"Digital" Mode of the Week: Slow-Scan TV

Like HF FAX, the rather misnamed "Slow Scan Television" mode is actually analog. However, it appears in most multimode packages intended for digital reception, and it is also one of the more fun things you can do with a radio and a computer.

I can't imagine anyone actually using SSTV to send continuous video frames, though the simpler black and white versions would at least be able to do a couple of these per minute rather than a couple of minutes per frame. The major use for SSTV is for amateurs to swap still photos. In fact, I can't think of any commercial applications for the SSTV mode on the radio. The closest would be the pictures sometimes sent down by ham radios on the International Space Station.

Old B/W SSTV was a kind of scaled-down (320x240), sped-up version of FAX. It, too, used frequency or audio-frequency modulation of a single carrier or tone over 800 Hz, with black at 1500 and white at 2300. For digital, the grey scale is quantized to 256 levels (8 bits), which are plenty.

One of the first innovations was to add color. This was first done by sending the three color channels sequentially in full frame. This didn't show the true colors until all three frames were received, a rather slow process. A later innovation was line-sequential mode. It typically sends each line three times, with one 1200-Hz, 5-ms, sync pulse at the start of the red line.

One of the early color modes was Robot, as originally done in dedicated hardware boxes with a huge "ROBOT" logo on the front. It really looked like something from science fiction, and cost like it too.

Out of the 30-some common SSTV modes still in existence, nearly all transmissions are in Martin 1 (114 sec for a 320x256 frame), Martin 2 (58 sec), Scottie 1 (110 sec), Scottie 2 (71 sec), and Scottie DX (268 sec). Martin 1 and 2 were developed by amateur Martin Emmerson, G3OQD, and they are common in Europe. Scottie was developed by another British ham, Eddie ("Scottie") Murphy, GM3SBC. Its modes are dominant in the United States and Japan.

SSTV can be tuned the same way as FAX, by centering the audio between the high and low lines on the computer display. It's trickier, but you can also center the sync pulse as close to 1200 as you can get it. Most programs have AFC if you're off a few hertz. The little picture will start to scan down its part of the screen. One of the nice things about most analog transfer modes is you can start in the middle, and that's possible here too.

SSTV software can auto-start, and also it can auto-mode, either by measuring the interval between sync pulses or reading a start burst called a VIS signal. The VIS consists of a 1200-Hz marker followed by a mode designator in FSK between 1100 and 1300 Hz. A similar optional code is sent at the end of a picture.

SSTV, like FAX, will be slanted proportional to the difference between your clock frequency and that of the sending station. Various programs cope with slant correction in various ways, usually by buffering the picture and allowing its adjustment in real time during reception. This can be manual or automatic. One program, MMSSTV, allows you to click on a happy face when you get a straight picture from a "trusted" station, locking in this correction.

By far the most active SSTV frequency is 14230 kHz USB, which really lights up on weekends, going also to 14228 when there's QRM. When 20 dies, there's lesser activity on 7171 kHz LSB, +/- a kHz or so. There's also spotty activity on 6 and 2 meter VHF. Oh, and some parts of the world have pirate SSTV networks outside the amateur bands, which exchange pictures that are far more X-rated than anything amateurs can get away with.

You might have heard of the even more misnamed "digital SSTV." It uses file transfer software to exchange data, which in this case just happens to be pictures. It's typically on 14233 kHz USB, using one particular, rather fussy and esoteric program. It looks great, but it requires a very high signal to noise and almost no fading.

One of the programs designed for this purpose is the same DIGTRX that the Cuban SK01 numbers station uses to send its weird little files. Woe betide any ham who transmits with this one on 14233, though, since it is not the OFFICIAL program mentioned above. You'll think you had a visit from the Wouff Hong. Don't hate ham radio, just come back to good old grungy noisy easy and fun analog.

Cuban DIGTRX Numbers 9/13/07

The Cuban numbers stations, always good for a surprise, are at it again.

On September 10, Tom Sevart heard a new digital mode (for them) on 17478 kHz USB at 1600 UTC, and again on 17436 kHz USB at 1700. This is an 8-tone modulation mode with distinctive beginning and ending signals. It was quickly identified by Tom and others as Redundant Digital File Transfer (RDFT), a mode with several submodes of varying transfer speeds. It's used by hams for transferring files, usually slow scan TV (SSTV) pictures. We've written about this before.

Today, September 13, Tom and I both heard transmissions at 1700 on 17436. They were in RDFT and another digital file transfer mode called HamDRM, an amateur version of the Digital Radio Mondial broadcast standard. The program in use identified itself as DIGTRX version 3.11, a free download available on all the usual ham radio sites.

The sending station ID in the DRM transmission was RAD01. The file being transferred was called nen12.txt. Its contents are a standard V02a/M08a three-message encrypted whatever, but without the formatting into 5-figure groups. We thank Tom for sharing his decode with the Internet.

It is interesting to speculate on whether RAD01 and nenxx.txt are internal names used in the Cuban numbers operation, or just special test names. Probably ultimately futile, but interesting.