Path: utzoo!attcan!uunet!super!udel!rochester!cornell!mailrus!purdue!decwrl!nac.dec.com!plouff
From: plouff@nac.dec.com (Wes Plouff)
Newsgroups: comp.sys.amiga
Subject: Re: VBlank interrupt servers
Message-ID: <8809131559.AA01218@decwrl.dec.com>
Date: 13 Sep 88 18:51:00 GMT
Organization: Digital Equipment Corporation
Lines: 25

Steven Vermeulen (svermeulen%Janus.MRC.AdhocNet.CA) writes:
>I am busy writing parts of the next AMUC Demo...
> 
>As part of this I needed an exact, stable, time base that would track
>across 9 Amigas for an extended period of time without drifting
>between machines.  To do this I am proposing to use the VBLANK signal
>and essentially keep a count of all the VBLANKS that occur.
> 
>First question:  how much drift between VBLANK counts between machines
>am I likely to see over 15 minutes, 1 hour, 8 hours?

You can get the drift to zero by genlocking Amigas 2 through 9 to 
Amiga 1, or all of them to a commercial television station.

A little back-of-the-envelope math says that if VBLANK counts occur 
every 1/60th second, and the Amiga clock oscillator has a tolerance of 
+/- .005% (just guessing here), then any two Amigas can drift a maximum 
of 2 x tolerance, or 21.6 counts per hour.
-- 
Wes Plouff, Digital Equipment Corp, Littleton, Mass.
plouff%nac.dec@decwrl.dec.com

Fantasy TV ad ending: "Except for my voice, every image and sound in
this commercial was created or processed by an Amiga.  Write us, and
we'll tell you exactly how we did it."