High Tones, Low Tones, Modem Tones What Are They and What Do They Mean? BY BILL HENRY*, K9GWT

HAL COMMUNICATIONS CORP. BOX 365 URBANA, ILLINOIS 61801 If you're satisfied with just learning buzz words, keep on going. If you would like to actually and quite painlessly learn what these words mean, then read K9G WT's informative article on RTTY tones. High Tones, Low Tones, Modem Tones What Are They and What Do They Mean? BY BILL HENRY*, K9GWT O Loop ur RTIY hobby has grown a lot in the current, ma past ten years, and we have added many Mark features and terms that, while they cer­ 60 . �-- tainly expand the usefulness of our equip­ I ment, also generate a certain amount of I I confusion. One of the most confusing I RTIY concepts for both newcomers and I old-time RTIY operators is this business I of RTIYtones: "high," "low," "modem," I Space and "IARU" tones. 0 I Start 3 4 5 Stop Time pulse l pulse Why Use Tones? Data pulses .I. I. 1 Baudot letter (Y) Next lett er - Many of the early amateur radio RTIY techniques were adapted from existing (AI RTTY PULSES SPACE standards developed for commercial MARK Vo Vo printing telegraph systems. The use of (M) audio tones and the frequency of the I _. Shift= 170Hz tones evolved from existing commercial I standards. The RTIY data from the key­ I board is actually a series of pulses of cur­ I rent or voltage. The "standard" Baudot I teleprinter hook-up uses a 60 milliampere (21251 2295 current loop, the Mark or "rest" machine 2125 f(Hz) f(Hz) condition being with current on and the (B) AUDIO TONES Space condition with current off (some­ Vo Carrier Vo what "upside down" from what you might think). On a telephone line, this in­ LSB USB formation can be transmitted simply by LSB USB (M) (M) opening and closing a series circuit. A I I printer's selector magnets are connect­ I --t !-Shift= 170Hz I I ed at the other end and the RTIYcircuit is I I complete. However, when long lines are 51,000,000 f(Hz) used, it is much more convenient to send 50.997,875 50,997,705 I 51.002,295 a.c. signals over the telegraph lines be­ (C) AM AFSK RTTY (51 MHz, 100%- A2) cause of the relative simplicity of a.c. am­ Vo Vo plifiers as compared to d.c. amplifiers. (M) Therefore, most long-distance RTIY cir­ I cuits use two different audio tones to rep­ -+1 -.Shift= 170Hz I resent the Mark and Space RTIY condi­ I tions. Early amateur RTIY operators I used this same tone technique for v.h.f. RTIY communications. A typical RTIY f(Hz) 50,993,625 51,006,375 50,993,115 pulse waveform and a frequency dia­ 50,995,750 51,004,250 50,995,410 gram of the audio tones used are shown 50,997,875 51,002,1 25 50,997,705 in figs. 1 (A) and (B). (D) FM AFSK RTTY (51 MHz, 5KHz DEV- F2) When we transmit RTIY on v.h.f., we Vo Vo (M) use these tones to directly modulate the I f.m. or a.m. transmitter to produce the F2 t- Shift = 170Hz I or A2 type emission. (F2 emission is audio I tone modulation of a frequency modulat­ I ed signal; A2 is audio tone modulation of I an amplitude modulated signal.) Two dif­ 14,099,830 ferent audio tone frequencies are used to 14,100,000 f(Hz) f(Hz) (E) FSK RTTY (14.1 MHz- F1)

Fig. 1- A visualization of Audio Frequency Shift Keying (AFSK) and Frequency Shift *Box 365, Urbana, Illinois 61801 Keying (FSK) RTTY. See text for details. represent the RTIY mark and space con­ use the same unit for reception of both we will discuss it in greater detail shortly. ditions; the difference between them is v.h.f. and h. f. RTIY signals. Note that al­ First, however, we need to consider just called the "RTIY Shift Frequency" (Shift though the tone frequencies produced by how h.f. FSK RTIY is generated. for short). This technique is commonly the h.f. receiver can be set to most any called AFSK (Audio Frequency Shift Key­ convenient frequency by adjusting the Transmitting H. F. FSK RTTY ing). Notice that you need two electronic tuning, the v.h.f. receiver does not have As we discussed above, our h.f. ama­ interfaces to the v.h.f. station: {1) a tone this capability. Since F2 or A2 modulation teur RTIY must be transmitted using type encoder (audio oscillator) that converts is used for v.h.f. AFSK, the receiver de­ F1 emission, frequency shift keying the transmit mark and space pulses into