From: znpt01@trc.amoco.com (Norman P. Tracy)
Newsgroups: rec.audio
Subject: Re: optical vs. coax digital interconnects
Date: 13 Oct 92 22:05:31 GMT
Organization: Amoco Production Company, Tulsa Research
Originator: znpt01@gpss43


The story thus far.............................
+
+I'm considering upgrading my audio system with a new CD player and external
+D/A, but I'm not sure if I should get a player/converter with optical or
+coax digital interconnects.  I've heard that the coax interconnects are
+susceptiable to picking up ambient RFI from other electronic devices such as
+computers and VCRs, and this will affect the soundstage and transient
+response of my setup.  However, I've also read in this newsgroup several
+months ago that optical interconnects must be routed correctly, i.e. no
+sharp bends that are at a harmonic of any frequencies in the audio range
+because this will cause "flattening" of the sound since the digital bitstream
+will reflect inside the optical fiber and cause reflections which will cause
+the D/A converter to mistrack.
+
+I have noticed that in my fiber optic ethernet network whenever the fiber
+was bent or twisted inadvertantly, the display on my network connected
+Xterminal would appear "squished" and the rendered fonts would not be as
+crisp as they are when the fiber is undisturbed, so I would assume that the
+same thing would happen to the digital audio data coming out of a CD player
+and going to an external D/A converter connected by fiber, but manifesting
+itself as a decrease in auditory definition.
+
+Any ideas would be much appreciated, as this is a fairly substantial
+investment for me.
+
+mike
+
+--
+>: >I'm considering upgrading my audio system with a new CD player and external
+>: >D/A, but I'm not sure if I should get a player/converter with optical or
+>: >coax digital interconnects.  [...]
+>: 
+>: Now, did you want Toslink optical, AT&T glass fiber, or a proprietary
+>: optical interface?  Unbalanced coax or balanced?
+>
+>I'm concerned that if I choose glass fiber, it might eventually crack (as
+>glass does), so I think I'd prefer plastic-based fiber.  If I decide on
+>coax, I assume balanced coax, so that my CD player and converter won't have
+>problems with common-mode noise (hence, have good common-mode rejection).
+>When I used to work on Prime computers, they used twinax, i.e. two center
+>conductors wrapped in a braided shield, for their network connections.  If I
+>decide on balanced coax, I would rather twinax cable than two seperate coax
+>cables.
+
+Just a few more comments--note that Toslink and AT&T glass fiber are two
+entirely different transmission schemes, not just a difference in cable
+materials.  (I believe it's pretty much universally accepted that Toslink
+is considerably inferior to coax while AT&T is usually slightly better 
+than coax.)
+
+Incidentally, if you are going to work with coax, unlike unbalanced
+interconnects and speaker wires, impedance-matching is important; I've
+occasionally tried connecting D/A converters and transports with 
+connectors that weren't 75 Ohms with fairly poor results.  (BTW, I
+use twinax for most of my line-level interconnects, myself, but it
+seems much easier to get ahold of 50 Ohm twinax than 75 Ohm twinax.)
+
+Trygve
+
+
+Now this really confused me. Are not the digital-to-digital
+interconnects bit streams? I don't understand this. How come a cheap
+$25 doller SCSI cable gets every bit right between the hard disk and
+the computer but all these options don't seem to do it.
+
+Am I missing something obvious?
+
+This also applies to transports. Four hunderd doller CD-ROMs seem to
+do fine reading every bit of a disc correctly. Why pay >$3000 for some
+esoteric transports for the same behavior. Is not digital, digital?
+
+boris
+
+
+I have no view on the views I will express :-)
+
+In article <BP.92Oct12234246@hobbes.cs.brown.edu> bp@cs.brown.edu (Boris Putanec) writes:
+> $25 doller SCSI cable gets every bit right between the hard disk and
+> the computer but all these options don't seem to do it.
+
+The SPDIF format, it is claimed, suffers from the fact that the receiver
+has to recover the clock and the word-select from the data stream.
+Jitter in the recovered stream will equate to jitter in the recovered
+clock.  The high-end DAC designer upstairs claims that jitter in the
+receiver ASIC, for example, is a Bad Thing.  All this tends to imply
+that rise-time is an issue in the link.  Note how sensitive Ethernet ---
+another self-clocking protocol --- is to poor terminations.
+
+Note that several transports now support using the clock from the DAC
+(Linn, new Arcam) so that this problem is attacked from a different
+direction.  You know the clock that the SPDIF was built with, so don't
+have to receover it.  Word select is less of an issue.
+
+> This also applies to transports. Four hunderd doller CD-ROMs seem to
+> do fine reading every bit of a disc correctly. Why pay >$3000 for some
+> esoteric transports for the same behavior. Is not digital, digital?
+
+CD-ROM uses a much stronger Error Correction strategy, I believe.
+
+ian
+
+>> This also applies to transports. Four hunderd doller CD-ROMs seem to
+>> do fine reading every bit of a disc correctly. Why pay >$3000 for some
+>> esoteric transports for the same behavior. Is not digital, digital?
+
+>CD-ROM uses a much stronger Error Correction strategy, I believe.
+
+I don't understand what difference that makes.  If it can get all of the
+bits right for $400, why not do the same in audio?  Even on a CDrom errors
+are not what one would consider frequent in the audio world.  This has
+always confused me.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

And now my $0.02 worth of theory and experience.
These questions and discussions of S/PDIF cables and data sources seem
to appear on the net at regular intervals, hopefully we can shed some light
on this FAQ in spite of the fact we do not yet know all the intricacies of
digital audio reproduction.

