Most amateurs have some knowledge of SSTV by now. It appeared on the 20M Band about 40 years ago. Since 1990, as the Personal Computer began to appear in many forward-thinking Ham's shacks, SSTV has either captured your interest or  -- more than likely -- you hate it!  Until you see Ham Radio Video, you will never appreciate it, nor understand the tremendous enhancement it can provide in your contacts with other people.

Say what you will, Analogue SSTV is pretty neat stuff and a lot of guys are having fun with it. But the new DIGITAL system offers  "forward error correction" such as you have on your computer and this means the picture will be as colorful and clear as the original picture being sent -- if you ever get all of it down to the last BIT! I say that with tongue-in-cheek, however, since I have yet to see pictures on the PC that equal the brilliant color we enjoyed on the R-G-B monitors when we were using the ROBOT 1200Cs.

What do you think of Digital SSVideo -- now heard daily on 14.233 KHz, 7.228 KHz and 7.173 KHz?  Digital is a different ballgame and a bit more complex to understand. Whereas Analogue SSTV sends information a line at a time (like Facsimile) and color information is sent sequentially, the Digital system sends blocks or segments of information.

There are five programs available for Free on the Internet, namely:

1. DIGTRX/ HamDRM by PY4ZBZ Roland M. Zurmely

2. WinDRM by HB9TLK Francesca Lanza   (digital voice. pictures & text files)

      Note: The difference between WinDRM and HamDRM  -- as best I can determine -- is that WinDRM includes Voice, Pictures and any digitized file. HamDRM  is limited to pictures  & text files (i.e., NO VOICE).

3. REDUNDANT FILE TRANSFER (RDFT) by KB8VAK Barry Sanderson
    (the first to appear)

4. DigiACE by Martin Emmerson

5. SSTVPAL MULTIMODE & DIG-PAL by  VK4AES ERIK

We have been monitoring the progress of Digital as a participant and more as an observer for about 15 months. Here's what we think about it at this point. The following comments relate only to the specific programs listed and does not consider  other programs  that  have become companion routines that offer  certain special enhancements.  Those programs will be discussed in the Final Review in October 2005.

First, we should note that Digital Slow Scan Video is nothing more than an adaptation of what the Personal Computer does routinely when sending your E-mail or when you are downloading a program.  By using "error correction" technology, if you can receive every bit that has been transmitted, you will get a picture! If some "segments" or "blocks" are not received, those missing parts have to be retransmitted. You, the operator, will seldom see a picture if it is not perfectly transmitted and received. There lies the trouble with Digital at this point, in our view!

The early Digital programs required that the operator codify the picture file, then transmit it. After it was received, the receiving operator had to Decode the file. These two steps consumed some time; not a lot but it became a holdup once you got familiar with the program. About six months ago, WinDRM (adapted by Francesca Lanza HB9TLK from the DREAM system developed by Volker Fischer and Alexander Kurpiers) appeared.  DREAM is the basis for a new digital broadcasting system being used experimentally in a number of countries.

WinDRM is a barebones program that has no GUI (Graphic User's Interface); it eliminates the waiting time for Coding and Decoding.  In recent weeks, Roland PY4ZBZ, whose DIGTRX has a very nice GUI, has incorporated the WinDRM system without the voice capability into his program. It is called HamDRM. They operate separately, of course. This means you can run the RDFT system or the newer and faster HamDRM merely by selecting one or the other. Obviously, the faster processing of the pictures has become very popular.

The setup for digital is slightly more mysterious than Analogue SSTV.  The adjustments for the drive in the Sound Card menu (for Playback or Transmit) must be set low enough to avoid excessive sidebands just as is experienced when running PSK31. Although you have a 100 watt transmitter, the right balance of the amplitude and phase shift modulators forces you to run barely more than 30-40 watts. The system's "throughput" of 2400 bps is quite impressive. A linear amplifier is needed to multiply the signal power once the transceiver is properly tweaked. But don't expect to see the linear output -- based upon meter readings and output readings -- look very robust.

