Date   : Thu, 19 Jan 2006 18:22:18 -0000
From   : "Mike" <profpep@...>
Subject: Re: Master to SCART

Thanks to Sprow for pointing out a mistake in my earlier post. Sync is into
a composite input, thus the sync component is only  0.3V. Thus my resistor
value for sybc is wrong!

Another look at the diagram made me realsie  need to make another point
too - the BBC sync output comes out of the end of a 74LS86 - not buffered
like the RGB's. LS TTL can sink quite a few mA, but can only source 800uA,
and even 0.3V across 75R needs 4mA! I think a sync buffer is in order here,
though 1 transistor and a couple of resistors
should fit inside the SCART. I wonder if the originator of the diagram had a
TV with a high Z sync input?

It's one of those things that seems to depend upon the set. The RGB's can
source enough current for the 75R termination, the sync output cannot, and
will be running limited by it's own output impedence, which might cause the
chip to fail eventually. TTL input, as on a Microvitec or a Philips CM8833
is no problem. It is only a bad case running into a properly terminated
SCART.

Ideally, I think what is needed is a lead with a small box in the middle,
with some diode clamps to the RGB's and a sync buffer. Putting a variable
voltage on the clamps would allow the user to match brightness with their TV
set. A switch could be added to allow change back to TV without unplugging
or turning the BBC off.



Mike