From: summer@mullian.ee.mu.OZ.AU (Mark Summerfield)
Subject: CAV variable frame rate? (was Re: CAV vs CLV)
Organization: Computer Science, University of Melbourne, Australia
Date: Mon, 28 Sep 1992 22:37:39 GMT

Here's one thing I didn't see mentioned in any of the articles on CAV vs CLV
here. This may be because it's not true :-) Can anybody independently verify
this? Here goes:

According to an article in a local newsletter recently, CAV discs can be played
at any frame rate, i.e. they are not restricted to 25/50 frames/sec PAL and
30/60 frames/sec NTSC. The article claimed (on the basis of stopwatch trials
with a computer controlled LD player playing a defined number of frames from
a CAV disc) that some Criterion CAV titles actually ran at 24 frames/sec.
This is the frame rate used for film, of course. The implication of this
is that CAV discs may be mastered direct frame-for-frame from the original
film, the reduced processing resulting in better picture quality. If true,
this would give CAV (particularly letterbox format) a distinct advantage
of CLV, since there would be virtually no processing of the image in
transferring to disc.

The article claimed that "most" modern TVs would have no problem with this
since they sync automatically to the incoming frame rate. Personally,
I don't belive this -- *I* would certianly know it was happening (if I
had any CAV discs to try it on :-). My (Hitachi) TV (remembering I'm
in a PAL country here) syncs perfectly to 50 frames/sec on all its
RF channels, but rolls if fed the 60 frames/sec PAL-colour converted
NTSC from my LD player. A vertical hold control would probably fix this...
if it had one. Instead it has a (undocumented!) AV channel which syncs
perfectly to 60 frames/sec (and a good thing, too!), but rolls if
fed the 50 frames/sec PAL video from my VCR! This means I have to select
a different channel depending on whether I'm viewing a PAL or NTSC LD
(which is more convenient than playing with a vertical hold control,
but less convenient than having a more expensive TV which would do the
job automatically). It also means that if I stuck in an NTSC disc
running at 24 frames/sec, I would probably be surprised to find that
I had to select the channel I normally use for watching PAL to get
it to sync properly.

What I'm trying to say is that many people with TVs that cost them less
than an arm-and-a-leg (or just with older TVs) would be aware that some
LDs were running at a different frame rate, and some of them would
not be able to watch the discs at all without the image rolling.

Can any body out there confirm or deny any of this?

Mark
          --------------------------------------------------------
              Mark Summerfield,  Photonics Research Laboratory
Department of Electrical and Electronic Engineering, University of Melbourne  
                ACSnet[AARN/Internet]: summer@ee.mu.oz[.au] 
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