NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/electron/13430

Internet Message Format | 1992-07-27 | 3.4 KB

Xref: sparky sci.electronics:13430 rec.audio:10911 alt.folklore.computers:11257
Path: sparky!uunet!munnari.oz.au!yoyo.aarnet.edu.au!sirius.ucs.adelaide.edu.au!cs.adelaide.edu.au!francis
From: [e-mail] (Francis Vaughan)
Newsgroups: sci.electronics,rec.audio,alt.folklore.computers
Subject: Length of CDs (was Re: Life after CDs)
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Date: 28 Jul 92 01:50:40 GMT
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The subject of the variation in length of CDs seems to
crop up with regularity.

CDs are played using a focussed laser beam with an effective
beam spot diameter of 2um.  Effective because at these sizes
there is no edge, rather the intensity drops to a level at
which energy outside the defined spot diameter is low enough
that the signal to noise is good enough to recover the data.
There are diffraction rings of significant amplitude outside
the spot.

The standard track pitch for a CD is 1.6um.  The tracks
themselves are 0.5um in width.  This gives the laser beam spot a
reasonable margin between tracks.

The tracks themselves are made of pits and bumps, the pit
being one quarter of a wavelegth of of the light used in height.
The lasers used are 780 or 790 nm, and with a refractive index
of 1.55 for polycarbonate this yeilds a wavelength of ~500nm
and a pit depth of 125nm.

The pits encode the data by their length.  The coding is a 8/14
modulation that is encoded onto the CD with a non return to zero
inverted modulation that results in pits with a minimum length
of 3 times the base clock rate (T) and a maximum size of 11 time
T.  The actual physical length of the pits is limited by the
optical system, which must be able to detect the length of a pit
or bump reliably.

CDs play with a constant data recovery rate.  To make the best
use of the storage the CD the data is encoded with a constant
density, requiring that the CD be rotatated at a different rate
depending upon where on its surface it is being played.  Disks
are in general encoded so that the effective velocity of the
track is 1.4 metres/second.  Thus they vary in rotation speed
from 200 to 500 RPM.

CDs may be encoded with a linear velocity of anything between
1.2 and 1.4 meters/second.  This results in pits with a minimum
length of 0.833um if a velocity of 1.2 M/s is used.  CDs may be
recorded with a playing time of up to 74 minutes and 33 seconds
with the slower velocity.

However it is also possible to drop the inter track gap.
Dropping the spacing puts greater strain upon the tracking
system (which must be able to keep the beam on the same track)
and also increases the noise.  However by doing this CDs with
playing times in the vicinity of 82 minutes can be made.  Older
and poorer quality players can have problems with these CDs as
they are really not quite within spec.

Finally, CDs play from the inside to the outside because the
optical system needs to get a lock onto the track so it can
focus the beam.  The CD surface is the most stable in the inside
(where it is supported) so the system stands a better chance of
getting its initial lock.


                    Francis Vaughan