n)
Walt, you wrote,
>Since audio CD as played includes a step to deal with any missing bits,
>playing is not a perfect reproduction.
You're talking about error concealment, I think. To clarify:
The audio CD system assumes that not all the bits will be read
correctly from the disc. This is the -normal- state of affairs, a
certain error rate. Same for DAT and any digital medium! Audio CD
playback has three levels of dealing with errors:
1) Error correction: The player uses redundant coding to reconstruct
missing data. This normally happens all the time. The result is
perfect reproduction.
2) Error concealment: When the error rate rises so high that the
redundant coding can't correct it, the player interpolates samples,
i.e. makes up data that smooths over holes in the incoming stream.
This process can be very subtle and impossible to hear. On my
instrumented player, I only see error concealment flags where there
is gross damage to the disc. I have a disc that I found in the street
that has around 1000 error concealments per second. It still sounds
like music.
3) Muting. This is the zipping, ripping sound that means the holes in
the data are so big the player gives up.
To repeat myself, any commercial pressed CD in good condition can be
captured by a digital link with perfect reproduction. CDR playback
may be compromised into error concealment or even muting by the low
modulation level of the dyes used today. You can proof your system by
reading a CDR back into the workstation digitally (synching to the
digital input), slipping the files into bit-synchronization, and
doing a subtraction test. If the files subtract to dead silence, the
CDR is good.
-Dan Dugan
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