CHAPTER 13

Writable CDs Chapter 13
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It would be much better if you could read the original digital data directly off the disc. That was not
possible with older CD-ROM drives, but newer drives can do what is called digital audio extraction
(DAE). This is a process in which they read the digital audio sectors directly and, rather than decode
them into analog signals, pass each 2,352-byte sector of raw (error-corrected) digital audio data
directly to the PC’s processor via the (ATA, SCSI, USB, or FireWire) drive interface cable. Therefore, no
digital-to-analog conversion (and back) occurs, and you essentially get the audio data exactly as it was
originally recorded on the CD (within the limits of the CD-DA error-correction standards). You would
have essentially extracted the exact digital audio data from the disc onto your PC.
Another term for digital audio extraction is ripping, so named because you can “rip” the raw audio
data from the drive at full drive read speed, rather than the normal 1x speed at which you listen to
audio discs. Actually, most drives can’t do DAE at their full rated speeds. Although some are faster (or
slower) than others, most perform DAE at speeds from about one-quarter to about one-half of their
rated read speed. So, you might be able to extract audio data at speeds only up to 20x on a 40x rated
drive. However, that is still quite a bit better than at 1x as it would be on drives that can’t do DAE
(not to mention skipping the conversion to analog and back to digital with the resultant loss of information).
Virtually all newer CD/DVD drives can perform digital audio extraction on music discs. How fast or
accurately they do this can vary from model to model. One might think any extraction (digital copy)
of a given track (song) should be the same because it is a digital copy of the original; however, that is
not always the case. The CD-DA format was designed to play music, not to transfer data with 100%
accuracy. Errors beyond the capability of the CIRC in the CD-DA format cause the firmware in the
drive to interpolate or approximate the data. In addition, time-based problems due to clock inaccuracies
can occur in the drive, causing it to get slightly out of step when reading the frames in the sector
(this is refered to as jitter). Differences in the internal software (firmware) in the drive and differences
in the drivers used are other problems that can occur.
Positioning can also be a problem because the CD-DA format was designed to stream (play continuously)
and not to read individual sectors. CD-ROM sectors are 2,352 bytes long,and these bytes are
further divided into 2,048 bytes of data plus 304 bytes of syncronization, header, and additional ECC
information to control positioning and allow for error-free reads. No such syncronization, header, or
extra ECC information exists for audio sectors; instead, all 2,352 bytes are used for pure audio data. To
address an audio sector, the Q subcode information is used instead (see the section “Subcodes,” earlier
in this chapter). Most audio players position to within only 75 sectors (1 second) using the Q subcode
information. CD-ROM drives that can perform digital audio extraction are usually much more accurate
than that, but because of how the subcode works (as well as the cross-interleaved way audio data
is stored), designing a drive that can position every time to the precise audio sector that starts the
track can be difficult.
All of this conspires to cause inaccuracies or slight differences in multiple extractions of the same
track (song). Perfect extractions are possible, but making perfect extractions is difficult to achieve for a
lot of reasons. For example, even a slight amount of dirt or scratches on the disc has a great effect on
the quality of your extractions, so be sure the discs are clean. As a test of your drive’s capability to perform
DAE, try extracting the same track (song) multiple times from a new, clean, scratch-free disc,
using a different filename for each extraction. Then, bring up a command prompt and use the FC (file
compare) command to compare the different files to each other. If they compare exactly, you have a
combination of hardware and software that can do perfect or near-perfect extractions.
If you intend to do a lot of extracting, you should ask around to see what hardware and software others
are using for this purpose. As a general rule, SCSI drives work better than ATA, but some ATA drives
are just as good as the better SCSI drives. Plextor is well known for drives that are excellent when
it comes to digital audio extraction, and I’ve always had good luck with Toshiba drives, too.