Richard R. Parry W9IF 38W2SS Deerparh Road Batavia IL 60510 So You Want To Get Into RTTY? gation con di t ion s ? No problem. If the other sta tion has an answer-back system, all you need do is type the sta tion's access code. The transmitter is then auto matically turned on and a short message is broadcast. If you are of an artistic bent, exchanging RITY pictures shou ld fulfi ll you r desires. If you enjoy music, composing songs to be played on the telepri nte r wil l prove fascinat ing. If building is your bag, you have come to the right p lace . Whi le commercial equipment is available, build ing your own equi pment should prove rewarding, not to ment ion the pecuniary savings. If contests are your main interest, RTTV has many throughout the vear. You say collecting award certificates is your thing? Then try working all states or all continents on RTTV for a challenge. If you li ke to rag chew, you have come to the right place. RTTV is a very relaxing way to communica te. Si nce the conversation is printed on paper, you can go get tha t cup of coffee or glass of soda pop while the other fellow is talking withou t missing any of the conversation. It is also possible to start answeri ng the other fell ow while he is still ta lking by punching your message in to paper tape. When the other fellow is fi nished transmitting, you can send your prepared message back. If you want to keep informed on diffe rent aspects of ham radio, Wl AW broad casts RTTY bulletins daily. Info r ma t io n on OSCA R crossings, propagation, and general comments about ham radio are given. You say you like to keep abreast of the news and weather? No pro b lem . Man y co mme rcial stations such as the Associat ed Press broadcast news bull etins throughout the day. Other commercial sta tions broadcast weather condi tions. This type of info rmation could be most useful dur ing winner • example, how would you like to be able to turn your friend's teleprinter on and leave a message for him when he gets home? You can do it; it's called a selective calling system. Or how would you like to turn on a transmi tter thousands of mi les away to get an idea of current propa- • •, C J J J '; '; ,J';\J~ " il • "• ..... • • •• • " 0 .J • , ted circuits over the last decade and the rapid decrease in the cost of these circuits have made it easier than ever before to get into RITV at a minimum cost. It is this pro liferation that has also aided the development of RTTV systems that were heretofore impossible or im practical. For "Call For Papers"- - Photo 1. The author and friends. The basic RTTY equipment includes a Model 15 teleprinter, an ST-6 demodulator, and an AFSK modulator. The amenities include an electronic keyboard; a CR T RTTY tuning indicator; a UT-4 for siqnal regeneration and electronic speed conversion; a digital date, time, and message generator for the Sefcal and W-R-V system; and a cassette tape recorder for storing and transmitting data. (Photo by Anthony R. Donaldson.) s o you want to get into radioteletype? Congrat ulations, you made the right decision . Radioteletype (abbreviated RITV) is the most interesting, fascinating and rewarding mode of communication aro und today. The burgeo ni ng of integra' --I I S£l £ t Tl)fl I" AG~ ( '_ _ J At the heart of every amateur RTTY station is the teleprinter. The teleprin ter is quite si milar to a typewri ter. mark of t he Teletype Corpor ation. As such, it shou ld always be capitalized and used as an adjective rather than a nou n. The na me Tele type has come to be used erroneously due to the popu larity of the Teletype Corpor ation products. There are, however, other manufacturers of t e le pr in ters including Kleinschmidt, Lorenz, Mite, and ITT Creed. Therefore, to be correct, I wilt use the word teleprinter to describe this family of machines. •LOOP tURR E ~T r - I e e I PO*1:R '--'OURt£ r------ I 111 111 ,,, get down to basics. There are actuall y on ly three compo nents necessary to get a RTTV station on the air - a teleprinter, a modu lator, and a demodulator. Fig. 1 shows these three basic components and their interconnection in a typical RTIV sta tion. PART 1 TH E BASICS l.n eOARO i CO-.,..-, ___ .