an introduction to amateur radioteletype

Irwin M. Hoff, W6FFC

NOTE

Mr. Hoff is an active radio amateur and a recognized authority in the field of amateur radioteletype. Articles by Mr. Hoff have ap- peared in leading publications for several years providing major contributions to the technical advancement. and ever. increasing popularity of RTTY. As a service to the ama- teur* Frederick Electronics Corporation has persuaded Mr. Hof to prepare the material in this booklet, thus making available a refer- ence of considerable value to the novice and experienced radioteletype enthusiast

frederick electronics corporation

frederick, maryland

An Introduction to

AMATEUR RADIOTELETYPE Irvin M. Hoff, W6FFC

Amateur radioteletype is one of the most FORMS OF AMATEUR RADIO fascinating hobbies available today. Watch- There are nearly as many different as- ing a machine print a message automatically pects of amateur radio as there are amateurs from some remote point thrills the average with imagination. Some individuals like to person immensely, whether young or old. handle traffic from the servicemen overseas; This is true whether the message is coming some are interested in Civil Defense work; from the next room or from around the some see how many foreign countries they world. Most people have visited at one time can contact; some like to use Morse code or other a newspaper office or radio station while others prefer voice communication; where the Teletype machines were clacking some like to buy all their equipment and a away with incoming news from various few build everything; some fellows are pri- "wire services" such as Associated Press and marily interested in technical subjects while United Press to mention a couple. Persons others prefer talking about the weather; who normally have little or no interest in some branch off into associated fields such other forms of commercial or amateur com- as moon bounce, v.h.f. research, amateur munications will stare at such a machine for television and of course our present discus- extended periods of time, captivated by the sion – amateur radioteletype. knowledge it is constantly printing informa- tion of interest. Even if a machine is con- This list is far from complete but shows stantly repeating test information such as the versatility that is available to the enthu- "THE QUICK BROWN FOX (etc.)", indi- siast interested in radio communic ation. viduals will watch the printer indefinitely. Eventually such a person will necessarily develop an interest in one of the aspects of As is true with any hobby, amateur radio- amateur radio perhaps more than the others, teletype will probably appeal to only a lim- particularly if that aspect offers at the same ited number of individuals who will wish to time an opportunity for personal pleasure as obtain the equipment and use it on amateur well as advancement of skills and knowl- frequencies to "talk with" other equally- edge. interested individuals. Just picking up this publication shows 1 don't know what type of individual this some interest in the field of radioteletype by person may be. but this booklet is being the reader, so perhaps it is now time to men- written for him-to give an overall picture of tion a few things that can be done with ra- what is being done with amateur radiotele- dioteletype, type today as well as background informa- tion which will enable this person to assem- THINGS THAT CAN BE DONE WITH ble, connect and operate the necessary RADIOTELETYPE equipment to not only print incoming radio- 1. Amateur Two-way Communication. teletype signals but to transmit as well, as- While it is always interesting to watch a suming he is already a licensed radio ama- Teletype machine printing general in teur. formation such as newscasts, it is much Perhaps we should point out that it takes more interesting to have it print a per- no license to merely copy signals from the sonal message intended for you alone. air, as it is possible same readers may wish Amateur two-way radioteletype is little only to receive and not transmit – several of different from normal voice or Morse my friends are not radio amateurs at all, but code contacts, other than it is an entirely are still capable of printing conversations different form of communication. Many from amateurs. little "tricks" can be accomplished that

2 cannot be done with voice or Morse same time which puts the incoming sig- code and these will be mentioned nal on tape for later replay. It is also shortly. possible to "retransmit" the in. coming information directly on another fre- 2. Unattended Reception Via Automatic quency, thus relaying the signal instan- Means. Out of the most fascinating taneously to a second station. We might things a machine can be fixed to do is say at this time that the field of traffic - operate even though a person is not pre- handling has the greatest potential yet to sent. On voice or Morse reception this be developed in amateur radioteletype, would not be possible and the transmis- since it offers many advantages that nei- sion would be "lost forever" if the re- ther voice nor Morse. code can make cipient were not present and listening available. carefully. However, a Teletype machine prints on regardless. Associated with 6. Foreign Countries. Although a major- this is an additional advantage that al- ity of radioteletype operators are in the lows the operator to leave the equipment United States, a surprising number of running indefinitely on a specific fre- foreign amateurs are already on radio. quency and his friends can leave a mes- teletype with more doing so all the time, sage anytime during the day or night- particularly in Europe. The DX- thus offering the advantages of sched- enthusiast will find ample opportunity to uled operation without presenting the contact many countries, and WAC restrictions of both operators being pre- (Worked all Continents) is easily, possi- sent at the same time, or even. any spe- ble-in fact one of the more active DX- cial time.". enthusiasts has worked over 53 coun- tries on two-way radioteletype. 3. QSL Cards. Many amateurs in their spare time type up tapes which can then 7. MARS Transmission. Military Affili- be played at automatic speed later. ated Radio Stations are turning more These tapes often contain "QSL Cards" and more to the use of radioteletype for which have their call letters and a quick general message and traffic -handling as run-down of the station equipment as more affiliated amateur stations are ob- well as a typed-in verification of the taining the equipment. Certainly there contact and date. It is always interesting will he some readers who have picked to watch these being transmitted, and of- up this public ation in order to assemble ten if there are visitors present. the op- a station for MARS use. erator will request if you have any such 8. Technlcal. The individual in. terested in tapes to send for their interest and en- the technical side will find ample oppor- joyment. tunity to develop new circuits for his' 4. "Pictures" of Famous Persons. Some station; to program his equipment to of the more talented operators make print only those messages directed to' "picture portraits" on tape for later hire; to improve reception through transmission. Ralph Larsson of the Tele- methods not yet developed; to incorpo- type Corporation has perfected this abil- rate advanced' ideas which may not ap- ity to a high degree and many of his pic- peal to the average enthusiast, and in tures have been made available to vari- general to advance the fate of the art. ous operators who them pass them along 9. Civil Defense. Many counties are turn- vita radioteletype to interested parties. ing more to radioteltype for primary de- Again, when visitors are pres. cut, the fense systems, since more reliable operator often asks if you would please communic ations are possible while at send any such pictures for them to see. the same time some privacy is offered 5. Traffic Handling. Teletype certainly on the frequency. Again, radioteletype offers an ideal means of quickly and offers an ideal means of traffic - accurately handling traffic, as not only handling. does the machine print at 60 w.p-m. but 10. Record Copy. After the conversation also gives a copy of what is being has been terminated, the operator may transmitted. Clever operators can have a either discard or retain the- printed "typing-reperf", machine running at the copy. This offers many advantages, par-

3 ticularly with respect to handling traffic. 2. Tape reader, often called a "TD" to play However, even on general con- versa- the tape back automatically. tions, information may have been re- Such additional equipment also allows the ceived which the operator may want to operator to call 'CQ' automatically. keep in his files for law reference. Also the newcomer may frequently monitor CONNECTING THE EQUIPMENT conversations by individuals much more Many amateurs add all sorts of things to expert than he, that he wants to study their normal voice or Morse station such as over later. rotary beams for better reception; oscillo- These are but a few of the many things that scopes to show the proper modulation; may he accomplished on radioteletype – phone patches 'for public service work; others being equally possible. Hardly a week Morse monitors to hear their keyed signal; goes by but what some enterprising individ- s.w.r. bridges or wattmeters to observe the ual thinks up a new use to which the ma- condition of the antenna and to assure opti- chines, can be put. A majority of these mum loading of the transmitter; timer clocks schemes are involved with some form of to make legal 10-minute identific ations - the automation or traffic -handling, list could continue for some time and is lim- ited primarily by the operator's available Many of these ideas would appeal to you space, finances and enthusiasm. Adding ra- and we if only somebody took the time to dioteletype equipment, then, is really no publish some of them. Fortunately, most more difficult than adding most of the items amateurs are quite proud of their achieve- just mentioned. ments and are quite willing to tell others about them in the Topes the other person No attempt will be made in this public a- might be able to use the idea or even im- tion to present a reference text containing prove it. Such exchange of information is information on each machine or each possi- unique to amateur radio where few individu- ble way to connect a station–rather we shall als profit financially to any extent in the include a minimum of information in the process. In this manner we all benefit. hope that more clarity will result. After the WHAT IS NEEDED? reader has digested the information con- tained, be then can tackle some of the more To assemble a station for receiving and advanced material available with the knowl- transmitting radioteletype, the following edge that has at least one good working would be required; method available. 1. A decent antenna for the frequencies In the process of writing a simplified used. condensation with specific recommenda- 2. A good communications receiver with tions, the author's preferences must neces- b.f.o. (beat frequency oscillator) for sarily be presented. We hope the reader will Morse reception. not object if something he previously has heard or read is not included. There are other 3. A stable transmitter suitable for Morse methods of doing things as well, but always transmission. in the background are basic facts upon 4. A teleprinter machine. which the subject is centered. Do not de- velop a closed mind that the methods pre- 5. A radioteletype demodulator to change sented here are the only way to do things, the output of the receiver to d.c. pulses but keep in mind they are a simple and yet to operate the teleprinter, effective method that works. Now systems This is the minimum needed, and perhaps are being developed and you yourself could half o£ the stations in use today do not have easily be one of the individuals coming up more than this. The well-equipped station with the next idea we all would like to hear will eventually add tape equipment for mak- about, ing and playing tapes automatically. Tape OBTAINING ADDITIONAL equipment would consist of the following-, INFORMATION 1. Typing-reperforator to cut. the tape ei- Several periodicals are published primar- ther from a local source or from off-the- ily for those interested in amateur radiotele- air. type. One such booklet is published monthly

