 5 shows the clock-divider can be seen from the waveform diagram and tens respectively and these circuit diagram, and Fig. 6 the associat- in Fig.3. that this waveform also goes to together with the three bits allocated to ed waveform diagrams.

40 Wireless World, January 1976

neg framing code Fig. 5. The clock-divider circuit. IC17 is a 74121 monostable, IC1, IC10 arid IC19 are pulse from IC23 pin 8 binary counters, ICII is a 7474 dual D-type flip-flop and IC42, 47 are 7442 b.c.d.-to- 5V decimal decoders. lines 11-21 WFM from IC12 pm 6 VR2 25k 111

IC2 2 from IC47 2n2 pin 4 1 data allow' IC9 WFM

4--- lines 11-21 WFM 5 from IC12 ICie from IC3 pin 6 pin 1

neg mixed syncs R4 470 112 ICg IC26 from ICB display line b ankmg pm 8 from O IC3 ICie pin 1 waveform

IC6

CLK Ro1 Roe Rol Ro CLK clock 9 IC11 IC19 input ICio QB QC 00 QB QC

9 IC26

IC13 13 1

12

A B C B C C IC42 IC47 outputs outputs o 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

to IC1a

outputs to character strobe outputs for column address outputs and graphics generator detecting Hamming bits for data store

Fig. 6. Waveforms seen in the circuit of Fig. 5 during data lines.

Hamming coded groups 32 display characters FC CRI IC RAG 1 I I I 1 1 I I I I data line 1 14115 L framing code I I I I detections 5 IC17 —1 differentiated output data allow waveform L line blanking waveform L

6 bit column address

reset pulse

IC12 pin 12

IC47 pin 4 Wireless World, January 1976 41

Write mode. Detected framing code pulses, which are one clock pulse wide (144ns) and negative-going, are invert- ed in gate (22,10) and fed into gate (9,12) together with the differentiated output of monostable 17. This is the "framing- code allow" monostable and its differ- entiated output is about 2µs wide and timed to occur at the time of the frame code, ie, about 15ps after the start of line syncs. In this way, only valid framing codes are allowed through gate (9,12). This method was used to avoid allowing through random framing codes detect- ed in the colour burst, which can occur A second binary-to-decimal decoder The final prototype decoder, which with certain types of data-slicing cir- IC47 is connected to the outputs of the measures about 40 x 22.5 x 6cm. The cuit. The monostable triggering is second clock-divider i.c. after they have ROLL switch enables pages to be inhibited by means of the B input on all passed through the four AND gates of viewed in rapid succession. We except lines 11-21. The valid framing IC2. These gates serve to allow the understand that a complete kit of parts codes are then used to trigger the "data outputs of IC47 only during the "allow" for the decoder will be available from allow" flip-flop made up from gates (9,8) waveform at the output of gate (6,12). Catronics Ltd, 39 Pound Street, and (9,6). During lines 11-21, the first This waveform consists of narrow, Carshalton, Surrey. two clock-divider i.cs are held in the positive-going pulses derived from reset condition until the DA-waveform, decoder 42, gated with a waveform goes to "1" (18,8 and 18,11) i.e., until the which only allows the pulses for the first frame code occurs. It should be noted 13 bytes of data. The output pulses of that i.cs of the 74H series are specified decoder 47 then become narrow, nega- for gates 22 and 9. Although all the tive-going pulses, timed to occur one prototype decoders worked satisfactor- during each byte of data from byte 3 to The newest character code, which is ily with ordinary gates here, it would be byte 12: i.e., decoder 47 output 1 occurs now in use Reference 1 indicates that possible under worst-case conditions during byte 4 — output 9 during byte 12. these undefined control characters will for the time delay between detecting the These pulses are used to time each be allocated in order of decreasing framing code and removing the reset group of Hamming coded bits in the row serial number. Codes referenced 2 are from the dividers to exceed the maxi- and page recognition circuitry, and for to assist compatibility with standard mum permissible time of 144ns. By presetting the line divider i.cs as data codes All character rows start in using 74H series gates, the time delay mentioned earlier. the "steady, alphanumeric white" will always be less than 144ns, even condition, unboxed and unconcealed under worst-case conditions of supply Read mode. Resetting of the clock unless control characters indicate the voltage and gate delay. dividers in during the "read" mode is contrary.

b7 jp. 0 0 0 1 0 1 Bits b6 0 C 0 1 0 1 0 0 0 1 1 b5 ► 0 Column b4 b3 b2 bl 0 1 2 3 4 5 6 7 Row is g, P H P I .1.mm 0 0 0 0 0 NU L D L E I--, .... 0 re: @ 1 'CD I " I t I in I Alpha" Graphics , I a I vo q 1 --.1 0 0 0 1 1 Red Red ! [ moo 1 I.:mi A I A 0 1 0 m _ • Alpha^ Graphics 2 I --MI B 1 licl R I R b 12.. r I •— C 0 1 0 2 Green Green " I -- 1 m l an Alpha" Graphics £ I -- I c s I -- 0 0 1 1 3 Yellow Yellow 3 1 . C S I S c WEI I Al pha^ Graphics $ I mo. 1 'HZ D , n -m 0 1 0 0 4 Blue Blue I NomN. 4 1 N. 1 - T T d IHJ 1 1 i I I in En Alpha^ Graphics 0/0 I . LI 5 1 ri E I E U 1 U 0 1 0 1 5 Magenta Magenta on e 1°. u I I 1-1= Alpha^ Graphics im I • • 0 1 1 0 6 Cyan 8, I1 mism• 6 I m F F V I V v I Cyan I I f Alpha" Graphics i I ii. 7 1 . 0 1 1 1 7 White White m w I I ij G I G W 1 W .r g I- I Concealed =MI UMW I •==i 1 0 0 0 8 Flash Dsplay ( I:. 81%, H IH X I X h I „„• x I m I m I I I .- IIIII 1 0 0 1 9 Steady ) 1:m 9 I mmo I I. Y I , . Y I ' YI I End Box CD Z I Z z I 1 0 1 0 10 * I aim: I mm j I j I I 2 .% '4 i . 1 0 11 Start Box E S C -I- 1 mai'11 - I is. K K "4---- I'll— ° k I ' 1 1 1, 1 mom 1 1 1 1 IUD= — 1 1 0 0 12 ® I :: < 1 L I L 2 2 I i II I 1 .. I I 1 a in um 3 I ow 1 1 0 1 13 ® - I1 in. = I .. .KA v i - 'I Nil —111.- t - --illii. , m I m 4 I mr 1). 1 I I. . I IM • I saw > I 0 N 1 N 4" I f n I - 1 1 1 0 14 S 0 , • -I SI I 1 0 10 0 I 1 15 ____ / I " ? I . 1: I I Wireless World, February 1976 47 *Wireless World Teletext decoder 4 — Framing code detector, error circuits and storage

by J. F. Daniels

Discussion of the serial-to-parallel latches, as the fan-in requirement of 16 when the eight outputs of the shift convertor, framing code detector, data could not be met by the more common register are in a condition that causes all latches, Hamming corrector, data store 7402, two-input NOR gate. The timing "1"s to be present at the gate input. It is and code converter completes the of the strobe pulse is quite critical and it possible, with the use of more complex description of the circuitry contained on must, of course, occur only when the circuitry, to detect the framing code in digital board one. eight bits of data in each byte are the presence of a single error, as was Fig. 1 shows the serial-to-parallel correctly positioned in the 74164 shift explained in the introductory article. converter, data latches and framing- register. The detection of the framing- However, this added complication will code detector. IC21 is an 8-bit, serial-in, code pulse initially sets the timing of the also increase the number of false parallel-out shift register, which pre- latch strobe pulses, and subsequently framing code detections, and this can in sents all eight bits of data in one "byte" they are derived from a =8 output of some circumstances be more trouble- to the inputs of the data latches at the the clock divider circuit (IC42, pin 1). some than the occasional missed fram- same time. A strobe pulse is applied to ing code. This simple detector was the data latches, which causes the eight Framing code detection found to be perfectly adequate. bits of data to be stored in the latches The framing-code detection circuit until the next strobe pulse arrives, eight consists of only four inverter gates and Error detection and correction clock periods later. A 7428 buffer i.c. is a single 8-input NAND gate (23, 8). The Before continuing with a description of used for driving the strobe inputs of the output of this gate goes to "0" only the parity and Hamming-code correc-

these 12 outputs IC4 also go to from IC42 pin Hamming corrector clock Input (see 119.3)

• Q

Q

Q 0 0

to data store inputs ( see fig. 3)

►

—I 3

IC22

Fig. I. Circuit diagram of serial-to-parallel converter, framing 3 11 ► to IC22 pin 11 code detector and data latches. IC4 is a 12 7428, /C2/ is a 74164, /C22 is a 74H04, IC23 2 a 7430 and ICS.30 are 7475. IC23 48 Wireless World, February 1976

tion circuits, it would be as well to instance, a framing code detection will D B A C explain in more detail some of the result whenever the sequence of data 0 error free bits is 11100100. This will occur more 0 0 0 ) reception effects that can be caused by noise and often than might be expected since this 0 0 0 1 distortion in the received signal, and what can be done about correcting sequence may occur across "byte 0 0 1 0 them. barriers" e.g. the letters p , r will be 0 0 1 1 Below I have listed five different represented by the data bits 00001110 reject even types of possible error in the data 01001111. It can be seen from this that 0 1 0 0 order error display, and I have put them in what I the last four bits of the first group and 1 1 0 0 would consider to be in order of the first four of the second group 0 1 1 0 decreasing annoyance value: combine to form a framing code a) a row of complete "rubbish", i.e. sequence. 0 1 1 1 random characters across a complete This simple example shows how 1 0 0 0 invert 171t 7 display row; frequently these framing code 1 0 0 1 1 b) a row which begins correctly with detections might occur. The false detection, does not, however, cause any 1 0 1 0 0 3 intelligible data but which becomes rubbish somewhere across the row; serious problems, as there are two fairly 0 8 1 0 c) a row which is correct in terms of the simple ways of overcoming it, both of 1 1 0 0 0 5 data it contains but which takes up the which are used in this design. Firstly, position of another row in the display; the output of the framing code detector 1 1 0 1 " 6 this may leave a blank row where it is gated with a pulse, derived from 1 1 1 0 4 should have been situated; monostable 17, which only allows 1 1 1 2 d) a row which should contain informa- through framing codes that occur at the tion, but which is displayed blank; expected position on the television line. e) words which have individual letters This gating pulse is sufficiently wide to A,B,C & D are parity checks over incorrect, or missing — other than those allow for expected differences between the following bit no's caused by typing errors! TV channels, but not so wide as to allow, through framing codes caused by byte A checks on bits 1 2 6 8 Apart from the Hamming correction barriers. Any framing codes which may B . II ii 2 3 4 8 circuits, which can be considered an occur before the correct one are also C ll ii . 2 4 5 6 instantaneous form of correction, eliminated in this manner. (It is possible D II x . 1 2 3 4- 5 6 7 8 another type of correction can be used which relies on the fact that the pages of • that framing codes may be detected in = parity check correct information are transmitted cyclically, the colour burst with some types of data 1 = incorrect repeating every 15 seconds or so. If separator circuit.) Secondly, the reset information is written into the data waveform to the clock dividers consists Fig. 2. Table showing the results of four store every time the selected page is not simply of framing code detections, possible parity checks carried out over received — and not just the first time it but of the Data Allow latch waveform, the Hamming coded groups, and the arrives after being selected — then some which is derived as explained last action required for each combination errors such as missed rows, missed month. This allows only the first of results. letters, etc., can be corrected. Bearing framing code detected on_ any data line this in mind, we can now consider the to set the initial timing of the latch types of error mentioned earlier, their enable pulses and ignores subsequent causes, and whether they can be framing code detections on that line. corrected by subsequent detections of False framing codes which may be the correct page. more of a problem, are those that can be Rows of complete rubbish are almost detected on lines other than 17, 330 and Fig. 3. Circuit diagram of the Hamming always caused by the detection of false 18, 331. It has already been pointed out corrector. IC7 is a 7404, IC8 16 are 7410, framing codes. These arise in several that data is only "looked for" during IC24, 31.32 are 7486 and IC40 is a 74180. ways. On normal -data rows, for lines 11, 21, 324, 334, and in this way

bit no s from IC 7 pin 10 even error reject output 1 IC > \ 11 B (during row address only) tram IC29 pm 16 2 10 15 odd® IC29 0E E 3 to ICg pm 4 1C30 10 19

