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.

14/06/2021, John's reply...my scan converter generates a 262 line non-interlaced at 60 Hz vertical, 15.734 kHz horizontal that's close enough to the NTSC standard to display properly on every U.S. TV set and monitor I've tested.

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

Steve A.

As with most diagrams posted left-click the in-line image for a better resolution version, browser 'back' button to return...at least in Chrome it works...

SSTV v5 Demodulator 1.gif


TriplePIC_article.pdf
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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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Re: SSTV Demodulation.

Postby Steve Anderson » Tue Jun 15, 2021 6:43 pm

I'm thinking of adding a spectrum display to this where you'll be able to see exactly where the sync frequencies fall and also the video. akin to some of the software demodulators 'out there'. But it needs some thought. For example, just considering full cycles, at 2300Hz will generate 2300 samples in one second, whereas 1500Hz will just generate 1500. Double if using half-samples.

So a straight-forward graphical representation would not be linear. A 'fudge-factor' would need to be applied...more in due course...the X-axis needs to be frequency, not time/duration. One is the inverse of the other, so not impossible..

Not having a reliable source of demodulated SSB I need some way of simulating that...complete with off-tuning...but I have an idea already...simply make an off-tune SSTV modulator...I'll have to look into the version I use to see if it's possible...it's software defined, so it should be possible...

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Re: SSTV Demodulation.

Postby Klaas Robers » Tue Jun 15, 2021 8:15 pm

Steve, it is remarkable that John Magliacane's, KD2BD, did his FM-detection and low pass fitering of the video signas, as well as the sync detection completely in the analogue domain. When I read his description he overthought this carefully.

I also observed that his filtering in the sync channel is much more advanced than I did in my L-C pot-core design in 1973. That is something that is better than mine, and now and then I missed that extra performance.

As you absolutely have read, in the past I did an effort in digital filtering and digital FM-detection, but got lost in the problems of a-synchronity. May be that John's choice of doing the analog world in the analogue domain, is not so bad.
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Re: SSTV Demodulation.

Postby Steve Anderson » Wed Jun 16, 2021 12:41 pm

Well I guess you could say that a zero-crossing arrangement is digital, its output is a string of one's and zero's. But as we've seen it's not so good for sync detection (due to sync-jitter). So as I have mentioned above the video demodulation will be via zero-crossing, whereas sync detection will be good old-fashioned linear...as per above. I'm hoping this works better than both previous attempts - not that either of them were 'bad', just some improvement needed - time will tell...

The sync 'side-chain' in the circuits above are basically a copy of John's adapted to this system. The filters are a copy except for some minor changes. All kudos to John...

It's most unlikely that the video and sync will be exactly co-timed so one of them will need to be delayed, I suspect the video...however, you never know...

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Re: SSTV Demodulation.

Postby Klaas Robers » Thu Jun 17, 2021 8:31 pm

Steve, I expect that the sync detection uses more time, as there is the double swinging choke and the double low pass, both with a rather narrow bandwidth. However, I don't know at which point in time the sync is detected. If you use the down going edge, you are faster.

I see that you changed your avatar. Who is this girl / woman? Mine is from an EBU test slide that was used in the beginning years of (PAL) colour television. At that time, 1967, before the start of the real program, there was first the SMPTE black and white test chart and then, during 5 minutes before program start, a series of these test slides in colour were transmitted. This was done to allow CTV technicians to tune the RGB colours to an accepable colour reproduction. This "Lady With Roses" was for me the most characteristic one.

I have had the complete original set of EBU test slides, but I donated them to the audio-video museum in NL. That is better than here somewhere in a cupboard, where nobody besides me knows the historical value of them.
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Re: SSTV Demodulation.

Postby kd2bd » Fri Jun 18, 2021 4:07 am

That is correct, Klass. Sync detection will be delayed about 5ms through the bandpass filter alone. The output of the bandpass filter will reach its maximum amplitude just as the 5 ms sync pulse ends. The ideal voltage threshold for the comparator following the LPF is half the peak voltage of the pulse coming from the LPF. Of course, feel free to experiment. :)

In my TriplePIC unit, I pass the digitized video through a queue (serving as a delay line) to align it with the delayed horizontal sync. That complexity alone is probably why we never saw sync detector bandwidth exactly matched to the bandwidth of the 5ms horizontal sync pulses in the past, especially in purely analog circuit designs.

