[linux-dvb] Re: Help with J2 RGB

[Michal Dobrzynski <michal_dobrzynski@mac.com>]

On 30/04/2004, at 12:37 AM, Ragnar Sundblad wrote:

> --On den 29 april 2004 20:05 +1000 Michal Dobrzynski 
> <michal_dobrzynski@mac.com> wrote:
>
> > 10. About 40 hours ago I started to attempt to make my own J2 to SCART
> > socket. Destroyed a perfectly good (and unique) VGA cable based on 
> > tips
> > on here. The entire cable is shielded (is this ground?) with the R, G 
> > and
> > B wires independently shielded (is this R, G and B ground?).
> Unique VGA cable - if you needed it, why did you use it for this? :-)
Yes indeed. Why? I guess I imagined that I could simply reconnect the 
connector making a shorter cable. But after all the difficulty of 
soldering I fear even trying it. It was so neat before and I have no 
hope of getting anything close to it. 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. 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 shield is only ground if you connect it to ground, but that
> is normally a good idea.
J2 ground or SCART ground? Or both as I have?

> > At the J2 end I connected the ground to the shielding and each of the 
> > RGB
> > grounds to the shielding also. At the SCART end I connected a 75 Ohm
> > resistor between R, G and B and between their respective shielding 
> > (all
> > connected to ground at J2). R, G and B as well as the composite sync
> > signal and left and right audio are connected from J2 to the SCART 
> > socket.
> 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?

> This is also a common way to connect a buffer amplifier.
>
> Also, as you hopefully know, you must be very very careful if
> you don't use buffers, as these outputs seems to be very very
> easy to fry if you for example happen to connect the signal
> before the ground.
I still don't really understand what these buffers are or how they 
work. I have noticed what appears to be a burnt out trace on my FF 
DVB-S card but I don't know whether it was always there or not and it 
doesn't appear to affect the cards performance. The trace leads to one 
of the connections to the PCI slot.

> /ragge
Thanks for your input. Much appreciated.

Regards,
Michal



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