OPERATIONAL PROBLEMS

The main irritation with Digital, as I see it, is that many transmissions are not received by some operators. They have missed a number of segments or blocks and they have to be "fixed" by the Sender or a station that received the whole picture. Hams with marginal receiving conditions (e.g., local noise, poor antenna, faulty equipment and improperly adjusted Sound Card parameters) seem to have the most trouble. Sometimes, after the first "fix", several more segments are missing so they have to have another "fix".  Missed segments seems to be due to the program being very sensitive to propagation problems and interference. Not a surprise, actually, but the idea of a system that will not display the picture until it is "perfect" is expecting too much when the mode of transfer is by radio waves.

If you were using regular Analogue SSTV, you would receive a poor picture but at least you had something to show for your time and effort. With Digital -- due to its error correction feature -- if you don't get a 100% perfect picture, you get absolutely nothing but a "Sorry, Charley! In a Net operation, most of the time is spent requesting  missing Segments (BSR= Bad Segments Request) or Blocks (BBR=Bad Blocks Request) and then resending them. The sending of the missing segments is pretty neat and PY4ZBZ's GUI makes it very easy! But the number of pictures per hour transferred appears to be less than half that of the Analogue SSTV!

We have monitored the fellows operating on 7.173 KHz and I have come to the following conclusion: 1. Use of 100 watt transceivers that have to be set at less than     30-40 watts for Quadrature Amplitude Modulation just do not do     the job. YOU NEED A LINEAR AMPLIFIER CAPABLE OF ABOUT     200-400 watts! Those with marginal power do not override the     noise, fading and other propagation effects like multipath. 2. If conditions are Poor, if you have less then a solid S7 to S8     signal. don't bother sending pics. You can expect many     requests for "fixes". 3. While the Purists are ecstatic about Digital, I would suggest the     authors permit the operator to remove the "error correction"     requirement and let the picture be displayed as in Analogue     SSTV. The slightest imperfection (missing blocks or segments)     prevents the Operator seeing anything. A less rigid protocol     would allow the operators to see what got through to them.     This may eliminate the constant sending of BSRs or BBRs.     The use of IRFANVIEW and Progessive Receive will let you     see the actual picture with its missing segments. (Go to     KB4YZ's Website for details on this add-on feature.)

COMMENT:

The above raises a question I have been discussing for many years regarding how perfect a Xerox copy has to be or how perfect a picture has to be to convey to the reader/observer what he needs to know.  The issue is a subjective matter and the Purists who want everything to look beautiful and absolutely perfect may seem to be a pain in the neck but this is how a new technology is refined.

Most experienced Hams realize that when radio propagation conditions are poor, the use of Digital can be very problematical. The option is obvious: go back to SSTV Analogue! At least you will see something! Some new formats in MMSTV are especially designed for bad conditions and they seem to produce good pictures. The PAL MULTIMODE program by Erik VK4AES has both Analogue and Digital in one GUI, hence, it lends itself well to a quick change to regular SSTV.

For the above reasons, I urge the authors change the program to let the Ham operator see how bad the picture is before wasting valuable time trying to retransmit missing blocks endlessly. I predict that many pictures that have some missing blocks will be perfectly acceptable as a practical matter. As a technical matter, where there is no human judgment, Hams will be piddling away valuable time and burning up their equipment needlessly.

BOTTOM LINE: add an option that permits the operator to see what he received. Then he can decide whether he really needs "fixing"! The Purists will always want every bit in-place because they want to save only perfect pictures. Fortunately, most Hams tend to be more practical! Only DIGTRX/HamDRM by Roland (PY4ZBZ) incorporates the Progressive Receive feature. Other authors should take note!

A final thought, if I may: we note a number of new operators show up on the Digital channels and they require a lot of time for tutoring.  I respectfully suggest that they download MMSSTV, an analogue SSTV program by Mako Mori. This will help them learn a multitude of things such as setting the SOUND CARD - a major bugaboo with PC-controlled SSTV. Once you learn how to do regular SSTV, you will be prepared to move up to the Digital system. You have to creep before you walk!

While we have recommended this reasonable approach with Newcomers for many years, operators persist in staying on the Net frequency where many other operators are waiting their turn to send a picture. Having an experienced operator take the newcomer to another channel for some tutoring is the courteous thing to do for both the new operator and the others already familiar with Digital SSTV.

As usual comments and clarifications are always welcome in this Old Timer's Shack!

73,

Cholly- W3OHV

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