4 by Merrill L Swan, W6AEE. It is called Typical Teletype printers carry the THE RTTY BULLETIN, The majority of numerical designation such as 15, 19, 26, 28, ideas developed by amateurs themselves and 32. The 32 is the newest, but the 28 is appear in that publication. the most versatile for the advanced enthusi- ast, However, the model 28 is difficult to CQ magazine far a number of years has obtain at prices the usual amateur can afford, run a column by Byron Kretzman, W21TP, as the more simple version usually sells used each month in addition to occasionally pub- for at least $400. The model 32 can be lishing articles on some phase of the subject. bought new from the factory for as little as Fred DeMotte, W4RWM, publishes the $450. FLORIDA RTTY BULLETIN monthly, and The MODEL, 15. This is now the back- newsy items of general interest are contained bone of amateur radioteletype. The machines together with extensive classified ads of are rugged although somewhat noisy interest to teletype enthusiasts. and were intended for continuous 24-hour My own series of articles on radioteletype duty. Prices range from $25 from some so- started in the QST magazine published by cieties to $100 and more from specific indi- ARRL in January, 1965 and ran throughout viduals. the year. Other authors have and will con- tribute to that magazine as well. The MODEL 19 is a model 15 page printer plus tape-making facilities. A tape OBTAINING A TELEPRINTER punch works directly from the keyboard at Everything is dependent of course in get- speeds up to 125 words per minute. A ting some of the equipment in the first place. counter is included to inform the operator By getting one of the publications just men- when to start a new line. A heavy-duty tioned, the classified ads will usually include power supply is included to operate the tape available equipment and normally the price punch magnet, and a tape reader (TD) is also will be included. Certain organizations have included. This makes a complete station set- been incorporated in a few states such as up and is usually "snapped up" as soon as New York, Illinois, Michigan and California available. to name a few, where machines obtained The MODEL 26. This page printer was through the Bell System can be purchased at made for "light-duty" operation and was, very low prices-usually that of the equiva- never intended for continuous or heavy duty. lent junk weight price. A letter to ARRL These machines make excellent items for headquarters should also bring a list of most amateurs as they are relatively inex- available sources. The classified ads in other pensive and in comparison with the model amateur public ations often list equipment for 15 are rather quiet. Many amateurs also pre- sale and at times it becomes available fer the touch of the keyboard over other through surplus outlets to MARS members. items such as the 15 and 19 machines. They Occasionally surplus commercial stores have have some minor disadvantages, the primary used military items for quite reasonable one being the special table needed that con- prices. tains the roll of paper-the paper normally WHAT TO OBTAIN being contained in the printer itself on the 15 and 19. The main item needed will he the printer. There are two types – the "page printer" and The MODEL 28. The 28 equipment is the "strip printer". Since even a short con- actually a complete series of items with the versation would use a lot of tape on the strip designation 28. There is a 28110 printer printer, these are of almost no interest to the which is for Receive Only and has no key- amateur and can be obtained for as low as board; there is the 28KSR which corre- $10, Their primary value is for spare parts sponds to the model 15 with keyboard and for other machines. there is the 28ASR which has complete tape facilities and is similar to the model 19. There are several types of page printers by several different companies. Since only The 28 line contains such a fantastic list the Teletype Corporation printers are gener- of accessories entire catalogs are devoted to ally available, only those will be discussed. them. It is for this reason that the most ad- vanced amateurs eventually add 28 equip-

5 ment to their station. However for the casual while the perforators on the 19 and 28ASR operator or the newcomer, such a step can be can operate at speeds up to 125 w.p.m. or relatively costly–-the typical 28KSR costing more. $1200 new and the basic 28ASR costing The typing-reperforator, then, not only $2200 new. Even used the prices are quite prints from the incoming signal but types on high–around $400 or more for the 28KSR chadless tape as well. These machines are and around $750 or more for the 28ASR. highly sought-after also. However, there are a surprising number of these machines in use by amateurs, even so, The model 14 is essentially a printing me and certainly in coming years the price will for tape, and comes with or without become much more attractive. Some of these a key and with or without the typing part. beautiful machines are occasionally avail- The normal deluxe model 14 has a key- able to MARS members at no cost. board, out-of-tape warning bell, end-of-line The MODEL 32. Again there are several warning light and back-up mechanism (me- versions available-the 32KSR is similar to chanical) for observing and correcting er- the model 15 with keyboard and the 32ASR rors. is similar to the model 19 with full tape The TAPE READER, A tape reader equipment, These machines can only be pur- (TD) will he needed to play the tape that has chased new at present as they are too new to been prepared by the perforator or typing- be found used. The 32KSR is $450 and the reperforator. The model 19; 28ASR and 32ASR is $580. The cost has been held quite 32ASR already have such a tape reader. The low in comparison to the 28 equipment by model 14 TD may be purchased and used taking advantage of new lightweight inex- separately. pensive plastic components. These machines are intended for light-duty a few hours per TYPICAL STATION SET-UPS day at most. They are quite attractive, easy The most usual arrangement for a now to handle because of their weight and rela- station would include a model 15 or 26 page tively quiet. printer and the necessary demodulator to TYPING REPERFORATORS change the output of the receiver into d.c. pulses to operate the printer. (Model 14 items). There are several dif- ferent ways in which tape is cut for later re- The typical station with tape equipment play. One method punches holes clear would consist of the Model 15 or 26 printer through the tape. The chads (knocked-out plus a model 14 typing-reperforator and holes) are then collected in a little box which model 14 tape reader. is periodically emptied. A deluxe station might consist of a model Other units punch the hole in such a man- 15 page printer, a model 19 composite vet ner that it is not quite removed from the tape and a model 14 typing-re perforator. but remains on a little "hinge". This is called Anybody having model 28 or 32 equip- a "chadless" tape. ment would hardly be considered typical, Some of the machines will type charac- but a "first-class" station might consist of a ters in ink on the tape as the holes are being model 32ASR or a model 28ASR with some punched. These machines have the designa- of the other items already mentioned. tion "typing" perforators. HOOKING UP THE MACHINE The perforator is a fairly simple unit that Now that you have gotten your Teletype works directly off the keyboard, such as in printer (with keyboard) you will want to the model 19, 28ASR or 32ASR. The perfo- learn how to use it, Most printers that have rator cannot be activated from an incoming been removed from landline (the term used signal, only, the local keyboard. for normal circuits hooked to the phone lines The "reperforator' is actually a complete or telegraph lines) have three cables dan- receiving machine that works directly off the gling from them – one that goes into regular line and is activated by electrical signals household 115 VAC; one with a red plug on rather than the mechanical action of the local the end that goes to the printer and the third keyboard. The "reperforators" are limited to which has a black plug and goes to the key- the 60 speed that their gears are set for, board..

6 The first thing necessary is to determine To hook up the machine, the 115 VAC if the two selector magnets on the printer are plug is put in the wall socket If the motor is hooked in series or in parallel. Take an ohm- now turned on, the machine will run "wild" meter and measure the d.c. resistance (this is actually called running "open"( and between the tip and the shell of the red plug. no printing will result. A d.c. power supply If it is amend 50 ohms, the magnets are in will have to be constructed in order to oper- parallel and need a 60 ma, circuit. If it is ate the machine in a normal manner. This around 200 ohms, they are in series and need d.c. supply is shown in its most simple form a 30 ma. circuit, in Fig. 1.

1N2070 OR 22 O 2W F-4 ETC.

STANCOR PA-8421 R1 R1 FOR 30 mA - 5000O, 10W 60 mA - 2500O, 20W

80 µF 20 k 115 VAC 250V 5 W J1 PRINTER

J2 KEYBOARD

LOCAL LOOP SUPPLY FIGURE 1 The circuit shown in Fig. 1 is referred to ries for a model 28. for instance, for each of as a "local loop" – By Selecting R1 for 5000 the two coils.) When the keyboard is oper- ohms if the magnets were in series (200 ated it alternately opens and closes the cir- ohms) or for 2500 ohms if they were in par- cuit much the same as an on-off switch allel (50 ohms) you will be able to get things would. When the circuit is closed after hav- going, Insert the red plug in one jack and the ing been open, the inductance resists mo- black plug in the other. It makes no differ- mentarily letting this current flow. (You are ence, but drawings usually show the black probably familiar with the smoothing action one (keyboard) in the bottom where the offered by a choke in a power supply-this is voltage is closest to ground potential. a similar effect, or tries to be.) Unlike the power supply illustration, we Want this Cur- BASIC THEORY rent to flow to the maximum value as soon Now that you have the machine operating ae possible in order to get maximum per- you no doubt are already wondering what formance of the printer. Thus we "hit" the really makes it work and why does the coil with a much larger voltage than is printer have a separate plug from the key- needed in order to achieve this top perform- board. You are probably also wondering ance. Some military circuits use as high as why we use such a high d.c. voltage on the 300 volts, but 120-150 is "normal". Of local loop. course the selector magnets will work with less voltage, but with poorer performance. Let's take that parallel 50 ohms resis- tance. According to Ohm's Law: E=IR (It is this requirement for around 120 (60 ma. times 50 ohms equals 3 volts) we volts d.c. that until recently has made the use only need 3 volts to properly operate the of transistors somewhat difficult in optimum circuit. Here we are using closer to 150!. Of circuits, although some of these have course the dropping resistor "R" in Fig. 1 worked quite well in normal circumstances takes up the difference. but even so, why so where conditions were good. Newer units much voltage? are using 400 volt transistors recently devel- oped which remove earlier objections to The explanation is a fairly simple one. solid state units for driving the printer from The coils in the magnet of the printer have a external signal sources.) certain value of inductance, (Around 3 Hen-