IC30 00 IC40 5 15 parity check output 1C30 0 B bit 6 IC30 18 0 odd IC20 9 9. 7 6 inpu ,

IC29 10 7 8 , ror4 IC29 Pl Hamming In 14 corrected bits IC31 ICie \ IC32 9 111. bit 2 IC32 IC7 12 11 8 11 a morn IC30 pin IC32 4 IC24 IC18 \\_t 2 3 2 life bit 4 49, 8 IC31 2 IC24 7 12 IC 12 from IC 30 Pin 1 3 12 13 IC3 2 IC \\_ IC31 b I t 8 13 0 8 ► D 2 3 0 IC 24 IC7 from IC29 Pin 11 10 IC31 1%\ IC24 IC8 ( 9 -a, 11 r blt 8 5

Wireless World, February 1976 49

column address inputs row address inputs A

27,8 19,14 19,12 19,9 19,8 19,11 20,8 20,5 20,9 20,2 20,12

V V

11 13 9 10 12 f13

'(3ZIr") c) 11 IC27 IC27 IC25 1C25 1C27 IC27 IC 2 7

5 4

IC25 6 IC 25 IC25 IC 25

bit outputs 3 bit Q 1C39 no s from 1C42 Pin 7 from IC 2 9 pin 16 0. 1 e)1C34 12 2 IC29 15 li. () IC33 tc,3-1c39 12 IC30 10 Et 3 4 7 x 1024 bit random 1C30 9 la 'tIC35 access memories 12 (all pins except 11 &12 are \ 5 \ common to all seven i c's/ IC30 15 •1;11IC3, 6 IC30 16 —1. 1'11 ;i.k- 37 (> 1C29 " 12

IC70 " 8

detections during separated picture Fig. 4. Circuit diagram of the code decoders employing Hamming correc- information are avoided. However, lines converter and data store. IC25. 26 are tion, as single errors in the row address 16 and 329 carry another form of data, 7400, 1C27 is 7404, IC28 is a 74174, 1069 is will be corrected automatically. Even if and lines 19, 332 and 20, 333 carry a 7474 and the r.a.ms are 2602B or an even number of errors are present (2, insertion test signals. Despite the 2102B. 4, or 6), the row will still not be written precautions already taken it is still into the wrong position, as even errors possible under some circumstances to can be detected, and in this design they detect a framing code at the correct rause the Data Allow waveform to time on one of these lines. It is this type entirely different cause, is apparent in return to "U", preventing any data being of false detection that can cause lines of rows which begin correctly with intelli- written into the store on that row. This complete rubbish to be written into the gible data, but turn to rubbish at some means that only errors totalling 3, 5 or 7 display, and it is this type of error that is point across the line. For this to happen, bits in the row address group will cause most objectionable. The simplest way of the framing code must have been rows to appear in the wrong position on overcoming it in this design is to not correctly detected, but somewhere the screen, and this is rather unlikely, look for framing codes on lines 16, 329, across the line the divide-by-eight except in cases of severe interference 19 & 20, 332 and 333 and this can be counter generating the latch strobe (car ignition interference, etc). easily accomplished by inhibiting the pulses gets out of step with the data The types of error dealt with so far are entry of data into the 74164 shift register bytes, and this causes the information all very undesirable for the simple on these lines. This is achieved by to turn to rubbish. The most likely reason that they may occur in the feeding the QB output of IC5 into the causes of this are that the clock display at any time, even if the page is second data input of the shift register oscillator is slightly off frequency, or read out correctly the first time, and it is (shown dotted in Fig. 1). This solution to that some particularly severe noise in for this reason that the extensive the problem has the disadvantage that if the data line causes the clock generator precautions described above are taken the allocation of lines in the vertical to mistrigger. to prevent their occurrence. The interval is changed in the future, a The third type of error is the row remaining types of error to be described modification to the circuit may be which appears in the wrong position in are less annoying because they are self required. For this reason the connexion the display. This probably has the correcting, i.e., a page may be read out is not included on the p.c.bs, but may be secondary effect that the position it which has errors such as rows or letters added by means of a wire link if it is should have occupied in the display will missing, but these will be corrected found to be necessary. be left blank, thus effectively making when the selected page is next trans- A second type of error, which is also two rows of the display incorrect. This mitted in 15-20 seconds time. This is quite disturbing but which is due to an type of error is fairly uncommon in another good reason for having a short

50 Wireless World, February 1976 returns the Data Allow waveform to column addresses "0", thus preventing the writing of 19,9 information into the store on that row.

19,8 Data store and code converter 19,11 The data store consists of seven 1024-bit static random-access memories B (r.a.ms). These are m.o.s. devices, but x2 are extremely easy to use, as they have row fully t.t.l.-compatible inputs and outputs addresses — and require no external pull-up resis- Y2 tors. The printed-circuit boards are designed in such a way that holders may be used for these i.cs to reduce the possibility of damage by "leaky" sol- Z2 dering irons etc. The outputs of these i.cs are only capable of driving one t.t.l. load and must therefore be suitably unused z, part of i7 buffered to obtain the normal t.t.l. store ,r.-97 fan-out of 10. They also have a some- what variable access time, which 20,2 20.12 depends on device type, temperature, and the particular 'location' in the store which is being addressed. Although the specification states that the access time will always be less than 1ps, it is page-access time, since corrections will Fig. 5. Diagram showing the operation important in this circuit that the bits are be made faster. of the code converter circuit (see text all time coincident at the output of the A blank row in the display may be for explanation). store, and this is achieved by using t.t.l. caused either by an even error detected D-type latches on the outputs of the in the row address group, or by a missed r.a.ms. These D-type latches perform framing code. In either case this type of the dual function of providing bit error cannot cause an already correct over the byte and in this way it is outputs that are time-coincident and page to be made incorrect, and correct .possible to determine which one, if any, with a full fan-out of 10 t.t.l. loads. information will almost certainly be of the bits is incorrect. If one of the The read/write input of the store is taken the next time the page is trans- message bits is found to be incorrect, normally at the "1" level during the mitted. then it is simply inverted, and hence display of data on the screen. When new Nothing has yet been said about the corrected. The table in Fig. 2 shows the information is written into the store, parity bit contained in each of the parity checks that are carried out and display character groups. This is used in the action taken if one or more of the the following way: a parity check is checks fails. The actual circuit, shown made on each 8-bit data byte before it is in Fig. 3 does not need to be quite as written into the store, and if the check is complicated as the table suggests To help readers to buy components, we satisfactory the seven bits of the because there is nothing to be gained by print here a list of i.cs used in the character code are written in the correcting the odd numbered bits decoder. The full parts list will follow (parity bits) and in fact only the later. ICg5_90 are required for a two-r.o.m. normal manner. If, however, the check character generator, also described message bits are corrected. shows the parity to be incorrect, later. nothing is written into the store during Gates (32, 6), (24, 11) and (24, 8) that byte. (Anything already in the store indicate parity checks A, B and C 1 7493 31 7486 61 7412 at that address location will therefore be respectively as shown in the table, and 2 7408 32 7486 62 7410 unaffected, as only the action of writing the results of these checks are fed into 3 74121 33 2602B 63 7404 removes • already-stored information.) the four three-input NAND gates, (16, 4 7428 34 2602B 64 7474 This has the effect that when a page is - 12), (16, 8), (8, 12) and (8, 6). The output 5 7490 35 2602B 65 — first read out onto the screen, any condition of these gates determines 6 7404 36 2602B 66 7404 7 7404 parity errors will cause characters to be whether or not the bits from the Q 37 2602B 67 7404 8 7410 38 2602B 68 — missing from the display. These letters outputs of the 7475 latches will be 9 741-110 39 2602B 69 7474 will then be filled in 15 seconds or so inverted or passed through the. exclu- 10 7493 40 74180 70 7410 later, when the page is next written into sive-OR gates unchanged. (Normally 11 7474 41 7402 71 7400 the store. Incorrect characters will only the Q outputs are inverted by these 12 7427 42 7442 72 7474 be read into the display if more than one gates, but if an error is detected, the 13 7400 43 7402 73 2513 (u.c.) error is present in the data word in such errant bit is passed through 14 7493 44 7410 74 7403 a way that the byte-parity is still unchanged). 15 7400 45 7420 75 7403 correct The 74180 eight-bit parity checking 16 7410 46 7402 76 — This, then, is a brief resume of the i.c. performs a dual function in this 17 74121 47 7442 77 — more common types of error that can be 18 7400 48 7442 78 7474 circuit. Firstly it is used simply as a 19 7493 49 encountered in the teletext display, and 7400 79 7474 parity checker for the 150-7 coded 20 74177 50 7400 80 7474 we will now continue the circuit characters, to determine whether they 21 74177 51 7474 81 75107 description by looking at the operation should be written into the store. Se- 22 741-104 52 7474 82 7403 of the Hamming corrector circuit. condly, it is used in conjunction with 23 7430 53 7474 83 7400 gate (16, 6) to detect even-order errors 24 7486 54 7402 84 7410 Hamming correction during the Hamming-coded bytes. The 25 7400 55 7474 85 2513 (1.c.) It has already been explained that the input to gate (8, 8) from IC7, pin 10 26 7400 56 7474 86 7483 27 57 7400 87 7421 Hamming-coded bytes contain four restricts the detection of even-order 7404 28 74174 58 7410 88 7400 message bits and four parity bits. The errors to the row address group only. As 29 7475 .59 7404 89 7402 use of four parity bits enables four explained earlier an even-order error 30 7475 60 7412 90 7403 separate parity checks to be carried out detected at the output of gate (8, 8) Wireless World, March 1976 75 Wireless World Teletext decoder

