Date   : Thu, 16 Dec 1999 08:56:26 -0000
From   : John Simpson <Simpson.J@...>
Subject: Re: Video output on Master 

Taking on board various people's suggestions on how to link a BBC to my
telly using either the RGB or Video ports, I had a good look at my TV last
night and at my Beeb. Perhaps inevitably I now have some further questions
(sorry about this), and I'm afraid they are of an electronically naive
nature. If they're off-topic for the mailing list I apologise, please email
me directly if this is the case.

My telly has 2 Scart sockets and a "video in" socket on the front. The
manual indicates that the 2 Scart sockets are not identical though: AV-1 has
RGB inputs whilst AV-2 does not. Presumably then, if I wanted to use RGB I'd
have to move my existing video-to-TV cable onto AV-2. I noticed that the
AV-2 port has luminance and chrominance pins as well - does this mean it is
equivalent to the "video in" port?

The manual does not disclose whether the RGB signals can be TTL voltages, so
I'd be a bit nervous of trying this out - could the TV be damaged?

Finally I got out the BBC User Guide and had a look at the circuit diagram
for the display output hardware. As far as I could tell, the circuit
combines the Red, Green and Blue signals into one voltage (I assume that
this gives rise to the term "composite video"?) and outputs it to the
central part of the socket. What baffles me is previous posters'
explanations of how this port can give a colour signal. How this might work
is not obvious from the circuit diagram, because the Red Green and Blue
components have been combined. 

I realise that an understanding of this is not necessary in order to be able
to use it, but I'm curious. 

john