Slow Scan Television 1972

ijf Lai Ni W4 G TS of At/anM. as seen on Navassa.
Phil W4GTS .is seen by Allantes

reason for bringing the remains of that antenna back.

ihe ride back was i\ good deal easier than ihe trip to Navassa and many ol Lis were able to stretch out and gel some sleep, at least tor a couple of hours. No sickness this time. We did bum out one engine and IkllI to limp hack at half speed for the J:js1 fe w hours on the other engine. We just about ran out of fuel oil too. with only d spoonfull left when we docked, i he engine ijuit as we maneuvered info placei Luckily we had "borrowed" some f uel from a passing boat at Navassa in exchange for replacing a fuse in their radio, or we might have had to go into port out by the end of Jamaica instead of heading directly to Kingston.

We all had a quick dinner ai a local department store. the only restaurant open at 10 P\K and went off to a very deep sleep. Chris 6Y5CB and his attractive wife were there to meet us and guide us around. Most of us flew back to the U.S. the next morning, though a couple stayed on w It their wives for a few more days vacation. B\ Tuesday night ! was back at 73 six days away and another memory for a lifetime for me.

The trip to Navassa was so much fun that it put me back to the map of the Caribbean to see where else a small group might go that would be fun, reasonably rare, and not too expensive. I ill ink I've got it> Would any of you readers like tu fry your hand at a DXpedirion this fall? How about one to Baha Nuevo (HK0> in late October to coincide with the l)\ contest? I figure that the whole thing can be done for about S50D per person with ten going. We would leave from Kingston on October 25th and set up on the 26th- Ihe big push would be on the 2708th. and back on the 29th lo Kingston - to the U.S. the 30tlu One week of great fun.

This time we would have a place to sleep on the boat going over for il would take about 24 hours to sail that distance from Kingston. We would have two transceivers with beams, one with a good linear, and one spare transceiver with more modest anien-nas. i he beams would be complete with rotators and short towers, I think I can plan the trip and arrange for the equipment. It will be hard work al times, so strength and stamina are required! Any takers?

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Wb8dqt Televisor

You are about to enter the wonderful world of amateur slow scan television. Imagine yourself sitting in front of a radio receiver with a slow scan television monitor plugged into the headphone jack. The screen shows SM0BUO in Stockholm operating his rig from his lakeside cabin. You turn your dial to another frequency on the 20 meter hand and observe W0LMD exchanging circuit diagrams with W4TB in North Carolina. These things are now possible with slow scan televsion. 11 has not always been this way, so iet us turn the slow scan history pages back a few years io see how it all began.

In 1958, Copthorne Macdonald* a young engineering student at the University of Kentucky, began an investigation into the possibility of reducing the bandwidth of a wideband television signal to such a narrow spectrum width that it could be sent over a voice communication system. This meant that a 3 MHz television signal had to be reduced to 3 kHz signal a I 000 to I reduction in bandwidth! Macdonald eventually designed a television system that met these requirements. This system used an AM sub carrier, with picture information transmitted as amplitude variations of a continuous lone of constant frequency. The old 11 meter band was used to transmit test pictures since that was the only HF band authorized for picture transmission. The AM subcanier system proved successful, and circuit details were published in August, September QST (1958). Unfortunately, before the slow scan pioneers of that period could build equipment, the 1 1 meter band was allocated to the Citizen Band Service.

Slow scan television was now limited to frequencies of 50 MHz and above. However, under special authorization from the FCC, it was possible for Cop to perform tests on 10 meters for short periods of time, it was during one of these test periods that Cop WA2BCW, by transmitting 4' blind11 over a period of 30 days, was successful in sending pictures across the Atlantic Ocean to John Plowman G3AST. hese tests, and the pictures received in England, were described in March QST (1960).

i heir imaginations sparked by the transatlantic tests, other amateurs soon began to explore SS I V, Soon, a small group of SSTV enthusiasts were exchanging SSTV audio tapes, and investigating a variety of circuits and standards. An FM subcarrier system was developed (January, February

Sound Beam Frequency Khz

Fig. I-lp Picture of W9CNW, sent from W9NTP to VK3AHR, 10,000 miles, on 20m. 2.5kHz bandwidth.

QSI 1961), and it proved superior to the AM subcarrier system due to its greater immunity to interference from QRM and fading. Since none of the early experimenters were close enough geographically to exchange pictures on the 6 meter band, tests were confined to closed circuit systems, tape exchanges or transmission over phone lines. During this time the slow scan enthusiasts were attempting to get slow scan (SSTV) permitted on the HF bands. Petition after petition was written, but no FCC action resulted, Macdonald, now WA0NLQ, enlisted the help of several hams in Washington and succeeded in getting a special lest period set up at the Rocky Mountain ARRL Convention in Colorado to test the possibility of interference to other hams. W3LJV, W7FEN, WA0NLQ, W9NTP, and W0ITB participated in these tests, and for three days visitors at the convention saw themselves on slow scan television either going out over the air or their image being returned over the air from far-off places via retransmitted tape-recorded pictures, it was hoped that other amateurs would hear the signals, form an opinion about their nature and interference potential and let their feelings be known through proper channels. It was during tJ lis test that the well-known picture of W9CNW, Figure 1-1, was recorded in Australia by VK3AHR with the scheduling help of W9TCT. This test was described in September 1966 QST,

As a result of the Rocky Mountain Convention tests, Macdonald convinced both the Navy and the FCC to authorize special SSÏV tests on the HF bands. The primary goals of these tests were twofold: to improve the techniques and equipment developed for S i TV, and to provide visual communications for personnel wintering over at McMurdo Sound, Antarctica. Fifteen stateside amateurs were authorized to participate in these special tests and Macdonald provided a camera and monitor to KC4USA/N0ICE for use in Antarctica. Despite erratic propagation conditions, excellent pictures were often exchanged between Antarctica and the States, some of which were shown in QST Feb. 1967, page 77. It is interesting to note that the Navy to this day continues to support the develop-me ni and use o; slow scan, and has recently authorized W4ABY/N0AAJ and W4UMF/ N0XTV to exchange SSTV pictures with the S.S. Hope.