7 Another phenomena of inductive circuits moving tape for key down, was raised above is the "spike" that is generated momentarily the paper tape for key-up. This be. came when the circuit is abruptly broken. This known as "marks" and "spaces". The spaces occurs constantly as the keyboard is used in corresponded to an open circuit. Teletype circuits. For parallel circuits, this We use the same terminology in Teletype spike is typically 75 volts but for series cir- today. If the circuit is closed, this is called a cuits it can be as high as 150 volts. Thus if "mark" and if it is open it is a "space" several printers are used at the same time, this spike could be of some concern, The In Morse operation, one dash is three dots average amateur would probably never put in length; one dot length separates parts of more than 3 printers or reperfs in the circuit the same character; 3 dot lengths separates at one time, and in this case no problems characters in the same word; and 7 dot should exist, unless he was using an older lengths separates words. This is called an solid-state circuit of some type where the "uneven length code" since each character is transistor was only rated at 60 or 100 volts. a different construction; ranging from dots With newer circuits, this is no concern. for the letter "E" (including the space be- tween the proceeding character) to 22 dots This same inductive spike is utilized by for the numeral 0 (again including the 3 dot clever designers of radio receiving units for lengths from the proceeding character.) As Teletype printers to actually assist in cutting may be imagined, such an uneven length off the circuit more abruptly when the circuit system posed serious problems to automatic opens momentarily for a Teletype character. reception. Thus the Tele. type machine was Many of the older tube-type units did not based on an entirely different system–called take this problem into account and suffered an "even length" code. in performance as a result. In this method, each character has the RELATION TO TELEGRAPH LINES same over-all construction, differing only in Long-range communications via electri- the type of pulses contained, not in number. cal means are really not so very old. I be- You certainly have often heard the term lieve 1832 was the year given credit to tele- "digital" used in reference to computers and graph circuits and it was not until 1906 that other data equipment. Basically speaking the the first teleprinter as we know it, was in- Teletype machine is an early form of a digi- vented by Charles Krum. It was natural that tal data system. its use should follow in some respects the established practices of the telegraph system. In a digital system a pulse may he either The principal similarity was the use of open or closed, or more correctly on or off. "closed current" circuits. With such a sys- A Teletype machine then has 5 information tem, one operator could interrupt another for pulses carrying information for each charac- corrections, etc. by merely depressing his ter. It also has a "start" pulse which is al- key, opening the line. Of course if the line ways "off" (space or open circuit) at the be- inadvertently opened, no operation by any- ginning to start things going on each charac- one could result. This is actually an-ad van- ter. At the end of the 5 information pulses is tage, however, as the condition of the circuit a "stop" pulse, which is always "on" or could be immediately determined. closed (mark). You can refer to these in any way you like and probably he correct as long Thus certain telegraphic practices and as you realize the Teletype-machine works terms are still current in Teletype operation from on-off pulses. as well. When the circuit is complete, and no typing is being done, the machine idles, Each of the 5 information pulses which When the circuit is broken, there is no cur- follow the start pulse may be either on or rent to hold the selector magnets closed, so it off, thus giving some 32 possible combina- runs "open". tions. (For the mathematicians in the crowd, 25=32.) An "R" for instance has the 2nd and In early days of telegraph, various inven- 4th pulses "on" with the 1st, 3rd, and 5th tors tried to perfect systems which would "off". The "Y" character is just the opposite automatically copy an incoming signal for with the 1st, 3rd, and 5th being on and the later reading by the operator, or for verific a- 2nd and 4th being off. Pictures are often tion of transmitted text, Early attempts were drawn showing one of these characters, but centered around a pen whic h marked on a

8 perhaps Fig. 2 would explain it as well or while copying various commercial cir- better without the usual reference to actual cuits. (They often send these to keep a cir- pulse times. cuit running for adjustment purposes when no information needs to be transmitted.) Since the "R" and "Y" are exactly the op- posite from each other with respect to the 5 While speaking of circuit adjustments, information pulses, these two keys are often another popular test method is to use: alternated to test the circuit. You will often THE QUICK BROWN FOX JUMPS see a line of: RYRYRYRYRYRY OVER THE LAZY DOGS BACK 1234,567890

START 1 2 3 4 5 STOP

R OFF OFF ON OFF ON OFF ON

CURRENT IN LINE

Y OFF ON OFF ON OFF ON ON

CURRENT IN LINE

TYPICAL TELETYPE CHARACTERS FIGURE 2 We showed earlier that only 32 possible ers for the keyboard are available for the combinations of 5 pulses are possible. There model 28 and model 32 printer». are 26 characters in the alphabet used in the (2) Weather type-these are for Weather English language, so this only leaves 6 other Bureau use, and since they are usually pur- possible things. They are used as follows: chased by the government F.A.A., they sel- 26 letters in the alphabet dom show up in amateur hands. However, 1 carriage return the Bell System places some of these ma- 1 line feed chines on lease to the airlines and it is possi- 1 space between words ble you may get one- These machines have 1 figures key for upper-case certain unusual characters designating cloud 1 letters key for lower-case coverage and wind direction in the upper- 1 "blank" key case position. ___ (3) Communications type. This is the 32 total style that you will be interested in as an Thus there are no punctuation keys avail- amateur. In the event you should see a ma- able in lower-case. The upper-case has some chine with the bell being over the "J" key additional 26 characters available, then. rather than over the "S" key, this was proba- These additional characters fall into three bly a Westrn Union machine, in which case category", depending on what purpose the it will have a few minor differences on the Teletype machine will be used for. upper-case position. Again, these can be changed to correspond to other communic a- (1) Business type-the Bell system ma- tions machines. I should point out though, chines normally are in this category. Many that a few amateurs never bother with upper- of the upper-case characters are fractions, case, at all since they have never bothered like 1/4 1/2 3/8 etc. The characters can be changing their machine from business type, removed and exchanged with other types if etc. This person usually never even sends needed. The corresponding key tops on the commas or periods – usually using a few keyboard pull off readily on the model 15 extra spaces at the end of the line instead! and 14 machines and can be easily ex- changed with other types. Different key lev-

9 Refer to Table 1 for keyboard arrange- of course the selector magnets are hooked to, ments and 5 level code format. the loop supply.) When a "start" pulse comes through (a start pulse is always space or Now to explain why the Teletype ma- "off") the selector magnets momentarily re- chine has a separate cable coming out for the lease, tripping a "atop arm" on the main keyboard and the printer. In a normal type. shaft clutch, and the selector cam sleeve then writer, the keys are connected mechanically starts to rotate. On this cam sleeve are 5 dif- to the typing bar, so when a key is depressed ferent projections in consecutive order. If the it causes the typing bar to be thrown against magnet is open at the time the particular cam the paper. This is considerably different projection comes around, it will trip a selec- from the way a Teletype machine works, tor lever which in turn operates a code bar. although at the first glance some models As the cam sleeve finishes a complete rota- such as the Model 15 appear to be nothing tion, all five code hays have been positioned more than an oversized typewriter – the either left or right depending upon the char- keyboard is somewhat similar, there is a acter transmitted, With all code bars se- typebasket that looks quite like a typewriter lected, one of the printing bars can now fall and the unit seems to work about the same in for. ward a little. A hail bar then operates, general when used on a local loop. catching this printing bar and flinging it It would be quite difficult to explain ex- against the platen. Thus printing occurs. This actly how the keyboard operates on a Tele. entire operation occurs each time any of the type machine without involved diagrams and keys is depressed on the keyboard, although text. Essentially when a key is depressed, it same of the functions such as the carriage causes five long bars below the keyboard to return do not actually print any thing on the slide to the left or right a small amount the paper. direction moved depending on the character This general explanation is necessarily depressed. brief and perhaps even a little vague. The These bars then control whether the car. primary thing to remember is that the key responding pulse will be mark or space board generates electrical pulses correspond- (closed circuit or open circuit,) In the mean- ing to the key depressed, and the printer uses while depressing the key also trips a clutch electrical pulses to determine what character on a cam that works directly from to print. With this in mind, you are now in a position to fully understand why the key- the motor shaft. As this cam rotates a se- board may he used on a separate line to cut ries of contacts opens or closes consecu- tape while the printer re mains printing tively depending on the position of the slid- something different. I know this took me ing code bars set up by the key de pressed. quite awhile to fully understand, so I hope Thus the keyboard does not con. ml the printer mechanically at all, but consists pri- the explanation will make things more easily understood. As an example you may have marily of a group of electrical contacts that the printer copying an incoming signal while are opened or closed to pro. vide the infor- using the keyboard to cut tape for an imme- mation to the printer. diate answer at machine speed. (Most ama- The printer works in a somewhat similar teurs seldom use tape, and just type directly manner, changing these electrical pulses on the keyboard as best as they can, since a hack into mechanical motion to operate the relatively small number are able to type sub- typebasket The motor operates a abaft going stantially faster than the normal 60 speed to the printer as well as the cam for the key- amateurs are allowed to use.) board. As long as no on off impulses are fed to the printer, it will remain idling (assuming

