From: paul@gaitlab1.uwaterloo.ca (paul j guy)
Newsgroups: rec.audio.high-end
Subject: Re: SPDIF jitter
Date: Thu, 12 Nov 1992 03:54:43 GMT
Organization: University of Waterloo, Waterloo, Ontario, Canada
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

In article <1drc7iINNl9r@uwm.edu> jas@proteon.com (John A. Shriver) writes:
>A friend asked me for a better explanation of the whole jitter issue
>in SPDIF encoding and decoding.  I decided that this is worth further
>distribution.
>
>The problem with jitter is in the SPDIF format.  You are transmitting
>two things down the wire.  One is the data, the other is the clock.
>For SPDIF, this is done with Manchester encoding (to the best of my
>knowledge).  In Manchester encoding, at the center of every bit time,
>there is a transition.  If that transition is low to high, that bit is
>a 1.  If that transition is high to low, that bit is a 0.  At the ends
>of the bit cells, there may be transitions to reach the right state
>for the next mid-bit transition.
>

....The modulation scheme used in cd players is 'EFM' or eight-to-
fourteen code. This is more efficient than MFM, the density ratio
(bits/transition) is typically 1.25, a very high figure. A good
description of this appears in "Principles of Digital Audio", a
Howard M. Sams book.

>
>(Manchester encoding is designed for reliably transmitting data in a
>self-clocking manner.  It is not designed to preserve the timing
>information reliably, only the data.)
>

   What has me puzzled about all this 'jitter' talk, is what audible
effect does it have ? I don't have any reliable or trustworthy articles
that show any clear audible effects of jitter. I can understand that
if the PLL is not quick enough, data errors will occur. In the absence
of hard errors, the digital data stream is placed in a local memory
(at least in the block diagram of a Yamaha YM3616) that buffers 2k bytes
of data. The error correction is done as the data comes out of this
buffer, and the corrected data is crystal clocked out to the over-
sampling filter (Yamaha 3404b). The oversampled, filtered output
runs into the d/a, also crystal clocked.
    So apart from distinct data errors, what will jitter cause? I
think I'm missing something.....
    The hi-fi rags blame damn near every audio evil there is, on jitter.
    I have heard a number of CD players that reduce jitter (LINN 
NUMERIK, Meitner C-lock, etc) and I'm impressed with the sound. But
is the superior sound due to other design considerations? Changing
the analog circuitry on my own CD player (NAD 5100) as per Audio
Amateur's latest issues made an enormous difference to the sound.
Putting in ridiculously fast amplifiers MADE A DIFFERENCE! I 
measured a 30-fold improvement of IM distortion going from a 20v/us
amplifier to the 2500v/us AD811. I used to be non-believer of audio
madness, but now I listen more carefully.
   The problem with the jitter issue, is that it is very difficult to
measure, and I don't know what to listen for, assuming that I can
cure or change the jitter of a cd player.
   The values of timing jitter that are talked about are a few 
nanoseconds for el-cheapo players, and less than a few hundred
picoseconds for the super-duper players. I find it difficult to
understand how such small timings can affect the sound.
   In desperation, I may set up a circuit on my bench to make a
'precision' jitter so that I can listen to its effects, but
hope that some kind person can clue me in as to what it should sound
like, and spare me the agony of breadboarding some whacky circuits.
(The circuit would consist of two RC circuits, fed by audio signals
180 degrees out of phase, and combined differentially. The
capacitor in the RC network would be a DC isolated varicap diode,
modulated by an external signal. As the varicaps change, the group
delay of the RC filter will shift. The differential circuitry will
cancel feedthrough of the modulating signal. And there are many more
potential bugs to be solved...)
   So tell me.... (flames if necessary) what does jitter sound like ?

   -Paul
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