NBTV/SSTV-625 Up Converter MkIV

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 MkIV

Postby Harry Dalek » Tue Feb 23, 2021 7:47 pm

Oh ok Steve ...just wait and see this time
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 MkIV

Postby Steve Anderson » Thu Feb 25, 2021 1:41 pm

Updated design coming on nicely... now I have to turn my attention to demodulation. There's three (maybe more) ways to do it, 1) Zero-crossing, 2) FM demodulation, 3) In software using mixing, FIR filters and a bit of calculus...

The MkI used #2, the results I think where quite good considering the relative simplicity of the design. I did experiment with #1, it worked OK, better in some areas, marginally less so in others, sadly I can't find the code I wrote for it. or even the concept as to how I did it...
.... I've yet to try #3.

Two of these methods may require an external demodulator, #2 and #3. Even #1 may benefit by using an external arrangement. I'm still dwelling on this but I don't want to spend too much time on it.

I may go with #1 first, and try the others later...

Steve A.

A fourth method that slipped my mind #4) PLL (Phase Locked Loop). Either analogue (e.g. our beloved 4046) or an ADPLL (All Digital Phase Locked Loop). The latter holds a lot of promise, but the maths required, "Does me 'ead in"...translation from 'saaf londin' speak, it's somewhat challenging...

Another musing, a blend of means is also a possibility...more to come...
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Re: NBTV/SSTV-625 Up Converter MkIV

Postby Steve Anderson » Sun Feb 28, 2021 2:15 pm

Thought I'd hit another impasse this morning, but thankfully dodged around it. This VGA idea is testing this (and me) more than I expected! Though these micros can operate at a clock speed of 64MHz it's not compatible with a VGA 800x600 display, 40MHz it has to be.

I was hoping to have some simple screen-shots up this weekend, but I keep getting thwarted, plus I still have to do that outing to get some vital components...

Steve.
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Re: NBTV/SSTV-625 Up Converter MkIV

Postby Harry Dalek » Sun Feb 28, 2021 5:33 pm

Never fun when it does not go to plan !
So now your clock will have to run at 40MHZ crystal change i suppose /
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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Harry Dalek
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Re: NBTV/SSTV-625 Up Converter MkIV

Postby Steve Anderson » Mon Mar 01, 2021 1:08 pm

Harry Dalek wrote:...crystal change i suppose /

Yes, oscillator changed to 40MHz along with a whole heap of other stuff, there's little similarity between the early versions and how it is now. The major components are the same, micro, RAM, etc., it's how they're strung together that is very different.

The reason for the speed change is the chosen flavour of VGA uses 25ns pixels (the inverse of 40MHz). However here they'll be in blocks of four at first (100ns) with a change, perhaps, to a pair (50ns) later. Hence the requirement for 74ACxxx series logic chips...as I see it at the moment just a few (3-4) 74AC00 and/or 74AC08.

Steve A.
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