10 TABLE I - VARIOUS KEYBOARDS

UPPER CASE 5-LEVEL LOWER American Western CCIT No. 2 Bell CODE* CASE Communi- Union cations 11000 A - - - - 10D11 B ? ? ? 5/8 01110 C : : : 1/8 10010 D $ $ Note 2 $ 10000 E 3 3 3 3 10110 F ! Note 1 1/4 01011 G & & Note 1 & 00101 H # # Note 1 # 01100 I 8 8 8 8 11010 J ' Bell Bell ' 11110 K ( ( ( 1/2 01001 L ) ) ) 3/4 00111 M . . . . 00110 N , , , 7/8 00011 O 9 9 9 9 01101 P 0 0 0 0 11101 Q 1 1 1 1 01010 R 4 4 4 4 10100 S Bell ' ' Bell 00001 T 5 5 5 5 11100 U 7 7 7 7 01111 V ; ; = 3/8 11001 W 2 2 2 2 10111 X / / / / 10101 Y 6 6 6 6 10001 Z " " + "

00000 BLANK 00100 SPACE *1=MARK - 0=SPACE 11111 LETTERS 00010 CARRIAGE RETURN NOTE 1. Available as required 11011 FIGURES 01000 LINE FEED NOTE 2. Answer-back

11 One technical aspect remains-the range nent publications. However, the information selector and its adjustment. This is provided contained in this section should be complete on all machines in order to set the response enough to enable the beginner to form an of the particular machine to its optimum opinion of how the machines operate. point. The range selector is located in the SPEEDS vicinity of the selector magnets, and is usu- ally calibrated from 0 to 120. The operation The Teletype machines that are com- of the Teletype machine uses only about monly used for radio work normally run at 20% of each pulse to determine if it is a 60 w.p.m, although speeds of 75 and even mark or space pulse. Consequently about 100 w.p.m, are occasionally used. The FCC 80% of the pulse is not needed. The range limits amateur operation to 60 w.p.m. which selector is then adjusted so that the center is faster than all but a few operators can type 20% is used, giving maximum protection anyway. against an incoming signal whose pulses are The actual speed of the machine depends distorted. The range selector is adjusted on the pulse length which is determined while receiving copy from either the local primarily by the motor speed and gearing. loop or an incoming signal. A minimum set- The motors normally used are 60 cycles AC ting at which normal printing results is noted synchronous motors running 1800 r.p.m. and then the selector is moved to the other Appropriate gears are then used to turn the end of the scale and a maximum reading is main shaft at 420.5 r.p.m. on the printer and noted. The selector is then set in the middle the clutch then limits the number of opera- of these two extremes. Usually after this tions per mints to the 368 that are standard. optimum setting has been determined no This of coarse brings up a few more confus- further adjustment is needed unless the ma- ing terms and figures – referring to Table II chine receives some mechanical adjustment might help to straighten things out, due to replaced parts, etc. Since the Teletype machine as we know it This adjustment can he and indeed should is a "start-stop" system, every time a key is be made by the owner. A typical setting depressed on the keyboard, this trips off a would be around 60-65 on the scale. string of pulses automatically, commencing CHANGING TO PARALLEL with the "start" pulse. The "stop" pulse being mark information then determines how soon On many of the machines, a switch has another "start" pulse could be tripped off by been installed on the rear which allows the another key being struck. A stop pulse 1.42 selector magnets to he changed immediately times longer than the rest of the pulses is from series to parallel. Otherwise, a simple normally used. change with a screwdriver will place the magnets in parallel – once the chance has By adding the start pulse, the 5 informa- been made, them will be no reason to again tion pulses and the 2.42 stop pulse, we get change it for the life of the machine. A the "7.42" unit character. This is also re- check with an ohmmeter will determine if ferred to as a 5-unit (or 5-level) start-stop the wiring is correct. It is possible of course code with 7.42 unit character separation. to have parallel wiring and still get no results European machines use a more logical from the printer. In this case the coils are 1.5 unit stop pulse, as do the newer model connected out of phase and are bucking each 32 and 33 machines in this country. other, Reversing the wires to one of the two coils will provide normal results. The primary advantage to using a longer- than-the-rest stop pulse is for better syn- There are of course many other aspects to chronization in the event a. garbled signal is the basic theory that have not heart men. tio received. Some Western Union machines on the serious enthusiast will wish to pursue the other hand use a 1-unit stop pulse and them further by reading other publications operate at 390 operations per minute rather such as the series in QST; manuals accom- than 368. This prints normally, although the panying the Teletype machine; government machine actually seems to be running faster. training manuals that might be available through the MARS program and other perti-

12

TABLE II - SOME OF THE TELEPRINTER SPEEDS IN CURRENT USE W.P.M. Bauds Signal Unit Stop Pulse O.P.M. Character Length Code Interval

65 45.5 22 ms. 5 1.0 (22 ms.) 3903 7.0 62.5 45.5 22 ms. 5 1.27 (28 ms.) 375 7.27 61.3 45.5 22 ms. 5 1.42 (31 ms.) 3681 7.42 66.6 50.0 20 ms. 5 l.5 (30 ms.) 4002 7.5 62.5 50.0 20 ms. 5 2.0 (40 ms.) 375 8.0 75 56.9 18 ms. 5 1.42 (25 ms.) 460 7.42 100 74.2 13.5 ms 5 1.42 (19 ms.) 600 7.42 1 Standard American system used by amateurs 2 Standard international system. 3 Standard Western Union system in U.S.A. Since the stop pulse can vary considera- TYPING SPEED bly from one system to another, engineers The motor turns at a constant speed of have adopted the term "Baud" to designate 1800 r.p.m. When a key is depressed on the the pulse rate. On 60 speed a pulse would be keyboard this releases a clutch that trips off 22 ms. regardless of the length of the stop the keyboard distributor cam and a series of pulse following. This is 0.022 seconds and; pulses an generated. It makes no difference when a person is typing with "hunt-and- peck" technique at a slow speed or if a tape 1/0.022 = 45.45 Baud reader is sending a maximum ma. chine As a result, American 60 speed is called speed of 60 w.p.m„ the important thing to 45 Baud, while European speeds are called realize is that the time used to transmit one 50 Baud, since their pulse length is only 20 character remains the same. Thus even if the ms. operator is typing only at 5-10 w.p.m. each of those letters is being formed at the rate of As long as the Baud rate is not changed, a 60 w.p.m. specific machine can copy that speed whether the stop pulse is 1-unit; 1.42 units, Thus all receiving units are constructed so 1.5 units or whatever. Reference to Table 11 that optimum results are based on this 60 should then answer more questions than we w.p.m. figure. Any "hunt-and-peck" on the could do with further explanation. keyboard then stands a possibility at least of being received less well than normal maxi- It a machine is geared for 75 speed rather mum machine speed of 60 w.p.m. than 60 speed, the amateur must obtain new gears for the motor and main shaft. These This is not to discourage anybody who is may be obtained (as well as any outer parts a poor typist from obtaining radioteletype needed) from the manufacturer on a C.O.D. equipment. On the contrary, we all had to basis by the individual owner. Usually a start sometime-only a few amateurs have b phone call to the local Telephone Company lucky enough to know typing quite well will turn up an individual who is well versed prior to becoming interested in radio teletype with Teletype maintenance end repair who However, we at least wish to point out the may be hired after normal working hours to advantages in trying to improve one's typing under-take the modification if the owner ability-not only will you enjoy this hobby does not feel equipped to make the change. more, but you will get better results at the same time.

13 Thus the operator should endeavor to However, a few receivers will do better than learn the "touch" system in which all nine others and some or all of the following sug- fingers are used (we my all nine since the gestions will assist in letter printing. left thumb is rarely used at all!) and when 1. The receiver should be of the type properly placed on the keyboard, he may normally used for single-sideband re- type at a rapid rate without looking at the ception; that is with a "product detec- keys. It is not as hard to learn the touch sys- tor" rather than the diode detectors tem as one might at first think, and I know uses for AM and CW reception on several enthusiasts who have taught them- older units. This will allow use of the selves at home. The left little finger is "an- AVC as well as the BFO and at the chored" on the "A" key and the right little same time greatly reduce certain finger is anchored on the carriage return key. types of distortion. That's all there really is to it. The rest is sim- ple. 2. Stability – this is quite important to good reception, particularly if the op- The following table shows how the vari- erator prefers to copy in the Autostart ous fingers are then used: position where he walks away from Left Right hand the station after tuning in a signal. hand Again receivers ni tended for single- Index R,F,V,T, U,J,M,Y,H,N, sideband reception usually offer far B,B, greater stability than older receivers 3rd E,D,C, I,K,Letters Key intended primarily for AM or CW re- ception. 4th W,Sx O,L,Line Feed 5th Q.AZ P,Carriage Return, 3. Selectivity – an ideal band-width for Blank Key normal 850 cps radioteletype copy would be about 1.1 kilocycles. Few Thumb Not Space Bar receivers offer selectivity of this or- Used der; most sideband. receivers having The operator should practice some in his a 2.1 kc. position. For "narrow, shift spare time using the "local loop" until some reception, a receiver selectivity of progress is noted. We should like to suggest around 300 cycles would be desired. that until. the operator can type: One current receiver has a 1.2 kc. po- "THE QUICK BROWN FOX JUMPED sition and a 400 cycle position. It is OVER THE LAZY DOGS BACK" ideally suited for radioteletype recep- without looking at the keys or his fingers tion from this and other standpoints. that he will not really enjoy radioteletype as he otherwise might. 4. AVC – the receiver should have a fast attack, slow-delay AVC system USING THE PRINTER FOR RADIO (sometimes also called "AGC}. SIGNALS Again most single. sideband receiv- Now that you have the printer operating ers have this feature. normally on the local loop, you will want to 5. Variable b.f.o. or passband tuning, adapt it for radio signals. This involves four Audio tones are used from the re- items: ceiver to operate the f.s.k. demodula- 1. Receiving antenna tor which follows prior to the printer. These tones are normally 2125 cycles 2. Communications Receiver for mark and 2975 cycles for space. 3. F.S.K. Demodulator (for converting Many modern receivers will not pass the audio output of the receiver into 2975 cycles as they have a fixed fre- d.c. pulses to operate the printer) quency b.f.o. with no variable control on the front of the receiver. Such re- 4. Teletype printer (and loop power ceivers can be satisfactorily used for supply) radioeletype reception by either in- THE RECEIVER stalling a special crystal to move the selectivity curve further to one side Most modern communications receivers or else by selecting different filters will work satisfactorily for radioteletype. for the demodulator which would