modulator frequencies must match the (FSK). The classic way of generating the mark and space audio tones, and (2) a tones transmitted. Therefore, if we are to FSK RTIY signal is simply to use a circuit tone demodulator that converts the mark use the same demodulator for v.h.f. and that directly shifts the transmitter carrier and space tone frequencies back into h. f., we would want to choose a unit that frequency as the RTIY pulses change be­ keyi-ng pulses which drive the RTIYprint­ is compatible with the tones used by oth­ tween mark and space conditions. In er. The combination of the transmit tone er stations. Here is where the "great high fact, this is the way we generated RTIY encoder and receive tone decoder is of­ versus low tone controversy" starts, and for years, and it is still used in some com- ten called an RTIY demodulator, tuning unit (TU), or Modem (computer term for modu­ lator-demodulator). Frequency diagrams for typical 51 MHz a.m. and f.m. RTIYsig­ nals are shown in figs. 1(C) and (D). RTIY operation on the high frequen­ DATA TERMINAL FOR cies (below 30 MHz) is done in a slightly different manner. We are allowed to use MULTI-MODE COMMUNICATIONS only type F1 FSK (Frequency Shift Keying) for RTIY transmission on the lower fre­ quencies. (F1 emission is the direct shift­ ing of the radio carrier frequency by the ASCII, Baudot, or Morse - RTIY data.) The reason for this restric­ the DS3100ASR can do tion is to minimize the bandwidth required them all with a full comple­ by each station in this crowded section of ment of data rates and inter­ the spectrum. The FSK RTIY signal is transmitted so that the mark condition face standards. The DS3100 corresponds to one radio frequency and is designed with the oper­ the space to a lower radio frequency. ator in mind; keyboard con­ Note that only one radio signal, at either the mark or space frequency, is transmit­ trols and operating condi­ ted at a time. tions are clearly shown by To receive this RTIY signal we use a the triple-legend keytops communications receiver with a b.f.o. and on-screen status in­ (beat frequency oscillator) and adjust the frequency dial so that two different audio dicators. The very large display memory (200 lines of tones are produced, one for the mark 72 characters) and additional "mailbox" storage of the pulse and the other for the space pulse. MS03100 (32,000 characters approximately 450 more An RTIY demodulator is then used to re­ cover the RTIY data pulses from the lines) allow you to handle your traffic without resorting tones, just like for v.h.f. AFSK operation. to paper tape or retyping print-outs. Other outstan- Note that because we are using the b.f.o. ding DS3100 features are: on the receiver, the audio tone frequency produced at the receiver speaker can be controlled simply by turning the receiver • True "AS�" operation - pretype and «:d!t50 lines frequency dial. Any number of different of transmit text on-screen while rece1vmg mark and space tone frequency pairs • may be chosen for reception; however, 150 Line receive display buffer • they will all differ in frequency by the Baudot from 45 to 100 bd (US or CCITT #2) same amount-the original difference • ASCII from 110 to 9600 bd (full or half duplex) between the transmitted mark and space • Morse from 1 to 175 wpm radio frequency. This frequency differ­ • Current loop or RS232 modem connector ence is called the RTIY Shift Frequency, or • WRU, SEL-CAL, and Mailbox (with MS03100) Shift for short (as in the previous discus­ • sion of v.h.f. AFSK operation). A frequen­ Ten programmable HERE IS messages cy diagram of a 14.100 MHz FSK signal is • Available for 120V, 60Hz or 220V, 50Hz shown in fig. 1 (E). Note that none of the di­ agrams in fig. 1 present the full frequency Write or call for more details about the DS3100 and spectra; the sidebands caused by the dig­ MS03100. ital switching between the mark and space are omitted for clarity. From the previous discussion, you can HAL COMMUNICATIONS CORP. see that there are some similarities and BOX 365 differences between v.h.f. and h.f. ama­ URBANA, ILLINOIS 61801 217·367·7373 teur RTIY operation. Obviously, since both require use of a receive tone demod­ 00 ulation unit, it is desirable to be able to s

