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

[Michal Dobrzynski <michal@appl.com.au>]

Thanks Jon,

I'm not sure I understood. Do you mean:

J2 red -> resistor -> cable -> scart connector red
J2 green -> resistor -> cable -> scart connector green
J2 blue -> resistor -> cable -> scart connector blue

And why it be bad to do it this way:

J2 red -> short cable -> resistor -> scart socket red -> scart cable to 
tv
J2 green -> short cable -> resistor -> scart socket green -> scart 
cable to tv
J2 blue -> short cable -> resistor -> scart socket blue -> scart cable 
to tv

I ask because I am trying to make a J2 -> SCART socket adapter so that 
I can use off the shelf SCART cables.

Having described it those still look like what would be considered 
"parallel" termination but I can't figure out from your description 
what "serial" would look like. Mea Culpa. Could you maybe describe the 
series of connections verbosely? Thank again and Merry Christmas.

Regards,
Michal

P.S. I'm not perceiving any ghosting of vertical lines. It seems more 
like graininess introduced by the signals being too strong. It's as if 
the signal were being clipped. When I turn down the brightness the 
previously extremely white areas are still relatively bright while the 
rest of the screen is almost black.

P.P.S.

I just tried it and a bit as I expected it didn't work. After all if R, 
G and B are connected to ground you end up with wiring that connects R, 
G and B together. This resulted in a black screen though I could still 
hear the audio.

On 24/12/2003, at 1:42 PM, Jon Burgess wrote:

> 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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