Retro SSTV anyone?

Where it all started as far as most are concerned and saw heavy use from the 60s through to the 80s. Colour and Hi-res modes have unfortunately pushed this system into the backwaters of SSTV. Time to resurrect interest in this simple analogue system.

Re: Retro SSTV anyone?

Postby Steve Anderson » Fri Sep 18, 2015 6:23 pm

Klaas, yes I did notice your 'Testbeeld' file was a bit low in frequency but I was too polite to mention it. But now you have I attach the spectra...

The reason I even looked at the spectra was that on my demodulator the syncs seemed a little 'crushed' and I thought this was unlike Klaas.

The crushing is due to the sync frequencies are now in the non-linear region of the demodulator as can be seen by the V/F chart below. The voltages would now be different (see previous post) but the shape of the curve would still be the same.

I guess this should be quite a simple fix.

I have changed the values of the filter in the modulator and although there is an improvement in the ringing I now am suspicious of the filter in the demodulator (above). Changing it to a Bessel filter would probably let too much of the rectification products through, I need to muse on this...while I do I'll get on with the contouring problem.

Steve A.
Attachments
Testbeeld Spectra 1.jpg
Testbeeld Spectra 1.jpg (69.07 KiB) Viewed 5434 times
Robot 70 Linearity 2.gif
Robot 70 Linearity 2.gif (4.88 KiB) Viewed 5434 times
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Re: Retro SSTV anyone?

Postby Klaas Robers » Fri Sep 18, 2015 7:12 pm

Thank you Steve, that is something I wanted to ask you and now you did it unasked ! I got already the impression that my frequencies were not correct. Yesterday I got the sync detector working again, after > 40 years. It works with an LC tuned circuit on 1200 Hz, which is driven with the limited input signal and which amplitude is detected. Extra capacitors are added via a rotary switch, to tune the resonance to 1000, 1100, 1200, 1300 or 1400 Hz. Then, when using the now built in SSTV generator, it struck me that I had to tune the sync resonator to 1100 in stead of 1200 Hz.

I had seen as well that the output voltages of the video signals were too low, when compared to the tone generator. This is not too strange as the frequencies are dependent on the +6V supply voltage. I don't know any more which supply I used in 1973, but now I made a supply using a 7806, much better than in those days.

It is very simple to adjust the frequencies. The FM generator is a unijuction oscillator, with a capacitor that is charged via two adjustable current sources:
- one source gives a constant current and defines the frequency of the sync (1200 Hz),
- the other source is switched off during sync and is modulated with the video signal.
- This one has an offset for the 1500 Hz black level.
I will add the circuit diagram. It might be too wide for the web page, then please load it down for viewing, and a photo of the circuit board. The generator has 5 of these boards, standing upright, in a small rack.

So I can adjust the sync frequency, the black frequency and the white frequency. These potentiometers are small, era 1970 pertinax trimming potentiometers, so they may also need some adjustment. I will do that today and come back with an SSTV audio file. Then please measure the frequencies again for me.
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Schema AFM-oscillator.GIF
(10.92 KiB) Not downloaded yet
Foto AFM-oscillator.JPG
Foto AFM-oscillator.JPG (81.56 KiB) Viewed 5430 times
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Re: Retro SSTV anyone?

Postby Steve Anderson » Fri Sep 18, 2015 7:57 pm

Klaas Robers wrote:...I will do that today and come back with an SSTV audio file. Then please measure the frequencies again for me.

Sure, no problem at all. I'll also do a screen shot, with any luck the sub-carrier frequencies being a little higher the 1200Hz grating may be of a greater amplitude - but maybe not as much as we'd like.

Steve A.
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Re: Retro SSTV anyone?

Postby Klaas Robers » Fri Sep 18, 2015 10:19 pm

I readjusted the FM oscillator and that was needed.
- The sync was on 1074 Hz, that was adjusted first to 1200 Hz.
- Then still black was too low and white was too low as well.
It was quite some change. Both should be Ok now. Steve, surprise me.

Therefor I post two SSTV files in 8 bit 8 kHz:
- The well known Test picture
- a checker board. I will add a picture of it.
This is a headace file, as the line sync is preceded by a black line and followed by a white line, or reversed. So the sync will be moving, even the "center of gravity", and it is almost impossible to get nicely positioned rows of blocks. However it is a nice file to check the frequencies, as there is only sync 1200, black 1500 and white 2300.
Attachments
Testbeeld2.wav
SSTV black and white Test frame in 8-bit and 8 kHz
(234.5 KiB) Downloaded 298 times
Dambord.wav
SSTV black and white Checker board in 8 bit and 8 kHz
(234.5 KiB) Downloaded 291 times
Dambord.GIF
Checker board as it should look like. The grey border is NOT part of the picture!
Dambord.GIF (1.96 KiB) Viewed 5422 times
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Re: Retro SSTV anyone?

