Date   : Mon, 23 Apr 2007 14:43:22 +0100
From   : dm.hunt@... (David Hunt)
Subject: BBC to VGA monitor converter

> >> 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.
> >
> >   
> Thanks for that explanation Sprow. Having read your VGA 
> documentation, I think perhaps the easier solution would be 
> to build a stable well mixed RGB to composite converter then 
> and avoid the make shift one inside the B. Then use the 
> better composite in the scan converter I already have.
> 
> I see the Motorola MC1377 with two other chips and a few 
> components converts baseband RGB to composite NTSC/PAL. It 
> does require vertical and horizontal sync though which you 
> say in your documentation needs a tap onto the 6845 or the 
> use of a LM1881 circuit.  I found this Atari circuit by doing 
> a google. 
> http://www.preromanbritain.com/gwem/martbean/ataridiy/conv.htm
> The above circuit is for NTSC. The device itself can do 
> either. I think?? Perhaps the MC1377 *P*  version is for PAL 
> but I doubt it's that logical. I expect the crystal value in 
> the circuit would also need to be changed. Any ideas on what 
> would need changing before I get my vero board out and start 
> getting the components?

Have a look at http://www.datasheetarchive.com/datasheet.php?article=2247629

There's a circuit on page five for RGB->PAL/NTSC and it explains the
component differences for NTSC/PAL too.

Unless you're going to use a DC-DC converter, you're going to need to take
12v from the aux port.

I believe the Beeb outputs composite sync already, so you don't need to use
a sync separator to use this IC.

Cheers

Dave ;)