Slow Scan Television Experiments

[Steve Anderson]

Picking up where I left off....

A bit of time has become available to continue with the SSTV DDS modulator. The bulk of both the software and hardware is now complete, I'm just waiting for some 10k 25 turn trimmer pots to arrive to complete the hardware.

Even so it performs as well as could be hoped, better than I expected in fact.

I did mention I thought the micro might could end up being a 14 to 20-pin device. But I have squeezed into an eight-pin device, photo below.

The upper device is the 1.3kHz input filter to limit the baseband frequencies, the centre device is the micro, and the lower device is the filter which creates the sine-wave SSTV FM sub-carrrier from the PWM output of the micro. The transistor-looking thing is a 5V regulator for the micro, the board as shown only requires +12V @ 14mA.

The 6-pin header and the jumpers are only for programming, not normally required.

The next task is to generate some .wav files with video using this device. A few days should do it...

Steve A.

[Harry Dalek]

Looks good Steve nice and small !
Thats one thing all electronic thing will always have over mechanical is size unless your Mr Spock and can work will nano motors.
i will look forward to seeing the end result.

I was wondering how wide the bandwidth would be if there was a FM version of NBTV as in speeding up SSTV to a movement version ,i should not have these thoughts Sacrilege !

[Steve Anderson]

I was wondering how wide the bandwidth would be if there was a FM version of NBTV as in speeding up SSTV to a movement version ,i should not have these thoughts Sacrilege!

It can be done, technically. The problem becomes one of how to transmit or record/playback the resulting signal?

In very rough numbers SSTV requires 1kHz of video bandwidth, NBTV 10kHz. Keeping line-standards/frame-rates the same and crudely multiplying everything by a factor of 10, NBTVFM (we'll call it that for now) would have sync at 12kHz and white at 23kHz. It would be better though to raise these numbers somewhat, but we'll stick with them for now.

These days a PC sound card sampling at 48kHz could just manage it, far better at 96kHz. So for PCs - plausable to possible.

But transmitting? I don't think so. Another 'standard' - there are enough already. Special transmitters/receivers etc, but if the amateur community really wanted it...why not?

The great advantage FM as per SSTV is response down to DC. No more DC restorers, no more LF headaches, but standard domestic audio gear just cannot handle it...and that's in a way what configured NBTV as we know it and as well as the technology of the 30s era which defined it in the first place.

If two or more people wanted to develop this, it can be done, but without wide acceptance 'standards' die.

Steve A.

[Harry Dalek]

It can be done, technically. The problem becomes one of how to transmit or record/playback the resulting signal?

I couldn't help my self wondering i sort of knew the reason NBTV is AM for bandwidth and historical reasons but wondered a cross between the two systems would
be interesting.

In very rough numbers SSTV requires 1kHz of video bandwidth, NBTV 10kHz. Keeping line-standards/frame-rates the same and crudely multiplying everything by a factor of 10, NBTVFM (we'll call it that for now) would have sync at 12kHz and white at 23kHz. It would be better though to raise these numbers somewhat, but we'll stick with them for now.

I suppose a cross or a quicker frame rate for SSTV might have been interesting to see it evolve but as we know it went in a different direction higher definition
and colour as we know ,i think if it went into a slightly higher frame rate the monitor side of things would of had a longer life for the home constructor.
Those numbers sound good for pc thats interesting .

These days a PC sound card sampling at 48kHz could just manage it, far better at 96kHz. So for PCs - plausable to possible.

Possible is good

But transmitting? I don't think so. Another 'standard' - there are enough already. Special transmitters/receivers etc, but if the amateur community really wanted it...why not?

I think the ham radio guys theses days would want high definition digital but do they have NBTV don't use it and SSTV still wins out.
I sort of think these days the internet is the new ham radio for our time so never know .
The format would be good for VHF UHF but they again have other tv standards still it would be good for DX.

The great advantage FM as per SSTV is response down to DC. No more DC restorers, no more LF headaches, but standard domestic audio gear just cannot handle it...and that's in a way what configured NBTV as we know it and as well as the technology of the 30s era which defined it in the first place.

True theres a reason it is they way it is people are fickle i suppose the reason sstv still photos won over NBTV is picture definition.

If two or more people wanted to develop this, it can be done, but without wide acceptance 'standards' die.

Steve A.

OH i am sure it would not be needed but if some one asked the question in the early 70s SSTV could of been a little different perhaps .

[Steve Anderson]

Harry, can I ask a favour of you...

I've created the first wave-file generated by the DDS modulator. The video (baseband) was from the same development board I used earlier on when I started on this. When you listen to it all sounds fine. I've checked the line timings (US 15Hz, 120-lines) and they're spot-on.

I tried to use MMSSTV and using the auto-slant thing it sort-of achieved line sync but it never seemed to find the frame sync. The frequencies are 'close enough' in the spectum window.

I've never used MMSSTV before so maybe it's something I'm not doing right...very possible.

So could you have a look at this if you can spare some time? (But not too much). It's a fairly small wave-file due to the 8kHz sample rate, only sixty seconds or so and mono. Also, the best I was able to achieve in MMSSTV is below...

Steve A.

I've check the frame timing now, eight seconds it is....