The short answer is that the world is not as simple a place as we sometimes
wish it to be.

This argument that CD-ROM drives have a low enough bit error rate to be
used as computer peripherals misses half the requirement.  Two things are
needed for highest-fi digital audio: 1. the correct samples; and 2. the correct
time to convert the sample into an analog output signal.  It is #2 which seems
to improve as the quality of the drive and interface goes up.  As stated above
the instability or lack of accuracy in the clock is called "jitter", a commonly
dealt with phenomenon in computers, communication equipment, and now
digital audio.

Jitter levels are affected in the CD, LD, or DAT by the quality of the RF 'eye' pattern
signal output by the detector photodiode (or tape head), servo systems, grounding
scheme, PCB layout, and other factors some not yet known or publically discussed.
When the bit and word clocks are encoded on the S/PDIF (Sony/Philips Digital
InterFace) serial data stream to be sent to the outside world other sources of
error can creep in which (finally) gets us back to the original question.

The two channels of audio data, subcode bits, word clock, and bit clock are
transmitted as a serial biphase encoded signal in the S/PDIF standard.  This
signal has a frequency content of 1.5 to 7 MHz.  When an RF signals like the 
S/PDIF are sent down an incorrectly terminated wire some of the energy is
reflected back from the end of the wire and causes destructive interference
with the desired signal.  In the ideal case these reflections are controlled by
terminating both ends of the cable (75 ohms in the S/PDIF standard) and making
the characteristic impedance of the cable and all connectors equal to the terminating
impedance.  As is usually the case real world implementations fall short of the
ideal case leading to uncontrolled variables, performance variations, and
confusion among end users.  A key point to keep in mind is that over the short
lengths of home use cables the reflections do not cause data value errors but
rather tend to increase jitter.  Data errors sound like clicks, pops, or muting
(the audio DACs I've examined do not have data interpolation).  Increases in
jitter results in subtler problems as signal to noise ratio decreases and distortion
increases.

Using optical cables involves another set of tradeoffs.  The up side is a complete
break in the grounds between transport and DAC, immunity from RFI, and the
potential for a higher bandwidth data link.  I believe the less than excellent results
widely reported for the Toslink standard is related to two factors.  First most
Toslink transmitters and receivers seem to be rated at 5-6 MHz bandwidth.  Second
is the loose fit of the Toslink optical connector.  Misalignment is the greatest enemy
of signal strength in optical links.  Contrast Toslink to the ST (AT&T trademark) and
you'll find a tighter spec'd connector and a 50 MHz bandwidth.

The above tome barely scratchs the surface of objective considerations for highest
quality serial data transmission.  I believe generalities like "ST - best, balanced - better,
coax - good to excellent, Toslink - least" are based on conventional wisdom from a
shared experience base.  When the average parameters upon which the conventional
wisdom is based are violated the exceptions pop up which so confuse those who long
for a simple non-chaotic world.  I'll close with a personal subjective example of a violation
of the conventional wisdom yielding higher performance.  The point I hope to make
(sorry mail-orderers) is that there is still no substitute for trying out the exact set of
components you are interested in.  Failing that, keep your options open.

Last weekend I rented a JVC DAT recorder to hook up via S/PDIF to my Sony TCD-D3
DATman to make backup copies of some of my master tapes.  When the backups were
done I ran a comparison of the two units as digital sources for my homemade 18 bit
8x oversampling DAC.  The remainder of the system is a Counterpoint SA-3000 preamp,
Spectral DMA-50 power amp and B&W 802 speakers.  The JVC was interfaced directly
to the DAC via coax.  The Sony TCD-D3 has only the lowly Toslink outputs so it had to
be interfaced via. Toslink/coax adapter box.  So my expectations were that the bigger
more directly interfaced JVC would whip the little DATman.  Source was a 'greatest
hits' fusion jazz DAT made via S/PDIF from CDs.  It didn't take long for the Sony TCD-D3
to show that used as a digital audio data drive it is clearly superior to the full size JVC.
In A/B comparison the JVC had a leaden quality, rhythms seemed slowed, bass lacked
impact and highs were fuzzy.  Listening to the same cuts on the Sony TCD-D3 brought
the tape I know and love back to life.  In an audiophile context these were not small
differences.  I invited a friend over and he picked out the same problems with the JVC.

The moral of the story is, once again, LISTEN before you buy.

Norman Tracy (znpt01@trc.amoco.com)