Postby Steve Anderson » Sat Sep 19, 2015 1:17 am

I got those files, thanks.

A quick check with the scope verifies they are spot-on, see below - no time to re-name the files - tomorrow.

But on the screen there's something very wrong, and MMSSTV can't seem to understand the signal. It's the same with my files too. I suspect an earth/0v has dropped off somewhere or something else is wrong with the untidy lash-up here.

Ignore the cyan trace, I just forgot to turn it off.

I'll have a go at sorting things out in the morning, it's just after 9pm here...

Steve A.
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TEK0082.gif
TEK0082.gif (43.06 KiB) Viewed 5414 times
TEK0083.gif
TEK0083.gif (43.92 KiB) Viewed 5414 times
TEK0084.gif
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Re: Retro SSTV anyone?

Postby Klaas Robers » Sat Sep 19, 2015 1:46 am

Thanks Steve sofar.
I see that also on your oscilloscope the white for Testbeeld2 is slighly lower than for Dambord. I noticed that already here. Testbeeld comes from a different source than Dambord. The latter comes from my pure hardware generator, lots of DTL flipflops, no more than two in each dual in line chip. Testbeeld comes from the newly built 1 MB EPROM source. This one has a small cermet potentiometer to adjust the amplitude of white. I expect that MMSSTV will show that Testbeeld white is not 2300 Hz, but slightly lower. I have readjusted that small potentiometer already and now both are the same.

I see that you have lots of noise on the video. I don't see that on my analogue oscilloscope, but I haven't looked at my recording, only at the output signal of the generator......

Something to do.

1th edit: In the mean time I did some experiments.
- Indeed, when playing back my own files via the SSTV demodulation circuits, the demodulated video looks noisy as well.
- My first thought was: the lousy 8 kHz and the lousy 8 bits.... but
- when recording at 16 bits and at 44.1 kHz and playing back, the demodulated video looks exactly as noisy.
- I also looked at the played back FM subcarrier and it was also not as clean as the FM subcarrier directly from the generator.

I have no idea what happened here. The laptop was running free of the mains, on its built in batteries. Do you know how the recording level of a laptop is adjusted? I don't have a line input, so I have to use the microphone input. I have to experiment with that. May be THAT is too noisy.

2nd edit: It is not the 8-bits and 8 kHz. It was the attenuator to the microphone input. It was far too high ohmic towards the PC-input. Now I used 500K to 50 ohm and the 50 ohms connected to the microphone input suppresses the excessive noise. Here is a new 6 x 6 checkerboard with much less noise.

And there was no automatic volume control. I had to select the resistors by hand until the amplitude was good enough.
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Dambord2.wav
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Re: Retro SSTV anyone?

Postby Harry Dalek » Sat Sep 19, 2015 9:25 am

Hi Klass it is possible to use your laplops Mic input ....i have had lots of laptops depends what you have what you have to do .
https://www.youtube.com/watch?v=c7i6LJs3b6E

This above works if you are lucky and your laptop can record stereo...
\

IF only mono a litte circuit

http://www.epanorama.net/circuits/line_to_mic.html
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: Retro SSTV anyone?

Postby Steve Anderson » Sat Sep 19, 2015 2:02 pm

I love to hate those 3.5mm connectors used for audio I/O on PCs and other devices, about as reliable as the British summer.

The gain of the mic and line inputs (if any) is controlled within the codec so the gain is going to be controlled in software usually via an on-screen knob or slider. Even in the humble and now very dated AC97 codec there's a whole load of useful I/O that we never have access to unless you want to 'knife and fork' your mainboard.

Klaas, Dambord2 is much better but the spectra has a load of junk above 2300Hz - what that might be I have no idea. The waveform looks clean and the screen-shot is OK (ignore the sync-jitter and the junk top and bottom, my problem). I'm always suspicious of free third-pary software until I'm convinced its reliable, I would tend to ignore this.

It might be worthwhile re-doing Testbeeld again, the second version as you mentioned was very noisy in the mid-grey areas.

My problem late last night was as suspected, some earths/0v had drifted off into outer space.