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Fig. 2- Methods of generating an FSK RTTY signal. mercia! amateur transceivers (e.g., the (1) Separate audio tone frequencies however, we have a strong carrier on the TS820 and IC720). This technique is call­ are generated with an AFSK oscillator to air for the entire time we are transmit­ ed direct FSK and will give very pure FSK represent the transmit mark and spac;e ting-"100% duty cycle." Therefore, we. RTIY emissions. A simple "direct FSK" pulse conditions. need to be very heat conscious when us­ transmitter system is shown in fig. 2(A). (2) The audio tone signal is coupled in­ ing s.s.b. equipment for ATTY. Often, a fi- However, there are a number of prob­ to the microphone or phone patch input . nal amplifier stage designed for many lems with this technique that may not be of an l.s.b. transmitter. watts of output in voice service should be convenient for many of us. For example, (3) The balanced modulator stage of considerably derated for RTIY service. if the transmitter or transceiver does not the transmitter generates a double-side­ Particularly if the transmitter uses tubes include a direct FSK input connection, it band, suppressed carrier signal. not specifically designed for transmitter will be necessary to add a frequency (4) A bandpass filter stage in the trans­ service (so-called "TV sweep-tubes," for shifting circuit. Most of us are under­ mitter selects only the lower of the two example), we should decrease the output standably reluctant to modify our new ra­ sidebands and further suppresses the power from the normal voice or c.w. dio, particularly when a modification to carrier. values. Also, if a very strong audio signal the oscillator circuits could degrade the (5) The heterodyne oscillator and mix­ is introduced into the l.s.b. transmitter, frequency stability of the entire unit. Also, er circuits convert the l.s.b./FSK RTIY "splatter" will result. This may not be a changing the frequency shift to be trans­ signal to the desired band and output ra­ very serious problem for voice opera­ mitted involves further changes in both dio frequency. tions, but for RTIY, many spurious RTIY home-brew and commercial direct FSK Since the lower sideband was chosen, signals will be generated, all of which are circuits. For example, one commercial the lower audio tone frequency input illegal! Finally, consider the high-gain radio requires that you remove 37 (v.h.f. AFSK mark) will produce the higher audio input of most transmitters: if the screws, get out the counter, adjust a trim­ radio frequency output signal {h.f. FSK wires to the mike jack are not properly mer, change a jumper, put the screws mark). Thus, the two h.f. and v.h.f. tone grounded or shielded, 60 or 120 Hz back, and hope nothing has changed! standards are made compatible. The indi­ "hum" can be introduced and another A second technique which is much rect FSK transmitter system is shown in spurious RTTY signal will be the result! simpler for the operator to hook-up and fig. 2(8). These considerations are really just com­ use may be used to generate h.f. FSK Because we are using a transmitter mon sense, and most s.s.b. transmitters RTIY. This technique makes use of the designed for one serv1ce (voice s.s.b.) for can be used for h.f. RTIYif these precau­ l.s.b. (lower sideband) mode of the trans­ something completely different {RTTY tions are observed. mitter and the same transmit RTIY tone FSK), there are a number of precautions The technique of using audio tones encoder used for v.h.f. AFSK. This pro­ that should be observed. Foremost with an h.f. s.s.b. transmitter results in an cedure has become quite popular and among these is the consideration of the r.f. output that is a true F1, FSK emission, has a number of strong advantages, as 100% duty cycle nature of RTIY. In voice, the undistinguishable from a signal generat­ well as some potential problem areas to average transmitter power is only 50% of ed by shifting the transmitter oscillator watch out for. The technique works as the peak power developed while talking­ frequency. I recommend we call this tech­ follows: called "50% duty cycle." For RTTY, nique indirect FSK to reflect the fact that tones and s.s.b. techniques