Date : Thu, 16 Dec 1999 09:49:05 +0000
From : Tim Fardell <timf@...>
Subject: Re: Video output on Master
I don't know how many replies you've already had to this, so please feel
free to delete this one if you like!
John Simpson wrote:
>
> 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.
I don't think it's off-topic at all. I'm all ears...
> 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?
Yes, if you want to use the BBC's RGB output, you will need to connect
it to your AV-1 socket, which supports RGB signals. You shouldn't have
any problems with connecting your VCR to AV-2, but (unless you have an
S-VHS VCR) you may need to check that the input type for AV-2 is set to
PAL Composite Video, rather than S-Video or S-VHS. This setting is
probably accessible via your TV's menu system, if it is a relatively
modern set. (no damage will occur if the setting is wrong - you will
just see a black-and-white picture). The luminance and chrominance pins
relate to S-Video signals, which will only be of use if you have a
'high-band' VCR (like S-VHS) or something like a DVD player that squirts
out S-Video. I think that one of the S-video pins doubles-up as the
composite video pin on the SCART plug.
> 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?
It is possible that the TV could be damaged if it is not designed to
accept TTL input, so I would check with the manufacturer before trying
this out.
> 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'm no expert on video, but you are right that the composite video
signal contains all the picture information in a single signal. My
understanding is that a small section of the video waveform is reserved
for a short pulse containing all the colour information for the current
scan line. If this pulse is not present or suppressed, a black-and-white
picture is seen. This 'colour-burst' signal is present in the BBC for
the purpose of driving the RF modulator, but for some reason it is not
fed to the video out socket. By adding the capacitor to the circuit, you
are effectively adding the colour-burst signal to the existing
monochrome video signal, effectively by joining the wires together (the
capacitor filtering out other unwanted parts of the source signal).
>
> I realise that an understanding of this is not necessary in order to be able
> to use it, but I'm curious.
Hope this is of some help to you, John. Let me know how you get on!
Tim.
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