N-SSTV

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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N-SSTV

Postby Steve Anderson » Sun Jun 06, 2021 5:10 pm

...another version of SSTV I hadn't heard of until a day or two ago...N-SSTV (Narrow SSTV)...

It appears to be identical in all respects to conventional SSTV, whatever mode is in use, except that the usual 1200-2300Hz sub-carrier is 'squashed' down into 1900-2300Hz. That's an advantage on the noisy and crowded HF Amatuer bands, less so with what I'm generally interested in. But it's an easy modification/addition to existing systems.

Example off-air screen-shots seem much the same as the usual standard, no better nor no worse, maybe more so in marginal conditions. Start here if curious or interested...

https://narrowsstv.wordpress.com/

Steve A.

It seems that these N-SSTV modes have been around quite some time - years! - not come across them until two days ago!
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Re: N-SSTV

Postby Steve Anderson » Mon Jun 07, 2021 12:50 pm

As a quick exercise I revised my SSTV DDS modulator to generate both standard SSTV and N-SSTV. The only hardware changes required are the addition of one switch (or jumper) and one capacitor. The micro will need a software update though...

I very much doubt I'll be using N-SSTV, it was an interesting find, but the above shows how easy it could be...however, I doubt the Robot 70 (or similar) demodulators could be adapted without becoming overly complex...software (zero-crossing) would be one alternative...

I'm still having issues with generating decent drawings to post from Autocad...second attempt a bit better...

Steve A.
N-SSTV DDS Modulator 2.png
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Re: N-SSTV

Postby Klaas Robers » Tue Jun 08, 2021 10:43 pm

This implies that the frequency deviation is less, so you need more amplification following the FM-detector. This also implies that the signal to noise ratio of the video signal is worse.

An advantage could be that the low frequency response of the audio chain of the receiver is of less importance. Everything below 1000Hz is no more needed.......

And no, it is not true that you need just a bandwidth of 400 Hz (2300 - 1900). You still need the two side bands of both 900 Hz, in theory: 1000 Hz - 3200 Hz for a video bandwidth of 900 Hz.
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Re: N-SSTV

Postby Steve Anderson » Wed Jun 09, 2021 2:35 pm

Well, as I said, I doubt I'll be using N-SSTV, it was an interesting distraction for a while...and maybe of interest to others... but I'll be sticking with the McDonald standard.

Among the things on-hold here is a zero-crossing/software SSTV demodultor, an improvement on the one I tried for use with the first SSTV-625 up-converter - in addition to the updated Robot 70 version. It was an improvement in some areas, but maybe not others.

Steve A.
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Re: N-SSTV

Postby Klaas Robers » Wed Jun 09, 2021 6:39 pm

I did something in that direction. But be aware that this may interfere with the sampling frequency. I think that the best way is to use an internal timer and read/reset that when a zero crossing of the input signal occurs. Then you get the half period time. Using a look-up table you can convert this to the frequency = grey value. The table also serves as a scaling and black/white limiting device.

And then that value should be the input for the FIR-filter. The clock frequency for the FIR-filter should be high enough to prevent interference between that clock frequency and the read-out of the grey value.

Good luck!
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Re: N-SSTV

Postby Steve Anderson » Thu Jun 10, 2021 12:29 pm

Looking back over my notes for the zero-crossing SSTV demodulator I recall there was a minor (but annoying) problem with 'sync-jitter', which was not evident on analogue demodulators. If the sub-carrier were locked to the line frequency during the sync period then it should disappear. But that's a tall order for what should be a 'simple' system. If there were a front porch at black for a while that should help, but just like NBTV such things eat into valuable line-time.

Sync filtering at the output would seem to be one answer - but I don't recall trying it...more thought required...

The actual video portion of the recovered signal seemed quite good though...so there is promise in this approach...

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Re: N-SSTV

Postby Steve Anderson » Thu Jun 10, 2021 1:03 pm

Digging through screen-shots I found these two examples using two different demodulators. The first is the updated Robot 70 version, the second my zero-crossing try. You can see the evidence of the sync-jitter in the vertical of the lamp-post, also in the verticals of the ground-floor windows. Aside from that I think the zero-crossing version is the better of the two...also note much less overshoot on the extreme left caused by the sub-carrier filter...

Steve A.

Robot 70 The Albion 1.jpg
Robot 70 The Albion 1.jpg (344.19 KiB) Viewed 2065 times


MkII Demod The Albion 1jpg.jpg
MkII Demod The Albion 1jpg.jpg (331.29 KiB) Viewed 2065 times
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Re: N-SSTV

Postby Steve Anderson » Thu Jun 10, 2021 1:30 pm

One method to deal with the 'sync-jitter' is to process the sync extraction separately from the video...food for thought...

I'll continue this in a more appropriate thread...

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Re: N-SSTV

Postby Klaas Robers » Thu Jun 10, 2021 5:37 pm

I remember that in the very beginning of SSTV there have been systems that restarted the subcarrier phase on each line. That prevents sync jitter in case you have residual subcarrier in your video signal, but also causes a spike in the demodulated video at the start of a sync pulse. I never have done that. Rethinking the old system, it inserted a 1200 Hz burst in the FM signal, so it gave spikes at the beginning and at the end of the sync pulse.......

But I have also some sync-jitter, because there are no black porches before and after the sync. This is very good visible in the test chart, which has white / black rectangles at the end / beginning of each line. I planned a sync flywheel in my P7 monitor in Heathkit SB cabinett, but still have to program the PIC for that.
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Re: N-SSTV

Postby Steve Anderson » Thu Jun 10, 2021 6:48 pm

Klaas Robers wrote:....but also causes a spike in the demodulated video at the start of a sync pulse.

Yep, that's been my concern - and I can't see any sane way to get around it. So perhaps a separate side-chain for sync extraction is the way to go?

I'll start a new thread called "SSTV Demodulation" as this now has no direct bearing on N-SSTV.

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