are used to than low tones. Since the tone frequen­ dard amateur shift, 170Hz, will pass most generate the FSK signal. This technique cies are high, harmonics and distortion current receivers and transmitters (the is not AFSK as it has been mistakenly call­ products that may be caused by overdriv­ Collins S-Line is an exception). However, ed. AFSK is what we use for v.h.f. RTIY! ing the transmitter input occur at audio few pieces of equipment will pass the If all of this seems complicated, wait. It frequencies beyond the audio passband tones for both receiving and transmitting gets even deeper! To further add to the of the transmitter, and therefore should 425 or 850 shift with high tones (the confusion, there are now several audio not be transmitted. The rejection of the Drake TR-7 is an exception). The use of tone frequency standards that may be frequency components of the unwanted high tone demodulators for h.f. RTIY is used. In fact, there is one standard in uni­ sideband will also be greater for high therefore restricted to transmission of versal use by amateurs in the United tones than for low tones. just 170 shift, and only receivers incorpo­ States (high tones) and quite a different Disadvantages: rating either a variable b.f.o. or passband standard used by amateurs elsewhere in 1. The relatively high audio frequencies tuning will receive all three shifts. the world (low tones). Also, computer op­ used in the high tone set may not fall with­ erators now use entirely different tone in the audio frequency response of the re­ 2. High tones are not the IARU stan­ frequencies for phone-line and r.f. data ceiver or transmitter. In general, the stan- dard and will not be compatible with v.h.f. communications. The differences and uses of these tone frequency standards are discussed below. Low Tones or High Tones? PERFORMANCE The high tones (for higher frequency audio tones) are really the traditional VERSATILITY standard U.S. RTIY tones used since the early days of amateur RTIY. The low tones CONVENIENCE are the IARU international standard used extensively in most other countries of the world. When receiving (or transmitting) on the h.f. bands (3-30 MHz), either set of tones will work since you tune the receiv­ '-"'''"' 111� "1:06 lolA- I>.V'l't:>--· • • • • er to produce the desired beat note fre­ • • quency. However, when AFSK modula­ o;:: :-::� :.� -:::::· v::� �_, tion is added to an f.m. or a.m. signal, you must be prepared to receive the same tone frequencies as those used by the transmitting station; the a.m. or f.m. re­ The HAL ST6000 has them all: ceiver does not use a b.f.o. to produce • performance to copy the weak and distorted signal the audio tone. • versatility to match a variety of 1/0 interfaces In the United States, the long-standing • v.h.f. AFSK tone standard has been to convenience for simple but accurate operator use use the high tones (2125 Hz mark and All this at half the price of comparable-performance 2295, 2550, or 2975 Hz space). To be units. compatible with other U.S. v.h.f. RTIY stations, you must use a high-tone de­ PERFORMANCE: modulator! In Europe, in particular, the • Optimized active input, discriminator, and low-pass reverse standard is developing; the IARU filters. (1275 Hz mark and 1445, 1700, low tone • Crystal tone keyers match discriminator filters. or 2125 Hz space, depending upon the • Hard-limiting FM or AM types of operation. shift chosen) is the standard to be ob­ served. The two systems are basically in­ VERSATILITY: compatible for v.h.f. AFSK operation! • Interface current loop circuits; built-in loop supply. Due to low-pass filter parameters, use of • RS-232C and MIL-188 data 110 connections. data rates greater than 150 baud is not • CMOS pre- and post-autostart data 110 data connections. recommended when "high" or "low" • Available for low, high, marine-compatible, and special tone demodulator combinations are used. tones. Frequency relationships between "high" and "low" tones are shown in fig. 3. CONVENIENCE: Each tone set has its advantages and • Hard-limiting operation for simple but effective disadvantages. Some of the considera­ operation. tions for each tone set are as follows: • Tuning oscilloscope for precise receiver tuning.

