Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!aurora!labrea!decwrl!sun!pepper!cmcmanis
From: cmcmanis%pepper@Sun.COM (Chuck McManis)
Newsgroups: comp.sys.amiga
Subject: Re: Multisync interlace problem
Message-ID: <40648@sun.uucp>
Date: 1 Feb 88 22:28:03 GMT
References: <683@esunix.UUCP> <2582@swan.ulowell.edu>
Sender: news@sun.uucp
Reply-To: cmcmanis@sun.UUCP (Chuck McManis)
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Organization: Sun Microsystems, Mountain View
Lines: 43

blgardne@esunix.UUCP (Blaine Gardner) wrote:
> NEC Multisync monitor ... doesn't display interlace correctly.

Then, in article <2582@swan.ulowell.edu> (Bob Page) writes:
> Most of the multiscan/sync type monitors have problems with interlace.
> In fact, I think either NEC's or Sony's even says so.

Putting on my video hat for the moment, when you think about this for
a moment the problem becomes self evident. (Well, it did to me, and it
might to you too if you have read the Amiga Video tutorial :-)) The
answer goes something like this :

Monitors originally had fixed horizontal scanning frequencies, television
sets used 15,750 Hertz. What the Horizontal sync pulse does is tell the
monitor electronics that ``now'' is the time to start a new trace across
the front of the tube. Since the beam is moving down at the same time that
it is moving sideways, where the hsync pulse occurs determines where, relative
to the top of the screen or previous line, the new line will appear on the
screen. When the horizontal sync pulses are evenly spaced you get an even 
distance between lines. Those wizards that brought you Tv though, thought
up an even wilder scheme. They said "Gee if on the odd frames we make the
first horizontal sync pulse half as wide, then the lines from the odd frames
will appear *between* the lines of the even frames." And poof interlace was
born. And it was so, and this worked because the beam always traced across
the screen at the same frequency. 

Now, enter 'MultiSync' monitors. These monitors *vary* their own
horizontal frequency to match that of the computer. Very whizzy indeed
but what does that mean ? It means that there is a phase-locked loop on
the Hsync input. And that means that when the monitor sees this little
glitch in the horizontal sync pulse, which would start an interlaced
frame, it *tracks* it. Yes, that's right, it adjusts it self to get
'back in sync' with the computer. And when it does, by the next two
scan lines maximum, it has the display all lined up nicely again and is
probably feeling extremely proud of itself. And that means your interlace
pictures will probably look really weird on a good multisync. But of 
course, the FlickerFixer does indeed fix this and that's the end of the
problem.


--Chuck McManis
uucp: {anywhere}!sun!cmcmanis   BIX: cmcmanis  ARPAnet: cmcmanis@sun.com
These opinions are my own and no one elses, but you knew that didn't you.