5—Selection, control logic, control codes decoding and display

by J. F. Daniels

Last month's article concluded the are not fed directly by the Hamming explained in an earlier article, the page description of the circuitry on digital corrector output, the D input being fed is written into the store every time it is board I. This month we continue the via gates (71, 8) and (63, 8). This is received, and not just the first time, in description by looking at board 2, which necessary because of the Clear Page bit order that any errors may be corrected, contains the page and time selection which may occur during the minutes and also so that self-changing, and circuits, read/write control logic, con- tens group. If the extra gating was not updated pages will be automatically trol codes decoding, graphic and employed then correct time detection of written into the store. alphanumeric display circuits. pages which included a clear bit would • It must provide a constantly chang- not be obtained. The hours tens group ing time indication in the top right hand Page and time detection does, of course, also contain extra bits corner of the display, and also a This part of the circuit indicates when to indicate newsflash and subtitle continuously changing page header the selected page is reached in the pages, but it is unlikely that either of during the time between the "clear" transmission sequence, and it does this these types of page will ever need to be button being pushed and the receipt of by looking at the Hamming-coded time selected, and so no precautions the selected page. information contained in the page have been taken in this respect. • It should detect the clear-page bit, if it header (row address zero) of every The action of the chain of flip-flops is is present in the page header of the page. When a page header is found initiated by the preset input of IC55 selected page, and then erase the stored , which contains the same page number which is fed by the Data Allow wave- information before writing the new as that selected on the thumbwheel form. By doing this, only valid data rows page into the store. switches, an output pulse is obtained are interrogated. IC55, pin 8 goes to I • Finally, in the "auto newsflash" mode which lasts for the length of the data only during page headers and this of operation, newsflashes selected on line. If the time-coded mode of opera- waveform is gated with the output of the thumbwheel switches must not be tion is selected, a comparison is also the magazine number flip-flop, to allow written into the store until one is made between the transmitted time and operation of the fourth flip-flop, the detected that contains a clear-page bit, the setting of the time-selection page units detector. indicating an updated newsflash. thumbwheel switches. The pulse is fed The wiring of the thumbwheel switch The circuitry to achieve the above to the read/write control logic which for magazine number detection is functions is shown in Fig. I, together initiates the sequence of writing data slightly different to the rest of the with the page and time-selection cir- into the memories. Before we look at thumbwheel switches. This is because cuits. the writing operation in more detail we maagazine number eight is coded 000 — Page selection. The pulse from the page will consider how this pulse is obtained. or 0 in decimal terms, and there is no detection circuitry (which goes to I The method of achieving the detec- magazine number 0 or 9. This switch is during the page header of the correct tion of page number and time coding wired normally from inputs 1-7, hut page) is used to preset the D-type information is by means of a chain of input 8 must be fed from IC„, pin I, not flip-flop IC79 after being gated in (71, 6) D-type flip-flops, IC numbers 55, 79, 80, pin 10, and inputs 0 and 9 should be left with the output of inverter (63, 4). This 72 and 64. It should be remembered from unconnected, as these are unused waveform is an inversion of that an earlier article describing the clock- positions on the switch. From the page appearing at gate (12, 12) and was divider circuits, that IC47 produces clock units detection flip-flop through to the shown in the clock-divider waveform pulses timed to occur during each of the hours tens flip-flop the i.c.s. form a diagrams in Part 2 of the series. It only Hamming-coded groups in the page simple repetitive chain and the output goes to I after the Hamming bits of the header. (IC47, pin 2 is a strobe pulse pulse is obtained either from IC". pin 8 page header row address have finished, during the magazine number group - in the normal mode, or IC64, pin 8 in the and it is gated here with the page IC47, pin 10 a strobe pulse occurring time-selection, mode of operation. detection pulse in order to prevent the during the hours tens group.) Hamming bits from being written into • The first of the flip-flops (IC55) is used the store. 1 to define which of the rows are page Read/write logic The action of presetting IC79 sets the headers. The first half detects the least This part of the circuit performs a large Q output to I, and this is then gated in significant bit, and the second half the number of different functions, and (45, 8) with the output of the parity other four, by being fed with output before attempting to describe the circuit checker, the Data -Allow waveform and zero of the b.c.d.-to-decimal decoder, operation, these will be summarised as the line-blanking waveform. The output IC„. This i.c. is employed very usefully follows. of the parity checker is normally at I for to provide decimal outputs of all the • The basic function is to provide write valid bytes of data, but if a parity error is b.c.d.-coded addresses, and these out- "pulses to the random-access memory detected it goes to I, inhibiting the write puts may be fed to all the decimal store during transmissions of the pulses. The Data-Allow waveform is thumbwheel switch inputs in parallel. It selected page, in order that the page gated in at this point to ensure that only will be noticed that the inputs to this i.c. may be written into the store. As valid data lines are written into the 76 Wireless World, March 1976 store, and the line blanking waveform a "stop" signal after a count up to 24 (At the present time the BBC quite often further restricts the writing action by rows. It is possible that more than 24 transmit two page headers consecuti- inhibiting writing during the framing rows may be transmitted in any given vely, and the reason for this will be dealt code and control and row address group page, as there may be more than one with later in the article.) bytes. Gate (45, 8) will therefore go to 0 page header containing the correct page The only way to end the writing when we require to write information number transmitted at any time during sequence is to detect a "wrong" page into the store. This, in turn, sets (62, 6) the transmission of the page. This may header, meaning a header of any page to 1, which allows the write pulses (from be done to update the time information other than the one selected on the 66, 2) through gate (70, 8) and into the in the top right hand corner of the thumbwheel switches, and this is done read/write input of the store. display at regular seconds intervals. by clocking IC79 with page header This process initiates the writing of This last piece of information also detections, while the D input is held at 0. information into the store, but it must precludes another possible way of This returns the Q output to 0 and stops also be stopped at the end of the page, ending the writing sequence, that of the writing sequence. It may not seem an operation which is not quite as using the detection of the next page obvious Why this does not still stop the straightforward as it might appear. The header after the correctly detected one. writing action even if a second 'correct' difficulty arises because there is nothing page header is transmitted. The reason to indicate that the page transmission is that the preset input will always has finished. Detection of row number overide the action of the clock input 23 to stop the writing action is not (which only operates on positive-going feasible because this may be a blank edges), and the preset input will row, in which case it may not be Fig. I. Page and time detection circuits always be present during the correct transmitted. The same reason rules out and read/write control logic. page header detections.

IC 4 6 IC63 IC70 from ICg 10 19,11 IC62 66, 2 to IC39 pin 8 (9, 8) . 1 11 10 9 11 12,12 pin 3 3 10 (R/Cv) IC46 11 4 3 24, 3 y6 IC 62 IC55 IC45 3 4 9 47, 2 /fR • li.) CK -.0,6 b. 10 9, 8 12..- 13 00 0 0 IC71 48,1 0 D PR 18, 3 O mins 12 0 IC55 units IC72 (IC70 °00 47, 3-1.-"-(1)1 CK 0 47, 6 CK 2 1 13

IC63 00 00 0 o 0 PR 0 P 12 0 71 O 13 0 mag no. IC O mins. IC 62 On wiper ° tens IC79 11 0 1c72 12 00 2 13 47, 7 CK 55, 4 11 CK a 13 ao- 63, 4 10 13 O 00 IC46 0° IC46 1 0 IC63 0 PR IC70 0 0 D 8 9 12 11 0 page O hours 0 units 0 units IC64 A A 000 wiper °00 4 +5V 47, 4 47, 9 CK 0(6 13,8 14,1 20,2 20,12 ► R6 1k

clear 0 0 PR O PR PR page 0 1lio. D O D D 0 page tens 0 hours 0 wiper IC80 00 tens IC64 IC78 000 00 47, 5 IIR CK O 47,10 PR K CK time time CLR on off

+5V C9 iOn auto 32,11 V

norm newsf lash 24,6 7, 12 24,3 P.

32,8 I C 63 IC71 24,3 47, 7 -11ok- 1 47, 7

Wireless World, March 1976 77

It will be noticed from the circuit 42,4 header position on the screen. This diagram that the clear input of IC79 is waveform is gated with (78, 9) and also 42,5 fed from the output of (70, 6). However, (a) the inverse of the Data Allow waveform this only has any effect in the auto- 42,6 in (70, 12) to provide the Clear Page newsflash mode of operation which will 42,7 action. The inverse of the Data-Allow be dealt with later: During normal waveform is gated in at this point to operation the clear input is held at 1 45,6 prevent the Clear Page action taking because one input of (70, 6) is held at 0 place on valid data rows. The Clear Page by the auto/normal switch. action can also be initiated by operation bit bit of the Clear Page button which presets 1 2 IC78, which again sets the Q output to 1. Time display. Having looked at the 1C76, is cleared by negative-going frame operation of writing normal pages into bit bit sync pulses obtained from (7, 12) which the store let us now look at how the 3 4 stops the clearing action at the end of continuously changing time informa- the display period. tion is obtained in the top right hand bit bit 5 7 The operation of deriving the space' corner of the screen, and also how the r character codes to be written into the page header is made to "rotate" when 5,1 5,9 store is achieved in the serial-to-parallel the Clear button is pushed. The QD 5,8 5,12 convertor, Ic21 (74164). In fact, a space output of counter 19 is very useful in character code is not used in this design, this respect because it goes to 1 only 66,10 merely a set of eight zeroes, obtained by during the last eight bytes of data on 66,8 clearing the shift register outputs each line, which happen to coincide ( b) 66,6 during the display period. It will be seen with the point at which the time display from the code table that this is in fact a information is transmitted on the page 66,4 NUL control character, which is not header. 66,2 designated to any particular function, This waveform is fed through gates and is in fact inhibited from doing (46, 1) and (63, 10) into gate (62, 6) where anything in the control codes decoder. it causes the output to go to 1 (so (To be described later in this article.) It enabling the write pulses) only during will of course be displayed as a space parity-correct information. The action character, as are all control characters. of pushing the clear button presets IC78, which is a D-type flip-flop. This sets the Newsflash. The only other function of Q output to 0 which is fed to one input of this part of the circuit is to provide the the two-input NOR gate (46, 4). Here it Auto Newsflash facility, which is mere- is gated with the waveform which ly a method of preventing any news- inhibits writing during the Hamming- flash (or any other pages) being written coded bytes and is then fed into (46, 1) 5,8 4.10 5,9 into the store, until one is found which where it causes the whole of the page outputs contains a Clear Page bit. header information to be written, Fig. 2 (a) Graphic character segments When one of these "new news- instead of just the time information as are of unequal sizes to avoid gaps flashes" is detected, the decoder reverts before. between characters. Alphanumeric to normal operation, i.e., it writes all the generation is shown at (b), which does succeeding newsflash pages into the Page clearing. Another function not include the three vertical columns store as,well as the one containing the performed by this part of the circuit is for gaps b&ween characters. clear bit. In this way errors can still be the action of clearing the page when a corrected as described last month. clear bit is detected in the Hamming Normally the auto mode is inhibited bytes at the start of the page header. by means of gate (70,6), the output of The clear bit is transmitted during the the Hamming corrector is strobed by a which is held at 1 by virtue of the header of any page which contains new pulse from IC47, pin 7. The resulting, auto/normal switch. When the auto information, i.e. during automatically- short, negative-going pulse is used to mode is first selected the positive changing page, every time it cycles on clock IC76, pin 11, and if the D input is at transition at the clock input of IC69 sets to the next one of the group, or 1, which it will be during correct header the Q output to 0 and this still allows the whenever a single page is transmitted detections, the Q output will also go to first newsflash received to be read into for the first time with updated informa- 1. the store, regardless of whether it tion. Now there is one slight problem contains a dear bit. However, if the In order to effect the operation of which prevents us using this output Clear Page button is pushed, this presets clearing the page, the store must be directly to initiate the writing action IC69 Q output to 1 and the output of gate filled with 'space' characters, by writing during the display period. This problem (70,6) goes to O. This clears IC79 and thus them into the store at all the positions is that the header which contained the prevents any further pages from being used for display purposes. Now the most Clear Page bit will be erased from the written into the store. The Clear Page convenient way to do this is to use the store by the action of writing during the bit detection circuitry, however, still display period to write in these space display period and the page will then be works as usual and if a newsflash is characters as the store automatically displayed with no header, except for the detected which contains a clear bit, IC69 cycles through all its positions during time information which will, of course, is cleared by a pulse from C9. This sets this time. In fact the system specifica- reappear almost instantly. In this design the circuit back to normal operation, tion allows us to do this as it states that is prevented from happening by and all the succeeding newsflash pages that:L "A clear page command for a inhibiting the Clear Page action during are read into the store until such time as particular page; and new information the part of the display normally occu- the Clear Page button is pushed again. for that page will not be transmitted in pied by the header. that is. all the the same field blanking interval to allow displayed header row except the part Graphics generation time for the receiver store to be cleared" occupied by the Hamming coded bytes, Figure 2(a) shows the way in which (hence the need to transmit two page which are still erased. graphics characters are formed. Bits 1, headers in succession occasionally). This is achieved with gates (46, 10), 2, 3, 4, 5 and 7 each represent one sixth Detection of the clear page bit is (46, 13) and (62, 12) which gate out a of the graphic shape, and it is intended achieved in gate (71, 3), where bit 8 from waveform corresponding to the display that when a bit is 1 its corresponding 78 Wireless World, March 1976 section of the graphics rectangle should displays where large areas of colour gates are used, one for each bit of the be 'illuminated'. The decision as to how were intended to be shown. displayed character. One input of each much area shall be occupied by each of In fact the effect of having unequal of these gates is fed with the bit the bits is left to the circuit designer, size rectangles forming the graphics information from the output of the and a number of possibilities will be characters is far less disturbing than the r.a.m. store, and the other inputs are fed, considered here. If rectangles of equal effects caused by having gaps between one with the horizontal gating wave- size were chosen for each of the six bits them, and this design uses the format form, and the other with the vertical of information, then the best arrange- shown in Fig. 2(a) which uses four TV gating waveform. The six outputs are ment would be for the bits each to lines per field for bits 1 and 2 and three added together by the open-collector occupy three lines on each TV field and lines for each pair of the other four bits. connexion of the gates and the output three out of the four available vertical In the horizontal direction the bits are at this point is negative-going graphics columns. (The actual size of the com- equal in width, each one being formed information. The horizontal gating plete character box available is ten lines from four out of the eight clock pulses. waveform for the right hand half of the per field and eight clock pulses wide in The reason for making bits 1 and 2 graphics character is obtained at the the horizontal direction.) larger than the other pairs of bits, was output of gate (45,6) and the waveform This arrangement would, however, purely one of simplicity, requiring fewer for forming the left-hand half of the give gaps between each of the graphics gates than any other arrangement. characters is simply the inverse of this characters in the vertical direction, as The circuit for generating the gra- waveform. only nine out of the ten available lines phics characters is shown in Fig (3a). The vertical gating waveform for bits would be used, and in the horizontal Six, three-input open collector NAND 1 and 2 is obtained by gating two of the direction, as only six out of the eight line divider waveforms, (5,11) and (5,8) available clock pulses would be used. together in NOR gate (43,10). Bits 3 and Now, although this method would give 4 require the slightly more complicated characters with equal size components, Fig. 3(a) Circuit of the graphics gating arrangement, achieved with the effect of having gaps between each generator. The r.o.m. character gates (44,8), (50,3) and (59,6), and finally of them would be most undesirable for generator is at (b). the waveform for bits 5 and 7 is obtained from gate (50,6).

