Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!munnari.oz.au!comp.vuw.ac.nz!actrix!Bruce.Hoult
From: Bruce.Hoult@bbs.actrix.gen.nz
Newsgroups: comp.arch
Subject: Re: CD-ROM documents (was Paperless Office)
Message-ID: <1990Dec7.161340.4346@actrix.gen.nz>
Date: 7 Dec 90 16:13:40 GMT
References: <1990Dec5.105743.25693@actrix.gen.nz> <1990Dec6.154348.5206@d.cs.okstate.edu>
Sender: Bruce.Hoult@actrix.gen.nz (Bruce Hoult)
Organization: Actrix Information Exchange, Wellington, New Zealand
Lines: 38
Comment-To: norman@d.cs.okstate.edu

In article <1990Dec6.154348.5206@d.cs.okstate.edu> norman@d.cs.okstate.edu (Norman Graham) writes:

>I've seen a couple of postings like this, so I'll try to clarify things
>a bit.
>
>CD-ROM drives vary the rotation speed (AV) so they can _read_ the disk.
>This has nothing to do with the drive trying to maintain a constant
>data rate.
> 
>Let me explain: CD-ROMs don't actually store bits--they store transitions
>between bit runs. So the bit stream 00110000101110 has six transitions. 
>Transitions are recorded so that the _distance_ between transitions
>indicates how many bits are in the bit run.
> 
>Well, how do you measure the distance between transitions? With constant
>tangential velocity (or constant linear velocity (CLV)), the distance
>between transition A and transition B is a simple function of the time
>between transition A and transition B and the tangential velocity (which
>is a constant--remember?). Does the equation distance = rate * time 
>sound familiar?

Indeed it does.

The problem is that even in normal audio players, starting from the first
"track" and following the spiral to the end of the data, the linear velocity
doesn't actually stay exactly constant.  This would require that the drive
motor be infinitely controllable and infinitely accurate.

What actually happens is that the motor speed is fairly crudely controlled
and the data is read into a FIFO buffer, and clocked out at a precise rate.
That is why the wow & flutter for a CD is so much less than for an LP --
there is no way that the drive motors in the cheapest CD player are just *so*
much more accurate than the motor in a multi thousand dollar Linn turntable.

Have *you* heard of the term "phase locked loop"? :-)
-- 
Bruce.Hoult@bbs.actrix.gen.nz   Twisted pair: +64 4 772 116
BIX: brucehoult                 Last Resort:  PO Box 4145 Wellington, NZ