--r_._.,Qe _." ..,. , "A"" 5P AC[ T I.. E a telegraph key is nothing marc than a switc h, and a te lep rinter's keyboard is also a switch, why don't we su bs ti tu te t he keyboard con tacts fo r the te legraph key? With th is technique, t he mark may be thought of as t he transm itter's "on" condition, and the space the transmitter's "off" cond ition. This is called make-and-brea k keying and is sometimes used. However, a p roblem arises using this meth od . It occu rs while the transmitter is off. Suppose a static c rash occurs dur ing the space cond it io n (transm lt t er off); chances are the static crash may be misconst rued by the equi p ment as a mark, when it is really a space. The net out come under this method is poor reliability, and thus a wrong character may be printed. We don't have this problem with CW because the incredible mac hine, the brain, can discrimina te between a static cras h and a signal. So now what do we do ? The answer is use frequency shift keying (FSK). With this method, the transmitter is on during both the mark and space states. If the t ran smitter 60 wpm (actually 61.3). Since few amate urs are speed typists, this does no t usually limit one's typing speed. While it is tr ue that other speeds are used, such as 67, 75, and 100 wpm, amateu rs use 60 wpm almost excl u sively . It is important to note tha t each and every character is 163 ms no matte r who is typing or how fast. Contrast ing th is with CW, eve n if all ope rators sent CW at a specific speed, each character is of a di fferen t duration. Th is is a cha racteristic in herent in the Morse code. We now know the code that we must use to talk to the teleprinter. Our next task is to put t hese marks and spaces in a form that can be transm itted over the air . Th is is t he purpose of the modula to r. "4 ''. The Modulator Modulation refers to the process of transmitting the RITV signal. Le t 's return once aga in to our analogy between CW and RTTY. CW is transmitted by opening and closing a telegraph key. Since printer uses the "figures" and " letters" command. The upper case is attained by depressing t he figures key. The teleprinter moves to the lower case when the le tte rs key is depressed. Going back to our example, if t he tele printer is in th e lower case (letters), it wil l pri nt the le tte r "R". If a figures key has been depressed previous ly, the te leprinte r will be in the uppe r case (figures) and print the number "4". Since each character is of a finite d uration, there must be an upper limi t to th e num ber of words that can be printed in one minute. Adding t he duration of t he 7 elements of the code, we ar rive at a to tal t ime of 163 rns for each character (see Fig. 3). Since each character is 163 ms, t he number of characters that can be prin ted in one minute is 368 (60 seconds/. 163 seconds) . One word is typical ly considered to be 5 letters plus 1 space. Th us, the num ber o f words per minu te is equal to 61 .3 (368/6). Th is is t hen rounded off to 60. It would be nice to use machines that operate at higher speeds, but one shou ld remember that te lepri nters use mech anical parts t hat arc limited to relatively slow speeds. Video display un its are obviously not mechanical and hence are not li mited to such low speeds. Speeds of several thousand words per minute are not uncom mon. The 163 ms character length sets th e u pper t yping limit at It wou ld be d ifficult to design equi pment that wou ld have to copy correct ly at all typing speeds. Therefore, a special teleprinter code, referred to as the Baudot code, is used. This code was designed so that each character contains the same number of elements or bits, and each clement is of a fixed duration. One other important characteristic is required of our teleprinter code. When transmitting the code, the receiving teleprinter must be synchronized with the transmitting teleprinter, even though they may be thousands of miles apart. T o accomplish this end , a start pulse is added at the begin ning of t he characte r, and a stop pulse is added at the end. These pulses are used to insure synchron ization of the machines. Table 1 shows the Baudot code that is used by teleprint ers to comm unicate. All elements of the code are 22 ms (22/1000 seconds) except the stop pulse, which is 31 ms. Fig. 3 represents grap hically a teleprinter character. The entire 7 bit word for this character may be written as SSMSMSM. Since the first and last bits are the start and stop bits respectively, the character is reduced to the following 5 bit code, SMSMS. Referring to Table 1, we see the character is the letter "R", or the number "4". Why are there two possibil i ties you ask? For those of a mathmatlcal bent, you will notice with a 5 bit code that the maximum number of comb inations is 32 (25=2 x 2 x 2 x 2 x 2=32). The alp habet and ten digits require 36 (26+10) combina tions by themselves, and we still haven't allowed for punctuation and other special functions. To obviate this problem, the teleprinter is equipped with a lower and upper case character