SSTV Demodulation.

Forum for discussion of SSTV topics. Slow Scan television (SSTV) is a picture transmission method used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or colour.

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SSTV Demodulation.

Postby Steve Anderson » Thu Jun 10, 2021 6:52 pm

...continuing on from where the "N-SSTV" thread left off...

I'll give the 'side-chain' sync-extraction idea a try on breadboard before wielding the soldering iron. I've simulated it, but simulation can only go so far...

There's a chance there might be a timing difference between the video demodulation and the sync-extraction circuits, if noticeable I'll deal with it when I get there...a digital delay-line would allow re-timing of the signals to one another...or even something as simple as a monostable if it's the syncs that need delaying.

I've heard of 'flywheel sync' many times over the years, but searching on the 'net comes up with very little...it seems to have been used more in valve/tube circuits rather than semiconductors.

Steve A.

I've given 'SSTV demodulation' this thread of its own as it's perhaps the most difficult aspect of SSTV to implement and yield good results...
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Re: SSTV Demodulation.

Postby Steve Anderson » Sun Jun 13, 2021 1:03 pm

I've put together the 'side-chain' SSTV sync detection/extraction circuit. It uses the filters, (IC122a/b and IC121b) with permission, from John Magliacane's, KD2BD, Triple-PIC SSTV up-convertor. Website here with all the information, circuits and software for construction...

https://www.qsl.net/kd2bd/TriplePIC.html

I've also attached the complete article below...

I've made some minor changes to John's original, mainly to ease sourcing components, e.g. changing 68n caps to 47n, with appropriate changes to associated resistor values. Also to suit the +/-12v supplies I'm using.

I've also asked John if the Triple-PIC is 525/60 output only, though most modern TVs/monitors will sync to both 525/60 and 625/50. Update upon John's reply.

The zero-crossing demodulation will follow in due course...so far this has only been simulated, not built/tested.

Steve A.

SSTV v5 Demodulator 1.gif


TriplePIC_article.pdf
(3.3 MiB) Not downloaded yet
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Re: SSTV Demodulation.

Postby Steve Anderson » Sun Jun 13, 2021 2:44 pm

A few minor changes to the above circuit...

1) Change from 3xTL082 to 1xTL082 + 1xTL084.
2) Rearrange such that large amplitude/switching signals are only within IC121.

Steve A.

SSTV v5 Demodulator 2.gif
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Re: SSTV Demodulation.

Postby Steve Anderson » Sun Jun 13, 2021 4:06 pm

A slightly changed version of a demodulator from a couple of years ago...also untested, and I still have to write the code...

There are seven spare pins on the micro that can/will be used with an up-convertor, here it's just a baseband demodulator.

Note: There's no sinc filter on the D-A output, one could be added but I doubt it would make much difference in this application. If oversampling is used it might remove the requirement anyway. Also the output voltage will be less than the 4.096v shown.

Steve A.

Thankfully I created the look-up table required a couple of months ago, 6000+ bytes, 3000+ 16-bit values...there's still 10,000 bytes left for the program proper, should be more than enough! Only 12 bits of the 16 bit look-up data are used as it's a 12-bit D-A.

A few minor changes to the circuit below, not worth re-posting it at this stage...don't build this circuit...will update in due course...

SSTV v5 Demodulator 3.gif
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