Sunday, March 11, 2007

Digital SSTV Saga Ends

After another day of watching the grass grow, I have decided to go back to utility and leave the digital SSTV experimenters to it. I received a couple more nice pictures, with missing segments, from an amateur in Mexico, who was sending dramatic pictures of women in body paint obviously applied as some sort of art show. These were great stuff, and make me want to look around Google Images for more examples of same.

To see real content is, of course, is rather refreshing, considering the one-note samba that is American analog SSTV. You have the guy that wants us to get Jesus, the one who never met an airplane photo he didn't like, and the one with "YL" in his call who sends pictures of, uh, YLs in rather scanty clothing.

Come to think of it, that third guy can keep on with what he's doing. That scanty clothing thing is fine with me.

Must... think... about... radio... .

Digital SSTV, of course, is not a very good name for it. Digital file transfer would be better, since that's what's going on, and the data could really be just about anything. That's the advantage and disadvantage.

The advantage is that, if everything is received, you have the same file that left the transmitting station, not a grungy decode of a scan of its contents. The disadvantage is that Digital Radio Mondiale in its present state is an audio broadcast mode, and better ways exist to transfer files. There is no repeat procedure for missing information transfer units (packets, segments, whatever), because in audio broadcasting there does not need to be. Segments failing the CRC check are simply discarded, and you don't hear that part.

Of course, the receiving station of a file transfer can always ask for, and get, the missed segments. They've sort of automated this process, with bad segments being logged and requested manually by clicking a button for a "BSR" (Bad Segment Resend?) transmission. This is fine, but obviously the retransmission will probably not be the same segments that people listening on-channel missed. For that, you need redundancy, like in FEC mode. In digital SSTV, real redundancy consists of transmitting the whole file more than once, but that means very long transfer times.

There is also the problem that DRM might not be the best mode for amateur equipment, which at least in the US is not allowed to exceed 1500 watts peak envelope output power. DRM's peak to average ratio is awesome, something on the order of 10 dB. In amateur service, this means that your typical 1500 watt PEP linear cranked to the max will be reading about 120-130 average watts at "full legal output."

Somehow I doubt that we will be seeing whole screens of Internet SSTV widgets showing reception worldwide, as we've had in the analog mode for some time now. (A great one is Worldwide SSTV Cams.)

Now, back to utilities...