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

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

*Michal Dobrzynski wrote:

*> I'm not sure I understood. Do you mean:
..
>And why it be bad to do it this way:
...

I'm not sure what is the difference between the two you mention. There 
is nothing particularly wrong with adding any extra cable, it is just 
that the longer it is, the more distorted the signal will become. I 
don't have time or space to go into all the signal theory involved. I 
would send you some links on the the net, but I'm at home on a dial-up 
link and don't have the time to search around for you.

Signal termination is a subject which takes some time to cover on 
electronic engineering degree courses, so it takes a bit of effort to 
explain or understand all the details.

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

I see nothing wrong in making an adapter which connects to J2 on one end 
and a SCART socket on the other. I would just advise that you should 
keep the wiring fairly short and tidy. About 10 to 20 cm should be OK.

> 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.


I'll try a little ASCII art, but I'm not sure i'll be able to get it 
across.

Series termination places the resistor "in series" with the output i.e.

Signal source---[Termination resistor]------- Signal wire of cable 
--------- Signal receiver (TV)

Ground--------------------------------------- Ground shield of cable 
------- Ground of receiver


Parallel termination places the resistor between the signal and ground, 
"in parallel" with the output cable:

Signal source-------------------------------- Signal wire of cable 
------- Signal receiver
              |
              --[Termination resistor]--
                                       |
Ground--------------------------------------- Ground shield of cable 
----- Ground of receiver


I have delibrately ignored any specific mention of cable plugs and 
sockets - I simplified the description to just assume that they merely 
pass the signal to and from the cable without effecting it in any way. 
In you real setup, you'll have sockets and plugs on both ends of the cable.

If you take a look at the wiring diagram on the vdrportal page you'll 
see how the wire from J2 pin 2 has a 47R resistor "in series" between 
the J2 pin and the pin 16 of the SCART. The R G amd B pins should also 
have a resistor in series with them (although the resaon why the 5V pin 
has the resistor in series is completely different).

It looks like the guy who put together the wiring diagrams on that page 
knew what he was doing. He is suggesting wiring a 75R resistor to ground 
in a parallel termination, and adds a few extra components which will 
act as a low pass filter. If you take his word for it, then you need a 
parallel termniation not a series one.

It all depends on the circuitry driving the J2 output. Some circuitry 
will be designed to include the termination already, others to work with 
parallel termniation, others with series termination. I can only guess, 
and I believe that series termination is more common.

> 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.

Both types of termination will approximately halve the signal level.

>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.

I don't think I understand what you've done, R G and B should not be 
connected directly to ground or to one another. Using a resistor to 
connect them to ground should not kill all the signal.

   Jon




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