• High Tones (Mark = 2125Hz) ATC, DTH, KOS, autostart, and antispace features. Advantages: • Operate 120/220 VAC, 50/60 Hz; table or rack mount. 1. High tones are the U.S. v.h.f. AFSK • Solid state design with proven field-tested dependability. standard. Their use is required for com­ Write or call for more information on the HAL ST6000. patability when operating v.h.f. AFSK in the U.S. A high tone demodulator may be used for both v.h.f. and h.f. use in the HAL COMMUNICATIONS CORP. United States. BOX 365 2. When tones are connected to the audio input of an h.f. l.s.b. transmitter, URBANA, ILLINOIS 61801 217·367-7373 there may be less problems with spurious signals when high tones are used rather Vo up to 300 baud. The modems that are sold i t =::O shift ::: f include both transmit tone generation s s i and receive tone demodulation circuitry. !+425 Control circuitry is also included to permit

shift170 shi170t automatic send-receive control or f l l "handshaking" between two data termi­ � lrl nals; a touch-tone dialing circuit some­ M I s s s s times is also included. The 103 Modem is I I I I designed for full duplex (FOX) operation, I 1 I I I meaning that data or text may be receiv­ I I I ed and printed even while you are typing 11 I I and transmitting data. Our amateur com­ 1275 1445 1700 2125 2295 2975 I I 2550I I f(Hz) munications are usually carried out using half duplex (HDX); we take turns talking or "Low tones" "High tones" exchanging data. 3- Because the 103 Modem operates in Fig. Low and high tones. FDX, two sets of tones are used for data transfer between data terminals, one set AFSK in many countries of the world. Modem Tones for each direction of data flow. Since any Low Tones (Mark = 1275 Hz) With the increasing popularity of per­ terminal on the telephone circuit can sonal computing, more and more ama­ start or originate a data connection, a Advantages:Low tones are the IARU internation­ teur radio operators are combining the convention has been established to keep al 1.standard, and their use assures com­ two hobbies and using their radios to ex­ track of what set of tones is used with patibility with v.h.f. AFSK operations in change computer ideas and programs. each terminal. The terminal or station many areas of the world. Recent U.S. FCC approval of amateur that places the initial "call" is called the 2. Low tones can be used with virtually use of the ASCII computer code has cer­ originate station; the receiving terminal is all s.s.b. r�eivers and transmitters for all tainly encouraged this growing section of called the answer station. The ''originate'' three standard shifts. Variable b.f.o. or our RTIYhobby, particularly among v.h.f.­ and "answer" modes are set in each passband tuning features are not re­ f.m. RTIY repeater users. Some of us modem by control signals at the begin­ quired to assure good reception of 425 have used the "standard" high tones of ning of the data connection, and these and 850 Hz shift stations. 2125/2295 Hz for 110 and 300 baud modes remain in this condition for the ASCII communications and 2125/2975 duration of the exchange. However, after Disadvantages:1. Low tones are not compatible with Hz for up to 1200 baud ASCII. If the de­ the call has been completed and the con­ existing U.S. v.h.f. AFSK operations. modulator low-pass filters are readjusted nection broken ("hang-up" the line), the Since there is a great deal of high tone (and they must be on practically all good­ second station may now place a call to AFSK equipment already in use in the quality h.f./v.h.f. demodulators), these another terminal. Since it is now the origi­ United States and neighboring countries, tone pairs work well for ASCII. nating station, its modem will select the it is unreasonable to expect that the U.S. However, commercial standards al­ originate tone sets for use. The tone fre­ standard will shift to low tones. ready exist for ASCII communications on quencies used in 103 Modems are: telephone circuits. These tone standards 2. When low tones are used with an Originate Mode are particularly attractive for amateur l.s.b. transmitter to generate F1 RTIY Transmit data: 1270 Hz mark, 1070 Hz use since modems can often be obtained emissions, there is a strong probability space from surplus sources for low cost or can that overdriving the transmitter audio and Receive data: 2225 Hz mark, 2025 Hz be built with a minimum number of parts modulator stages result in spurious space harmonics and mixerwill products that will from diagrams published in semiconduc­ be radiated. Of course, these problems tor manuals. Since these tones are not Answer Mode will not occur if the audio drive level is the same as those we have used for 45 Transmit data: 2225 Hz mark, 2025 Hz properly set. baud Baudot for years, there is often little space interference caused between users of Receive data: 1270 Hz mark, 1 070 Hz Although you may develop your own ASCII and Baudot. Sometimes the same space personal preferences, I recommend that v.h.f. repeater can be used simultaneous­ Notice that the transmit tones of