Steve A.
Attachments
Dambord2 Screen Shot 1.jpg
Dambord2 Screen Shot 1.jpg (135.07 KiB) Viewed 5401 times
Dambord2 WFM 1.gif
Dambord2 WFM 1.gif (43.15 KiB) Viewed 5401 times
Dambord2 Spectra 1.jpg
Dambord2 Spectra 1.jpg (19.49 KiB) Viewed 5401 times
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Re: Retro SSTV anyone?

Postby Klaas Robers » Sat Sep 19, 2015 9:06 pm

Hi Harry,
this is more or less what I did, but I have to optimise it. My recording software (Gold Wave) has a constant recording level. In fact I like that, as it is now ME that defines the amplitude of the recorded signal. But i did it in a wrong way, that is in respect to noise.

Steve,
the screen shot looks very fine. And the squares are aligned not bad at all. Also the demodulated video wave form looks good. Are the levels DC-wise comparable with what you are used of?

Then I don't understand the "spectrum" of MMSSTV. The sync is a very narrow peak, this looks good, so why are the blacks and the whites so wide? I expect that the program is not a real spectrum analyser, but it measures the time between all successive zero crossings and makes a histogram of this. That is what I should have done. In a spectrum amplitude is also taken in aspect, but that is of no use for FM signals.

Steve, how much time of SSTV signal do you need to make a "spectrum? If it is short, one second or so, then I can send a short phragment at 48 kHz and 16 bits resolution. But no, this cannot be the reason. Have you looked at the wave form of the replayed audio? These should be sine waves or almost sine waves. That is what I see on my oscilloscope when I look to the headphone output of the PC.

Here is a new Testbeeld file, however in 32 kHz 8-bits.
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Testbeeld3.wav
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Re: Retro SSTV anyone?

Postby Steve Anderson » Sat Sep 19, 2015 10:36 pm

My fault again, overloading the input to the PC line in (again), reducing the signal to 1.0V p-p results in a much cleaner resultant spectra. The peaks at 1500 and 2300 are quite broad compared to the modulator I'm using here.

Looking at the sub-carrier sine wave of Danbord on the scope it is quite 'jittery', as if some noise is getting into the VCO control line or the unijunctions are being affected in some way.

It may be worth generating a file of a whole black frame to see if any harmonics of sync or black are creeping in too.

I'll have a go at Testbeeld3....

Around 10 seconds of 16-bit/48kHz mono should be enough, it should be a file of only around a megabyte.

The 'contouring' problem turns out not to be contouring from the multiply routine. I added the required hardware (one pot, one resistor), removed the multiply routine - and it's still there! There appears to be two factors, one is our good old friend RAM I/O timing differences, but it's the lesser of the two. The second one I'm still trying to pin down.

Steve A.
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Dambord2 Spectra 2.jpg
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Re: Retro SSTV anyone?

Postby Klaas Robers » Sun Sep 20, 2015 12:08 am

Nice picture Steve. It looks as if the frequencies are now much better.

If there would be "spurious" creeping into the VCO then this should be visible on the monitor screen, but even more on the oscillogram of the video wave form. I see nothing of that.

But what me also surprises is that the "spectrum" is roughly cut off at the upper edge. Wouldn't that graph go higher up? And how would the peak form be there? Is this the only setting for these graphs? Would it be normal that the graph is flat topped?

After better observing it looks that my sync is now slightly too high. I will have a more precise setting later in time. On the other hand, a VCO is always a somewhat unstable oscillator. On my oscilloscope the video line of a black picture is razor sharp, as is it on a white picture. Spurious in the VCO input would then be visible.
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Re: Retro SSTV anyone?

Postby Steve Anderson » Sun Sep 20, 2015 2:39 pm

I think the spectrum display in MMSSTV is to be thought of more as a tuning aid or tuning indicator when receiving via SSB, not as one would think of a real spectrum analyser. A guide if you will. It's accurate enough in frequency when fed a fixed tone but as for amplitude - well, anyone's guess.

I made up a chequerboard file similar to your own with eight squares rather than six, 256 doesn't divide by six easily. It's also easy to see where it came from.

The screen-shot, spectra and waveforms are below. The waveforms are the baseband input to the modulator (magenta) and the demodulator output (yellow), The overshoot is due to the demodulator output filter which I'm still musing over.

I'll get on with Testbeeld3 right now.

