NBTV/SSTV-625 Up Converter MkIII

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: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Sep 01, 2017 10:25 am

For the moment I wouldn't spend any more time on this illusive problem, it is doing the job even if not quite as well as hoped.

Now I'm back home I'll have a look myself before and after I have modified my build to the '3.3V version'. I expect some sub-carrier bleed-through on the sync-tips where the carrier is at its lowest frequency, but from black to white it should be somewhat cleaner.

Given the low resolution of the system generally this degree of noise is unlikely to be visible on-screen unless really trying to see it.

As mentioned before I intend to replace this demodulator at some point in the future, so I wouldn't worry about this too much.

Steve A.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Fri Sep 01, 2017 2:49 pm

Steve Anderson wrote:
As mentioned before I intend to replace this demodulator at some point in the future, so I wouldn't worry about this too much.

Steve A.


Steve is it worth waiting as i have not started on the demodulator yet mainly because a seller on Aliexpress had not sent the TL071 2 8 ics i ordered they say they had a problem didn't send them if i had not asked i would still be going to the letter box each day ....buggers !

Did you say your back from Europe now ?
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: NBTV/SSTV-625 Up Converter MkIII

Postby acl » Fri Sep 01, 2017 4:45 pm

Thanks for all the ideas lads. I will wait Steve.

Regards Chris Lewis
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Sep 01, 2017 9:11 pm

Harry, well there's not going to be a mad panic rush, so wait a while for the chips...make sure you've ordered the correct parts though. I wouldn't wait for the replacement demodulator design as that will happen after this MkIII version is complete along with the NBTV side of it. So that could be quite a way down the line.

There is also the chance that a better demodulator could be too complex and/or expensive, unlikely, but possible.

Steve A.

Yes, back in Bangkok now.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Fri Sep 01, 2017 9:38 pm

Steve Anderson wrote:Harry, well there's not going to be a mad panic rush, so wait a while for the chips...make sure you've ordered the correct parts though. I wouldn't wait for the replacement demodulator design as that will happen after this MkIII version is complete along with the NBTV side of it. So that could be quite a way down the line.

There is also the chance that a better demodulator could be too complex and/or expensive, unlikely, but possible.

Steve A.

Yes, back in Bangkok now.

OK i will get some Tesla DAC's off to you next week .
Sorry yes they were a mix the TL084 2 TL072 and 71s opamps some for this and stock up a bit .
Ok Steve i will make the original Demodulator understand no worries .
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: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Tue Sep 05, 2017 8:22 pm

Steve, I am trying to devellop a PIC software FM-decoder, with a counting FM-detector followed by a software 7-pole Butterworth FIR filter. This is something I never did before, in the past I only got something of the theory behind it. To begin with I plan to DA-convert the calculated video output, because then I can measure the frequency response and other characteristics and see principal (rounding?) errors.

Until now I plan a sampling frequency of 20 kHz for the FIR-filter and a cut off of 1 kHz. With a X-tal clock of 20 MHz I hope to have enough clock pulses to run the FIR-filter. The input of the PIC will be a limited SSTV audio signal.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Wed Sep 06, 2017 11:51 am

I too have looked at FIR filters for the demodulator, but so far only as a paperwork exercise. More to do here when I replace the Robot 70 demod.

The counting/zero crossing demod was what led to my problems with sync-jitter, so for the time being it's also on hold. But strangely, subjectively the actual video appeared better.

In the background I have another concept for this but first I need to review some theory which I have forgotten.

Steve A.
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