* "Timothy P. Stockman" <[EMAIL PROTECTED]> on Tue, 22 Aug 2000
| CDDA is *not* analog! It is as digital as CDROM, your computer hard drive,
| etc.
CD-DA is Compact Disc Digital Audio. That means Linear PCM (pulse code
modulation) 16 bits wide at a sampling frequency of 44.1KHz, which is a
digital signal. The ones and zeros are coded on the disc media as ones
(light reflects one way) and zeros (light reflects another way). But the
media itself is analog in that the intensity of a given "one" -- that is,
the ammount of light reflected from a given pit representing that bit of
data -- is variable within a given tolerance.
| CDs use a sprial track (similar to an LP) that was originally designed
| to be played continuously, not random accessed.
No. The Compact Disc has always been concentric tracks (rings) divided
into sectors, intended for random access, just like analog LaserDisc from
which it descends.
| Unlike computer disks, which have a header on each sector containing the
| sector number so the drive can be 100% certain which sector it just read,
| CDDA contains no such identifying information in each frame.
Wrong again. Each sector of a CD has a numerical sector count, which can
be used by the playback mechanism.
| As long as you play CDDA continuously (like an audio CD player does)
| there's no problem. Since the frames are in order on the disc, the data
| in the resulting audio stream will be in order. The problem comes in
| when one is extracting the digital audio information to a WAV file.
The problem with "ripping" audio tracks is that CD-ROM (CD data) has three
layers of inherent error checking, whereas CD-DA requires only two. That
extra layer is what makes CD-ROM useful as a data storage system. The lack
of that extra layer makes it difficult for CD-ROM mechanisms to read CD-DA
as data because the frequency of "soft" errors exceeds the mechanism's
threshold.
| In this case, the player does not read continuously but instead reads the
| data in "chunks".
Those chunks are called "blocks". Each block is a single sector of data on
the disc, or two sectors for some operating systems.
| After each "chunk" it has to seek back to where it left off before
| reading the next "chunk". Since there is no header to unambiguously
| identify the frame,
There is no header to identify the PCM frame because it is a data stream,
not block I/O. But there is the sector index counter of the disc itself,
and the ripper uses that to gauge it's progress.
| it seeks back to *approximately* the same place, reading some overlapping
| data. It then compares the data and discards the overlapping data.
Rippers re-read blocks to ensure that the lack of error correction does not
cause the ripper to get incorrect information. The player has little to do
with this aspect of the ripping process.
| Some CDROM drives are better at this than others.
Some CD-ROM drives are better at compensating for the lesser error
correction required by CD-DA.
| A few are very bad, causing the program to give up,
Which is ultimately a flaw in the program. Paranoia does not give up.
| but a few are perfect.
Plextor.
| Most are somewhere in the middle. But as long as the program can
| identify the overlap, the resulting WAV file will be a perfect copy of
| the audio data.
As long a) the disc is not excessively damaged and b) the CD-ROM mechanism
isn't a piece of shit, you can get a PCM data stream that is a copy of what
is on the disc. You can then swap the byte order and wrap inside a WAV
container.
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