+5V 42, 7 111. IC45 IC67 Alphanumeric characters 42,4 op R IC50 The generation of alphanumeric char- 1k 42, 6 9 acters is similar in many respects to the 4 28 2 • 1111 58. 2 graphics characters, the main difference 42, 5 11 being that each of the alphanumeric IC6 13 characters is formed from thirty-five 2 small squares situated in the character 1 IC43 28,15 1/0 cell. In fact, the character cell is divided 5,11 into a total of fifty squares, as shown in IC61 69, 9 Fig. 2(b). (This does not include the gaps 5,8 P• ► 10 e-e'N between characters in the horizontal 9 I C 50 IC59 direction, which take up three extra [C44 columns). 13 5,12 e". The top row of five squares is 5, 8 10 1 ):33 5 11 ._ normally left blank to give a space 5, 9 28,10 I/ 2 between rows of characters, and the IC61 two lines of five empty squares at the IC50 28,7 110 IC59 5 bottom of the character cell are used when lower case characters, which 5,11 (C60 normally descend below the line (g, j, p, q, and y), are generated. 5 3 Figure 3(b) shows the circuit diagram (a) 28, 5 P. of the alphanumeric character genera- tor. It is not practical to use the same 42,3 42,4 42,5 42,6 42, 7 +5V approach as in graphics generation using discrete i.cs to form the char- V acters, because this would result in an (o) F11 9 extremely large number of components. Special i.cs are in fact manufactured to j IC 66 R 8 1k perform the function of character IC67 6 4 -J generation, and these are called read- 13 12 only memories or r.o.ms. The one IC74 28, 5 o/p 2 shown in the circuit diagram is manu- factured by Signetics and contains 28, 7 IC74 57,9 information for sixty-four different 28,10 o/p characters, including all the upper-case 2 12.--\, 11 0 69, 9 alphabet and various other characters I C 74 ‘. 13 such as brackets, numerals etc. 28,15 I C73 0/p 3 10 There are five outputs from the i.c., 28, 2 9 one for each column of the character, as IC74 5, 8 o/p indicated in Fig. 2(b), and nine address 4 4 inputs. Three of the address inputs 5, 9 IC75 5 (called row addresses) are used to define 4 10 o/p which of the horizontal rows of the 5 11 12---.\ 0 character is to be displayed, and the other six inputs are used to decide which of the sixty-four characters will Wireless World, March 1976 79 be displayed. The row address inputs mation in five 2-input open-collector and also to blank out the control are fed from the line divider outputs NAND gates, where they are added characters, which of course are not (5,9) and (5,8), also (4,10) which has together to form negative-going intended for display. been gated to inhibit the Output during alphanumeric characters. There is also a problem caused by the the last two lines of the character cell. It fact that the standards of transmission does this by making the row address Control-codes decoding and output have been changed recently, in terms of "000" during the last two rows, and in This part of the circuit, shown in Fig. 4, the position in the code table of some of this particular r.o.m. the output is performs several functions. the control characters. (On February 2, always '0' when this row address is • To detect all the various control 1976, BBC1 began to transmit the new present. codes that are transmitted, for colour, standard, BBC2 reverting to the old The other six address inputs are fed graphics, flashing and boxing informa- standard.) This problem is overcome by directly from the r.a.m. store outputs. A tion and to switch the output gates to providing three links on the printed quick look at the code table might the correct state. board which can be changed according indicate that bit numbers 1-6 should be • To provide switching between the to which type of transmission is being used here, but as this i.c. is capable of graphic and alphanumeric information received. producing upper-case only characters, in the "alphanumeric blast through" (To be continued) this would result in lower-case alphabet mode. letters appearing as numerals and • To add line and field blanking Corrections various other odd symbols. If, however, information to the output waveform, In the list of i.cs, published last month, bit 6 is replaced with bit 7 inverted, the IC2, was incorrectly given. It should be desired effect of lower-case letters 74164. appearing as capitals is achieved. On page 48, Fig. 3, the connexion to IC40, The outputs of the i.c. are gated with Fig. 4. The control codes decoder and pin 8 should come from IC30, pin 9, not their respective vertical column infor- output circuit. pin 19.

IC59 IC49 8 44,12 28, 2 IC58

IC 59 2 8. 2 ► 28,10 I C 54 28, 7 —11i. red output 28,12 C41

28. 5 10

IC59 28.15 13 12 69,9 —110.--•--

green output

I C41 IC49 IC50 1 p IC 43 5 69, 9 1 28,10 1 a 28,10 ,. blue output IC59 42,2 IC43 11 10 3 28,15 PP IC 50 13 1 .67 11 5,_6 white 2 output

6 0, 8

13 IC58 I C 5 8 IC 87 1 4.12 11 28,12 IC57 2 10 8 11 10 28, 2 ► 11 9 IC57 IC57 3 10 74, 3 C14 41,10 IC 41 1n C10 470 P 1 18, 3 —111113 15..3 2

IC44 C2011r,

77,7 6 IC41 C12 100 'cut hole 1 output R10

I C67 64 Wireless World, April 1976 Wireless World Teletext decoder 6--Lower-case characters and analogue circuits by J. F. Daniels

The control-codes detection is based described for colours. Flip-flop (53,8) gates (57,8) and (58,12), containing the on six D-type flip-flops. The first three obtains its clock pulses from the same graphics and alphanumeric characters of these (52,6), (52,8) and (53,6) detect place as the colour changing i.cs and its respectively. A third input to this gate is codes which mean "go to red display, D input is fed either from (66.12) or fed with a composite blanking wave- green display and blue display" respec- (28.10) according to which transmission form which contains both line and field tively. The fourth one dictates whether standard is being received. blanking, and also information to blank the display should be in the alphanu- Flashing and boxing codes are dealt the control characters. Gate (44,12) meric or graphics mode, and the fifth with, again in a similar way to that adds together the line and field blanking one (51,6) indicates when characters already described. Clock pulses are only waveforms, and a third blanking input should be flashing. The final one is used allowed during the two code positions is provided here which may be used to to derive a wavefrom to switch between allocated to flashing and boxing blank the Teletext display output while the TV and Teletext displays during the respectively. The D inputs are fed with watching TV programmes. (It is possi- insert or boxed mode. bit 1 information, and in this way the ble that the Teletext waveform could The Teletext specification says that output of each flip-flop is set either to break through onto the TV picture all rows should begin in the steady, the on or off condition depending upon under some circumstances, if the leads alphanumeric white, unboxed condi- which of the control codes is received. were not properly screened, for tion, and this is achieved by presetting Normally, of course, the flip-flops are instance.) The output of this gate is the outputs of the six flip-flops to the set to the off condition by the line- delayed by capacitor CH to allow for required state with the output pulse blanking waveform, and then some- delays in the r.a.ms and r.o.m. This from (59,8), which is a combined line where along the character row a output is added to the control-character and field blanking waveform. flashing code may be received which blanking waveform in gate (41,13) and Considering first the codes which will set the flip-flop to the on condition. the output of this gate is then fed into indicate a change in colour of the A further code may be received to turn (58,8) where it serves to blank the video display, it can be seen from the code off the flashing or, if no code is received, display waveform. table that bit 1 is always at 1 when a red the line-blanking waveform will again The output of (58,8) is then inverted output is required, bit 2 is at 1 when a set the flip-flop to the off condition at and gated with the output of the green output is required and bit the end of the row. flashing oscillator formed from (67,2) 3 is at 1 when a blue display is required. Switching between the alphanumeric and (67,4). This flashing oscillator is Combinations of these three bits will and graphic displays (both types of allowed by the D type flip flop (51,6) and also give the complementary colours character are actually generated for gates the display waveform in (57,6). At correctly such as yellow when bits 1 each character box in the display) is this point the composite display wave- and 2 are both at 1, and also white when achieved with gates (57,8), (57,11), (58,8) form exists in monochrome form, and all three bits are at 1. The way in which and (58,12). Alphanumeric characters then the colours are incorporated by this is achieved in the decoder is to feed are fed into IC58, pin 2 and graphic gating this monochrome waveform in bits 1, 2 and 3 respectively to the D characters into IC57„pin 10. A feed of the 2-input NOR gates (54,4), inputs of the three flip-flops, (52,6), bit6 is also connected into IC58, pin 1 and (54,13) and (54,1). The three D-type (52,8) and (53,6) and feed the clock IC57, pin 13. This is to enable the "blast flip-flops enable or disable these gates inputs in parallel, with clock pulses through" mode of operation, in the to form the red, green and blue outputs. occurring only during the fourteen following manner. Normally when Finally the output of the flip-flop colouring codes in the code table graphics are being displayed, bit 6 is in (51,8) is gated with the line and field The clock pulse gating is achieved in the 1 condition. If, however, bit 6 is blanking waveforms to give an output (41,10), (49,8), (43,13), (44,6), (58,6) and made 0 as for instance in the transmis- which can be used as a switching (49,6). The actual clock pulses are sion of upper-case letters, this can be waveform when a "boxed" display is narrow negative-going pulses obtained made to switch away from the graphics wanted. from decoder (42,2) and are fed into gate mode, and into the alphanumeric mode (43.13). The other gates merely serve to for the duration of the character. In this inhibit the clock pulses at all times other way switching between graphics and Lower-case characters than when a change in colour of the alphanumerics can be obtained very The character-generation circuit display is required. The input to IC49, pin economically, without the need for a already described is capable of generat- 9 can be changed by means of the link, separate control character (which ing only upper case, or capital letters. from IC28, pin 7 to 1059, pin 2, altering would be displayed as a space). The Although this does not detract in any the Clock-pulse-allow waveform to slight disadvantage of this is that only way from the information-carrying cater for the new code allocations. upper-case characters in columns 4 and capabilities of the system, some people The changeover between graphics 5 of the code table may be displayed in may consider it worthwhile to add the and alphanumeric operation is obtained this way. extra circuitry required to display lower in a very similar way to that already Gate (58,8) is fed with the outputs of case characters. Wireless World, April 1976 65 The method is exactly the same as inputs are fed either with a binary zero, that described last month for upper case during normal characters, or the binary characters, except that some lower case NEE number fourteen during characters letters, i.e. g,j,p,q,y, drop below the line which require descending by two rows. of normal characters. Provision has U Sum outputs one, two and three are fed already been made for this as the MEM to the row address inputs of the r.o.m. character box is ten lines high and only and these will change for a descended the top eight lines are used for the ME El character in such a way that the upper-case characters and a space line character will be lowered by two TV between characters. This leaves two • lines. Sum output four can be used as a unused lines available to display lower- (a) (b) blanking waveform to inhibit the gen- case descenders. eration of characters during the top two The same type of r.o.m. is used to Fig. 1. Lower-case letters with "tails" are rows of the character box, when a contain the lower-case characters, the stored in the same memory rows as ordinary descended character is displayed. The only difference being that the lower- letters. Row addresses are changed during blanking is achieved by means of gate two rows. case memory only contains the thirty readout to lower the letters by (89,13) which switches the character two characters in columns six and seven generator to one of the blank character of the code table, the other thirty two The character generator, IC85 is of the spaces during the required blanking spaces being left blank. The complica- same type as the upper case one, a 2513, period. This method of achieving the tion arises from the fact that characters but its suffix CM3021 indicates that it is blanking is extremely useful, as the stored in the memory can only occupy programmed with lower case char- delay applied to the blanking waveform up to eight lines of the display, as the acters. (The upper-case version is will be similar to the delay of the row-address information to the r.o.m. suffixed CM2140). Switching between character read-out of the r.o.m. (about consists of only three bits of informa- the outputs of the two character 400ns). If the blanking had to be added tion. Fortunately, however, none of the generators is facilitated by their externally to the r.o.m. then some characters having descenders contains "tri-state" outputs. This means that as means of delaying the waveform by a information in the top two rows of the well as having the normal states of 0 and suitable amount would have to be character box, and this enables the 1 at the output pins, a third condition found. A useful feature of the lower- character to be stored in the r.o.m. two can be obtained where the output pin is case circuitry is that only one track on rows higher than its intended display effectively open-circuited from the rest the upper-case printed board has to be position, as shown in Fig. 1. This means of the i.c. This third state is controlled cut when adding the extra board (the ) and all the that when the row addresses are applied by the "chip enable" input, and by chip-enable pin of IC73 to the r.o.m. they must be changed for suitable control of this input, any connections to the existing boards may those characters which have des- number of 2513's may be connected to be made to the underside of digital cenders, in order to lower the display the same five output rails. In this circuit, board two, which entails a minimum position by two TV lines. switching between the two r.o.m.s is amount of disturbance to the existing Figure 2 shows the extra circuitry controlled by gate (90,3). The output of upper-case circuitry. For this reason needed to produce lower-case char- this gate goes to 0 only during columns also, I would suggest that the decoder acters, and in practice these additional 6 and 7 of the code table, enabling IC85, should be built and tested as an i.cs are mounted on a small board which and the inverse of this waveform is fed "upper-case only" unit initially. The fits above digital board I at the opposite to the chip enable pin of the upper-case lower-case board can be added later as end to the analogue board. I.cs 87, 88, 89, character generator IC73, enabling it there is no extra line-up procedure and 90 are used to detect the characters during columns 0-5 of the code table. required when this board is fitted. which require lowering by two rows, The row addresses to IC85 are fed from and this information is available at gate IC88, which is a four-bit binary full Analogue board (89,4) where a 0 denotes a normal adder, type 7483. The A inputs are fed This board serves three main functions, character and a 1 indicates a character with the four-bit row-address informa- namely to provide feeds to the digital that should be lowered by two rows. tion from the line counter 105, and the B boards of mixed syncs, separated data,