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Re: SSTV Demodulation.

Postby Steve Anderson » Fri Jun 18, 2021 3:15 pm

Simulating the second circuit of the 13th June above yields these results...
Fig 1-1a-graph.gif
Fig 1-1a-graph.gif (12.75 KiB) Viewed 639 times

The blue trace is the input, six cycles of 1200Hz, the green trace is pin 1, IC122a, and the red trace is pin 8 IC122c. The circuit works as advertised...

I've yet to figure out how to switch the simulator between 5ms of 1200Hz and the rest of the line at 1500Hz (worst case), but will check up to 2300Hz - but I don't see any problems arising. Even if the simulator refuses to cooperate, breadboarding will have the final say, but again, I don't see any issues arising.

I'm also thinking of adding a feature that MMSSTV has, they call it a spectrum analyser, to me it's more of a 'Tuning Aid'. I don't plan at this stage to place it within the screen display but use an external 'scope or a dedicated CRT monitor. Later I may have a go at putting on the main screen. It'll be not as 'pretty', but functional...

mmsstv-spectrum_analyzer.gif
mmsstv-spectrum_analyzer.gif (3.95 KiB) Viewed 631 times


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Re: SSTV Demodulation.

Postby Harry Dalek » Fri Jun 18, 2021 6:12 pm

Good work Steve ,and i can see you have had a make over :wink:
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: SSTV Demodulation.

Postby Steve Anderson » Fri Jun 18, 2021 6:26 pm

Here's an early version of the 'Tuning Aid', 73 Mag. Dec. 1972...nearly 50 years ago!!

Steve A.

73 SSTV Dec_1972.pdf
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Harry Dalek wrote:Good work Steve ,and i can see you have had a make over :wink:

Bangkok is known the world over for its skills in cosmetic and gender reassignment surgery...however, the hair in the photo is really mine, not retouched..
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Re: SSTV Demodulation.

Postby Steve Anderson » Fri Jun 18, 2021 7:02 pm

Another 1970s 'Tuning Aid' or 'Tuning Indicator' article...

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73 SSTV Jan_1972.pdf
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Re: SSTV Demodulation.

Postby Steve Anderson » Tue Jun 22, 2021 6:02 pm

Still conceptualising this, only around 5% of the time is for syncs, whereas it's possible that the rest may be all white, or all black. Perhaps that's why there's an apparent limiting function in the MMSSTV software. It seems that if the actual video portion is over 5% (the sync level) it gets limited. Which as a 'tuning aid' it does exactly what its intended to do. But it's not a spectrum analyzer in the truest of senses.

I'll try and do a true frequency analysis on some SSTV signals later...but then again, why? All we need to know (if using SSB) is the proper sync frequency of 1200Hz.

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Re: SSTV Demodulation.

Postby kd2bd » Wed Jun 23, 2021 5:26 am

Here are some ideas...

You want the demodulator in your scan converter to be optimized for the proper tuning and proper bandwidth of an SSTV signal. However, these criteria limit its ability to function as a useful indicator of off-tuned signals.

We know that the only reference we have in the video signal is sync at 1200 Hz. If we can tune sync to 1200 Hz, then everything else (that we have control over at the receiver) will fall into place.

So, how about using a separate, wide bandwidth demodulator for your tuning scope? It doesn't have to be sophisticated. You could use a single chip PLL (CD4046, XR2211) as an FM detector to do this. Follow it with just enough LPF to reduce subcarrier ripple.

Next, DC offset the LPF output with an OP-AMP so that with the PLL VCO centered at ((1200+2300)/2) Hz, a 1200 Hz input provides a 0v DC output.

Apply this to a DC coupled scope. SSB tuning is correct when the sync pulses fall directly over the X-axis of the scope. :D


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