14 match the receiver's characteristics. signals and back again for the mark signals. In my case. I have purchased and in- This is called "f.s.k." or Frequency Shift stalled in one of my receivers a modi- Keying, Carrier shifts of 850 cycles are con- fication kit sold by the receiver sidered "normal" shift and 170 cycle shifts manufacturer for radioteletype recep- are called "narrow shift". These two values tion. It was easy to accomplish and are more-or-less standard for amateur opera- allows me to use standard audio tion. Commercial operators use other stan- tones. However, it is probably easier dards ranging from 85 cycles to 850 and to buy a receiver in the first place that even more in some foreign countries. Ama- has pass-band tuning or a variable teurs are allowed to use any shift up to 900 b.f.o. that will allow normal reception cycles. This allows adequately for experi- of the 2125/ 2975 tones. mentation should the parties be interested. 6. The ability to readily change the re- An "F.S.K, Demodulator" then, is a unit ceiver to frequencies other than normal ama- that changes an f..s.k. signal into d.c. pulses teur frequencies for the reception of com- for the printer. mercial radioteletype can be quite interest- DEMODULATOR THEORY ing. Some receivers offer extra switching positions for unusual frequency segments. Until the past several years, only one type of basic demodulator design was known to 7. Other minor features, such as 500 ohm amateurs-the FM type using limiters. As a output to operate the fs.k. demodulator that result it really made no difference whether follows; headphone jack to deaden the loud- the operator knew anything about FM theory speaker when desired; dial accuracy for and operation or not. Consequently very lit- funding a station easily on another day, and tle has been printed regarding why a limiter other features such as general adaptability to is used and how it affects the rest of the cir- the operator's station, etc. cuit. These features should give the prospec- Recently, however, new techniques have tive operator some overall idea of what an been developed that use no limiters. Such excellent communications receiver can and systems are based on "AM" (amplitude should offer for optimum reception, modulated) concepts and have opened a new THE F.S.K. DEMODULATOR horizon to radioteletype reception. Some type of simple explanation should then be The communications receiver changes the included so the operator will know at least a incoming radio-frequency signal into an au- little about the advantages and/or shortcom- dio signal. This audio signal is then changed ings of both systems. The Frederick Model by the fs.k. demodulator into on, off dc. 1200 F.S.K. Demodulator offers both sys- pulses to operate the Teletype printer, There tems in the same unit, and by merely turning are two methods in current use today by the limiter switch on or off, the operator can which the demodulator can accomplish this select either basic system. task: Fig, 3A shows a simple f.s.k. demodula- 1. The "FM" system which uses a lim- tor block diagram, based on FM principles. iter, and A limiter is used to change the audio sine 2. The "AM" system which is new in waves from the receiver into square waves the last several years and uses no lim- of constant output voltage. The output of the iter. limiter then is split into either the mark sig- nal or the space signal by the channel filters Both systems rely on "f.s.k." transmission which follow. The signals are, then changed for best results. into d.c. pulses by the detector stage–usually WHAT IS "F.S.K"? a positive pulse for the mark signal and a negative pulse for the space channel, These We pointed out earlier that a Teletype pulses are then combined and fed to a trigger machine works from on-off d.c. pulses. stage which in turn operates a keying relay When transmitting Teletype signals on radio that will handle the 60 ma. current required frequencies, it is customary to move the car- by the printer. rier to a different frequency for the space

15 MARK FILTER DETECTOR

KEYING TELEPRINTER LIMITER SLICER RELAY LOOP

SPACE DETECTOR FILTER

BASIC FSK DEMODULATOR FIGURE 3A The trigger stage merely tells the keying little tolerance for signals that are mistuned relay to conduct when it gets a mark pulse or that drift during reception. You can also and tells the keying relay to shut off when a quickly see the disadvantages of such a sys- space pulse comes through. tem for round-table work, where various stations may be somewhat off the intended Since the limiter does put out a constant frequency. output voltage, then the mark pulse reaching the trigger stage will be the same voltage as Another serious problem (particularly to that of the space pulse a moment later, al- amateurs who must share their frequencies though of different polarity. This is called with all comers) results from strong nearby polar keying. signals. Without going into FM theory any further, we can say that the strongest signal If the incoming signal is not tuned cor- on the frequency will capture the limiter and rectly of course the output from the mark disrupt printing completely, even though that channel filter might be greater than that of signal may not be on the mark or space fre- the space channel filter. In this event, a tim- quency. There are several ways to minimize ing error can easily result at the trigger this problem, but no way to solve it other stage. This is known as "bias". The typical than abandoning the limiter. One method printer can accept ± 45% bias so this rarely would be to use a notch filter in the receiver becomes much of a problem. that would remove the offending signal. or at Where bias does become a most impor- least cut it down to where it is less strong tant problem, however, is when the signal than the desired signal. Another much better disappears momentarily into the noise level. method would be to use a pre-limiter filter Actually when the signal is weak. it is rare that would exclude as much as possible that that you lose both mark and space into the was not directly required for the mark and noise at the same time. Due to "selective space channels. The Model 1200 demodula- fading" usually one of the two signals has tor has such an optimum input bandpass fil- not completely faded. ter prior to the limiter to assist in this very thing. The Model 1200 is illustrated by Fig- The FM demodulator, due to its limiter, ure 3B. continues to have a constant output voltage although it will now consist primarily of In any event, FM systems fail when the random noise. These noise pulses will be signals are quite weak with selective fading separated by the mark and space filters and taking one of the channels into the noise he presented to the trigger stage. Since ran- level, and when strong nearby signals are dom noise has no intelligence, garble results. present. This combination is quite common on higher frequencies such as 20 meters Thus the FM system using limiters re- where one might be copying a weak Euro- quires that the signal be substantially above the noise on both the mark and space pulses pean with a c.w. station in the USA near the frequency. to print well. We can then say the FM sys- tem with limiters requires BOTH the mark and space signal for readable copy, It has

16 LEVEL LOOP METER

+ + + - MARK CHANNEL BAND-PASS DETECTOR

R FILTER

E O I V I D E U C A

E INPUT m R AMPLIFIER DECISION B M BAND-PASS LOW-PASS

d THRESHOLD

O LIMITER FILTER

0 FILTER

R COMPUTER F

MARK CHANNEL BAND-PASS DETECTOR FILTER

AUTO MARK/HOLD CIRCUIT MARK/HOLD

LOOP OUTPUT KEYER LOOP MODEL 1200 DEMODULATOR FIGURE 3B THE NEW AM LIMITERLESS The most advanced of these new circuits CONCEPTS is the DTC [patented] (Decision Threshold Computer) which automatically restores the Realizing that the FM systems offered se- balance between mark and space voltages vere limitations an radioteletype, several presented to the trigger stage. Other more firms both at home and abroad have been simple circuits have been patented in the working diligently to develop superior past, but not until the development of the methods for improved copy. Such efforts DTC has authentic balance been developed have been highly successful, although only which works equally well on slow keyboard- recently brought to the attention of the ama- speed typing; machine speed or periods of teur, These concepts are based on limiterless no typing as when a steady mark signal is use so that in the momentary absence of a received. A more comprehensive discussion mark or space signal, the noise would not be of threshold computers and their action has amplified by the limiter to the point it would been published by myself and WSSDZ, if cause the trigger stage to print an error, the reader is interested in the complete Merely turning off the limiter. or by-passing evaluation of the, system. [RTTY Bulletin, it is not the complete answer by a long way. Dec 1964] When the limiter was in use, it put out square waves of constant amplitude. If the In general, the DTC samples the mark signal was properly tuned, the d.c. voltages and/or space voltages to charge storage ca- presented to the trigger stage were then pacitors which in turn control the bias on the equal for mark and space, although of oppo- trigger stage to keep its operation symmetri- site polarity. As soon as the limiter is turned cal around the fluctuating mark and space off, however, this is no longer the case. First voltages. Another set of storage capacitors in of all, sine waves are now passed along, and an ingenious application quickly disconnects if any amount of selective fading (that is, the first set shortly after a Teletype character mark signal from the receiver is temporarily is received to compensate for optimum bal- less than the space signal in amplitude, or ance. It is this disconnect system that allows vice versa) is encountered, then the mark the DTC to work equally well on slow-speed and space volt. ages will not be equal at the typing, which in the past has posed a trigger stage and timing error or distortion partic ular problem to limiterless copy. will result. Thus it was necessary to develop In order to adequately use limiterless circuits which would restore the equality of copy even with a suitable threshold correc- the mark and space voltages at the trigger tor, great voltage variations at the trigger stage for proper reception, while at the same stage must be faithfully handled. This is time not resorting to any form of limiting known as "dynamic range". The Frederick which would tend to degrade the signal.