An FCC proposal of rulemaking permitting SSTV operation on the HF bands was published in November, 1967 QST with one month allowed for reply. After this reply period, several amateurs - W2PMV. W9VZL/3, WA0NLQ and W9NTP - went to Washington to review the 30 letters of opposition. Very few of these letters were strong in their criticism, but some letters expressed fears that needed to be studied and resolved. After eight hours of study and discussion these amateurs spent that evening at the home of W9VZL/3 where they wrote answers to the criticisms, i his small group felt sure that a slow scan television allocation was just around the corner,

At this time incentive licensing was getting quite a bit of attention by the ham world, ARRL, and the FCC. After about a year with no FCC slow scan television action, fears began to grow again. By making inquiry through the proper channels, we were told that SSTV would be a part of the incentive licensing proposal. The rest is history.

During the summer of 1968, while on a mobile DXpedition to Central America, one of your authors heard the famous words from W7FEN; "Slow scan is authorized on HF/' That is how it all came about. Those of you who now enjoy the luxury of operating your slow scan gear have these early pioneers to thank. Copthorne Macdonald is the inventor. and by his persistent interest finally perfected the system and obtained FCC action to use it.

Slow scan television can be transmitted in aii of the advanced and extra portions of the amateur bands. Its bandwidth is confined to that of a voice station which is interpreted to be 3 kHz for single sideband transmission, "he 160 meter band is excluded but all the phone portion of 10 meters and VHF and UHF can be used by General and higher classes of amateur licensees. It is of interest that SSTV received more band allocation from the FCC than any of the petitions ever requested.

This is the history of slow scan television. Today hundreds of amateurs throughout the world - in Russia, England, Sweden, Italy, Greece, South Africa, Colombia, Venezuela, Guadeloupe, Puerto Rico. Nicaragua, Canada, Alaska, Hawaii, New Zealand. Australia, Japan, Brazil, Southwest Africa, Belgium, Germany, the continental United States, a total of 28 countries - routinely exchange pictorial information via slow scan television on the 80 through 10 meter bands. And what started as a senior project for a young undergraduate student at the University of Kentucky is now one of the most exciting communications modes within the amateur service.

Basic Principles of Slow Scan TV

Let us explore this new amateur television that was specifically designed by hams for picture transmission on the high frequency bands. Instead of 3 MHz being available for ATY\ as in UHF, we are restricted to a 3 kHz voice bandwidth in the HF amateur bands, "he bandwidth of a conventional television signal must be reduced by a factor of over 1000 for slow scan. It becomes obvious that both I lie horizontal and vertical frame rate must be reduced to as low a frequency as possible in order to reduce the bandwidth from 3 MHz to 3 kHz. Resolution of the picture is a prime consideration in choosing the standards Home television viewers may feel that all 525 lines of commercial TV are needed for good PV pictures, but with narrow i-fsN poorly ad* justed interlace and other deteriorating fac tors the actual resolution of a home television receiver is reduced to about 250 lines. As an example, 80 lines per MHz is the computation formula used foi estimating the resolution of a 3 MHz i-f system. This is a resolution of only 240 lines. If we now consider that viewing will be done on a small cathode ray lube (available radar tubes), the tube screen size becomes a limiting factor due to beam focus. Everything being taken into consideration, 1 20 lines were chosen as the maximum number of lines for slow scan TV. It will be shown later that if 240 resolution lines are desired, they can be transmitted in an interlaced format for increased resolution, but such a picture cannot be viewed directly and must be photographed for the increased resolution to be realized in practice.

The vertical frame rale should be as slow as possible consistent with the storage time of the viewing tube. As far as the average ham is concerned, this means the use of a surplus P7 phosphor radar tube since available storage tubes are prohibitively expensive- Tests run over several years showed that 10 seconds was the maximum time of a frame that wouid permit viewing the frame on a P7 tube with a viewing hood in a normally lighted room.

it is a real advantage to use a horizontal line frequency that is related to the power line frequency. After sampling many viewer opinions and running laboratory tests it appeared tha! 15 Hz (i.e., 60 Hz divided by 4) was a good choice. 120 lines at 15 lines per second gives an eight-second vertical frame rate. Since these standards have been used by slow scanners in all 60 Hz power countries, it is hoped that they will be maintained indefinitely.

One thing that remains to be demonstrated is the video bandwidth of the slow scan system. Since an aspect ratio of 1:1 seems the most desirable to fit a round radar cathode ray tube, the horizontal resolution should equal the vertical resolution. There are 120 lines division in the vertical direction; therefore, there should be 120 division in the horizontal direction. This is shown in Fig. 1-1. For simplicity assume that every other spot is black alternated with white spots If these spots

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