|
The compact disc industry started in 1980 when Philips and Sony
introduced the Compact Disc Digital Audio Standard. This is commonly
called the Red Book. The Red Book describes the Audio Compact
Disc (CD) that you find in music stores today. It is the foundation
on which all other CD standards are built. Because audio discs
are manufactured per the Red Book Standard, all audio compact
discs will play in any audio compact disc player. The track type
defined in the Red Book is: CD-Digital-Audio (CD-DA), for audio
music. The Red Book specifies that the audio data are on the CD
in one or more tracks. Each track is normally one song. These
tracks are further subdivided into sectors (frames) that are 1/75th
of a second in length and contain 2352 bytes of audio data in
digital form. A maximum of 99 audio tracks may be placed on a
standard Red Book disc.
The process of direct reading the audio data is called Digital
Audio Extraction (DAE) or as "CD ripping". CDRoller
utilizes DAE to read the audio sectors (frames) of the CD disc
and write the resulting data into a WAV , MP3, MP2, OGG or RAW
file. The advantage of DAE above recording the file using your
sound card is that there is no loss of music quality, the data
in the WAV or RAW files are an exact copy of the data on the audio
CD. For the insiders, using your sound card, the digital data
from the disc are converted to an analog signal, which is then
digitized again by the sound card. Often the sound quality will
deteriorate when going this route, often producing noisy WAV files.
Furthermore, DAE is also much faster than ripping audio data with
your sound card.
However, Digital Audio Extraction is connected with more problems
then simple playing an audio track.
The first problem is that all 2352 bytes of sector contain audio
data - there is no positioning information returned in the block
read. When reading consecutive blocks, the new reading can start
within +-"N bytes" of the intended position. Generally,
"N bytes" value limits of 2352 bytes (1/75 of a second),
but we found that our Hitachi CDR-8130 "limited" this
value -21288 bytes below and +16824 bytes at the top! This effect
known as "jitter" results in the clicks and pops in
the output file produced by DAE. Some CD drives support a
feature known as "Stream is Accurate", it means that drive's controller
guarantees itself that new reading always starts where the last
one left off.
The second problem is that an audio sector as "raw"
sector has no auxiliary field for layered error correction (L-EC)
therefore reading block of damaged sectors can result in invalid
data recorded to the output file.
Finally, some CD drives simply don't support Digital Audio
Extraction - while most SCSI drives support DAE, not all IDE drives
do. In addition, manufactures of SCSI units sometimes utilize
vendor specific ("non-standard") commands and read algorithms
to implement DAE features. Some of these methods are undocumented.
To prevent the appearance of the clicks and pops in the output
file CDRoller utilizes the method known as "Jitter correction"
or overlapping. Generally, it involves using overlapping reads,
and attempting to match the end of one read with the beginning
of the next, so that there are no gaps in the data read. These
gaps really cause clicks and pops in the resulting output files
produced. For instance, the program might begin reading a block
at Logical Block Address (LBA) 100 and read 27 sectors total.
While next reading, it would read block starting at LBA 95 and
would try to match the end of the first read in the beginning
of the second read. Once it finds a match, it would cut off the
beginning of the second read, and use only the part after the
match. In addition to this way with Fixed overlapping (five
sectors) CDRoller also uses Dynamic synchronization - if
attempt of matching fails, the program re-attempts the process
with a slightly larger overlap area.
To check the errors CDRoller reads new block of sectors at least
twice in both methods mentioned above. If reading error occurres
even the synchronization has been applied successfully, the program
tries to read and synchronize current block again to obtain two
blocks of identical sectors finally.
The last method named Burst Copying can be applied to
CD drives supported "Stream is Accurate" feature. The DAE
produced by this method is the fastest, but the program doesn't
utilize "jitter correction" and no error checking can
be also performed.
To detect appropriate read algorithm for user's CD drive CDRoller
utilizes as commands described in SCSI-3 and SCSI-2 standards
as vendor specific commands found in available manuals.
To provide audio compression CDRoller supports a wide set of
audio codecs and wave formats via Windows ACM (Windows
Audio Compression Manager) and external libraries (DLLs).
The program converts audio data "on-the-fly"
without creating an intermediate WAV file.
|
