Update to SSTV demodulator

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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Re: Update to SSTV demodulator

Postby Klaas Robers » Tue Oct 29, 2019 11:29 pm

I would prefer the look-up table. In that way you can in the same operation adjust the output range to what you like.

As I said, there is no need to go deeper than 6 bits for black and white if you adjust 00 0000 to black, 1500 Hz and 11 1111 to white, 2300 Hz. See that undershoots and overshoots are clipped off. May be that you can allow some slight under- and overshoots. If the look-up table returns any way a byte, you may solve that in the table outputs. And who cares about the space of such a table, which resides in the progam space. You know how to do that?

In the past we did some experiments at Philips Research on the bit depth of video signals. The first thing that you see if you have too few bits is "contouring". You see then grey rings and areas in parts of the picture with almost equal grey level, e.g a back ground wall. This is no more visible if you have 64 shades of grey. But if you want to code the sync pulse in the digital stream you need 7 bits, because 44 shades is too few and contouring becomes visible. And if you want to digitize a composite PAL colour signal, you need 8 bits. This is also the reason that together with Vic we made the colour NBTV standard with 6 bits for Red and Blue and 6½ bits for Y, if I remember well.

May be that you can do the sync detection in the same way, may be it can be done in the same look-up table, in the two remaining bits. May be you get a slight jitter in the sync, but we can see if we can solve that further on in the chain. I think that a sync fly-wheel is any way a good idea. That will remove the jitter as well.
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Re: Update to SSTV demodulator

Postby Steve Anderson » Wed Oct 30, 2019 1:38 am

I used a look-up table in the PIC16F683 DDS SSTV modulator for storing the values to load the Frequency Registers, just 256 'words' of 14-bits, (the actual program code is only 83 'words' in addition'). The PIC16 devices have 14-bit 'words' per program memory location, the PIC18 series use conventional 8-bit bytes for a look-up table, stored as two bytes within a single 16-bit program memory location/address.

The previous (first) SSTV-625 up-converter I made used two processors plus the Robot 70 demod. Using the circuit above would make that three, so what I'm trying to do is shoe-horn the demod into the front end of the Write Processor, at the same time reducing the pin-count of both processors to 28-pins each, and the RAM to two 8-pin devices.

The devices I have in mind are 2x28-pin PIC18F25K20s and 2x8-pin 23LC1024 serial RAM chips. The processors will run at a 16MHz instruction rate (64MHz oscillator with PLL), both the RAM and the micros run on a 3.3V supply at those speeds.

As for the bit-depth used - well everything is a multiple of 8-bits, why not use them? No real increase in complexity, coding or cost that I can see.

Steve A.

I did make a start on this some time back, I may make some minor changes but basically this will be about the extent of the hardware, no separate demodulator required.

Yes, I am going to move that capacitor in the centre...

Also the two 14-pin sockets (and their devices) may not be needed either.
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Re: Update to SSTV demodulator

Postby Steve Anderson » Wed Oct 30, 2019 7:41 pm

Compare this (the Mki) to the photo above - and this first version doesn't include the demodulator...

Steve A.
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Re: Update to SSTV demodulator

Postby Klaas Robers » Wed Oct 30, 2019 11:22 pm

Well Steve, I had the idea that the 2 extra bits above the 6 bits for the video, can be used to decode the sync pulses. Then 1500 Hz to 2300 Hz is addressing the part for the video output signal, 00 0000 black to 11 1111 white, while around 1200 Hz two bits could be used to indicate the sync: 00 = no sync, 11 is bottom sync, and 01 and 10 for the edges, at frequencies around 1100 and 1300 Hz. I don't know yet how to extract the beginning and end of the sync pulse, but I have the feeling that quite some sync jitter can be eliminated by this.

So the bytes coming out of the look-up table are 2 bits for sync and 6 bits for the video. White clipping and black clipping can be done in the look-up table as well.

And then I still think that we need a video low pass filter at 1 kHz. Without that the bandwidth, especially in the white area (2300 Hz --> 4600 Hz sampling frequency) becomes too high and the video signal may become noisy. However you might start without that filter and see how it looks.
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Re: Update to SSTV demodulator

Postby Steve Anderson » Thu Oct 31, 2019 11:30 am

One thing I omitted to mention in my previous post...

This has developed into a full-blown scan/up-convertor, the original intention was just a standalone SSTV demodulator with a baseband output, maybe digital too. Once this up-convertor is underway I'll rip out the required code and simplify the hardware for just such a device, just one smaller processor, no RAM, D-A...that's a maybe.

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Re: Update to SSTV demodulator

Postby Steve Anderson » Thu Oct 31, 2019 1:55 pm

I'm going to continue this where it should belong in this thread..."NBTV/SSTV-625 Up Converter MkIII" in the folder "Copthorne McDonald's SSTV system." for the complete SSTV-625 converter, the stand-alone demodulator will continue here...

The NBTV-625 part hasn't been forgotten or dumped, it's a case of one thing at a time...

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Re: Update to SSTV demodulator

Postby Steve Anderson » Wed Dec 11, 2019 3:25 pm

Minor changes to the zero-crossing demodulator.

The part below the 'Optional' line is just that, if you wish to have an analogue output you'll need it, otherwise omit.

The Digital Output (Pin 10) is what will feed the up-converter proper...

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Re: Update to SSTV demodulator

Postby Steve Anderson » Wed Dec 18, 2019 2:06 pm

A note on the above...if you don't want/need the analogue output omit all below the 'optional' line as previously mentioned, IC401 pins 8,9 &11 can just be left open-circuit as they'e all outputs.

Steve A.

Potential major change...I may dump all the above and as I was hoping move this demodulator into the Write Processor of the MkIII SSTV-626 up-convertor...it does seem possible. Though whether there'll be an analogue output has yet to be seen...I'm running out of I/O pins on the Write Processor, and I'm resisting the idea of going to a 40-pin device...
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Re: Update to SSTV demodulator

Postby Harry Dalek » Wed Dec 18, 2019 3:47 pm

There it is i thought i saw that circuit some where before ! \
The analog output i understand but to a digital out put i see yet to be designed ...what would the display device be Steve if you go down this road also ?
PC ? i suppose new monitors these days also have digital inputs .
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Re: Update to SSTV demodulator

Postby Steve Anderson » Wed Dec 18, 2019 4:31 pm

OK - decision made, the above is dumped, gone, kaput, I can squeeze the demodulator into the Write Processor. I rearranged the I/O after deleting a few lesser used items. For the time being this makes it SSTV only, but the NBTV version will come along in due course...I simply ran out of I/O pins and I am reluctant to go to a 40-pin device which could easily do it.

So it retains the SSTV analogue output which is useful for viewing the SSTV baseband waveform on a 'scope or driving an anlogue baseband input SSTV monitor.

There is no digital output now. It was never intended to be used externally anyway...

So I guess this thread has reached its conclusion. To be continued in the up-convertor thread proper...

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