set. Th is ;s analogous to the upper and lower case of a modern e lectric typewriter. Ho wever, where the typewriter uses "shi ft" and "unshlft" to control the case, t he tele- 31 Photo 6. The TSR-500 is at the heart of all the special features of the Dovetron MPC-1000R and may be purchased separately. The error correction circuit allows one to erase the last word put in the memory by depressing a "blank" key. Information on this and other Dovetron products may be obtained from Dovetron, 627 Fremont A ve., South Pasadena, California 9 1030 or phone 213-682-3 705. (Photo courtesy of Dovetron.) is on during both states, how do we different iate between the two? Simple, we let the transm itter transm it one fre quency during the mark condition and transmit an other frequency during the space condition. Th us, we are shifting between two fre quencies, and hence the name, frequency shift keying. The difference between the two frequencies is referred to as the shift. Wh il e amateurs are allowed to trans mit RTTY using any sh ift less than 900 Hz, the amateur fratern ity has settled down to two stan dards, 170 and 850 Hz shi fts. The 170 Hz shift is used almost exclusively on the low bands, while 850 Hz shift is ty pically used o n the VHF bands. Two methods are used to s hi ft the t ra ns mitt e r ' s frequency - carrier frequen cy shift key ing (FS K), and aU~io frequency shift keying 32 (AFSK). With the f irst tech nique, the carrier frequency is shifted by pl acing a capacitor across the transmitter's osci l lato r ta nk circu it. This capacitor is connected by the contacts of the keyboard. w ith the ca pacitor d iscon nected, the transmitter is transmitting a mark signal. When t he capaci tor is connected by depressing a key on the keyboard, the tr a nsmitter's freq uency is lowered , and this re presents the space condition. Typical ly , the keyboard is buffered from the tan k circui t because the keyboard's contacts can not efficiently contro l these radio frequency signals, but the idea of connecting a small ca pacitor for space and disconnecting it for mark remains unchanged. Wi th the second met hod, audio frequency shift keying, audio tones arc generated and fed directly into the microphone jack of the trans mitter. For narrow shift, the AFS K unit gene rates 2125 and 2295 Hz signals (2295 . 2125 = 170 H, shift). The 2125 Hz signal denotes the mark signal and the 2295 Hz signal represents the space signal. When the keyboard contacts are closed, a 2125 Hz signal is fed into the microphone jack. The 2295 Hz signal is generated when the contacts are open. Note that with this method, the space freq uency is higher than the mark frequency. When using carr ier FSK, the opposite is true; the mark frequency is higher than the space freque ncy. Thi s apparent contrad ict ion can be explained by noting that the heterodyne system of genera ting a lower sideband signal has the effect of flip ping the frequencies over. Therefore, the final space frequency generated by the transmitter is indeed lower th an the mark frequency when using AFS K as it is when FSK is used. The AFS K method of generating a RTTY signal requires a stable audio genera tor. Its main advantage lies in the ease of interfacing the keyboard with the trans mitter. One need simply plug the outp ut of the generator directly into the micro phone jack of the transmitter. The De modulator At the receiving end, our job is to decipher the tones and reconstruct the informa tion in a form the teleprinter can handle. The re are basical ly two methods that are used to differentiate between th e two tones. The old technique, and still the most widely used, involves demodulation via two separate resonant LC (inductor and capacitor) cir cuits. In the case of 170 Hz shift, one LC resonant circuit is tuned to pass the 2125 Hz mark signal, and the other is tuned to pass the 2295 Hz space signal. These filter cir cuits, called discriminators, pass only their respective tones, mark or space, and drastically attenuate all other frequencies. Fig. 4 shows a simplified diagram of this type of demod ulator. After the two tones have been seg regated through the filter ing process, they are fed to a stage called a slicer, which drives the keying transistor. When a mark signal is re ceived, the slicer turns the transistor on, and we have a closed loop. With a space signal received, the slicer turn s the keying transistor off, and we have an open loop. Thus the key ing transistor acts as a switch. However, more to the point, it is simulat ing the keyboard switch contacts of the station transm itting. A modernization of this method employs active filters to replace the passive LC filters. The main advantage of this technique comes fro m the fact that fairly large inductors (typically 88 mH) are not required. Tuning the ..." .