17 Model 1200 has over 40 db. of dynamic LIMITER IN range in the limiterless mode. For autostart unattended copy; round. ta- One distinct advantage of using the DTC bles; monitoring while eating supper, etc. circuit in a demodulator is that normal copy For receiving in quite noisy conditions can be achieved with only one of the two such as summer thunderstorms, etc. channels operative. This is because of the automatic mark-space balancing by the For receiving during very rapid flutter threshold computer part of the DTC. Thus if conditions such as aurora either the mark or space channel fades inde- To tune the signal (gives a stable tuning pendently into the noise level, normal copy indicator display) results from the channel that is above the noise level. This is true to some extent even LIMITER OUT when using the limiter. due to the fact that For reception during interference by the threshold corrector's output over. rides strong nearby signals the output of the noise level at the trigger stage. This should be explained a little – in For receiving weak signals that exhibit the design of a demodulator, great care is selective fading taken to equalize the band-width of the mark For observing selective fading conditions and space channel filters. When random For general reception noise is fed into the limiter, the output at the trigger stage then is rather small as the posi- In any event, these are merely "hints" and tive noise pulses from the mark channel are the operator can quickly go from one system equalized for the most part by the negative to the other by merely throwing the limiter noise pulses from the space channel These switch. then tend to cancel out A good demodulator DIVERSITY RECEPTION such as the Model 1200 then has an opti- mum bandwidth low-pass filter just prior to Some amateurs may have at least heard of the trigger stage which tends to further re- "dual-diversity" reception. I doubt if any duce the output on random noise. Thus in a amateurs at present are using "fulldiversity" welt-designed system using well-balanced as this requires net only separate receiving channel filters as well as an optimum band- antennas but separate receivers as well, to- width low-pass filter prior to the trigger gether with rather elaborate demoduIators stage, the DTC allows excellent copy even which then take the signals from each of the with a limiter while tuned to weak signals receivers and combine them for optimum exhibiting selective fading. This is a new results, the antennas are usually rather elabo- concept and the Model 1200 exploits it fully. rate affairs and spaced that if a signal is "down" on the one antenna it will be "up" on WHEN TO USE WHAT SYSTEM the other one. Combining the output of the If the demodulator has been designed two then tends to produce a signal that is primarily for use with limiterless AM con- more uniform in output. In the past, military cepts, then the addition of a limiter offers and some commercial circuits have relied on tremendous versatility. So much so, in fact, this complex installation for reduced error that we are still trying to determine under rates. what conditions which of the two systems The Model 1200 with the Decision would be most beneficial. Certain conditions Threshold Computer actually makes a diver- seem to definitely require one method or the sity installation out of one antenna-receiver/ other, so in developing the Model 1200, both demodulator combination. As you can imag- systems were included. Thus the operator ine, the reception of such a system is a vast can choose either FM with limiting or AM improvement over the usual system that limiterless by merely flipping a switch on amateurs have been using. In fact, one the front panel. Model 1200 has actually given superior re- We can draw a few guide-lines for the sults to a full-diversity system using de- operator to consider; modulators based on older concepts. The use of the DTC has been a tremendous "shot in the arm" for commercial and government installations. and of course offers amateurs

18 an unbelievable improvement, partic ularly 7. Has a multitude of external connec- during conditions of strong nearby stations tions at the rear for adapting to nearly where in the past they had to quit for the day any configuration imaginable such as or else revert to narrow shift in the hopes the remote control; external scope reduction in the bandwidth would eliminate connections, etc. the offending station entirely from the re- 8. Has a lamp on the front that glows ceiver's output. when a signal is in the mark channel, I suppose I sound overly enthusiastic and a similar lamp that glows when about the value of the DTC in particular, but the signal is in the space channel. it is changing the concept of radioteletype as 9. Has a limiter in-out switch. did the development of the jet airplane change commercial and military air travel. 10. Has a "normal-reverse" switch if a station should transmit its mark and PRIMARY FEATURES OF THE space signals reversed. MODEL 1200 11. Has an adjustable control on the front While discussing demodulators, you may for matching the output of the re- be interested in some of the features offered ceiver to the demodulator input level in the Frederick Model 1200. for limiter out position. 1. All solid-state – 40 transistors and 44 12. Has all normally-used positions diodes are used. This offers long marked in red so that anyone can eas- trouble-free life; light-weight; small ily and quickly use the demodulator size (the Model 1200 is only 13/4 with no special instruction. inches high with standard rack size width) ; cool operation and low cur- 13. Has a "Mark-Both-Space" switch – rent drain. Indeed it can be left oper- when used on mark or space it auto- ating 24 hours a day for years with no matically inserts 50%, bias to the problems. trigger stage so that normal copy on one channel only will result while us- 2. Has DTC – this is the only commer- ing the limiter. cial demodulator on the market that has the patented DTC. 14. All printed circuit-board construction used. These boards are installed on 3. Has a beautiful metering system that binged connectors so that either side can be switched for several quite im- may easily be worked on instantly portant and beneficial readings as should the occasion ever arise. well as used for a novel new type of tuning indicator that exhibit great ac- 15. Uses an input band-pass filter, mark curacy. channel filter, space channel filter and post-detector low-pass filter-all 4. Has a minimum number of controls of which are optimized for the keying while still offering utmost versatility. speed and shift used. All filters are on 5. Has a new autostart that works very one plug-in circuit board that may be effectively, such as 80m where auto. rapidly interchanged with other start has not been used until now (ex- optionally available plug-in boards cept by a few experimenters using with narrow shift filters or whatever expensive character recognition sys- other requirements the operator has. tems based on digital computer cir- 16. Operates on 50 cycles as well as 60 cuits); this autostart also can control cycles. the printer motor; needs no special call-up technique or shut-down 17. An optional visual indicator is avail- method; and due to a clever concept able using a new audio spectrum in- will not respond to c.w. or voice dication unlike any previous display should it come on the frequency. with which amateurs are familiar. 6. May be used for "retransmit" if de- 18. Optional filters available for various sired (where an incoming signal can shifts and various printer speeds be concurrently retransmitted onto other than 60 words per minute. another band for relay purposes)

19 No doubt there are other features which Position 4 – this reads loop current di- would appeal to the operator that I have not rectly, with the meter having markings for mentioned. One of these certainly would be both 60 ma. (amateur) and 20 ma. currents. its instant adaptability to unattended This allows the loop current to be set accu- autostart frequencies, thus opening a new rately without resorting to external meters. potential activity for amateurs on H.F. as SUMMARY OF THE MODEL 1200 well as on V.H.F. where autostart has been used to a small extent previously. This all solid-state demodulator is based on the latest principles of radioteletype A final word about the Model 1200 might communication to be found anywhere in the be of interest and that is the unusual switch world today. It uses the exclusive, patented positions for the meter. DTC circuit offered by no other manufac- Position 1 – Input-in this position the me- turer. It is based both on AM as well as FM ter indicates input level from the receiver. principles, and this together with several The front panel gain control is set so the me- features offers the greatest versatility ever ter indicates a 0 dbm input level, incorporated in an f.s.k. demodulator. Its over-all simplicity plus attractive design and Position 2 – +/- position – this reads the small size puts it in a category by itself- It output of the detector system for correctly will outperform any other f.s.k. demodulator setting the b1o. or passband tuning for opti- available commercially to the amateur today. mum noise balance. The receiver is tuned to a no-signal frequency, and then the bfo. or HOW TO CONNECT THE MODEL 1200 passband tuning is adjusted so the meter INTO THE LOCAL STATION reads mid-scale. The receiver is now cor- The Model 1200 is normally delivered rectly adjusted. with no local loop supply so that the opera- Position 3 – +/+ position – this is the po- tor may incorporate whatever system he sition normally used (marked in red., there. might already have available, or prefer to fore) for tuning the receiver correctly. This use. An optional loop supply is available at is a new concept that works so well that the extra cost, however. normal oscilloscope display is not needed, Fig. 4 shows a loop supply which has and actually would not work as well in many been designed specifically for the Model eases, The signal is tuned for maximum de- 1200, and is an adaptation of the outlines flection of the meter and minimum flicker- well-known "Mainline f.s.k. System". This ing as the f.s.k. signal is received. When the not only provides the needed voltage to meter stands motionless, it shows that the drive the printer, but also includes a method mark output voltage equals the space voltage of providing a signal to key the transmitter. output and thus the receiver is tuned cor- This loop supply can he built for less than rectly. In the past some meters have been $10 if all items are purchased new. Fixed- used in the output of the detector (plus- value resistors are used for simplicity and minus position) for tuning in the f.s.k. sig- lower cost. This circuit will give around 60 nal, but even when correctly tuned the meter ma. current to the printer, depending a little would he of little value on keyboard speed on the local line voltage level. A few copy, since the meter would favor mark so milliamperes variation plus or minus will not heavily. The "+J+" position is a different affect the performance of the printer. concept and works incomparably better with mucks greater ac. curacy, This same indic a- If a typing-reperf is to he used at the same tor works astonishingly well to copy shifts time, it would be placed in series with the other than 850 cycles, as it allows quite ac- printer and keyboard. curate "straddle tuning" of shifts less than Switch S-1 not only places 110 volts on 850 c.p.s. The signal is tuned until the meter the local loop but at the same time discon- remains motionless as the f.s.k, signal is re- nects the f.s.k. voltage m the transmitter ceived, although now it will he deflected to which prevents any possible hum loops from the right much less than before. existing during voice operation of the sys- tem.

