8-bit Computer TV Channel Use

tony duell ard at p850ug1.demon.co.uk
Sat May 23 12:41:26 CDT 2015

> The output of a single-chip tuner might also be at IF. The Maxim part (which I will not use) outputs at 36 MHz, I 
> think. Can't tell the output of the SiLabs part without more info. Hopefully it's either baseband or a lower IF 

36MHz does sound like the standard TV IF frequency.

> frequency that I could sample with a cheaper ADCs for digital down-conversion. Needing to support a 36 MHz 
> IF would probably increase ADC cost vs. using ones that just need to sample baseband or a low IF.

> SAW filters are also black magic, and nowadays they are TINY!

Problem with TINY parts is soldering them :-). The SAW filters I used (with a conventional tuner module) were
round metal cans about 0.5" in diameter with 8 pins on IIRC a 0.1" spacing. Very easy to handle. If you are 
designing something for others to build (even in principle, i.e. you are making it an open design) then using 
impossible to handle parts is a bad thing if there are alternatives.

> > Oh yes, in the UK the sound carrier was 6MHz offset from the video carrier, in the rest of Europe the spacing
> > was 5.5MHz. So if you want to handle sound (some computers sent their sound output over the RF output) you
> > may need to cover both.

> And US NTSC puts the sound carrier at 4.5 MHz, so there's another thing in favor of using SDR techniques for 
> some portion of the demodulation if I can't find a Magic Chip that does the work more cheaply. The chroma 
> subcarriers are also at different frequencies in the various standards.

And IIRC US NTSC uses AM sound (Europe uses FM). I think you can forget about stereo sound, since
I doubt any home computer had a stereo RF modulator.

Be warned that there are many versions of PAL. PAL B/G and PAL I are the ones used in Europe and the UK, and
are basically compatible apart from the sound carrier offset (there are other differences, but they are unlikely
to matter here). But there is also PAL M and PAL N. at least. I forget which way round they are, but both have
a colour carrier around 3.58MHz. One is 625 line the other is 525 line. I think one was used for TV in South
America, did any home computers there use it? 

I doubt you would have to support system A (405 line) or system E (819 line), both AFAIK were only ever used
for monochrome signals. I can't think of a computer that would use them.

> > At the output of this section you had composite video and line-level audio. What you do with those is up to
> > you....

> And that's where the fun begins! The plan is to infer what color the vintage computer was trying to display at 
> any given pixel, with knowledge of the dirty tricks it used to get that color cheaply. Then cram that inferred pixel 
> into the frame buffer, and convert the video format on the other side of the frame buffer.

Sure. I think that is the interesting (and complex) part of the project. If you can get that right, you could even
just tell users to either tap the video off at the input to the RF modulator, or use an old VCR for the tuner/IF

Evil thought (and I have not worked this out yet). You are going to be connecting the RF signal straight from
the computer to this unit. Do you really need a _tuner_? You have essentially one strongish signal. What about
an untuned receiver and demodulator? At VHF totally possible, UHF might be a lot harder...


Mark J. Blair, NF6X <nf6x at nf6x.net>

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