- ----- - ----• • • U ... RVD-~OO8I:n. __ •__• •,._.- ..u~ f 'lTEII t KE "~ ~ .~O " f"---. TIIA~S 'STOII Ill'CE ,Y(1I ~, ~f' ) V SP&C£ f 'lTill 1± Fig. 4. Simplified circuit diagram of a RTTY demodulator using passive filters. Photo 7. The RVD-l005 visual display unit is the modern answer to the teleprinter. The unit accepts demodulated Baudot signals at 60, 66, 75, and 100 wpm and display s the information on 25 lines with 40 characters per line. (Photo courtesy of Hal Communications Corporation.) active fil ters for the exact frequency during calibrat ion is also simpler, since it typically requi res an adjust ment of a resistor rather than the trial and error technique used in trimming an LC filter to the exact frequency. The second method that is used to dem odulate a RTTY signal uses a phase locked loop (PLL) integrated circuit. Despite its inferiority to the method described above, it is cheap and easy to build. As such, it makes a great demod ulator to build to get on the air in a hurr y. PART2 THE AMENITIES As previousl y stated, all that one needs to get a RTTY station on the air is a tele printer, a modulator , and a demodulator . Ho we ver , severa l accessories may be added to a sta tion to really increase the joy of RTTY and o pe rat ing ease. Amenities available to the RTTY aficio nado include an electronic keyboard, a vidio display, tape distributor, tape reper forator, magnetic tape record er, a CRT RTTY tuning indicator, a Selcal, and a W-R-U answer-bac k system. The Electron ic Keyboard Typing on a typical te le printer keyboard will require some practice and patience, especia l ly if y ou a re accustomed to typi ng on a modern electric ty pewrite r. Refering back to Table 1, yo u can see that the alphabet is printed if the typing unit is in the lower case, and punc- tuation s and numbers are printed if the typing uni t is in the upper case. Thus, ty pi ng a station call or an address, which conta ins both numbers and letters, wou ld require extra operations to put the teleprinter into the upper o r lower case as desired. An elec tronic keyboard, designed for the Baudot code, automat· lcallv inserts upper and lower case commands to insure the proper character is printed without the operator directly requesting the up per or lower case. An electronic keyboard has ot her, more SUbtle, advantages. In addit ion to bei ng attractive and quiet, the action of the keys is much smoother than a con ve ntional teleprin te r keyboard . These features dec rease operator fat igu e and increase th e ease of operat ion. The Video Display For th ose who don 't like the noise and maintenance of a teleprinter, state of the art technology has made video displays possible. An old black and white TV, coupled with a video display interface module, makes an attractive display. Two notes of caution . Except for the Hal Corpora tion unit (see Pho to 7), these video display units require a special code called ASCII. ASCII is the acronym for USA Standard Code for Infor ma t ion In terchange. Therefore a unit capable of translating the Baudot code to ASCII is required. The second shortcoming of a video display deals with the maximum amount of information that can be d isplayed on the screen. Wh ile the format of charac ters on the screen varies from manufacturer to manufac ture r, 1000 characters is typical. This 1000 characte rs may seem like a lot, but it reall y isn 't. A typical QSO consis ts of several thousand characters. Th is means th at you cannot look bac k at the content of a previous trans mission or leave t he ham shack for more than a few minutes. Both are onl y slight disadvantages, but, nonethe less, something to con sider before purchasing a video display. The Reperforator If one wished to record a message to be played back at some later date, he wou ld use a reperforator. The reperfora tor records a message by punching holes in a narrow strip of paper tape. Messages ma y be recorded either locall y , by ty ping on the local keyboard, or straigh t off the air. The Telety pe Corporation Model 14RO is a reperforator typically used by amateurs. Of parti cular importance is that once the paper tape has been made, it may be trans mitt ed at full machine speed (60 wpm ), thus exped it ing the transmission of informa tion . A tape reperforator is used mos t ofte n by amateurs to make and record RTTY pictures. More on this ex citi ng facet of RTTY later. The Tape