23 May
2015
23 May
'15
9:57 a.m.
On May 23, 2015, at 09:25, tony duell <ard at
p850ug1.demon.co.uk> wrote:
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.
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.... 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 <nf6x at nf6x.net>
http://www.nf6x.net/