Date   : Tue, 16 Dec 1997 20:52:52 +0000
From   : Mike Tomlinson <jasper@...>
Subject: Re: How do I make Beeb video stable on 60 Hz?

In article <199712161656.QAA01508@...>, Arlen Michaels
<amichael@...> writes

>Can anyone help with this one?  My BBC Model B makes a slightly jittery
>display when I connect my RGB monitor.  The Beeb was originally used in
>Scotland but now I'm running it here in Canada where we have 60 Hz mains
>power, and I suspect there's a 50 Hz timing dependency that I've got to
>change. Is there a software or hardware patch I need?

The mains frequency won't make any difference when running on a RGB
monitor.  What you are probably seeing is the Mode 7 interlace, which is
worse on some monitors.  Some top-of the range CGA monitors even had a
control to reduce the jittery effects caused by interlace.  If you type

*TV 0,1
MODE 1

does the jitter stop?

You can turn off interlace in Mode 7 with the following:

VDU 23,0,8,144,0,0,0,0,0,0,0,0,0,23,0,9,9,0,0,0,0,0,0,0,0,0 

but that spoils the nice rounded Mode 7 font.

-- 
Mike Tomlinson