> On May 23, 2015, at 09:25, tony duell <a...@p850ug1.demon.co.uk> wrote:
> 
>> 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....

Yup. I'm a radio amateur and an electrical engineer working with GPS stuff, so 
I understand the pain! But nowadays, a VHF/UHF tuner is a single IC, possibly 
surrounded by a small handful of fixed inductors. The problems are:

* Might not be able to buy it in small quantity.
* Might need to read Chinese to understand the datasheet.
* Datasheet might simply be unavailable to individuals, even if the part is 
available.

I haven't begun trying to crack that egg yet, but there may a successful path 
there. With modern parts, even a dumb digital designer like me can successfully 
design RF front ends operating at 1.5 GHz, and cram an entire GPS receiver onto 
a fingernail-sized PCB. It's black magic if somebody makes the right chip. And 
it's still do-able if they don't make the magic chip... it's just a lot more 
expensive than consumer electronics have conditioned us to expect.

I think there used to be a rule of thumb for microwave work to not try for over 
10dB of gain per inch of circuit, or something like that. But now, slapping 
down a 20dB or greater gain LNA circuit the size of a barley corn is no big 
deal. Being able to make the whole circuit a lot smaller than a wavelength 
eliminates a lot of the pain.

As an example, Mouser carries a single-chip tuner for $2.32 at quantity 1. It's 
by Silicon Labs (I think I mistakenly states ST earlier). But only a short-form 
datasheet is available openly, and I need to contact SiLabs to see if I can get 
a full datasheet. I also don't know whether this analog TV tuner chip will 
remain in production, vs. Mouser buying some TV manufacturer's leftover parts.

> 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.

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 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!

> 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.


> 
> 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.

-- 
Mark J. Blair, NF6X <n...@nf6x.net>
http://www.nf6x.net/

Reply via email to