Newsgroups: rec.audio.tech
Subject: Re: Why are Audio CDs different from Data CDs?
References: <[email protected]>
"Scott Boham"  writes:

> I have heard people say that the digital output of a CD player can
> vary in quality, and you don't always get exactly the same bit-stream
> that was written to the disc.  

That happens sometimes for example in cases that you have very
bad quality CD (scratches) or baddly alligned CD player.
In good quality CD on audio CD player you woudl expect
something like every bit coes out as recorded or only
very few (least significant) bits on audiodata changed in
the hole disk.

> What I don't understand is how
> my �20 PC CD-ROM drive can read data from a CD 100% correct
> 100% of the time.  Every single BIT of the data is correct, no mistakes
> AT ALL.  

Those CD-ROM drives make mistakes sometimes in reading. But when the error 
correction notices it and can't correct the block, the drive tries to reread
that block which was read wrongly again few times until it gets it
right (or shows an error message).

In normal CD player you can't read the blocks many times, because
at 1x speed you have to output audio data blocks at the same
'speed as you read them. If there is a reading problem then first
CD player tries to use the basic error correcting code to correct
the mistake. Usually this solves the problem. If the data is
so much damaged that it can�t be complettely correctly recodered, then
it uses an extra error correction code so that it tries to get at least
the most significant bit on audio samples right (this effetively plays
back sound, only some fine details are lost and nosie is added).
At the las case if the block is completely damaged, the CD player
tries to interpolate the missind data (foe example copies the las
block and modifies it so that it "fits" to the place where sound
is missing). Those CD player error corrections which don't completely
restore the sound come into picture in cases where the CD is baddly
scratched or somehow other way damaged  (propably to case where
it the disk were a data CD-ROM it would give you an error and can't
get the data out at all).

> And it can even do this when the disc is spinning 24x normal
> speed.  Why can't an audio reader do the same, surely it is just reading
> the 1's and 0's exactly the same way as my PC CD-ROM, so there should
> be no errors at all.

In most of the cases the CD players and CD-ROMS read every bit
right from the CD. So no errors on output at all. One hifi magazine
(called HIFI and published in Finland) did once a test on some
CD product which laimed to help to get better sound quality from CD
(some pen with green in in it and somethign else). They run a test
where they read the audio CD twice (before and after tweaks).
THe results was that there was no single bitr difference on the read
data from the disk.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web pages at http://www.hut.fi/Misc/Electronics/