Date   : Mon, 23 Apr 2007 07:36:11 +0100
From   : info@... (Sprow)
Subject: BBC to VGA monitor converter

In article <1177303782.17612.1.camel@...>,
   Thomas Drage <drage@...> wrote:
> What is the difference between the RGB output on a BBC and the VGA
> standard used by computer monitors?

Mainly signal level (assuming the VGA monitor can handle PAL line and field
rates), the BBC outputs 5V TTL logic compatible signals - even on the sync -
and VGA monitors expect to see 1V p-p terminated into 75 ohm.

There are a multitude of pages about changing the level either using diodes
or resistive dividers.

There's some hand wavey stuff at
 http://www.sprow.co.uk/bbc/howto.htm#vgamonitor

In article <462BDAC2.2090503@...>,
   Mick Champion <RS432@...> wrote:
> I have found the composite output from my BBC B's to be very poor 
> compared to the RGB output. The Master seems to have a 
> far more stable comp output. I wonder why?

Try removing the colour burst link on the BBC B so you get black and white
output on composite, if that radically improves the output you know where
the problem lies.

The Master mixes its chroma inside the chroma chip, whereas the BBC B mixes
it just before the picture leaves the building. The result is the BBC B
barely outputs what you could describe as PAL - it's certainly on edge.

I did look about 18 months ago at making a beeb RGB to DVI converter, which
included a small framestore to allow the output rate to be much faster so
that the monitor didn't need to support PAL rates, plus it could be a shiny
all digital link. 
Like a lot of ideas I decided not to persue it, I seem to remember I was
missing a crucial signal (I can't think what now) which would have required
a flying wire into the case which would put a lot of people off,
Sprow.