IC, pm + 5V Fig. 2. Extra circuits to obtain lower-case +5v 73, 17 letters. 73 18 IC, pin 73 19 0/P IC ptn 4 11. 73 , 4 44, 9 ► 73 20 OA5 73 , 5 44, 11 ► 71 21 —•—• A6 IC88 44, 10 IC BD A7 3 73 , 6 IC, pm Pot 5 IC 8 13 5 6 43, 9 • 1 '10 AB 73, 21 —11. IC90 IC85 3 5 go. 73, 7 A9 bit 2 8 73, 18 IP 10 At 73, 8 73, 19 ► A2 Da 3 C89 A3 Pot 4 CE 73, 20 ® O 0 I C 88 I Csg bit 1 12 8 73, 1 7 IP. 11 0 P qy , 6 — 4 -av 88 —12V + 5V 13 0 bit 5 C9 r R2 IC87 13 1k 66, 12 IP 11 IC Pm bit 6 13 58, 1 bit 4 + 5V 59, 2 ► 12 67, 13 Pa 7 IC go R1 IC.Pn brt 4 1k 59,12 IC 89

6 tO IC73 pin 11 I Cgo I 7 66 Wireless World, April 1976

+5V

VR3 —Ve mixed syncs 1k +5V +5V R9 to IC3 pin 3 Co100n 4k7

C12 C17 R13 100n R10 R8 15k 100k 3k3 1n

C11 331-, —5V -5V comp video input &Ai. 15V Tr3 .C16 R14 BC109 22k 4n7

from IC81 pin 4 C14 C9 4,u.7 -5V -5V 100n (—)e mixed syncs ) data to IC21 pm I 2 00

C10 R6 10n 1k

13 5 R16 IC84 IC84 Tr2 C3 390 7410 7410 BC109 150p • 7577 R, 470 data from -1-1)e clamp lines 11-21 WFM pulse +5V IC81 pin 9 from IC 2 pin 6 IC82 7403 12 C 5 R4 1 C13 11 47p 12 470 1 100n C4 4 IC83 150p D" 13 IC83 IC 11 7400 83 +5V 11:2 7400 7400 470 IC83 R5 7400 470 7 10

11 10 clock C6 IC84 to IC21 pin 8 IC82 7403 7403 C2 220pIC1 Li 30t Poly T Poly560P

and a switched clock waveform which is Fig. 3. The analogue circuitry to produce great detail, however, it would be as suitable both for the writing of data into syncs, data and clock. well to discuss some of the problems the store and for reading it out during involved in successfully slicing the data the display period. The input to the signal. analogue board, the circuit of which is course, be obtained from the TV re- shown in Fig. 3, should consist of a ceiver in which the decoder is to be Data slicing composite positive-going video wave- installed, but it was felt that it would be In the simplest possible system, the form of between about 1 volt and 5 volts better to include one in the decoder if video waveform could be fed via a peak to peak. Tr1 is an emitter-follower only to 'reduce the number of connec- capacitor into the positive input of a buffer which provides a suitable low- tions to the TV set. It also has another differential amplifier and by varying the impedance source to drive the d.c. advantage in that the decoder may be direct voltage level on the negative input restorer formed from C8, Rig and D1 . fed from a "video ring main" where a by means of a potentiometer, sliced This restores the sync tips of the video separate feed of syncs may not be video and data would be obtained at the waveform to a potential of about —1 available. output. The fact that picture informa- volt. Chrominance information is then As well as feeding the digital boards, tion is sliced, and present at the output, removed by the low-pass filter R9, C7, the feed of mixed syncs is used to is immaterial as precautions against this and the remaining video signal is fed to generate a clamp-pulse waveform causing trouble have been taken else- the positive input of a difference which is used both on the analogue where in the digital circuitry. However, amplifier. The negative input is con- board and the video switching interface because of the nature of the video nected to a potentiometer which con- board in the TV receiver. The positive waveform, the varying picture infor- trols the point at which the video going trailing edge of the line sync mation will cause the average voltage of waveform is sliced. The best setting will waveform is differentiated by C10 and the signal to vary, and thus alter the depend on the amplitude of the video R16 and coupled into gate (84,12). The position at which the video (and data) is waveform, but the range of the poten- resulting negative-going pulse is sliced. If the data information were tiometer should be great enough to inverted by gate (84,6) and a positive transmitted as perfect square-shaped cover the whole of the sync portion of going clamp pulse is obtained which is pulses this would not matter because ,the video waveform and enable separ- about 4ps wide, and occurs during the the output mark/space ratio of the data ated syncs to be obtained at the output back porch of the video waveform. information would remain unaltered. of the difference amplifier. This i.c. is in This pulse is used in the analogue However, the data cannot be transmit- fact a high-speed dual line receiver with board to clamp the video waveform ted in this way because the bandwidth fully t.t.l.-compatible outputs and is before slicing the Teletext -data in a requirements would become infinite, ideal for use in this type of circuit. similar way to that used in the sync and the transmission system must be A mixed sync waveform could, of separator. Before describing this in tailored to suit the normal TV band-

Wireless World, April 1976 67 width of 5.5 MHz. The data is in fact transmitted in the form of raised cosine pulses, and this implies that the data must be sliced fairly close to the halfway point between its positive and negative peaks, if the mark /space ratio of the received data is to be close enough to the original for correct decoding. This is the, case even if the received signal is completely undistort- ed by the receiver tuner and i.f. strip. In cases where the receiver i.f. amplifier has insufficient bandwidth or large group-delay errors, or the aerial is mismatched into the receiver, the 1/ setting of the slice level will be even 2w more critical and in very severe cases of "ghosting" or i.f. misalignment it may Fig. 4. A suggested power supply circuit. The system described so far, consist- be impossible to find a suitable point at ing of a clamped video signal, sliced by which to slice the video waveform and turned on during the back porch by the means of a differential amplifier having obtain error-free readouts. The simple clamp pulse, thus holding black level to a variable voltage level at its second system described above could only be approximately 0 volts regardless of the input, is capable of giving very good adjusted to give satisfactory results average level of the signal. Purists may results under most reception condi- during periods of static transmission point out that the clamp action will be tions. The slice level setting can be fairly such as test card, where the picture upset by the colour burst. In practice, critical under adverse conditions, how- information is constant and the slice however, this only becomes a problem if ever, and although the black level end of level would remain unaltered. large variations in level of the the waveform is maintained at a fixed One way to overcome the problem of chrominance information occur, which level by the clamp, variations in ampli- changing level of the video waveform will in turn cause the clamping point to tude of the video signal will also cause would be to use the d.c.-restored video vary slightly. If this is really a problem unwanted movement of the data level. present at the cathode end of D I . that cannot be solved by improving the Small variations between the signal However, a better system is to clamp aerial installation, then a 4.43MHz amplitude of different TV channels the video during the back porch, and tuned circuit should be included in which may occur with some types of i.f. this method is used in this design. Tr2 is series with the collector of Tr2. It has strip can be enough to prevent correct not been included as a standard feature decoder operation. A more likely cause in the circuit, partly because it has not of video amplitude variations will occur proved niecessary, but mainly because in TV sets where the contrast control 9 it would be rather difficult to set up operates in the a.g.c. line. In this type of. 8 set the detected video information will 7 correctly without expensive test gear. 6 vary as the contrast control is changed to thumbwheel 5 switch contacts and nowhere in the circuit will a video 4 in parallel waveform be available that does not 3 Fig. 5. Function switching. vary with contrast control changes. It is 2 The "CLEAR" switch is a obviously undesirable to have to alter 1 push, non-locking type, the slice level whenever the contrast 0 the "TIME" button is a control is adjusted and some form of push-push, locking switch 7-1—Ilik green output ) to video and the rest are interlock- compensation must be provided to r--1-1. red output 1interface board ing. Further construction- automatically keep the slice level in tv receiver r--1-1. blue output al information appears in correct. One method would be to have a the next article. variable gain video amplifier which was hours tens automatically adjusted to keep the data hours units amplitude constant. However, this digital minutes tens to thumbwheel board minutes units > Switch wiper two page no. tens contacts page no. units magazine no Output blanking control I► to video switch interface board time 'of f time Select time 'on clear page 'cut hole' 'roll' auto newSf lash sel

