From: znpt01@trc.amoco.com (Norman P. Tracy)
Newsgroups: rec.audio
Subject: Re: How can different digital cables sound different?
Date: 7 Jan 93 03:53:47 GMT
Organization: Amoco Production Company, Tulsa Research
Originator: znpt01@gpss43


In article <1993Jan6.151540.16595W@lumina.edb.tih.no>, ketil@edb.tih.no (Ketil Albertsen,TIH) writes:
> 
> Maybe you didn't hear my sarcastic voice... If the part of the signal
> chain chain following the digital cable (DAC, amp, speakers ect.) is
> not changed, the sound produced depends solely on the input. For a 
> digital signal, the input is nothing but 0s and 1s having no other
> qualities than being 0s and 1s. If the same 0s and 1s come from two
> different sources, but are the same 0s and 1s, the input to the
> DAC/amp/speakers is identical. Unless the phase of the moon has
> some effect on the electronics, the sound will be identical, too.
> At least if analyzed by neutral means. I mean, this is a fact -
> you can't argue a fact.
> 
> Then comes a certain group of audiophiles, saying like that scientist
> a while ago (I'll find a reference to the source if anyone cares!):
> "Yes, I know it is a fact, but at the moment I prefer not to take it
> into consideration". And they add that there still is a quality 
> difference of the sound that, although not supported by facts,
> nevertheless is audible to them. OK, I don't believe in them, but
> I don't believe in angels or ghosts, either.

Oh good, a man with the facts. Well some of the facts. Ok, half the facts.
Here are some more facts for consideration:

1. The point of the exercise is to reconstruct an analog signal, not just
move bytes from file to file or onto a display.

2. To reconstruct an analog signal from digital data two things are required,
the data (as in the above facts) AND a clock signal (not mentioned in above
facts).

3. In the S/PDIF serial audio data interface used to get data from CD, LD,
DAT, DCC, MD, and PC soundcards to external DACs or digital recorders the
clock is transmitted along with the audio data as part of the bi-phase mark
encoding used.  The absolute stability of this clock from the source varies
depending on how good the designers were, the parts are, and the adjustments is.

4. Popular S/PDIF receiver chips like the Yamaha YM3623B and Crystal CS8412
are NOT crystal controlled but rather recover the necessary clock from internal
Phase Locked Loops (PLL) locked onto the incoming data stream.  The simple
two pin can crystals often seen directly attached to 3623's and 8412's are
optional. The 3623 uses the crystal clock to quickly lock onto the S/PDIF
signal. The 8412 uses the crystal clock to deterime and display the sample
rate and jitter level of the S/PDIF signal.  Both parts ignore the local crystal
clock once locked onto the S/PDIF signal.

5. Better (i.e. more expensive) outboard DACs use additional tighter PLLs after
the receiver chip to further cleanup the clock.  Generally there is a trade off
between low jitter PLL and wide locking such that low jitter PLLs may result
in a DAC being unable to lock onto a high jitter S/PDIF input requiring the
use of a low jitter Class 1 source as defined in the 'Red Book' spec. from
Sony/Philips.  This lack of universality and the fact that parts budget spent
on low jitter PLLs (like individual crystals for each sample rate) reduces
the parts budget for everything else (like filters, DACs, analog circuits,
power supplies, and case) leads many designers to leave well enough alone
and use the clock straight out of the receiver chip investing the saved resources
elsewhere in the design or lowering the cost.  This allows the DACs master
clock to be strictly a function of the source and interface.

6. As stated above accurate reconstruction of the analog waveform requires
both correct samples and a accurate clock.  A correct sample at the wrong
time is just as much an error as an incorrect sample at the right time.

7. Incorrect samples are rare as the system is quite robust. A few DACs
contain digital filters which interpolate across bad samples, most simply
mute or allow the data through.  Uncorrected bad samples sound like 'clicks'
or 'pops'.

8. A mistimed sample due to clock jitter is a subtle effect which usually
goes unnoticed by casual listeners using low resolution systems.  Critical
listeners report a variety of sonic effects related to the subtle details
of reproduced sound including but not limited to high frequency details,
the nature of reverberation, ambient sounds, and 'grain'.  Information theory
characterizes jitter as noise accompanying the clock signal.  The variety
of sonic effects reported by listeners may be accounted for by the fact that
the jitter noise spectrum effects how its error is superimposed on the
desired signal.  Jitter with a white noise spectrum results in the systems
noise floor being raised.  When jitter noise has specific frequencies present
these frequencies tend to show up as side bands on the desired output. See
reference #1.  In reference #2 Dr. Hawksford reports that a 1 LSB error of
a 15kHz signal results from only 0.324 nS of clock error.  In reference #2
Steven Harris states "For example, 2 nS peak white noise clock jitter will
degrade a perfect 16-bit ADC from a dynamic range of 98 db to 91 db. To
reduce clock jitter effects to less than 0.5 db impact on dynamic range, the
peak jitter amplitude has to be less than 400 ps."  The Harris paper is on
jitter effects on ADCs using both measurements of actual devices and computer
simulations.  I believe the results apply to DACs as well.

9. The threshold of audibility of these effects is a matter of considerable
controversy.  Seventy years after the introduction of electrical amplification
of musical signals a consensus has yet to be reached throughout the audio
community as to the threshold of distortion in its many forms.  I fear it
will take that long again for consensus to develop regarding the distortion
mechanisms unique to digital audio.  Compounding the difficulty is the fact
that a set of instruments to measure jitter at the levels of interest and
analyze its effects has an entry price tag into five figures in US $.

10. Then comes a certain group of audiophiles who declare digital audio by
its very nature is already perfect.  They agrue this point using a selected
subset of the facts and tenuous analogy to other usually purely digital
technologies.  Their belief system includes a form of audio egalitarianism
which has latched onto digital audio as a means to bring ultimate hi-fidelity
down to the lowest common denominator.  Anyone questioning this belief system
is to be labled self-delusional or a charlatan and immediantly challenged to
a duel in the form of an ABX test.

references:
#1 The Jitter Game, Robert Harley; Stereophile January 1993
#2 A Digital Discourse, Dr. Malcolm Hawksford; HiFi News & Record Review Feb,
April, June, Aug, 1990
#3 The Effects of Sampling Clock Jitter on Nyquist Sampling ADCs, and on
Oversampling Delta Sigma ADCs, Steven Harris; Journal Audio Engineering Society,
July 1990

Norman Tracy
znpt01@trc.amoco.com