20 10 O 1N2070 S1 A 2W

PRINTER 20 k 5 W

120 100 + KEYBOARD VAC 250V - 15 k 5 W TYPING-REPERF. 1 / 2 A

STANCOR PA-8421 2500 8200 TAPE READER 20 W 1 W

MODEL 1200 500 O 1 2 3 4 5 C.W. S1 B KEY FOR ID 500 O TO TRANSMITTER RECVR KEYER (FIG 6)

TYPICAL STATION LOOP SUPPLY FIGURE 4 TRANSMITTER CONTROL transmit-it not only puts the Model 1200 in standby so that the transmitted signal will Fig. 5 shows a very simple means of not attempt to key the printer, but at the automatically controlling the station's opera- same time places the transmitter on the air tion from a remote location-usually the two via its push-to-talk connection. If the trans- switches shown are placed quite near the mitter does not have a push-to-talk feature, printer for most convenient operation. the switch may be wind in parallel with Switch S2 places the model 1200 in standby whatever switch the operator normally while the receiver is being tuned, or when no closes to transmit. printing is desired from the receiver. Switch S-3 is used when the local operator wishes to MODEL 1200

4 8

SWITCH S2 & S3 MAY BE LOCATED NEAR THE PRINTER FOR MAXIMUM CONVENIENCE

S2 IS FOR MANUAL STANDBY S3 IS FOR TRANSMITTING STANDBY LINE

TO TRANSMITTER CONTROL (PUSH-TO-TALK ETC)

STATION CONTROL FIGURE 5 CONNECTING THE TRANSMITTER eration can be quite easily adapted for radio- FOR RADIOTELETYPE teletype transmissions. Some manufacturers even include provisions for radio. teletype in Most transmitters-whether intended pri- their units, but almost without exception marily for single-sideband or for Morse op-

21 these systems do not offer as effective or as ing the keyer voltage to the proper unit for simple a method as we shall shortly explain. the band in use. If a small capacitor - say 10 pf. (mmfd.) It is customary to transmit a lower fre- were connected from the cathode of the quency for the space signal. However this v.f.o. tube to ground, it would change the seemingly is backwards and somewhat con- frequency of the transmitter by perhaps a fuses everybody at times, since the lower thousand cycles, more-or-less. The fre- sideband position of the receiver is used for quency may go higher or lower, depending normal reception. Thus when listening to a on how the vfo. (or "p.t.o.") is heterodyned f.s.k. signal on the receiver, space goes in the transmitter. It makes no difference higher in audio tone – this in reality means whether the frequency goes up or down, that the transmitted frequency has actually since a correction may easily be made when lowered-you can see how this confuses peo- constructing the keyer that will give "nor- ple! mal" operation. At any rate remember three things and Many individuals have invested large you will never be confused again (?) sums of money in their transmitters and do 1. Mark is normally 2125 cycles not wish to make any type of change what- soever with respect to drilling holes or mak- 2. Space is normally 2975 cycles ing any type of modification. The author has 3. . Lower sidelined the receiver is nor- developed a system that adapts in a few mally used. moments to nearly any transmitter and no changes or modifications are needed. A little If you build the keyer as shown, and the keyer is constructed such as is shown in Fig. signal is reversed from normal, just reverse 6. the direction you have mounted the diode on

2.5 mH 3-12 pF the keyer – in fact, a good construction hint TO CATHODE would be to merely "tack in" the diode tem- PIN OF V.F.O. TUBE porarily until you get an opportunity to de- TO STATION LOOP (See Figure 4) 0.001 1N270 termine if it is normal or reversed. It may then be installed permanently when correctly positioned. F.S.K. KEYER FIGURE 6 Some older transmitters use v.f.o, units These components may be mounted on a that will cause the shift to be much greater e small terminal strip with a connecting pair of one band such as 20M from that on a lower wires long enough to reach the local loop on band such as 80M. If this is the case, addi- the output of the Model 1200. They need not tional keyers could be added and then preset be shielded since these wires are completely for each band; a switch selecting the proper by-passed. The little terminal strip may be keyer for the hand in use. There are usually quickly mounted under an existing nut or enough places to mount these tiny keyers in screw in the vicinity of the v.f.o. or p.t.o, in the vicinity of the v.f.o. tube –-in my own a few moments. The output of the keyer may transmitter I have mounted four such units then be connected around the cathode pin of and there is probably room for at least two the v.f.o. or p.t.o. tube which is then re- or three more without getting more than 5-6 placed in its socket. If care is used, the inches from the v.f.o. tube. shield for the c.f.o. or p.t.o. tube (if one is used) can even he replaced A great number of amateurs are begin- ning to use 170 c.p s. "narrow shift" at times This is all that is necessary to transmit ra- in addition to the more usual 850 c.ps shift. dioteletype. The small trimmer capacitor is In this event an additional keyer may be adjusted for the proper shift. Once the proper constructed in a few minutes and quickly setting is achieved, it will never need to be added to the basic keyer as already ex- adjusted again unless a different band or plained. The correct keyer then may be portion of the v.f.o. is used. In this case, the quickly selected with a switch at the Model particular transmitter may sometimes invert 1200. Being able to switch immediately the signal on some bands. A second keyer from one shift to another and then back may easily be constructed and added to the again has many advantages over older sys- first; a switch at the Model 1200 then direct- tem which used adjustable potentiometers

22 for changing the shift – in this case much pose. Many transceivers art not designed for confusion existed in trying to set the shift, the continuous carrier output required for and additional problems were raised as regu- radioteletype, and few if any transceivers lated voltages were necessary and the key- have any fans or blower to circulate the air board had to be placed in a circuit by itself. around the output tubes. In addition, few if any transceivers normally accept the stan- If a narrow shift keyer is added, the ca- dard audio tone of 2975 that is used for the pacitor may then be a 1.5-7 pf. rather than space signal to the f.s,k. the 3-12 shown for normal shift. demodulator. In addition, it is quite diffi- We have merely touched upon various cult to use a transceiver on radioteletype methods that may he used to hey the without resorting to the use of A.F.S.K. key- transmitter. The system described however ers with all their inherent problems. As long is the latest to be developed and has only as the limitations are realized, it is possible recently been brought to the attention of to modify them for satisfactory use, but very most amateurs after extensive development. few individuals will be interested in such a It adapts quickly and easily to nearly all move. transmitters with a minimum of effort and no modific ation of the existing transmitter. The average enthusiast can buy a used If the reader wishes to delve into other types transmitter in for Morse operation for well of f.sk. systems, some of these were covered under $100 and thus circumvent the need for in complete detail in the May and June 1965 using the transceiver for transmitting. issues of the QST Magazine in the author's In general, the transceiver has the follow- Radioteletype series. ing serious disadvantages: A.F.S.K. 1. Will not receive the 2975 space tone Eventually the term "A.F.S.K." will he without modification, 1275 and 2125 heard – this stands for "Audio Frequency are then sometimes used, but this Shift Keying". On the higher frequencies normally produces second-rate re- above 30 megacycles, it is legal to use audio sults – if an A.F.S.K. keyer of 1275 tones rather than carrier shift. Thus A,F.S.K. or 2125 is then used. the original keyers having audio output are used and fed problems with A.F.S.K, keyers are into the micro input at low level. greatly compounded, as the second harmonic of 1275 is not far from Some individuals possessing single side- 2125 and will be normally passed by band transmitter have using similar audio the transmitter. keyers on lower frequencies such as 80M. We should like to point out, however. that 2. A.F.S.K. is often used with trans- the indiscriminate use of A.FSK.. keyers on ceiver, and few units currently being single sideband transmitters may very likely used have acceptable characteristics, cause the user to be cited by the FCC for although this unfortunately does not improper operation, as almost none of the seem to bother the typical operator: inexpensive A.F.S.K, units constructed by 3. Reduced power normally must be the home enthusiast have the proper charac- used on Radioteletype since the typi- teristics for such use on the lower frequen- cal transceiver was never intended cies. A.F.S.K. keyers can he built which are for continuous carrier operation. satisfactory and one such was published in the June 1965 QST Magazine. However. it WHERE TO FIND AMATEUR was relatively expensive and we would sug- RADIOTELETYPE SIGNALS gest that you stick with the cheap, simple There are perhaps between 6,000 and and highly satisfactory keyer (or keyers) 8,000 licensed amateurs with Teletype shown in Fig. 6. equipment. This is still a "drop in the TRANSCEIVERS FOR bucket" when compared with those on voice RADIOTELETYPE or Morse operation. Thus it is easily possible to took on all the proper frequencies and not If the reader has a transceiver he wishes find anyone on the air at that time. In gen- to use on radioteletype, he should first con- eral, the 20M band is the most active and tact the manufacturer to determine if it most reception is concentrated around would be acceptable to use it for that pur-