white output +5VO—AAA, Of required) 470 - L .-0.****41111 'roll' Switch

CLEAR TV TELE SUB NEWS TIME TEXT TITLE FLASH SELECT 68 Wireless World, April 1976, would undoubtedly be expensive, and a providing the spikes derived from much simpler way of achieving the positive-going data transitions, and Who thought up the same end is to alter the slice level gate (82,6) spikes derived from nega- synchronous satellite? automatically, to enable it to follow the tive-going data transitions. Although varying data amplitude. This is done by the oscillator circuit may appear rather detecting the data amplitude with a crude, it has been found to give peak-detector circuit and then using excellent results in practice. The main this information to set the slice level point in its favour is that it is extremely midway between the data peaks and easy to set up, as there is only one black level. adjustment, that being LI. The specified Dr Harold A. Rosen, a vice-president Tr3 forms the positive-data peak coil former is the Neosid A6 assembly, of Hughes Aircraft Company and a detection circuit with the decay time set but in practice equally good results will pioneer developer of synchronous by the time constant R15, C15. This time be obtained with any former of approx- communications satellites, has won the constant is made fairly long compared imately 3/16in diameter, containing an first L.M. Ericsson International Prize to the data bytes to prevent too much adjustable ferrite core, so long as it can for "proposing the introduction of decay during the worst case condition be tuned in to the correct frequency of geostationary communications satel- of fourteen consecutive zeros. TR4 then about 7MHz. The frequency stability of lites and for his scientific and techno- serves to offset the base-emitter voltage the circuit has been found to be perfectly logical contributions to their develop- and the shorter time drop of Tr3 adequate so long as polystyrene capa- ment, design and operation." The prize, , increases the rise time constant R2, C16 citors are used for C1 and C2. The of 100,000 Swedish crowns (about of the peak detector circuit to reduce preferred method of adjusting the £11,000) will be presented by King Carl the effect of large noise spikes. The oscillator frequency will be dealt with XVI Gustaf at ceremonies in Stock- actual slice level is adjustable by means later in the article, as will the rest of the holm in May. To be awarded every of VR2 over the full range from 0 volts to decoder line up. three years, the prize honours the the positive data peak level. Although memory of Lars Magnus Ericsson, theoretically this potentiometer should founder of the L.M. Ericsson Telephone be in its mid position for correct data Power supply Company. slicing, non-linearity distortion intro- The power requirements of the decoder Dr Hakan Sterky, chairman _ of the duced by certain types of vision detec- are fairly modest. Five volts for the t.t.l. prize committee, says "Dr Rosen tor circuit will mean that the best circuitry is required at approximately proposed that a single satellite could be results may be obtained if the slice level 1.3A, and this can most conveniently be orbited at an altitude where it matches is not mid-way between the positive and obtained from a three-terminal regula- the earth's rotation and appear to be negative peaks of the data. The differ- tor of the LM309K variety. The input stationary, thereby simplifying con- ence amplifier used to perform the voltage to the regulator must not be too nection with earth stations and pro- actual data slicing is the other half of great, however, as the device will be viding 24-hour-a-day service". British the dual line receiver, IC81. working fairly clOse to its limits and readers, in particular, will be surprised may exceed its maximum power dissi- that no acknowledgement is made to Clock generation pation figure. For this reason, the Arthur C. Clarke, who is widely regulator must be mounted on a suit- considered to be the originator of the It has already been explained that the able heatsink. idea of satellites in synchronous orbit. clock waveform generated on the A negative five-volt supply is required Clarke pointed out in the October 1945 analogue board consists of the outputs at a current of about 25mA, and a issue of Wireless World (eleven years of two oscillators, one locked to the negative 12-volt supply for the char- before Dr Rosen joined Hughes) that a incoming data, and the other adjustable acter-generator i.c. at only a few space-station orbit with a radius of in frequency to enable the display width milliamps. Both negative supplies may 42,000km "has a period of exactly 24 to be adjusted. Switching between the be derived from simple zener-diode type hours. A body in such an orbit, if its two clock generators is performed by stabilizers, and a suitable power supply plane coincided with that of the earth's the 'lines 11-21' waveform. Gates (83,11) circuit is shown in Fig. 4. equator, would revolve with the earth and (83,6) are cross coupled to form a Constructors should see that the and would thus be stationary above the free running oscillator. Oscillation is connecting wires between the power same spot on the planet. It would inhibited during lines 11-21 by the supply and the decoder are of a suitable remain fixed in the sky of a whole waveform present at gate (83,3), and by gauge to prevent excessive voltage drop hemisphere and unlike all other also inhibiting oscillation at the start of of the plus five-volt rail. At a current of heavenly bodies would neither rise nor each TV line — Q of monostable 3 is fed something greater than one amp, it only set." Further, a satellite in this orbit into gate (83,11) — the oscillator is takes a few feet of thin connecting wire "could be provided with receiving and phase-locked to the TV lines. This to cause a voltage drop of greater than transmitting equipment . . . and could ensures that the characters will have 0.25 volts which will be sufficient to act as a repeater to relay transmissions "clean" verticals. If the oscillator was bring the five-volt rail outside the between any two points on the hemis- merely free running the phase would recommended specification for t.t.l. phere beneath . . ." (See "Extra-terres- alter at random from one TV line to the devices. tial relays", October-1945, pp. 305-308). next, and ragged verticals would result. (To be continued) Hughes Aircraft state that all of the VR1 adjusts the frequency of oscillation Intelsat communications satellites "are and forms the display width control. a result of Dr Rosen's synchronous- Gate (84,8) forms the active part of orbit concept." This is strange in view the data clock oscillator. The frequency of the fact that it was a vice-president of oscillation is determined by LI, CI and of Hughes Aircraft Company, Dr F. P. C2. The waveform on pin 9 of the i.c. Adler, who gave public acknowledge- only allows oscillation to take place ment to Clarke's proposal more than a during lines 11-21, and at all other times decade ago (June 1965 issue, p.269). the gate output is held at 1 allowing the Moreover, L. M. Ericsson have indicated display clock through gate (83,8) to the The next article in this series will give that they know of Clarke (although output line. The oscillator is locked to constructional details of the teletext they do not seem to be aware of the the incoming data by means of narrow decoder. Subsequently there will be an thoroughness of his 1945 proposal). negative going spikes fed into the third article on interfacing the decoder with Readers may be forgiven for question- input of (84,8). These spikes occur at various colour television sets in com- ing whether it really was Dr Rosen's every data transition, gate (82,11) mon use. concept. 64 Wireless World, May 1976 Wireless World Teletext decoder 7 — Construction and interfacing with the television receiver by J. F. Daniels

Construction techniques is probably the safest, if slightly more any faults will be located more easily if Many different methods of construction expensive solution. the lower case board is not present. are possible because the circuit is fairly Capacitors, resistors and preset non-critical in terms of i.c. layout. potentiometers can then be added. Video switching and interface High-frequency decoupling should be Some of these components may requirfe This board is mounted in the TV used on the power supply rails, and soldering on both sides of the board and receiver and must be capable of switch- about one 0.047p.F capacitor for every the best rule to follow is: wherever there ing the red, green and blue outputs of ten i.cs should be adequate. Extra is a roundel, solder it! the Teletext decoder into the receiver in decoupling should be employed close to When both of the digital boards have place of the TV picture. Before describ- the two clock oscillators and also on the been completed they should be joined ing some typical receiver and video plus and minus five volt rails close to together by wire links along the rear switching circuits, it would be as well to IC81, the dual difference amplifier. edge of the boards. These links are best examine the problems which will arise, For those constructors who intend made from lengths of insulated strand- and describe ways of overcoming them. to use the p.c. boards available from ed wire, each one being about two Let us assume that we have an "ideal" Catronics Ltd, the following hints on inches long. This enables the boards to receiver which we wish to modify. This construction may be useful. The first be "opened out" if access is required to receiver will have three identical video thing to do is to make up the through the i.cs on the lower board. At this amplifiers for the red, green and blue stage the four power supply leads may connection holes using tinned copper signals and the amplifiers will have wire. As there are a great many of these be connected to the lower board. (Note capacitor-coupled inputs, i.e., the to do, the following method will prob- that the two leads for the —5 and —12 amplifier will be internally biased and ably be found to be the quickest. Support volt supplies only go to the lower board, clamping of the three signals will take the board,component side down, about and no links are used to the upper place at the c.r.t. cathode. The ampli- digital board at this point.) fiers will require an input of about 4V 1/,in above a flat surface. Take a long length of suitable tinned-copper wire The analogue board may now be pk-pk and have a flat amplitude and push it through a hole that requires constructed. This is only a single sided frequency response up to at least connecting through until it rests on the board and a number of wire links are 5.5MHz. surface under the board. Solder it on required as shown on the layout If a receiver with this type of amplifier the upper surface of the board and cut diagram. The analogue board is intend- were being modified, a simple method off with side cutters. Continue by ed to be mounted above the upper would be to use a three-pole changeover soldering all the wires on the same side digital board at the right hand end. relay to disconnect the amplifier inputs of the board, and then turn it over and Three wire links are neeaea at the upper from the red, green and blue picture solder the other side. The underside of (short) edge to the lower digital board, information and connect the outputs of the board is done first because it is for the plus 5, minus 5 and 0 volt rails, the Teletext decoder. Although this easier to differentiate between con- and five more links at the top end of the method would produce a perfectly necting holes and i.c. holes on this side. right hand edge are connected to acceptable Teletext display, it would Holes for capacitors and resistors are adjacent pads on the uppermost digital have the disadvantage that newsflashes distinguished by the fact that they have board. These links to the analogue and subtitles could not be shown in larger "roundels". board should be long enough to allow boxes, as intended. After the connecting process has removal of this board, to allow "unfold- In order to display boxed information, been carried out on the two digital ing" of the two digital boards. the video switches must operate very boards, the i.c.s. may be mounted, and Connections to all the decoder func- rapidly, preferably with a switching care should be taken here to ensure that tion switches are made to the right- time of less than about 300ns. This no pins are left unsoldered. Only some hand edge of the lower digital board and however, is not the complete answer, pins require soldering on the upper this board projects further than the because if switching is attempted surface of the boards and these are other boards at this end to facilitate between the decoder output and capa- indicated by roundels. Where space connection of the leads. The suggested citor-coupled or "floating" picture permits these roundels have extra tabs method of connecting the function information, the background brightness to increase the soldering area. Great switches as shown in Fig. 5 (April). and colour of the display box will care should be taken when soldering the If lower-case characters are being change, depending upon the average m.o.s. random-access memories on included in the decoder the extra board level of each of the three picture board one, and similarly the read-only should be made up in a similar way to waveforms. This can be overcome fairly memory on board two. Although all the the two larger boards. The extra board easily by clamping the three TV wave- inputs are protected to a certain extent is intended to be mounted above the forms and the three Teletext output against static charges, care should be upper digital board at the left-hand end, waveforms to the same potential prior taken to ensure that the soldering iron opposite the analogue board. Wires to switching. The output of the video tip is adequately earthed. The printed- should then be connected from this switches can then be capacitor-coupled circuit boards have been designed in board to the underside of the lower into the output amplifiers as before. such a way that i.c. holders may be used digital board. It is advisable, however, Figure 1 shows a basic video switch- for the m.o.s. devices to avoid soldering to get the decoder working and lined up ing circuit, and this will be described on the top surface of the board and this before adding the lower case board, as initially. Alterations to the basic circuit

Wireless World, May 1976 65 will then be considered in conjunction lector t.t.l. gates, because \correctt effectively increasing the width of with different TV receiver designs. The operation of the switches requires that character verticals.) actual switching elements are con- the control-input voltage must The three capacitor-coupled video tained in IC1 ,2, and these are c.m.o.s. approach the supply voltage. The input signals are clamped to the potential CD4016 analogue switches, each with. from the decoder function switches is which is set by R1, VR1 and R3. Similar, its own control input. When the control normally held at 0 by a 470S2 resistor, the Teletext signals are clamped to a input is at the 0 level, the input and ensuring that the switches are left in the voltage set by R7, 128 and VR2. In this output terminals are effectively open TV position if the decoder is not particular arrangement the actual circuited, and when at a 1 level, the connected to the switch board. Resis- clamp voltage is arbitrary but would input and output terminals represent a tors R4,5,6 are chosen to reduce the probably be best set to about 3V. Two resistance of approximately 300 ohms. amplitude of the Teletext signals to the separate potentiometers are used so The device will pass frequencies up to same as that of the TV signals, and that the black level of the Teletext about 10MHz, which is ideal for our capacitors C4,5,6 may be added to boost signals can be varied independently of purpose, and as long as a fairly high the h.f. response if this is found to be the picture brightness, and thus the supply voltage is used, it is quite linear necessary. (Because of the high fre- background brightness of the inserted in operation. The slight disadvantage is quency components contained in the box may be adjusted. The reason for not that, being an m.o.s. device, the switch verticals of the alphanumeric char- clamping to 0 volts is that the c.m.o.s. time is not particularly fast, and this can acters, some commercial receivers may switches are not particularly linear cause slightly coloured edges on the not have a sufficiently good h.f. when the input signal approaches the inserted box. However the advantages response to display them adequately. extremities of the supply voltage, and of simplicity and cheapness outweigh Later in this article a simple modifica- raising the potential of the signals this slight disadvantage. tion to the Teletext circuit will be ensures linear operation of the switches. The typical switch arrangement described which reduces the h.f. After the video switches, capacitors C1 , shown in Fig. 1 uses two CD4016 i.cs. requirement of the video amplifiers by 2, 3 remove the d.c. component of the The control inputs are fed from the waveforms before they are fed into the outputs of two high-voltage open-col- Fig. I The basic video switching circuit. receiver video output amplifiers. (It

15-30V Y from tv 10 +15V from 1110 tv set +15V IC 1 12 5011. 1 see 25V text

.1 Y to tv VR3 IC2 12 capacitor In tv set red from tv

+15V C4 1k 'Cl • I.I • • — • R red output I " 10p. I I from +C 1 red to teletext I h—opoo output T amp decoder IC 2 1k 1k +15V 3 VR 1 green from tv — — — — Tr3 IC1 C5 1k - - green output - C2 from green to teletext I1--- output decoder Tr4 ► amp 1k 1k IC2 VR2 blue from tv

T 1k IC1

Ce r- 4 k7 blue output 10 11. IC2 C3 from blue to teletext output Tr decoder amp. 1k 1k 3k3

4k 7 +15V IC3 13 12 11 1 0 By • from decoder 100n function switches 3k3 clamp pulse 470 1k from analogue board -II 100n +5V +15V

66 Wireless World, May 1976

insert video switches here + isV 6µ4 ttt I—° 3C54 to c r t red cathode

to green , r- 21s2 amp '`-53

Fig. 2 Video output circuitry of the Bush CTVI82S, CTVI84S and CTVI87CS

5 insert video switches here 2k7 to red cathode L 122

I 5k6 I .c: 10k `', resistors `, added to 41 maintain load I on IC I

„Tr

to brightness control voltage

Fig 3 The BRC 8000 shown above, while below is the Philips G8 video output.

