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 » Thu Aug 10, 2017 4:38 am

Andrew Davie wrote:I'll join the crowd, though I have little idea what it does :p

Andrew, you'll probably be more interested in the NBTV-to-625 side of it. It takes a standard NBTVA analogue video signal and up-converts it to be displayed on a standard 625 TV with a baseband 625 AV input, most do.

The input can come from a PC playing back a .wav file, an audio CD player/DVD player, any analogue source of NBTV. Though it MUST be to the NBTVA standard.

In addition it up-converts the original Cop McDonald FM SSTV standard to 625, again the source is any analogue audio device.

A single pin on the board determines which mode it is in, if not interested in SSTV you simply wire it appropriately, ditto NBTV. If you want both (not at the same time though!) a simple front-panel switch is all that's required.

For NBTV an off-board DC-restorer/clamp derived from Klaas's design is required, for SSTV you'll need an SSTV FM demodulator as well. These are both quite simple and I plan to improve on the SSTV demodulator in the future.

OK, so that makes five interested bods so far.

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

Postby acl » Tue Aug 15, 2017 8:24 am

Thanks to Graham Lewis for details of source for chips used on the processor board. Progress is slow here due to real work but managed to get around to testing the demodulator,

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

Postby Steve Anderson » Tue Aug 15, 2017 5:55 pm

Looks good Chris, keep us updated with your progress.

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Thu Aug 24, 2017 12:14 pm

Just an update on the parts collection for the project from my side of things ,i have the 23Lc1024's and DAC's i still need the TL072/ 4/8 to turn up last thing i ordered for the demodulator..
I am thinking of working on the power supply and work out board positions now i have the case .
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A resounding success

Postby acl » Fri Aug 25, 2017 5:48 am

Just finish testing demodulator . It appears to work as described by Steve.
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NBTV/SSTV-625 Up Converter MkIII

Postby acl » Fri Aug 25, 2017 5:51 am

Test set up
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Aug 25, 2017 6:19 am

Great Chris. Though I'm a bit surprised by the amount of what appears to be residual carrier on the output waveform. My results were somewhat cleaner but they still had the ringing/overshoot also evident in your waveforms.

I'm assuming you have incorporated the modifications for the 3.3V processor system rather than the 'original' 5V processor system. If that's the case I need to find out why there's this residual ripple.

The sort of results I got on the original system were typified as below...yellow trace is the input to modulator, magenta trace is output of demodulator...

Steve A.

Footnote:- I'll not be able to do anything on this until early next month when I return to Bangkok, then I'll have access to my workshop.
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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Klaas Robers » Fri Aug 25, 2017 5:42 pm

Steve, it can be too that what you see as residual carrier is just noise. However with a signal of 3 volt peak to peak one should not expect that. It was also the first thing that struck me when I saw the screen shot of Chris' oscilloscope.

Chris, can you make a screen shot of just one stair case horizontal and a larger amplitude, comparable to the screen shot that Steve made? That will give us better information on the details of your signal.
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NBTV/SSTV-625 Up Converter MkIII

Postby acl » Fri Aug 25, 2017 8:51 pm

Hi there Steve and Klaas,

I will send a more detailed waveform screen shot soon.( I need to find out how to drive the USB interface to the 'scope.) I have not done any adjustments of the two 'on board' potentiometers at this stage. I will optimise the settings on the scope probes as well.

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

Postby Steve Anderson » Fri Aug 25, 2017 8:55 pm

In terms of performance there should be no difference between the two post-rectification filters except for the gain. In the original 5V arrangement the filter had a gain of unity followed by a x4 gain stage whereas the 3.3V version does have some gain eliminating the gain stage.

I need to double-check the calculated values and if they turn out the same I'll breadboard it and test.

Should it be noise - I wonder where that might be coming from?

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Steve Anderson » Fri Aug 25, 2017 9:31 pm

Although I'm in the UK I've been able to at least simulate the two versions of the filter, the only difference is the gain, as expected. But I will admit I haven't built the 3.3V version yet I don't see any reason why it shouldn't perform any different to the 5V version. We'll get to the bottom of it eventually!

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Re: NBTV/SSTV-625 Up Converter MkIII

Postby Harry Dalek » Fri Aug 25, 2017 11:11 pm

Steve Anderson wrote:In terms of performance there should be no difference between the two post-rectification filters except for the gain. In the original 5V arrangement the filter had a gain of unity followed by a x4 gain stage whereas the 3.3V version does have some gain eliminating the gain stage.

I need to double-check the calculated values and if they turn out the same I'll breadboard it and test.

Should it be noise - I wonder where that might be coming from?

Steve A.


Steve on chris and the demodulator noise i recall you mentioned when i built mine for the Slow Scan monitor that you use resistors of 1% tolerance even so good to check ..looking i think he has used these... what i do see construction looks to be strip board so leads are long .
As you always point out to me decoupling caps are a good tool to stop noise and keep all part connections short .
Chris do you have a analog scope i am wondering if you are picking up noise from the digital scope or your computer computers are noisy things also signal audio level to high or low ?
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 » Sat Aug 26, 2017 3:08 am

Thanks for your advice lads.


I'll try more decoupling to see if that clears it up and adjusting the audio level. I can post it to you in the UK to save you time building it up . What is the procedure for setting the pots up Steve?

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

Postby Harry Dalek » Sat Aug 26, 2017 8:31 am

acl wrote:Thanks for your advice lads.


I'll try more decoupling to see if that clears it up and adjusting the audio level. I can post it to you in the UK to save you time building it up . What is the procedure for setting the pots up Steve?

Regards Chris


Chris towards the bottom of the link page Steve explained the procedure to me and kindly posted the frequencies to test the demodulator read also the next page or 2
viewtopic.php?f=19&t=2080&start=60
I have to make mine again for this soon so might be in the same boat with a noise problem never know till you switch it on and test ,i do know the circuit works well in my p7 SSTV....its more than likely a audio level thing.
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 Steve Anderson » Sat Aug 26, 2017 7:03 pm

The two pots are to set the sync-tips to 0V (VR201) and VR202 sets the overall amplitude to a nominal 3.0V p-p (sync tips to white). Don't forget to set your scope input to DC-coupled and ensure you've allowed for a x10 probe if using one.

The more recent photo of the waveform does look somewhat better but not as clean as I would have hoped for. Some of this may be down to the layout which is quite 'loose' for want of a better word. Not a criticism but with these higher-speed devices things do need to be compact with short and fat ground and supply lines (sorry to harp on about this again). This becomes especially true when we get to the main processing part where we have 16MHz running around.

My prototype demodulator is below, it's a mess - yes - but has undergone several revisions, chips removed, values changed and will undergo one final revision to the 3.3V arrangement soon. Ignore the two jumpers, they're just for development...and the hair bottom centre!

Steve A.
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