23 14.090 to 14.100. There is little operation (When sending by Morse, only the trans- prior to the middle of the afternoon when the mitting stations call need be sent – this al- European countries are often heard. Activity lows automatic Morse keyers to be pro- continues until the band closes late at night. grammed with the local station's call.) Very little activity is heard around 15M, At the end of the transmission, identify although it is an excellent hand with little with the Teleprinter, turn up one or better interference. Typical frequencies are 21.090 yet two new lines and then send the Morse to 21.100. There is little activity on 40M, identific ation after which the transmitter also, although again it is a good band. may be turned off. A tremendous amount of activity exists On most machines, hitting the space bar on the 80M band from 3600 to 3640, with automatically places the machine hack into most activity being centered around lower case. This is a most advantageous fea- ture to have an radioteletype, since during The activity on 2M and 6M will d primar- marginal conditions a lower-case letters ily on the location of the operator. CQ and character may be missed, or an erroneous QST Magazine can be checked for those upper-case Figures character may be re- frequencies as a list is published monthly on ceived. If the machine does not have "un- the operating page of QST and in the RTTY shift-on-space" it may remain in upper-case column of CQ, for extended periods of time, ruining the OPERATING TECHNIQUE copy. The FCC requires that a Morse identific a- In any event, remember that most ma- tion of the station be made at the beginning chines use "unshift-on-space" but some op- and end of a normal-length transmission. If a erators have blocked off this feature for rea- straight c.w. transmission is made, it will sons known only to them. print garble on the printer at the outer end Thus any time you are in upper-case and unless the operator is present to turn off his have occasion to use the space bar, remem- printer. If "narrow shift c.w. identification" ber it will automatically cause many (but not is used, the printer normally stays on mark all) machines to go to lower-case. Therefore and prints no errors. The local loop supply he sure to type a new Figures key anytime shown in Fig. 4 for the Model 1200 has a you want to remain in upper-case after using narrow shift system for Morse identification the space bar. included. Also other individuals often use the space Using the narrow shift Morse identific a- bar to come out of upper-case, which again tion has an additional advantage – since you prints errors on many machines not equipped do not cut the carrier to send interrupted c.w. for unshift-on-space. Examples: the operator knows you have not stopped transmitting – further when he hears the One chap always types his call letters in change in shift as the Morse is being trans- such a manner they would appear as: K8 mitted, he knows he can then put the de- DKC K8 DKC K8 DKC (I used my own to modulator in standby so the printer will not not embarrass anyone) since he hits the run wild when the transmitter carrier is cut. space bar rather than the letters key after the numeral. When starting a transmission, the Morse identification should be sent first. Then al- When saying good-bye, same people who low a few seconds of carrier for the other want to type: 73 73 73 73 actually type 73 party to activate his printer, followed per- UE UE UE since they do not think to put a haps by a few "Letter Keys", then turn up new Figures key after each space key. one or better yet two new lines in the event Some attention to these rather small de- he had let his machine run during your tails will please those whom you contact Morse identification, This assures you that immensely, besides showing that you are a his printer has not been left in the middle of good operator, a line accidentally. Then be sure to send the identification again on the Teleprinter – this TURNING UP A NEW LINE is known as "dual identification" and is re- When turning up a new line, there is only quired by law. one "correct" way to do this. However, some

24 operators invent methods all their own-these Due to recent developments a new form quite often wasting paper for some individu- of autostart is now available that works ex- als who have added automatic carriage re- tremely well even on 80M, as it takes an turn-line feed systems. Some individuals authentic f.s.k. signal to keep it triggered. indiscriminately add extra carriage return Even so, many will profess to have no inter- keys throughout or at the end of their new est thinking it would be used for fixed- line to assure that you will receive it. They frequency unattended operation. Well it can do not realize that this wastes paper on many he used for this purpose, and in fact several advanced machines since they themselves do of us are doing so – my job takes me out of not have such protection. At any rate, the the city quite frequently and this system following system is considered standard by works so well, I leave all the equipment run- most press, commercial and military typists: ning while I am gone – when I return, there usually are several messages from other sta- Carriage return Carriage return Line Feed tions in various parts of the United States. Letters Key (one or more) (no more car- This system successfully ignores Morse riage returns) signals, the bug-a-boo of other autostart sys- It is courteous to turn up two new lines at tems for use on the lower h.f. bands. the beginning and end of the transmission so Even though you may never get quite so the tear will he "inside" any garble that may involved with autostart as to use it while not be printed during the reception of the Morse at home, don't overlook the tremendous identification should the operator allow his advantages of having a system that will machine to run. enable you to tune in an interesting AUTOMATIC CARRIAGE RETURN roundtable and then do other things around AND LINE FEED the house. Certainly radioteletype offers the ability to monitor your friends (or even Kits may he purchased for nearly any newscasts on commercial frequencies) Teletype machine so that when the printer reaches the end of the line it will automati- without being present at the time. One's imagination can run wild at the possibilities cally turn up a new line and at the same time of a good autostart system-new frequencies return the carriage to the left margin, should may be monitored automatically over short a normal carriage return be missed. This or extended periods; MARS operators can prohibits "pile-ups" at the right margin in accept traffic automatically without being mediocre conditions. With. out such a sys- present; messages may be received from tem. the operator either misses what was your friends who know what frequency you typed at such a time, or else must remain to have left your receiver on-the list grows manually return the carriage himself. It endlessly. In fact, with the new system should not be difficult to realize the immedi- ate advantages of such a system. Most model incorporated into the Model 1200 (the only other demodulator utilizing this new system 28 and 32 machines already have such modi- is the Mainline TT/L unit which is not sold fications as standard, while kits may be pur- commercially) it is entirely possible that a chased from various surplus stores and ama- new concept of automatic reception has teurs who stock them for model 15 and 26 begun, for commercial, military and amateur machines. Check the classified ads in ama- applications. teur publications. (We should mention that no special start or stop technique is required for autostart AUTOSTART operation in this system, thus giving the first When the term "autostart" is mentioned, really effective autostart system that does the average amateur often hastens to say he not resort to expensive digital computer sys- has no interest in such a thing, as he imme- tems to recognize Teletype characters on the diately thinks of 2M or 6M operation where air.) a friend can call him automatically and have FIXED-FREQUENCY OPERATION it turn on the printer. Many amateurs do not have V.H.F. equipment, and thus assume Several of us have been operating unat- autostart would merely be something extra tended 24-hour autostart on the 80M band to purchase which they would never use. Not for several years now. We use 170 cycle so. shift, primarily to -investigate the problems involved, which so far have never material-

25 ized. We have put "locking" crystals across rect frequency, the other station need not be the v.f.o. of the receivers by merely hooking "exactly" on your frequency for normal a little wire from a crystal socket under the reception-however, if he also fixes his grid pin and the other wire under the screen receiver like that just mentioned, and then grid pin of the v.f.o. tube, which was then refers to his tuning indicator for best replaced. Such receivers stay within 10-15 transmitter adjustment, excellent results map cycles by the month of the intended fre- be obtained for literally no expense at either quency if the room temperature does not end. This may not appeal to you, but on the vary excessively. Since mark tone is 2125 other hand, we assume many people will be cycles higher than "zero-beat", the crystal interested in such a possibility when they should be purchased for a frequency 2125 realize that it is easily possible and for little cycles higher than the actual frequency of additional cost to the equipment they already the transmitter. For instance if you were go- own, again assuming the Model 1200 will he ing to monitor 3600, you would use a used in connection with the other station 3602.125 kilocycle crystal. The main tuning equipment. dial of the receiver then becomes a very CONCLUSION broad fine tuning to touch up the actual fre- quency slightly-5 kc, on the receiver tuning The information contained in this publi- dial now affecting the receiver perhaps 50 cation necessarily is far from complete. cycles. With this type of precision and However, we hope it gives a good overall stability, autostart on 80M suddenly boils picture of what the requirements for radio- down to spending less than $4 for the teletype are and how to easily solve them. - crystal, assuming the Model 1200 was being The in. formation of course has been fash- used to monitor the frequency. ioned around the Frederick Model 1200 f.s.k. demodulator, and shows how that unit Such a deal does not tie up the receiver surpasses any other commercial demodulator except during those periods you are known available to amateurs for both convenience to be monitoring – like when away at work, and performance. for instance. The crystal can be removed by merely unplugging it from the crystal socket If questions concerning radioteletype (while leaving the socket itself connected to have been raised, perhaps they will be an- the v.f.o. tube) – normal receiver operation swered in one of the other amateur maga- then resulting. The crystal can be replaced at zines mentioned such as our own technical any time by merely putting it back in and the series in the QST Magazine that ran receiver again placed on the proper setting– throughout the year 1965, this setting is not critical and the "resetabil- We hope you have as much fun with ra- ity" is excellent. dio. teletype as we, have had, and are certain With the extreme latitude offered by the the use of the Model 1200 will enhance your 1200 for copying signals that are off the cor- pleasure.

26