23 May
2015
23 May
'15
10:41 a.m.
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
section.
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...
-tony
--
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/