From: lstowell@pyrnova.mis.pyramid.com (Lon Stowell)
Newsgroups: rec.audio
Subject: Re: How can different digital cables sound different?
Date: 5 Jan 93 19:24:01 GMT
Reply-To: lstowell@pyrnova.pyramid.com.pyramid.com (Lon Stowell)
Organization: Pyramid Technology Corporation

In article <1993Jan5.070435.16297W@lumina.edb.tih.no> ketil@edb.tih.no (Ketil Albertsen,TIH) writes:
>
>Sure, and the margins between 0 and 1 are VERY wide. You really have a
>distorted signal if that is a significant problem.
>
    Highly dependent on whether you choose to push the limits or
    not.  On any given media the limits are usually chosen to
    provide sufficient margin between the REAL limits and the
    operating limits.

>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.

   I hate to burst his bubble, but as a data communications
   consultant with over 20 years experience, errors DO occur on
   optical media.  What counts is whether the actual errors are less
   than the "span" of the error detection and correction
   algorithm...or whether a higher layer is able to correct the
   error with sufficient speed to paper over the errors which do
   occur.  "errors don't occur" and "errors are truly random" are
   famous last words in data comm.   

>
>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.
>
   If and only if the span of the error is less than the corrective
   capabilities of the forward error correction.  And since the
   discussion was about CABLES, not pulling data off the media, I
   fail to see what a scratched CD has to do with it.  If you are
   silly enough to push the cable length or use improper interface
   driver/receivers, you could be operating in the "grey zone" and a
   retransmission MIGHT get good data on a retry.  As noted, I do
   not know if digital audio uses an ack/nak scheme with
   re-transmissions or relies solely on forward error correction and
   trusts engineers to stay well within the boundaries.   Of course
   there is always some dweeb that wants to save a few pennies or
   just fell asleep trying to read "Pulse, digital, and switching
   waveforms"...


>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.
>
    Yup.  Digital Audio can probably work ok with interpolation.
    Digital DATA and computers tend to take offense...  Hopefully
    the difference between a concealed error (correction=100%) and
    an interpolated sample (close enough for MOST ears) need not be
    gone into.

>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.
>
   OK, I won't.  But I would be willing to believe that there are
   some people who can hear the artifacts of INTERPOLATED
   outputs...where the original waveform is NOT accurately output.
   Granted it would probably be more audible on test signals than
   music.  

   Of course there are also folks who claim to hear the difference
   when an error is concealed...and the RS code is able to 100%
   correct a data error due to the forward error correction in the
   data...   


>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.


  You lost me here....could you expand a bit?  Do you mean DAC's
  sound differently, or that identical signals into the SAME dac
  would sound different?   
Newsgroups: rec.audio