Date   : Mon, 11 May 2015 00:24:45 +0100
From   : tom@... (Tom Seddon)
Subject: The mythical chunky graphics mode

On 10/05/2015 21:31, Julian Brown wrote:
> Our suspicion is that something's breaking the display-enable signal 
> coming out of the CRTC and fed into the video ULA via IC38/IC41: that 
> might mean that RGB data is being output whilst the sync pulses are 
> being emitted, when for correct operation it must be zero (i.e., 
> black). With the RGB output, it's not so important if those channels 
> are non-black, because the combined sync signal is separate from the 
> colour information anyway.
I just tried chunky Mode 4 on my BBC B with a UHF cable, and it worked, 
for whatever that's worth. (When I discovered this - probably getting on 
for 25 years ago now... - I was using a Model B, but I would only have 
tried it with RGB monitors.)

Your theory certainly sounds plausible... the 6845 timing properties are 
identical whether you're using ordinary or chunky mode 4, so there's not 
many other ways for things to go awry.

--Tom