8-bit Computer TV Channel Use

[tony duell]

> Handling the VHF/UHF tuner economically may be another sticky point. Maxim makes a tuner chip that's available 
> at Digi-Key, but I refuse to design Maxim parts into anything on account of off-topic reasons. Mouser has stock of 
> a very inexpensive ST tuner chip that looks very promising, but the full datasheet isn't openly available. I need to 
> contact ST to see if I can talk them out of it. Their site mentions an NDA for the eval board, so it might be tough, 
> particularly since my intention would be for my design to be open to allow off-label uses.

What I say below covers how it was generally done in the UK/Europe. No idea about elsewhere.

Building a UHF tuner (even assuming you have a working design) is non-trivial. To give you some idea, the 
inductors are often straight metal strips, layout and length is critical. If you bend one, you throw the tuning
way off....

In the days of analogue TV, a lot of manufacturers had a tuner module. This was not regarded as field-
repairable. Some companies (e.g.  Philips) published data sheets on said tuners, and of course you could
buy the tuner as a spare part for the TV. It was typically a flat metal box with an aerial socket on the side. It took
a 12V supply and a tuning voltage (normally 0-33V, applied to varicap diodes in the tuner). It outputed an IF
signal (the receiver was a superhet, of course) around 35-40MHz. To control the tuning you either used a 
votlage stabiliser and a potentiometer (or several that could be pre-set for various channels and switched
between) or a frequency synthesiser type of circuit, the local oscillator from the tuner (divided down in
a lot of cases) was brought out for this. I remember a chip called the SAB3035 CITAC (Computer Interface
for Tuning And Control) that was used here. 

The output of the tuner fed a suitable IF filter (a SAW device was about the easiest to use) then the IF
amplifiers (video and audio) and then detectors. There were ICs available for this. If you chose wisely the
only adjustment was the quadrature coil in the FM audio detector (in Europe, video was AM, audio FM on
analogue TV). Making the IF strip and detector was certianly possible, you had to do it on a ground-plane
PCB, and take some care with the layout, but it wasn't that bad. At least one UK company sold a kit for
this, I built a couple of them and they worked first time. Again, Philips were one of the companies who
made ICs for this, and data sheets were easy to come by.

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.

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

[As an off-topic aside : Philips (again) sold a range of solderless educational electronic kits, rather better
than the 150-in-1 ones normally encountered. I only had the basic kit and a couple of the add-ons when
I was a kid, they let me make radios (including superhets), Wien bridge oscillators, etc. But I wish I could
have afforded the EE1007 and EE1008 (manuals are on-line in German). They culimated in making a monochrome TV, yes, the tuner and IF strip were ready-built modules, as was the CRT PSU, but you still got to do a lot 
yourself and, learnt a lot in the process. Perhaps looking at said manuals will let you know what you 
up against.]

-tony