Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!uwm.edu!microsoft.UUCP
From: brianw@microsoft.UUCP (Brian WILLOUGHBY)
Newsgroups: rec.audio.high-end
Subject: Re: Armor-Alling CDs
Message-ID: <2572@uwm.edu>
Date: 24 Feb 90 13:51:23 GMT
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Approved: tjk@csd4.csd.uwm.edu


In article <2375@uwm.edu> jas@proteon.com (John A. Shriver) writes:
>The "it's only one's and zero's" argument doesn't quite hold.  It
>would (perhaps) hold if there was a seperate clock signal on the CD.
>There is not a separate clock however, the signal is "self-clocking",
>like a local area network.  The clock has to be restored from the
>edges of the bits.  If the transitions are sloppy, the clock recovery
>will not be perfect.  As Keith Johnson explained in the interview in
>The Absolute Sound, 100 picoseconds of jitter (timebase error) is
>roughly equivalent to a one bit error in amplitude.

I can't let this go by without comment.  You can't have "one
bit error" when reading from a CD.  There is an error correction
layer which detects errors and attempts to correct them.  If a CD byte
were one bit off, then the player would refuse to convert this byte.
It would instead either correct the bit perfectly or generate a new
value by interpolation.  You'll never be one step off in amplitude
because of anything incorrectly read from the CD (unless the
interpolated value is coincidently very close ot the adjacent value).

Besides, the designers of the CD medium have already taken the "self-
clocking" aspect into consideration.  Each 8 bit data is actually
written onto the aluminum platter as 14 bits.  Thus a "one bit error"
has no correlation whatsoever to the data value.

Thirdly, the samples are 16 bits values and are stored as pairs of
8 bit bytes.  Thus, if Mr. Johnson's explaination were applicable
to the CD medium, then you would have a 50% chance of getting an error
of 256 amplitude steps!

>(I presume he
>assumes some typical rate of change.)  Needless to say, 100
>picoseconds is a damn small time.  (The 350 MHz Tektronix 2564A
>oscilloscope here only goes down to 5 nanoseconds/centimeter.  You
>would need a sampling scope to see it.)  The jitter is obviously
>greater than this.  Anything that reduces it will help.

Who cares?  You can't hear this!  Why do you think digital is better?

>Another thing the Armor All might do is reduce the surface roughness
>of the CD.  One very interesting point that I learned from a series of
>articles on how CD works in Hi-Fi News and Record Review is that the
>surface roughness of the typical CD is 10 times greater than the depth
>of focus of the laser.  Needless the say, the laser focusing servo is
>working real hard all the time.  Since most players use the same power
>supply for the laser focus as for everything else (digital, DAC, and
>analog), and the laser servo is probably not Class A, the laser servo
>is putting lots of crud into the power supply to contaminate the other
>stages.  This is the same basic argument of why damping rings, etc.,
>work. 

The CD is designed such that any (small) particles on the surface
are actually out of focus to the laser, which focuses on the aluminum
platter inside the relatively thick plastic encasing.  Large particles
can totally block the laser, but small particles can be "seen around".

Generally what happens in most CD players in that the disc itself
resonates and moves closer to and further away from the laser.  This
causes many players to stay in the error correction mode all of the
time.  Damping rings reduce the resonating of the CD, but any decent
CD player would have a damping clamp built into the unit (i.e. even
Pioneer had a unit with built in damping before the gimmicky adhesive
plastic shit showed up in yuppie stores).

Brian Willoughby
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