[linux-dvb] Re: DVB-S Rev1.5 RGB out nightmare

[Jon Burgess <mplayer@jburgess.uklinux.net>]

Emil wrote:
> I mean between red and ground, green and ground, blue and ground.

I haven't tried wiring up this RGB lead myself but I've done a fair bit 
of work with ananogue transmission line theory which explains why this 
termination is required to get a good signal.

I believe the answer is a series termination, not a parallel one. You 
should try adding a 47 Ohm resistor in series of each of the R, G & B 
lines. For each of the R, G & B signals, connect one end of a 47 Ohm 
resistor to the pin on J2, and the other end to the wire leading to the 
SCART connections. Do not connect anything between the signal and ground.

This should reduce the the ringing on the signal and get rid of the 
ghosting which appears when displaying on sharp vertical lines.

The composite/luminance pin should also have a series termination, buth 
this pin should not be required in RGB mode. I think i've seen a TV 
which got confused if it was presented with both signals at once. Try 
disconnecting this pin and see if it helps.

Try to make the leads as short as possible. Make any long leads from 
proper 75 ohm coax cable.

You might ask why 47 Ohms and not 75. The answer is that the chip 
driving the pin probably has a source impedance of around 20 ohms, so 
adding another 47 ohms should make the combined resistance close to the 
75 ohms which is the ideal value for driving the cable with least signal 
distortion.

The usual caveats apply, use this information at your on risk.

   Jon






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