Distr ibutor The tape distri buto r is used to read th e punched paper tape made on the tape reperforator. The unit trans lates the holes in the tape into pu lses so that they may be pri nted locall y o r sent to another station. The Teletype Corporation Model 14TD is a popular tape distributor used e xte nsively on the ham bands. The Magnetic Tape Recorder Si nce the RTTY signals sent to the demodulator are in the audio range (2125 and 2295 Hz), why not record the information on a magnetic tape rather than on paper tape? After all, a cassette player is quieter , cheaper, and more convenient than a mechanical paper tape reper fora tor. The answer is that we can do just that. In fact, th is technique is used extensively by computer hobby ists to reco rd data. However, some problems can arise. For e xa m ple , when using a narrow shift (1 70 Hz), th is meth od be come s very sensi t ive to fluctuations in 33 Photo 8. Shown here is a small sampling of some of the RTTY pictures thot are exchanged on tlJe ham bands. The small uncomplicated pictures require approximately 10 minutes to reproduce at 60 wpm, while some o f the large pictures with overlining can require an hour or more. The RTT V Journal sponsors an annual RTTY art contest for those interested in this facet of R TTY. The pictures were stored on the cassettes shown in the pho to. (Photo by Anthony R. Donaldson.) the tape speed. The mar k and s p ace tones will chan ge frequency as the tape player changes speed. Th is probl em can be rem edied by using a high quality tape player with a low wow and flutt er spec ifi cat ion . There is st ill o ne addi ti o n al c o m p l icatio n, t hough. It would be difficult to find two machines that turn at exactly the same s pee d. Therefore, th e cassettes could not be inte r changed between mac hines. Both problems are solved by using a wider shift (850 Hz). For a mar k ton e of 2125 Hz to be misconstrued as a space tone of 2975 Hz, the ta pe player wou ld have to be in very poor shape indeed. Photo 8 shows several RTTV pictures, all of whic h have been recorded on the cassette tapes shown. The CRT Tuning Indicator Several methods for tuning in a RTTV signal have been tried over the years. However , one of the oldest, and perhaps the best, makes use of a cathode ray tube (CRT). The mark and space signals 34 are de picted on the screen of a small CRT (see Photos 4 and 5). The mark is typically a line di splayed horizontall y, and the space signal is a li ne shown vertically. With such a unit, the operator tunes the receiver until t he horizontal and vertical lines arc of maxi mum length. The Selcal The stat ion eq uipped with a selective calling [Selcal] svstem can be turned on and off by typing a predeter mined access code. Wh ile a stat ion may be programmed for any access code, the sta ndard has beco me the use of the last three letters of the station's call. For example , if you r frie nd 's call were W9I UV, typin g the last three charac ters of h is call, IUV, at your station would turn on the teleprinter a t the station of W9 IUV. You can now leave a message. When you are fin ished, ty ping fo ur N~ will shut your friend's tele pri nter off. The four N~ at the end of the message are a uni versally accepted shut down code. Such a system is a grea t way to keep in touch with fr iends. Of particul ar importance is that t here is no need fo r a person to be on the premi ses. Therefore , the equ ip me nt may be left runnin g continuously. I st ill get a kick out of getting home to a few notes sent by friend s. The dist inct advan tage of this system is that only notes addressed to you are received . In an all-call system, you would receive everyone's notes. The W-R-U A W-R-U (who are you) an swer-back syste m is one step beyond a Selcal system. Leaving a note fo r a frien d can be a hit or miss affair unless you are sure his equi p ment is on. A W-R-U sys tem allows one to asce rtain just that. By ty ping a predete r mined access code, one can in essence inte rrogate the condi tion of the equipmen t. Adding the three characters figu res, blank, and H after the last three le tte rs of the station's call has become the customary code used to gain access to the equipment. For example, su ppose you wan t to leave a note fo r W9YPS, bu t you are not sure if his equ ipment is up and running. To ascertain the condit ion of the equipment, you wo uld type the six characters Y, P, 5, figures, blank , H. Typically , the message sent after the W-R-U is tri pped has been punched on paper tape an d Is played back on a tape di strib utor. Having tripped the W-R-U an d, thus, confirmed