0 :o._ v

39k insert video switches here 1k „._ to red ,1.- cathode 10n 11, 11 0— -0 BD115 10 39k 8----,\AA„—___. + 25V 1k 184530

R-G- B-Y 470p matrix 92 I 10n i100 1k2 T..3.3p

115 Wireless World, May 1976 67 should be noted that the polarity of c.m.o.s. switches must not.be allowed to, purpose we can use the two remaining these and the input electrolytics will go outside the limits of the supply c.m.o.s. switches. By connecting them depend on the individual receiver voltage to the i.c., as incorrect operation as shown in Figure 1 it is possible to design and may. not be as shown.) will result, with possible breakthrough insert the switch in series with the Figure 1 shows a fourth pair of switches on switches which are supposed to be in luminance chain at some suitable which are not required when modifying the off condition.) low-voltage point. VR3 may be used to a receiver of the type described. They Other receivers using the BRC8000 adjust the d.c. conditions during are useful, however, when modifying a chassis are the Ferguson 3712, Alba 8000 switching to keep the black-level cor- set which has colour-difference ampli- and Marconiphone 4712 to name a few. rect. fiers (R-Y, G-Y and B-Y), as they can be Similar output stages are also used in Other sets using this type of output used to switch off the luminance signal, Decca models CS2030, 2230, 2630 and stage are as follows: Bush CTV25 and which would otherwise be present while 2631. CTV167; Murphy CV2510 and CV2511; watching the Teletext display. The three types of output stage just Baird 700 series and 710 series; Decca Figure 2 shows the circuit of one of described cover the majority of the CTV25; Pye CT79, CT152 and CT153; three identical output amplifiers used in more modern British-made sets. How- Dynatron CTV1, CTV1CH and CTV2; Ekco CT102 and CT104; Ferranti Bush models CTV182S, 184S, 187S, 192, ever, there are still many older types of 194, 196, 197C, 199 'and 1026. For sets receiver giving good service and it was CT1166 and 1167; Invicta CT7050; GEC which have output amplifiers of this the tendency until fairly recently to 2040, 2041, 2073, 2100, 2103 and 2107. type, i.e., capacitor-coupled, positive- drive the grids of the TV tube with Before leaving the subject of modify- going R, G and B video inputs of less colour-difference, valve video output ing TV receivers, the BRC2000-3000 than about 5 volts peak to peak, the stages. (All the circuits so far described series of receivers, which do not really circuit just described will be adequate. feed the cathodes of the tube with the fall into any of the previous categories, Other receivers using similar amplifiers red, green and blue signals.) should be examined. Figure 5 shows a are: Murphy CV1916S, 1917, 2211S, 2212, This older type of circuit had three video amplifier used in the earlier 2213, 2610, 2611 and CT2516CS; Alba identical valve output stages, each one 2000-series chassis, but the later 3000- CS1919; Decca CS1910, 2211, 2213, 2611; usually consisting of a triode-pentode and 3500-series used a configuration ITT/KB CK600, CK500, CK701, CK 822, valve of the PCL84 type. The pentode very similar to this and for our purposes CVC5, Colourscope 20 and Studio 100, section was used to drive the tube grids can be considered the same. The three and probably quite a few others. with either the R-Y, G-Y or B-Y video amplifiers are basically identical, signals, and the triode section was used with one important difference. The R-Y Next will look at some receivers to clamp this signal at the tube base. and B-Y circuits are both as shown in which differ from those just described The cathodes of the tube were effecti- the diagram, but the input capacitor of by virtue of the fact that the output vely joined (sometimes potentiometers the G-Y amplifier is connected to the amplifiers are directly-coupled. The were used to vary the amount of drive +30V rail, effectively earthing this amplifiers still carry the R, G and B to each cathode), and fed with the point to a.c. and turning the first signals, but instead of being clamped at luminance output signal from a separ- the tube cathodes, the signals are transistor into a common-base stage. ate pentode valve. The emitter of this transistor is fed with clamped earlier in the circuit and A typical circuit used in the Pye CT70 directly-coupled from this point different amounts of R-Y and B-Y series chassis is in Fig. 4. All three signals to form the G-Y signal. through to the tube cathodes. Two output stages are identical, and they are The best approach to modifying this circuits are shown in Figure 3; the first fed from three transistors, two of which type of receiver is to treat the three is a type used in the BRC8000 chassis, are used as R-Y and B-Y amplifiers, the amplifiers as identical capacitor- which is used by a number of different third being used to derive a a G-Y signal coupled amplifiers and put the three manufacturers, and the second is a from the other two. Although it might video switches in the base circuit of the circuit using Mullard i.cs used in the be possible to drive the basis of these input transistors. Extra capacitors Philips G8 chassis, which is also quite transistors, the three circuits are not should be used in series with the inputs widely used. identical at this point and trouble might to the video switches, since the colour- Because these sets have directly- be experienced with the green channel. difference signals are close to the 30V coupled amplifiers, the easiest course of The best place to feed the signal in is rail at this point and outside the range of action is to remove the capacitor almost certainly at the grid of the the 15V supply rail permissible for the coupling in the original video switch pentode valve as shown in the diagram. c.m.o.s. switches. The luminance chain circuit (remove CI, 2, 3) and this means The only disadvantage of feeding in must also be broken before it feeds the that in the TV mode the only change in here is that the Teletext signal must be bases of the output transistors and this operating conditions will be the addi- negative-going at this point to produce can most easily be done at the input to tion of the effective resistance of the the correct display. the luminance emitter follower (not c.m.o.s. switch, about 30052, which will There is no reason why $the Teletext shown in the diagram). As it stands, this almost certainly be negligible. Since the signals should not simply be inverted, circuit will then produce reasonably TV signal is effectively clamped at this using three ordinary t.t.l. inverters, acceptable results although, because a point in the circuit, we no longer require prior to the attenuating resistors R4, R5 certain proportion of the red and blue Tr3 and Trs. The Teletext input and R6. The six capacitor-coupled signal is still being fed into the G-Y circuitry can remain unchanged and signals can then be clamped using Tr" amplifier, the green Teletext display VR2 should be adjusted to make the as before, but the clamp potentials appears rather too bright in relation to Teletext signal black level about the should be greater than before, at about the red and blue signals. The solution to same as that of the TV signal at this 6V, as the signals are now negative- this problem is to use two extra c.m.o.s. point. VR2 effectively forms a Teletext going at this point. Capacitors C1 , C2 switches, one in the R-Y feed to the brightness control. and C3 should be used to couple the G-Y amplifier and the other in the B-Y It should be noticed that three 10kS2 switch outputs as before, and these feed to this amplifier. These switches resistors are required on the output pins capacitors can be considerably smaller should be wired so that they are in the of the matrix i.c. in the BRC8000 circuit. than before as they are feeding valve open-circuit condition during the Tele- These resistors form the emitter load for grids. The Teletext tisplay will now text mode of operation. Resistors R4, 5, 6 the output transistors in the i.c. when appear on the screen, but it will be on the switching board will need to be Teletext is selected, to prevent the superimposed on a black and white TV about 101(2 for this type of receiver, and signal voltage at this point rising picture because the luminance signal is C4, 5, 6 about 10-15pF. towards the positive supply rail and still connected to the tube cathodes. The 12-15 volts required for the upsetting the operation of the c.m.o.s. Some means must be found of switching c.m.o.s. switches can easily be obtained switches. (The input signals to the off the luminance signal and for this from the video amplifier power rail in 68 Wireless World, May 1976

Fiji. 4 Video output stage of the Pye +225V C770/71. 3k9 to other anode circuits

12k 8M2 680p R -Y output Insert video to c rt grids switches here

470 ,---- 100 to other grids 100 9 R-Y from 22n 47n colour demodulator line pulse

2M2 p. to other 8k2 cathodes 4- 270 1n5 8k2

to other a& screen grid circuits

the TV set, using a simple zener diode other filters. The signal should To round off the series of articles, the stabiliser if the rail is higher than 15V. preferably be taken from a low-impe- final part will include a description of Most sets have a supply of between 12V dance point in the circuit, to reduce any setting-up the decoder and a suggestion and 30V and the current drawn by the losses in the coaxial cable feeding the for reducing the effects of restricted switching circuit is minimal. signal to the decoder. A large number of bandwidth in the television receiver. We This concludes the desciiption of modern receivers employ i.c. synchron- also hope, at a later date, to publish a colour-receiver output stage modifica- ous demodulators of the MC1330 type; modification to allow a new, combined tions, but there remains the problem of these are ideal, providing around 2V of upper- and lower-case character gener- obtaining ,a suitable video signal from positive-going video output signal. ator to be used. In addition, we will try the set to feed into the Teletext decoder. Some older sets such as the BRC 3000 to include information on modification There should be no serious problems series employ separate luminance and to the circuit to enable it to be used with here, but since the signal that is fed into chrominance detectors and in this type the Post Office Viewdata service. the decoder will determine whether or of receiver the luminance detector Lack of space will prevent the publi- not it operates correctly, it is rather output should be used. This is not cation of printed-circuit board layout important to get this bit right. The normally restricted in bandwith suffi- and component disposition, but we have signal into the decoder should be a ciently to upset the operation of the them at this office and will send copies positive-going video signal of between decoder. If a suitable positive-going to anyone who cares to write and ask about 1V and 5V peak to peak, not video signal does not exist, then a small for them. Please send a large, stamped limited in frequency by chrominance, or transistor invertor may be needed. and addressed envelope.