one sta­ you consider the following operating con­ ly for both applications! For these rea­ tion correspond to the receive tones of ditions as a starting point: sons there is a growing popularity in the the other station, assuring simultaneous In the United States use of two commercial data set stan­ data flow in both directions (full duplex). Use high tones for all v.h.f. AFSK ama­ dards for v.h.f. amateur operation: the Amateur radio communications are teur communications and for normal 170 Bell Modem and the Bell 202 Modem generally conducted using half-duplex Hz shift h.f. operation. Use low tones standards.103 communications. Also, when we are when receiving h.f. commercial RTIY The Modem is used for telephone transmitting, we prefer to transmit the stations unless you have a receiver with a line data103 communications at data rates same set of tones that we use when re- variable-pitch b.f.o. or with i.f. passband Vo tuning. The exception applies to use of Recommended for the older Collins S-Line equipment: use amateur HDX use low tones for all h.f. operations, amateur s or commercial. S M M I I In Europe (and all otherareas where iARU stan­ I dards apply): I I I Use low tones exclusively for both v.h.f. I • AFSK and h.f. operations. The only ex­ 1070 1270 2025 2225I f(Hz) ception would be when you communi­ Originate: Transmit Receive Answer: Receive Transmit cate with another v.h.f. AFSK station who is using high tones. "103 Fig. 4- Modem"I tones. M s 1275 Hz mark, 1700 Hz space-11 0 I through 300 baud (ASCII)

I C.W. ID = 1175 Hz (all shifts) I Vol I V.H.F. RTTY: I U.S.A.: I • 1200 2200 f(Hz) 2125Hz mark, 2295Hz space-45 to 74 baud Baudot Fig. 5- "202 Modem" tones. C.W. ID = 2025 Hz 1270 Hz mark, 1070 Hz space-110 to ce1v1ng. It is recommended that ama­ be over v.h. f. -f.m.links, signal-to-noise ra­ 300 baud ASCII teurs use the originate-transmit mode set of tios will generally be quite good and high­ C.W. ID = 1370Hz 103 Modem tones for v.h.f. ASCII trans­ performance demodulation circuitry is 1200 Hz mark, 2200 Hz space-300 to missions (1270Hz mark, 1070Hz space). not needed. A good phase-lock-loop cir­ 1200 baud ASCII These tones agree with the above com­ cuit will give very satisfactory perform­ C.W. ID = 1100Hz mercial standards and are sufficiently ance at 300 or 1200 baud over an f.m. Europe: removed from the standard Baudot tones communications link. Often, a satisfac­ 1275 Hz mark, 1445 Hz space-45 to (2125 Hz and 2295 Hz) to prevent inter­ tory demodulator can be constructed 100 baud Baudot ference. A frequency diagram of the 103 from diagrams given in a manufacturer's C.W. ID = 1175Hz Modem is shown in fig. 4. application notes and may involve only 1270 Hz mark, 1 070 Hz space-11 0 to The 202 Modem is used for data trans­ one or two integrated circuits. Such cir­ 300 baud ASCII mission at rates up to 1200 baud. The 202 cuits are available for the 565 PLL IC, the C.W. ID = 1370Hz Modem is a half-duplex device, and there­ 6860 IC, and the XR-2211 IC, for example. 1200 Hz mark, 2200 Hz space-300 to fore only one set of tone frequencies is 1200 baud ASCII commonly used: 1200 Hz for mark and Recommended Standards C.W. ID = 1100Hz 2200 Hz for space. Here again, these tones are different from those used by for Amatelir RTTY Tones As our small section of the amateur amateur Baudot stations. The 1200 baud Based on the previous discussions of hobby continues to grow, it is important data rate is certainly attractive when a the features and reasons for the various that we establish some compatible stan­ large amount of data must be sent and RTIY tone frequencies used, the follow­ dards so that we may continue to com­ the communications channel has little ing standards are recommended: municate with each other. V.h.f. repeater noise (typical v.h.f.-f.m. situation). How­ groups in particular are presently isolat­ ever, the tone frequencies of 2125/2295 H.F. RTTY {Mark = higher radio frequency) ed groups who may choose their own Hz and 1200/2200Hz are not easily sepa­ U.S.A.: standards with little concern about tech­ rated, and it probably is not feasible to 2125 Hz mark, 2295 Hz space-45 niques and procedures used elsewhere. use the two simultaneously on the same through 110 baud (ASCII or Baudot) However, this isolation is probably short­ channel as may be done with 1270/1070 2125 Hz mark, 2550 Hz space-110 lived, particularly if current trends for re­ and 2125/2295Hz tone pairs. A frequen­ through 300 baud (ASCII) peater linking and satellite development

cy diagram of the 202 Modem is shown in C.W. ID = 2025Hz (all shifts) continue. A little preplanning and agree­ fig. 5. Europe: ment now can prevent a very big compat­ Since the major application for use of 1275 Hz mark, 1445 Hz space-45 ibility problem later when the hobby has 1200 baud data exchange will probably through 110 baud (ASCII or Baudot) grown and these techniques are in use.

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(Reprinted from CQ, November 1982, with permission.)

HAL COMMUNICATIONS CORP. BOX 365 URBANA, ILLINOIS 61801