Erm... Well, I think the point Eiron was making was that the audio CD
format does, indeed, include various forms of redundancy which is then
checked and used to correct errors. The main layer of this is CIRC. This
is
rather more complex than plain CRC, but one of its functions is equivalent
to CRC.

I have no problem with that - always willing to learn.

I am talking about CRC - Cyclic Redundancy Check. CRC is employed on
data files. It is a check sum if you like. It ensures that data
retrieved from CDs or harddisks is 100% accurate.

Erm... no error detection and correction system can do that. With all real
systems there is a finite chance of an undetected error or errors.

My poor English! I didn't mean errors can be corrected - If you can't read a
data disc without error it's game over.
It's purpose is to 'know' when read data is bad. If it were possible to
correct then it would have been done.


If a sector read produces an incorrect CRC then it is read again.
Multiple attempts will be made before the source is considered
unreadable.

The same approach can be employed when replaying or reading audio CDs.
This is a function of the playing system, not the disc format.

This is *not* employed on audio CDs - the data is simply streamed - good
or bad.

I am not certain which of two meanings you have in mind above when you
use the phrase "employed on audio CDs".

If you are referring to the format in which data is represented on the
audio disc:

It is not correct to say that the information recorded onto audio CDs is
"simply streamed". Audio CD employes quite a complex and powerful set of
systems for redundancy and error detection and correction. If you don't
understand CIRC then you may not know the details of this, but it is
present. Indeed, this is why you can drill a 'small hole' in the
information layer, yet then be able to read the audio information with no
actual errors or loss to the audio data.

No actual errors or data loss????? Clearly there are errors and data loss.
I assume you are suggesting that the loss can be 'rebuilt' to be 100% as the
original? I feel if this were indeed possible the computer industry would be
using it. What may be possible is interpolation. This may be acceptable for
audio but disaster for computer data.
In the case of audio, we are still left with corrupt, inaccurate
information. Clearly then, this is of no help in the search for High
Fidelity.
It is interesting to read the CRC from an audio CD. Play a track and note
the CRC value. Play the same track again and see if the CRC is consistent. I
use Audiograbber to do this little test.


But if you are referring to the method used by the player to read that
information pattern from the disc:

Note that some audio players use the same method as you describe for data
discs. They read at high speed, and re-read if they detect an error they
decide they can't correct without more reliable information.

The original Philips documents specified that a hole up to 2.5 mm in
diameter in the information layer of an audio CD would result in no actual
audio data loss or errors - provided that the disc was otherwise fine. If
you go above 2.5, and up to a maximum of 7.5 mm then some data will be
'lost' and the reader will have to interpolate. But that isn't nominally
required for Red Book discs for a hole of less than 2.5 mm track-length.
For a small hole, therefore, the audio data recovered should be the same
as if no hole were present.

However Philips/Sony didn't have drilled holes in mind when they
arranged this. :-) In part, they wanted to cater for poorly made
discs, partly for specs of jam placed on the disc by the user, and
partly for brief losses of tracking or focussing whilst the player
reads a disc.

Hence even though most domestic audio players read the disc at x1
rate with no 're-tries', they still do not have to actually
read every single channel bit from the disc. They can accidentally
'miss' bursts of discs *and the result following error detection
and correction will generally be the intended audio data with no
errors.* This is the purpose of the systems Philips/Sony built
into the Red Book specs.

'generally be the intended audio'? Not good.

FWIW I had thought that data discs would be similar in this respect. The
point being that the data is redundantly interleaved along quite a length
of 'track' - thus catering for 'bursts' of data loss at the information
layer level. However I've not seen specs for this for a Data disc, so
can't comment on the details. But in both cases the channel bit stream
isn't simply the 'streamed' audio/computer data.

Again, if there was any way of reconstituting lost or corrupt data then the
computer industry would be using it.
While there is a lot of data interleaved on a harddisk (timing data for the
motor etc) I do not know if this is true for CD.
However, I don't think there is duplication of 'real' data on CD.


Note that one of the problems for computer data discs is that the user
generally wants to read data from them at far higher rates than are
required for audio CD replay. This requires high disc speed, and hence
much shorter response times for the track and focus servos in the
reader. This in turn means that reading the disc reliably becomes
harder than at a slow speed. And this may be one reason why for
computer data use, re-tries are useful.

Computers have the luxury that, within reason, it doesn't matter how many
re-tries are made to retrieve data accurately. Computer data must be
correct. Interpolation in this context would be catastrophic. Our ears, on
the other hand, will not accept a pause in the music while multiple retries
are performed. I would imagine that a not inconsiderable buffer would be
required to allow for re-tries.




No need to do this. We know it is not a clone or anywhere close. It
has been butchered by the RIAA filter in an attempt to make up for
the inability of a mechanical system.

The RIAA filter is reversible, as is done in any phono preamp, and is
therefore not butchery. What is your point?

If it isn't broken, why fix it? My point, hopefully, is crystal.

Afraid not - at least to me.

Ok.


FWIW The use of pre-emphasis and de-emphasis is quite common in
communcations engineering. It does not signify that a system is 'broken'.
Just that pre-whitening according to the noise and maximum levels as a
function of frequency can optimise the available dynamic range, and hence
the available level of precision. FWIW some audio CDs also employ
pre-emphasis as this is part of the Red Book spec. This does not mean the
resulting CDs are 'broken'.

As Keith G would probably be happy to confirm, I am not personally that
much of an enthusiast for Vinyl LP. In practice I prefer CDs for various
practical reasons. So the above is not intended to 'defend vinyl', just
to clarify the situation wrt CD.


Phew! To clarify my position. I want The Highest Fidelity that technology
will allow. I have never claimed perfection for any method of sound
storage/retrieval/presentation. My quest would be over and I could sleep
peacefully if I had.
As far as retrieval goes, I do know which gets me closest though.

Paul