12 4- 30V +270V 56k Op.5 6k

MN= from O BA148 R-Y 5k6 O demodulator --WV"' to red cathode • 14 111. 68p 68k insert 33k video Tr11 0A91 switches BF178 here line pulse 68k

Tr7 E5024

(0Tr10 2u. 11 1k5 4P-2:130p BF178

50 82

• from luminance emitter 860p oil ower

-Y to other to G amps Fig. 5 BRC 2000 series output. matrix Wireless World, June 1976 53 Wireless World Teletext decoder

8 — Modifications, and lining up

by J. F. Daniels

Most constructors will probably want to appearance of the characters is The circuit arrangement using the use the decoder with a colour television obviously distorted. A suitable control new i.c. is shown in Fig. 2 and is almost receiver, but for those who use a setting should be found which gives the identical to the original circuit using the black-and-white set, a few words on best character appearance. upper-case 2513. The differences are interfacing are needed. that all seven bits are now fed to the Upper and lower-case characters character-select inputs and the four-bit Monochrome receivers Texas Instruments have recently row-select inputs are fed from the QA , If a black-and-white receiver is to be announced a new character generator QB, Qc and QD outputs of IC5 directly. modified then there are two options i.c., the X887, which has been developed The Y1 -Y5 outputs feed, as before, into open to the constructor. The decoder as part of their Tifax decoder module, the five open-collector gates, although may be wired in, in a similar manner to and as the i.c. was specifically the timing of the strobe pulses from IC42 that described for the colour-receiver, or designed for Teletext decoders, it has a has been changed slightly to allow for a separate tuner and i.f. strip may be number of advantages over the 2513. the faster access time of the X887. used, the output of the decoder being Firstly, it contains a complete set of The cost of the X887 is around £20, fed via a u.h.f. modulator into the upper and lower-case characters, which which makes it rather expensive com- receiver's aerial socket. This second includes all those characters defined in pared with the 2513, but if a decoder method has the advantage that the the latest Teletext code table. Secondly with the lower-case facility is required, decoder may be electrically isolated it has provision for a four-bit row-select then the X887 is the obvious choice. from the TV set. Whichever system is input code which means that all ten used, the best display will be obtained rows of the characters may be by mixing the red, green and blue addressed independently, and no shift- Line-up procedure outputs of the decoder together by ing of address codes is required for The following line-up procedure has means of variable resistors, the resis- characters with "tails". Thirdly, it only been devised after lining up six tances of which may be varied to give requires a +5V supply, which means ' decoders, but it does assume that the the desired grey levels for different that the decoder power supply may be decoder has no faults and is being fed transmitted colours. Mixed syncs must simplified as the —12V rail is now with a suitable video signal as described also be added at this point to lock the unnecessary. earlier. It would also be advisable to scan circuits in the TV set. The problem check that data is actually being most likely to be encountered with transmitted on the selected TV channel, black and white sets, especially the since the system is still in its experi- older ones, is lack of bandwidth in the Fig. 1. This modification reduces the mental period and transmissions may be i.f. strip — a good reason for using the degrading effect of poor video interrupted occasionally. To check the extra i.f. and modulator approach. frequency / amplitude response in the transmissions are present, reduce the television receiver. The effect is to height of the T,V picture, or offset the Character verticals broaden vertical components of vertical-hold control and look for lines The following modification to the characters.- of constantly-changing white dots and decoder circuitry has been found advantageous because it reduces the h.f. response requirements of the receiver video amplifier output stages. Fig. 1 shows the circuit, which uses existing "spare" i.c. gates. The extra gates are wired in series with the output of gate (57, 6). Gates (75, 8) and (75, 3) pass the signal through unchanged, but gate (75, 6) adds to the original signal a pulse derived from the trailing edge of the display character components. VR3 can he adjusted to give the best character appearance, and the setting of this control will depend on the fre- quency response of the receiver video amplifiers. At zero resistance setting, the signal passes through this stage unchanged, but as the resistance is increased the width of the character verticals gradually increases until the

54 Wireless World, June 1976

dashes across the full width of the TV coil for the most consistent 14. The rolling-through action will line in the field blanking interval. rolling-through action. If the coil probably cease. Adjust VR2 on The line-up procedure requires no former being used is not the speci- digital board 1 carefully, until the test gear, but the power supply rails fied one, then the oscillation fre- rolling-through action begins should be checked with a multimeter quency can be checked as follows: again. This control has an "on-off" prior to following these instructions. — disconnect the uppermost wire action, unlike those previously Set the six preset potentiometers link to the analogue board, (lines adjusted, and is necessarily quite (three on the analogue board, two 11-21) and adjust L, to give a critical in its setting. The adjust- on digital board 1 and one on digital display of approximately the cor- ment should be checked on differ- board 2) to mid-position. A screen rect width. (Disconnecting this link ent TV channels, since there may full of random characters will inhibits the display oscillator and be slight differences in the position probably be obtained, because the makes the data oscillator operate of the framing code between dif- flip-flops in the r.a.m.s take up the whole time, so the characters ferent channels. random states at switch-on. Do not, will appear rather ragged). If this is Switch off the roll mode and select a however, push the clear-page but- outside the range of adjustment of clock-cracker page (at the time of ton at this stage. L1, adjust the value of C2 until the writing page 899 on Oracle or page Adjust VR3 on the analogue board width of the display is approxima- 140 oh Ceefax). This page is used to (set sync separator) to give a tely correct. Reconnect the link to accurately check the frequency of display that is free from vertical the analogue board. the data oscillator, and consists of jitter. Turning the potentiometer Adjust VR2 on the analogue board 14 consecutive Is followed by two too far in one direction will cause (slice level) to improve the rolling Os repeatedly across each row of the display to jitter vertically and through action and some correct the page. This means that the data move down the screen. Turning it pages may now be observed oscillator gets the least possible too far the other way will make the changing very rapidly. Continue amount of locking information on display disappear altogether. adjusting L, and VR2 alternately each line and so must free run at Adjust VR, on the upper digital boatd to give the best rolling action. (N.b., the correct frequency for almost (horizontal shift), and VR, on the when the roll mode is selected the two bytes of data. If L, is not quite analogue board (display width) to decoder reads out each data row as correctly adjusted, a regular pat- give a correctly-centred and it is transmitted, and this enables tern will appear on the left-hand suitably proportioned display. the effect of each adjustment to be side of the screen which will Connect a link between pins 13 and 14 observed, instantaneously, rather gradually disappear or turn to of IC9. This disables the "framing- than having to wait for up to 15 "rubbish" on the right-hand side. L, , code allow" monostable, and this is seconds, which would be the case if should be adjusted a quarter of a essential during the first part of the pages were selected in the normal turn at a time, until the page setting up procedure. way. readout produces a pattern which Switch the decoder to the "roll" mode Remove the link from IC9, pins 13 and is even over the whole screen. The and adjust L, to give the maximum two photographs in Fig. 3 show a amount of "movement" of the correct and incorrect clock-cracker display. The characters should start readout. These photographs were changing at random and this may Fig. 2. The revised character generator, taken from the output of a decoder appear to be happening on conse- which uses the Texas X887 to display which had a lower-case character cutive rows from top to bottom of both upper and lower-case characters, generator installed, and the char- the screen, giving a "rolling-thro- as well as the complete Teletext symbol acters displayed when an upper- ugh" appearance. If so, adjust the set. case-only decoder is used are

42.2 42,3 42,4 42, 5 42, 6 5,11 V 5,8 +5V 13 n 11 9 5 +5V I 5, 9 IC 67 ) IC 66 z:3 5,12 12 10 6 1k

bit 1 28, 2 NI 57, 9 28,15 z 7 69, 9 / 5 z xz 28,10 •-• z IC 75 z 13 28 7 z e`s z 2 28,12 V V 28, 5 zz

IC 74

Wireless World, June 11376 55

Fig. 3. The "clock-cracker" page, with L, (April issue, p.66) correctly adjusted Is shown at (a), Mule the result of incorrect tuning is seen in (b). Cirlirak 146 Thu 14 Rub 10 111/40 i( f21 31.3riltolM033 ' -11.11.1111111,11.....M.-11.-. 11.-.-1freoll-w11.111511:1:151n:eitilt - ,11011.1110 '' -...11*O.15-11011.11.11.-1111-6-11-.O.-11,2,11,1,...... s..,,,s,...... 0,0,s,s,...,wil-.11.410111.11-11-11,1110 ...... ,s,...... g,../...... different to those shown. The ...... ik...... s...... ,...... ,...... principle of adjustment is, of _•-•-•11.11011fre.-11.-11-11.11-'11-11-'11.-11-'1,r1-111-11-11ele course, the same. ii ...... -a-,...... ra...w....s...... g...... 11,..... Finally adjust the slice level control 111., .....,..c...... ,...... s...... E...... ,...... ,•••• (VR2 on the analogue board) to give 111-111.1-0-1e2.-11-41-11.-11.-2-11.-•-1-11-111-11011.-11-410111fr the most error-free page readouts, ...'O-110.64-06.1.-11-11-1-.S.-0,-M-1101110O-1-11..W.110 selecting pages' in the normal II -11--0-11--0-011-••.-0.-0-1.-1.-14.-0•0-,...11-1,•0011-110 0.-0...0.-6diED011.4,O.0.0,-.0-'0-004.-11-1101101-11.11.011.6.' manner If the contrast control on 0 ..0-14.-O.11.04-11.00.-0.-0.-11.-11.-.4.-0.-0.-11.'0-,2.1100.' the TV set alters the amplitude of 11 .-11..4•6111...116-1100-.M....0.-0.-0.-4.-0•11.-0.-0,-0.11.,0-11,- the signal being fed to the decoder, 111.- O-11-.-,1-11100-11.-64-11-.M.11.-M-110205-11, 111 -,11#11.11-411.11-1k-O-.14-11...2...M.-4011-.110A-111-08-4.1110111.. check that both the sync separater ,...... 111,8„.11-11,2,10...p...,B,...... ,...... s.s,S•11.-2•• and the data slicer work satisfac- O-11-2411.11.4,11•.1-1-4-1.11-11,, 11 ....-1,,m-o-11,.....,s,...... ,g,..11,11.WiliwO.-O-110111P1. ,.. torily over the required range of the 1 control. -...... s.ripli-s-a--9...... ,...... e. P. IP Check that the decoder tunction 1 .-.11.-41-11S.-4-1•0-.11-.- 11...-S..11..0-11.-11-.114-11-11.. •I ... switches operate correctly and adjust VR3 on the lower digital board to give the most pleasing character appearance.

Fault finding Experience with a number of decoders has shown that faults are almost invariably caused by i c pins being left CUP** 144 Thu 14 4u. SO 4801 unsoldered, especially on the top side of 71sniterine 'Os -G till' 4 i a...... s.e...... R a i the p c. boards, or missed connected- ■ ( C C through holes, so it is best to check the 4 C i ::11:1:121 .1; 2 C boards very carefully before attempting g10qp0.a..s.s.,.. yg G C G the line-up procedure 8.0.14-..., m up G 7 Faulty or suspect t t 1 gates can be 0...... ,411,..., Ix 0 c a,..p.....,D. 1.2..L. ... checked fairly easily by means of d.c. a ...s04000 ..0... tests (remember that t.t 1. outputs can low.. 0.0 X III dm q C c be shorted to the OV rail for "in-circuit" 111.,-.... Mx q G 111,4 111.111.-11-• Im 1 c testing of following gates, but must not 111..111-1fr '# G 1 imi be connected directly to the +5V rail.) ....r e st.....060...... -0.1.-8-45-e-r-o-s ex q If an oscilloscope is not available, s...... g...... -0- suspect counters can only really be G 6 checked by substitution Faulty r a ms . ..1.0110 0. 4x C ....p...... • C can easily be detected by spotting an .,011110.-g..... X C 6 abnormally large number of spelling .0. .._ 0, 0.0...... 11 mistakes on each page and by consult- • 0 0 04000.080.0. 11* ing the code table, the bit which is common to all the incorrect letters can usually be located

Installation A few final hints on wiring the decoder into a domestic receiver may be of some receiver, to An sure that every possible Norman Riggs and Arnie Tidder for use. Coaxial cable must be used for the safety precaution is taker.. The decoder their helpful suggestions and help with video input feed to the decoder, and this, box must be effectively earthed, not building the prototype decoders should be kept as short as is practical. connected tc, the TV chassis, and there The other feeds to the decoder will be must be no exposed connections that +5V, —5V, —I2V and OV From the are at anything other than earth decoder to the receiver there will be red, potential. An .solating transformer green and blue output signals, the video (which allows the receiver chassis to be switch control wire, and the clamp pulse earthed) must be used while lining up feed. These last nine feeds may be the decoder, but if the abo‘e precau- combined in a single multicore type of tions are observed. this will not he screened cable. Screening is necessary necessary once the decoder is installed to prevent pick-up of the high fre- in its box. quency display components of the output waveforms in the receiver sound or video stages. Acknowledg Ern era t s Finally it must be stressed again that 1 would like to thank Mullard Ltd for great• care must he taken when con- their help witl-r character generator i.cs necting the decoder into the TV and also my ciffleagLes, Dave Hea \ an