Steve A.
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Chequerboard 1.jpg
Chequerboard 1.jpg (28.6 KiB) Viewed 5639 times
Mod-Demod 1.gif
Mod-Demod 1.gif (53.07 KiB) Viewed 5639 times
Chequerboard Spectra 1.jpg
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Re: Retro SSTV anyone?

Postby Steve Anderson » Sun Sep 20, 2015 9:57 pm

Testbeeld3, screen-shot and waveforms below, I wasn't able to get a spectra for it, I think the Windoze PC needs the usual early evening re-boot. Well you can't expect the thing to run for more than six hours without some limb falling off...

You'll notice the sync detect output (cyan trace) is late, this is the result of making sure that it really is syncs. Seven samples in a row need to be below the set threshold before sync is acknowledged, this stops runt pulses falsely triggering, the actual trigger comes from the trailing/rising edge of the sync pulse. Non-standard I know, but it works.

Also the 1200Hz grating is of a greater amplitude.

Ignore the usual crud top and bottom of the screen-shot.

The other screen-shot is of my local when I'm in the UK. Ye Olde Kings Head, Church Street, Epsom, Surrey. A lovely little hole-in-the-wall.

Steve A.

One thing that might be 'educational' is to do a baseband to baseband screen capture and see how much the mod-demod process impairs the picture...
Attachments
Testbeeld 3.jpg
Testbeeld 3.jpg (43.82 KiB) Viewed 5632 times
Testbeeld 3 WFM 1.gif
Testbeeld 3 WFM 1.gif (50.91 KiB) Viewed 5632 times
YOKH Epsom UK 1.jpg
YOKH Epsom UK 1.jpg (45.04 KiB) Viewed 5632 times
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Re: Retro SSTV anyone?

Postby Klaas Robers » Mon Sep 21, 2015 2:54 am

Steve, nice pictures and oscillograms. I see that your synchronisation improved a lot. It is also good to see that the small arrows on the Testbeeld3 are fully visible. And I see too that the 1200 Hz has a larger amplitude. It is comparable to what I see on my oscilloscope. I think it very difficult to make a photo of those particular lines as the exposure time of my camera is at least several lines, so I have to take it on the guess.

What strikes me is that your demodulated and filtered oscillograms show no ringing at the towards white going edges. They show ringing (overshoot) on the down going edges. That is normal and cannot be avoided if you want to have a rather steep cut off. Only in Bessel filters there is no ringing, but they almost don't filter.

With an allpass you can spread the ringing to 50% before the edge and 50% after the edge, but the ringing remains, as well on the upgoing edges as on the downgoing edges. I have seen that in extenso on output signals of CD-players. There the ringing can be enormeous long, but fully symmetrical left and right of the edge or pulse. It looks as if your active filter suffers from overload to the positive side..... Could that be possible? I have seen that too on CD-players of "unknown" brands.

In the last days I did a lot on the monitor. Almost all circuits got their place on the chassis, or below. The FM demodulator above the chassis is running, you have seen the demodulated oscillogram. The powersupply, below the chassis is also running. Today I installed the transformer and the on/off circuitry. No external power supply is needed anymore, it's running now directly from the 230V AC mains. I should make a photo of the current set-up to show here and for documentation. I am missing still a crucial stud to mount the picture tube, so I can't show light off the screen.
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Re: Retro SSTV anyone?

Postby Steve Anderson » Mon Sep 21, 2015 11:51 am

Thanks Klaas. The reason I started on the sync arrangement where seven samples were required to say it was truly syncs was precisely due to the undershoot in the demodulated signal causing mis-triggering. The number seven is nothing special, it just comes out as 1.5ms of continuous sync before anything happens.

Why the ringing is generally on the -ve going edges and not the +ve I'm not sure. There is some on the +ve going edges, but much less so. I very much doubt it's overload of the op-amps. The signal is only around 4V p-p and the supplies are +/-12V, 24V total for the demodulator output filter. In some places just the +ve supply is used, even so I don't think this is the root cause.

It's a shame that there's no arrangement which is half-way between a Butterworth and a Bessel filter, well none that I know of. A Bessel filter to remove demodulation ripple would possibly require 10 or more poles. It may be possible to build a suitable FIR filter in software but it could suffer the same result, but perhaps I'll look into it.

I'm hoping that all this effort pays off when I start on the NBTV version of the software. I'm hoping that it's mostly going to be a matter of adjusting timings and reading/writing to/from the RAM backwards. (I can dream!)

Seems you're making good progress on your monitor...

Steve A.
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