From: shetline@bbn.com (Kerry Shetline)
Newsgroups: rec.audio
Subject: CD/LD jitter: I certainly do get it! (Was: Kerry doesn't get it...)
Followup-To: rec.audio
Date: 1 Jan 1993 03:16:21 GMT
Organization: BBN
Distribution: world
NNTP-Posting-Host: bbn.com

PART II
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Given the basic data stream stability provided by servo regulation, very
small buffers are quite sufficient to produce a regular clocked output. As
long as the drive slows down the disc as the end of its buffer space is
approached, (rather than waiting until the moment it is reached), overflow
is no problem. As long as the drive speeds up the disc as the buffer runs
low, (rather than waiting until it is empty), underflow is no problem. This
simple scheme guarantees (short of complete mistracking or intentional
cueing) that a steady, clocked device can always have data available to
transmit at a precise rate. The earliest and the cheapest CD players have
all had enough memory for this, partly because data has to be buffered for
error-correction. This takes two forms -- a group of data words (96 bits?)
is collected and an error-correction code is computed and compared to a
code from the disc. If there is a mismatch, error correction or
interpolation is performed. Then and only then does the data go on to the
DAC (or out the coax or optical digital port).

In addition, data words are interleaved -- that is, the physical word order
is different from the logical word order. This makes it less likely for two
adjacent data points to be simultaneously disrupted. The effect of this is,
however, that if a CD player ever were to become so desperate that is was
shoveling data straight off the disc into the DAC, the data would not only
be jittery, it would be in the wrong order! Not to mention all of the
subcode data, sync bits, and error-correction data that there wouldn't be
time to filter out. (Need I bring up 14-to-8 modulation?) You wouldn't hear
anything but a very ugly noise!

In other words, if you are hearing CD (or LD) digital audio, and it sounds
anthing like it should, the sound must, by the very nature of the process
that makes it listenable, be as free of jitter as any quartz-crystal timed
device can make it. The claims that Theta makes about its mechanical
transport qualities can not be responsible for any perceived improvement in
sound quality beyond an obvious lack of mistracking, clicking, and outright
cacaphony. Note, I do not contend that the Theta doesn't sound better (I do
doubt it, however). I merely claim that THE REASONS THETA GIVES FOR
IMPROVED PERFORMANCE ARE UNSUPPORTABLE.

In article <1992Dec31.093238@trc.amoco.com>, znpt01@trc.amoco.com (Norman
P. Tracy) wrote:
  ...
>many causes.  Anyone at all interested in this topic NEEDS to get the latest
>issue of Stereophile and carefully read the Robert Harley article measuring
>jitter levels in a variety of products.  And then order the AES reprints he
>references to learn even more, I know I will be.

I have found Stereophile interesting to read in much the same way I find
tabloid headlines entertaining at the supermarket. I may be occasionly
amused, but I'm still not going to let either of them make a dime from me.
I might be convinced to look up the article you mentioned in a borrowed
copy, however.

> or two to be learned.  Continuous improvement rules.

Continuous snake-oil rules as well. The need always exists to know the
difference.

-Kerry