the status of t he equipment, you can now proceed as with a Selcal system and leave a note with the assurance the note has been rece ived . PARTJ SOME UNIQUE ASPECTS OF RTTY For those of you who are connoisseu rs of art, you sho ul d enjoy d esigning, ma k ing, receiving, and sending RTTY pic tures. The numbe r and kinds are tru ly prodi gious, ranging from angels to nudes. Making a RTTY pictu re req uires a reperfora tor (paper tape puncher ) and a tape dlstrlbu tor (paper tape reader ). For those wishin g more lnforma tion on this fascinating aspect of amateur RITY, consult any of the RTTY handbooks li sted in the annotated bibli· ography. RTTY Music I got home late one night during the Christmas season and thought I would listen to a RTIV QSO or two before reti ring. Instead of hear ing the customary clatte r of old fait hful (my Model 15). my ears were treated to a chor us of Jingle Bells fo llowed by Noel and other Christmas favorites. This aspect of RTTY ma kes use of a tape distributor and tape reperfor ator. Instead of printing a character, the bell of the tele printer is actuated in time to the me lody. Conclusion It has been the purpose of this a rticle to give the reader an int rod uct ion to the fasci nat ing wo rld of RTTY and to whet his appit ite. For th ose wishing to pursue this mode of communication further , I have accumu lated an annota ted bibl iography list ing books and articles to read for further info rmatio n on this subject. Good luc k and MMSMM MMMSS MSSSS.• ANNOTATED BIBLIOGRAPHY Periodicals and Books Devoted to Radioteletypa American R a d i o Relay League . Speciali~ed Communica tions Tech niq ues for the Ra dio A ma teu r. New ington . Conn., The American Radio Relay League, 19 7 5. Only a portion of this book is devoted to radioteletvpe. O t her topics include slow scan tele vision, facsimile, and space comm unication . However, t he ap proximately 70 pages that are devoted to racnotetetvpe are well f illed with usefu l information . I recom mend th is book highly. RTTY Journal. Beginners R TTY Handbook. RTTY J ournal , 19 75. If you want the most fo r the leas t , t h is is it . As t he t it le in di cates, t he handboo k is designed spec if ically tor the beginner. I find it a n excellen t reference bo o k and recommend it hi gh ly. Send $2.50 to RTTY Jour nal, PO Box R Y, Card iff-by-t he-Sea CA 92007 . RTTY Journal This short but useful periodi cal is devoted exclusively to GLOSSARY power and a allows AFSK Answer-Back (W -R·U ) S yste m Autostart (AII·Calll System ASCII Baud Baudot Discriminator FIFO 'SK KS KOX Local Loop Abbreviat ion for audio frequency shift keying. With this method of modulation, two tones {mark « 2125 Hz, space" 2295 Hz} are fed directly int o t he m icrophone jack of t he transmitter. A system capable of being remotely controlled by another station. When tripped by a unique access code, a s hort predetermined message is broadcast. A n eutostart circu it may b e t hought of as a squelch circu it. If an authentic RTTY signal is detected, t he tele printer motor is turned o n a nd the message is printed. Unlike a selective calling system t hat turns the tete p unter on onl y when a particular access code is rece ived, au tostart circuits act as an all-call system a nd, t hus, prin t a ny RTTY signal . The cessatio n of t he RTTY signal causes t he telepri nter motor to turn off. Acrony m for t he USA Standard Code for Informat ion Inte rchange. At p resent, this code can not be used on the amate ur ba nds without special FCC p erm iss ion. Unlike the 5 level Bau dot code, this is a 7 leve l code used e xtensively in the electronics industry. T he basic unit of speed derived from the duration of the shortest code element. For 60 wpm operation, the baud ra t e is 45.45 (1/22 ms l. T he code used by amateur RTTY stat io ns t o excha nge information. It is a 5 leve l code plus o ne start and one stop bit. A disc rim ina t o r, as used In RTTY circuits, cons ists of a fi lter, eit her p ass ive or act ive, to pass one fre q uency an d discrimina te against all others. Acronym tor first-in/ttrst-out. A spec ial integrated circuit memory device that will accept parallel d a ta and retransm it it o n a first-i n, first out basis. Abbreviation fo r frequency shift keying. With this method of modu lation, t he frequency of t he trans mitter's crystal or vto is shifted. Abbreviatio n tor keyboard. Acronym fo r keyboard operated trans missio n. A station equipped wit h a KOX system can turn on t he t ransm itte r and turn ott the rece ive r simply by t yping on the station's keyboard. When t he operator ceases