[linux-dvb] Re: Help with J2 RGB

[Jerico Webmail <jerico@jerico.demon.co.uk>]

I haven't delt with the electrical principles of analogue video in some years,
but I'll try and remember as accurately as possible, its all SDI video around
here before we MPEG it. :)

RGB is rarely used professionally because its not very reliable and supprisingly
historically alot of equipment doesn't use it natively. Only with the rise of
SCART has RGB become more popular. Much better to use luminence based YUV
(YCrCb), its not so subject to timing errors (especially with PAL and SECAM).
It is correct to say that S-Video (YC) is about as good as it gets, because in
the case of MPEG, becasue of the 4:2:0 sampling method used in MPEG coding alot
of chroma detail is discarded and even with professional 4:2:2 coding you loose
about half the chroma. "Your only as good as your source material."

There should be little need to shield inside the connector, although as an
academic exercise we once made a SCART connector with matched length (quite
important) BBC PSF1/2 cable and BNC connectors on the other end. It made for a
respectable picture on a presentation monitor, and when combined with a set of
professional PMC monitor speakers fed from the SPDIF we nearly blew out some
windows with a Dolby trailor. PSF 1/2 is expensive, heavy duty and low loss
cable for broadcast use. If your really serious and need some lenght on the
cable there are a number of professional type SCART designed cables with all
the correct shielded cores. Sold in the UK/EU by 'Canford' (www.Canford.co.uk)
P/N: 36-901 at £4 per meter.

If you have access to S-Video then its best to use it, because it will preserve
the phase integrity and be more robust than RGB over distance. Canford also
sell a nice shielded Y/C cable.

The correct termination of RGB is tricky, it depends on what if the source is
expecting to drive into a 75ohm load and if the destination is already
terminated. If the destination is already terminated then double termination
won't do you any favours, but if you say you had too much level that could
indicate that with no termination you were driving it too hard.

By the way you need to ensure the cable you are using has a 75 ohms impedance,
even a decent RG-59B/U type RF cable is good for carrying video and is very
cost effective at under £0.50 per meter. BBC PSF1/3M is under £1 per meter and
BBC PSF1/2M is over £1 per meter.

The best tip for soldering is to remember that your putting the solder on the
wire, not on the iron. Don't put depostits on the tip and try and wipe them on
the braid, that won't help. make sure you put a good solid twist in the braid
before to form the wire. Then tin the cable, this involves heating the wire to
the melting point of the solder and then wipe the solder on the wire (no need
to touch the iron with the solder.

A decent fine guage self flux'ing solder is better than the 15 year old heavy
and abused fluxless solder that some of us still have in our tool kits. A nice
clean, fine tip on the iron is essential, have the sponge wet to clean off
excess from the tip. You can clean a iron bit using a fine guage sandpaper to
wipe off solid deposits when the iron is hot if your careful. Sometimes to aid
soldering in the beginning it does help to add just a touch of solder to the
tip to help conductivity and flux the tip.

I don't do alot of soldering here, but at home I use a gas torch for working
with DC and AC power, but for fine electronics I only use a mains iron with a
fine tip.

If anyone wants an old PAL to YUV converter I have a 1U rack mount rather
battered unit that might still work I am will to sell for nearly nothing and I
also have a PAL digital time base corrector that I have no use for (it might be
used to remove macrovision?).

Perhaps I should have cut this email in two!

Bob

--
The first person to violence is the last person to think.



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