[Harry Dalek]

Hi Steve just a quick one here to let you know viewing the wav file on mmsstv the bandwidth looks a little wide going below sync and above white so causing noise in the video .
It is slanted i will do a screen grab and show you in the next post.

[Harry Dalek]

Harry, can I ask a favour of you...

This is no problem I will have a good look at the wav test pattern now.

I've created the first wave-file generated by the DDS modulator. The video (baseband) was from the same development board I used earlier on when I started on this. When you listen to it all sounds fine. I've checked the line timings (US 15Hz, 120-lines) and they're spot-on.

So far its looking like theres noise either side of the gray bars i will check its not play back problem.

I tried to use MMSSTV and using the auto-slant thing it sort-of achieved line sync but it never seemed to find the frame sync. The frequencies are 'close enough' in the spectum window.

I will check with my wav file i never tried seeing if it would play and display should show if its a play back thing or not

I've never used MMSSTV before so maybe it's something I'm not doing right...very possible.

I have never played around with settings either init apart from selecting 8 sec sstv or auto ,playing live to it i found no need.

So could you have a look at this if you can spare some time? (But not too much). It's a fairly small wave-file due to the 8kHz sample rate, only sixty seconds or so and mono. Also, the best I was able to achieve in MMSSTV is below...

Steve A.

I've check the frame timing now, eight seconds it is....

Just to get this to you quick i will have another look and get back to you .

[Harry Dalek]

Steve i am getting noise as well playing back my file ! i will look into the player side .

[Steve Anderson]

Harry, I think it may be that used a sample rate for the wave file that MMSSTV doesn't support. Let me look into it further, don't waste any more time on that version..

I'll delete the previous attachment...done...

I'll do another one and check the results by playing it back in MMSSTV first...

Steve A.

Ah!! MMSSTV can't play wave-files it seems, only its own format (.mmv), at least in the version I have, 1.13A.

[Harry Dalek]

Harry, I think it may be that used a sample rate for the wave file that MMSSTV doesn't support. Let me look into it further, don't waste any more time on that version..

I'll delete the previous attachment...done...

I'll do another one and check the results by playing it back in MMSSTV first...

Steve A.

I tried it on a few players its bit better on a higher quality ones, but as you say i think you hit the nail on the head it was the recording side that caused it reason mine is also giving same result .
I also tried it on Chroma pix sstv and same result i will await the next recording .

[Steve Anderson]

Ah!! MMSSTV can't play wave-files it seems, only its own format (.mmv), at least in the version I have, 1.13A.

But thanks for your effort Harry...shame it didn't work out....

Steve A.

[Harry Dalek]

Ah!! MMSSTV can't play wave-files it seems, only its own format (.mmv), at least in the version I have, 1.13A.

But thanks for your effort Harry...shame it didn't work out....

Steve A.

Steve i changed the wav file to mmv and i got the below result looks good on the water fall display frequency meter the display side is nothing good but does show its not noise .......sample rate might cause the display result since its a file playing back the other way was sound play back via some thing else as in if it was played live.

IF you play it via a windows player it works but i am sure your right in that it was recorded at the wrong sample rate the programs can't handle/

[Steve Anderson]

What I'll do tomorrow (Friday) is knock-up a simple decoder on the breadboard and confirm or otherwise that what I'm generating is valid.

After that, if you can tell me the correct sample rates for .wav files that can be viewed by the software you have (or converted) we'll have another try. Please don't spend any more time on that original file...

Thanks Harry.

Steve A.

[Harry Dalek]

Been thinking the last few days on the line clock side of things with out building its clock yet ...just been pinching the frame stepper motor clock adjusting the line speed to see ....any case .
I wanted to see what the clock frequency needed to be to get 15 hz out the 4 sided mirror on my stepper motors opto switch,it was the top limit for the pot setting reading was around 565hz

I think i will go with a 4060 adjust it to get this .

I am not worried about the stepper changing speed that much comes down to how stable your clock is i suppose, what my problem may be is the mechanical side not starting a line in the correct position telling the system where the mirrors start position is .

I want a hybrid motor control or a better work is a start position for the mirrors as i posted a few posts ago with the clock and the opto switch as feed back control feeding a bistable ,the output clock pulse from the bistable is the line stepper motors clock pulse.

Now unless the 15 hz opto switch pulse and the 565 hz clock fed to the bistable will sync work mmmm may be or use a few 4046 ics as a frequency multipliers for the opto switch pulse and try and get around 565hz.

i also had the idea of using the pulsed 5 volt voltage from the opto circuits lm311 output and feeding a 4046 as a VCO, if the pulses are stable the VCO just needs to be adjusted to 565hz then again i have a match that might help with some sort of mechanical feed back .

I like the VCO idea less electronics but whats more important is what works .

[Harry Dalek]

What I'll do tomorrow (Friday) is knock-up a simple decoder on the breadboard and confirm or otherwise that what I'm generating is valid.

After that, if you can tell me the correct sample rates for .wav files that can be viewed by the software you have (or converted) we'll have another try. Please don't spend any more time on that original file...

Thanks Harry.

Steve A.

NO problems Steve post a some up when you can and i will check them out i am pretty sure your right just a digital recording problem more than likely an analog systems revenge on the digital age