Date   : Mon, 23 Apr 2007 10:03:30 +0100
From   : RS432@... (Mick Champion)
Subject: BBC to VGA monitor converter

Sprow wrote:
>
>  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.
>
>   
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?

Cheers,

Mick