typing, the transmitter automatically turns off, and t he rece iver turns on after some preset delay. The circuit containing a source, the selector magnets key board This connection Mark Modllm PLL RTTY Selcal Selector Magnet Shift Space Stunt Box Tape 0 istributor Tape Reparforator TU UART W· R·U local copy on the te lepr in ter (see Fig. 21. In RTTY appl ications, the mark is one of two states. The mark con d ition is characterized by a closed circuit . The space, the o ther State, is characterized by an open circuit con dition. Acronym for modulator, demodu lator. A modem is typically used in a system where tones are transmitted and received over telep hone lines. It may be t hought of as t he equivalent t o a RTTY station's t e rmina l unit. Abbreviation tor a phase locked loop . A PLL integrated circu it can be employed in a RTTY demodula tor to disc riminate between t he mark and space t o nes. Abbreviation for reotcretetvoe. Often pronounced "rittv." Acronym tor selective call ing system. This system allows t he te lepr inter t o be remotely controlled by a un iq ue code. An el ec tromagnet tha t senses current flowing in t he local loop and picks a character t o b e printe d in respo nse to the c urrent pu lses . T he d iffere nce between the ma rk and space frequencies. For ex amp le, if t he mark freq uency is 2125 Hz and t he space frequency is 2295 Hz, t he d ifference of 1 70 Hz is referred to as the shift. The 170 a nd 850 Hz sh ifts have beco me t wo widely used stan dards. One of two states describi ng the cond itio n of a te leprinte r' s loo p. The space is characterized by an open loop cond ition. T he ma rk state indicates a closed loop . A mechan ical un it t hat a llows t he operator to add control features to the teleprinter; fo r exam ple , an auto ma tic "no n-o v er I i na" cont rol whic h prevents print ing over a m essage a lready type d. Controlling a remote device is another use for t he stu nt box. A machi ne that reads prerecorded paper tape. A device t hat records data by punc h ing holes in a paper tape. Abbreviation fo r termi nal uni t. This is t he RTT Y equival ent to a mode m. It contains a modulator, demodu la t o r, and loop power supply. Acronym for un iversal asynchronous rece iver transmitter. A sop hist icated integrated circuit t hat accepts serial da ta and retransmits it as pa ra lle l data and vice versa. Abbreviat ion for who are yo u. See Answer-Back system. 35 amateur r a dto r e te tvoe. To subscribe send $3.50 to RTTY J our na l, P.O . Box RY, Card iff bv -tbe-Sea CA 92007. Pr ice d iffers outs ide the U.S . , Canada, and Mex ico. 73 R TTY Handbook There are two ATTY hand books, both of which are, or have been, dist ribu ted by 73. The old ATTY handbook, wh ne probably a fine book several years ago, is currently ou tdated . Being aware of th is shortcom ing, 73 h as re cently released a new handbook called, appropria t e ly enough, The New RTTY Handbook. Teleprinters McNatt, M.S. "A Guide to Baudot Machines: Part 1, Description of Avai lable Devices," Byte, A pril 1977,p.12. An excellent, interesting, and succinct article describing many of the Teletype Corporation mach ines u.e., Models 1 1, 12, 14 , 15, 19,20,26,28,29,31,32,33, 35, 37, 38 , and 40). O n page 158 of this article is a list of sources for Baudot equ ipment. Mod ulators Roos, J .C. " U niversa l AFSK Genera tor ," 73, July 1974, p. 37. This art icl e d escribes a RTTY modulat or uSlOg the AFSK method of modulation. The unit is somewhat advanced for t he beginner, but it does a very nice job. This is the modulator I have been using fo r several years, and I can speak highly of its effect ive ness and re liab ility . Demodulators Grossman, B., and John S. Reid. " B u il d ing the Safari RTTY Termin al ," 73, August 19 76 , p . 122. Unl ike most demodulators that us e passive fi lters (inductor and capacitor) t o discriminate between the m ark and space frequencies, th is un it uses active fi lters. Inherent in ac t ive fi lters is thei r ease of calibration . A resis tor is used to t rim the filte r to the exact frequency , rather than the Iterative method used 10 LC fi lters. Hoff , I. M. "The Mainline ST~ A TTY Demo dulator," Ham Radio, January 1971, p . 6. T he author, Irvin Hoff, has probably done more for ATTY within the last 15 years than any other RTTY aficionado. He is respo nsible for many demod ula tors , o f which this article describes t he most advanced. T his unit has become a standard of ex c el lence in the R T TY community . For t he amateur who is earnest about ATTY, t his is t he demodulator to get. Joffe, A.S. " A id icu lo usly Simple RTTY System," 73, September 19 76 , p . 