[linux-dvb] Re: Help with J2 RGB

[Ragnar Sundblad <ragge@nada.kth.se>]

--On den 30 april 2004 07:42 +1000 Michal Dobrzynski 
<michal_dobrzynski@mac.com> wrote:

> All other VGA cables I have appear to
> not be specially shielded (they are too thin and flexible) and certainly
> wouldn't have separately shielded R, G and B.
I think they all have individually shielded R, G and B coaxes.
A thinner cable may have higher risk to only have foil
screens which may be tricker to connect.

> They do not sell such cable
> here either and in any case I could actually check the construction of
> this cable BEFORE cutting it up so at least I knew it was the type of
> cable I needed rather than gambling by buying expensive VGA cables just
> for cutting up.
The idea was to use a spare VGA cable instead of bying expensive
special and possibly bad ones like SCART cable. There is
absolutely no need to use VGA cables. Besides, VGA cables are
often very cheap (when compared to "high end" scart cable).

> J2 ground or SCART ground? Or both as I have?
You typically connect it to ground in the J2 end and shield
in the scart end.

> > I think that if you don't use a buffer (amplifier), you should
> > rather serial terminate the signal, like so:
> >
> > <J2>-----<75 ohm resistor>-----<cable>-----<scart connector>
> >
> > I am not sure about this, comments are welcome!
> So you think the termination should be applied at the J2 end rather than
> at the SCART end?
I still haven't found a specification for the AV711x outputs,
but they seem to be current controlled which would mean that a
serial termination like this is good for protecting it at a
short circuit and for minimizing reflections in the cable.
I am not sure about this though since I haven't found the
specs, and I haven't tried it myself.

You can also skip them, just make sure that you don't short
circuit it. In any case, I am pretty sure that you _don't_
want to have parallell termination in the scart connector,
so I would cut those and try again.

> > This is also a common way to connect a buffer amplifier.
> I still don't really understand what these buffers are or how they work.
The buffers are amplifiers, often either 1:1 amplifiers or
2:1 amplifiers for use with serial termination like above
(these are normally voltage outputs and not current outputs
like the AV711x _may_ be).

They may be good to use after a filter circuit since the
output (or input) impedance of a filter may not be 75 ohms.
The filter circuit that someone pointed out is probably good
for instertion also without buffers.

They are also hopefully better protected than the AV711x
so that you don't fry them as easy, and if you fry them
anyway they are cheaper to replace.
They aren't strictly speaking needed since the AV711x can
drive a cable itself.

The filter curcuit is for cleaning up the signal after the
D/A conversion process. You can build this on a piece of
circuit board, cramming it into the scart connector is
probably very tricky.

Note - I haven't tried my J2 connector, I am still waiting
for parts to arrive. You should better listen to those that
have tried it. But since you mentioned the VGA cable
thing which was my tip I felt I head to answer.

Again - be careful so you don't fry your J2 output. If you
do you may very well loose you composite output too since
they use the same pins.

/ragge



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