Date   : Thu, 16 Dec 1999 10:12:05 -0000
From   : "Fraser, Colin J" <Colin.Fraser@...>
Subject: Re: Video output on Master

> -----Original Message-----
> From: Tim Fardell [mailto:timf@...]
> Sent: 16 December 1999 09:49
> To: John Simpson
> Cc: bbc-micro@...
> Subject: Re: [BBC-Micro] Video output on Master
> 
> 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 SCART connections are described nicely at
http://www.par.univie.ac.at/~bob/video/scart/scart.html
S-video uses the composite pin for luminance, and the red pin for luminance
signals.

Composite video is just a mix of the luminance and chrominance signals -
except in the BBC, where the standard video output carries luminance only.
They occupy distinct frequency bands - a composite video input has to be
filtered to separate the two signals for decoding.
The filtering inevitably introduces some distortion and 'ringing' in the
signals.
This causes noticeable effects on the screen, especially where strongly
defined edges are present eg a typical BBC micro game!

For this reason, S-video looks much better than composite video.

The signal that is coupled into the composite video output on the BBC by the
capacitor mod is the chrominance signal, presumably on its way to the UHF
modulator.
It should be possible to wire up a S-video output on the BBC that will
provide a high quality display on any TV that handles S-video.
I suspect more TVs have S-video inputs than RGB.

Also using S-video will save any worry about compatibility of RGB levels
(TTL on a BBC, 0.7v max on SCART RGB)
Next time I'm in Maplin, I'll buy a 4 pin mini din socket for my Beeb, and
report back if I get S-video output working.


Colin f