From: ketil@edb.tih.no (Ketil Albertsen,TIH)
Subject: Re: How can different digital cables sound different?
Organization: T I H / T I S I P 
Posting-Front-End: Winix Conference v 92.05.15  1.20 (running under MS-Windows)
Date: Tue, 5 Jan 1993 07:04:30 GMT

In article <184766@pyramid.pyramid.com>, lstowell@pyrnova.mis.pyramid.com (Lon Stowell) writes:


>In article <1993Jan4.215906.29420@news.ysu.edu> joe@avs.com (Joe Peterson) writes:

>    In the REAL world there is absolutely no such thing as a perfect
>    1 and a perfect 0.

True, but the nature of digital encoding allows you to reconstruct as
perfect 0s and 1s as your equipment is capable of, even if the source
is somewhat distorted. Errors don't add up, and you can correct 
(moderate) errors at a later stage, without the risk of removing any
information content.

>    What is real is that there are defined
>    boundaries for transmitters AND receivers such that a 1 is
>    beyond a certain threshhold and a 0 is not.  There are also
>    specifications for rise and fall times as well as maximum
>    jitter.  

Sure, and the margins between 0 and 1 are VERY wide. You really have a
distorted signal if that is a significant problem.

At a presentation of B-ISDN and ATM technologies, I queried one of the 
senior research scientists at the Norwegian Telecomm Adm about the
lack of net level error detection in ATM. He promptly replied, with a
smile: "Because errors don't occur in optical fibers!" Then he moderated
that statement slightly, but the spirit was the same: That simply ain't
a problem. And, that is with 30-50 mile fibers or more, and bitrates
approaching the gigabit/s range.

>   [...] If the protocol on the cable lacks error recovery and
>   retransmission capability, then you must be sure to specify the
>   maximum allowable noise (which would include jitter) within which
>   a given signalling scheme and bit rate will operate at 100% data
>   integrity.  [...]
>   I don't believe the digital audio standards include a
>   retransmission scheme as a Token Ring, etc. do....

There are lots of error correcting encodings that do not rely on
retransmission, but on extra data transmitted as part of the normal
data exchange. Lots of this is built into the CD format itself - if
the disk has a scratch, rereading ("retransmitting") it a hundred 
times won't remove the scratch. But error correcting codes can get 
around it.

Sure, every now and then, a bit error occurs. Could someone produce
figures for the error frequency, at the bit rates used in audio
equipment? I would be surprised if a bit error occurs more often than
once every several seconds. And *when* that happens, it certainly would
be detectable through some sort of error detecting code. And then the
DAC (or some digital stage ahead of the DAC) should interpolate between
the neighbour samples. In all but extreme sound patters, an interpolation
will be within a few percent of the correct value. Jitter has a different
cause and looks physically different at the receiver, but to the DAC
it all boils down to bit errors, anyway.

To evaluate the audible difference, you don't need a cable at all, if
you have bothered to register the bit errors that do occur (if any),
you simply compare the output of the player and the input to the DAC 
(each end of the cable) with suitable digital logging equipment. 
You may feed either of these bit patterns (the correct one and the the
one with errors) from a computer into the DAC. Then judge.

Don't try to tell me that you can hear a lot of difference from a
single sample being a few percent off every several seconds.

The true question is: Given two digital signals that are *exactly
identical* at the point where they are joined (e.g. at the DAC input),
can a trained ear *still* hear the difference? Evidence is that
that is possible.