70. For t hose w ho want to ge t on RTTY in a hurry, this is t he article you want. The demod ulator, while lacking sophistica tion, works, and you can't argue with success. Stinnette , N. "Phase Locked Loop RTTY T erm ina l Uni t ," Ham Radio, February 1975, p , 36. This articl e describes a simple demodulator using the phase locked loop principle . T his is p robab ly the best demodulator for the neophyte to ATTY since it is cheap and simple . SelcaL Answer-Back, and Relat ed Eq uipment Kelly, B. " Mon it o r Receiver for ATTY Au r o s t a r r ;" Ham Radio, December 1972, p . 27. This article describes a fixed fre quency receiver for the ATTY a fic ionado in te res ted in autostart operation. Unattended autostart operation requires more t han just a receive r with good sensitivity and selectivity . It requires t he receiver t o be ultrastabte since, as previously stated, the station is unattended . For this reaso n , t he rece ive r described in t his article is crystal controlled. Lichtenwalner, B.D. "The Com puter eso Machine," 73, January 1976, p. 80. T h is unit is capable of sending the t ime (hours and minutes}, the date (year, month, and day), a pre program m ed message, and a CW 10. This u nit is typically used in an answer-back system. When t ripped by an access code, the ti me, date, message, and ew 10 are broadcast, indicating the equ ipment is up and run ning. Sandel'S, L.W. " RTTY Autocall t he D igita l Way," 73, February 1976, p. 76. T h is article describes a selec t ive calling unit Isercen. Of part icu la r note is that t he uni t uses integra ted circuits rather than a mechanical stunt box . AdViinced Projects Guthrie, R.o. "ASC IIfBaudot with a PROM," 73, June 19 76 , p . 114. T here are many commercial video display units on the market for those who don't like the clatte r of a teleprinter. Unfortu nat ely , these commerc ial uni ts a re geared for the computer hobbyist and, as such, accept only t he ASC II code . This art icle describes a circuit that will convert the Baudot code to ASC II, so an amateu r RTTY station can u se one of the commercially manu factured video display units. Hoff, I. M. "The Mainline UT-4," RTTY Journal, February, March, Apri l, May 1974 . T his series of articles describes the UT-4 which is a UA AT / FI FO combination capable of regenerat ing t he received signal and elec t ronic speed conversion. It 's an advanced project but most worth while to the senous ATTY afic ionado . Hutton, L. 1. "Build This Excit ing New T VT ," 73, March 19 76 , p . 76. This artic le describes two projects. T he first is a hom e brew e lec t ron ic key boa rd that generates the Baudot code. The second po rt io n of the article describes a Bau dot to ASCII con verter and a TVT characte r generator . Levy , S.P. "A Morse to ATTY Converter," 73 , J une 1976 , p . 106. If you like sending c:w but not receiving, th is art icle is for you. The circuit m a kes use of a m icro processor to convert ON to ASC II or Baudot. It sure would b e nice to have t he nex t t ime you have to t ake a code test a t t he FCC. Mooring, E. E. " Phase-Sh ift RTT Y Mo n ito r Scope," Ham Rad io , August 19 72, p . 36. There are many d ifferent methods that one can u se to tune in a RTTY signal properly . This unit uses the phase shift method and a CR T d isplay. Th is tech nique is superior to most other methods, since t he CRT d isplay gives a great deal of info rm at io n. It is possible t o tell at a glance if the o ther sta tion is transmitt ing wide or narrow shift , if t he mark and space signals are reve rsed, and much, much m o re . This is a nice amenity to add once you have a station running. Miscellaneous Alexander , O. " T he First Com put er-Co nt ro lled Ham Station," 73, August 19 76 , p . 82. This very interesting artic le describes t he amateur RTTY sta t ion of the grand prize winner a t the 19 76 Altair Computer Con vention. If you want t o get some idea of t he flexi bility of a com puter-contrctled amateur RTTY stat ion, th is art icle is for you. Brehm, R.C . "R TTY Goes Modern," 73, February 19 77 , p. 82. T his article is a must for t he beginner who wants to know t he history of amateur ATTY. The author co ncl udes with a progno sis for ATTY and its ever increasing comradeship with microproces sors. Green, W. "Thirty Years of Ham ATTY," 73, November 19 76 , p . 11 0 . An interesting article describ ing the h isto ry of amateur ATT Y and is by one who was around during its infancy . Also inc luded are p ic tu re s of antique ATTY gear . 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