ProfitPress Mega CDROM2 …eeware (MSDOS)(1992)(Eng)/ProfitPress-MegaCDROM2.B6I/TEXT/INFO/CDROMSUM.ZIP/CD.3

Text File | 1991-03-12 | 5.3 KB | 101 lines

CD Summary Part 3

First Level Error Correction

Data errors can arise from production defects in the disk itself, defects
arising from subsequent damage to the disk, or jarring during reading.  A
significant characteristic of  these errors is that they often occur in
long bursts.  This could be due, for example, to a relatively wide mark on
the disc that is opaque to the laser beam used to read the disc.  A system
with two logical components called the Cross Interleave Reed-Solomon
Coding (CIRC) is employed for error correction.  The cross interleave
component breaks up the long error bursts into many short errors; the
Reed-Solomon component provides the error correction.

As each frame is read from the disc, it is first decoded from fourteen
channel bits (the three merging bits are ignored) into eight-bit data
bytes.  Then, the bytes from each frame (twenty-four data bytes and eight
error correction bytes) are passed to the first Reed-Solomon decoder which
uses four of the error correction bytes and is able to correct one byte in
error out of the 32.  If there are no uncorrectable errors, the data is
simply passed along.  If there are errors, the data is marked as being in
error at this stage of decoding.

The twenty-four data bytes and four remaining error correction bytes are
then passed through unequal delays before going through another
Reed-Solomon decoder.  These unequal delays result in an interleaving of
the data that spreads long error bursts among many different passes
through the second decoder.  The delays are such that error bursts up to
450 bytes long can be completely corrected.  The second Reed-Solomon
decoder uses the last four error correction bytes to correct any remaining
errors in the twenty-four data bytes.  At this point, the data goes
through a de-interleaving process to restore the correct byte order.

Subcoding Channels and Blocks

The eight-bit control and display byte in each frame carries the subcoding
channels.  A subcoding block consists of 98 subcoding bytes, and thus 98
of the 588-bit frames.  A block then can contain 2352 bytes of data.
Seventy-five blocks are read each second.  With this information, it is
now straightforward to calculate that the CD data rate is in fact correct
for CD digital audio (CD-DA):

Required CD digital audio data rate:  44.1 K samples per second * 16 bits
per sample * 2 channels = 1,411,200 bits/sec.

CD data rate:  8 bits per byte * 24 bytes per frame * 98 frames per
subcoding block * 75 subcoding blocks per second = 1,411,200 bits/sec.

The eight subcoding channels are labeled P through W and are encoded one
bit for each channel in a control and display byte.  Channel P is used as
a simple music track separator.  Channel Q is used for control purposes
and encodes information like track number, track type, and location
(minute, second, and frame number).  During the lead-in track of the disc,
channel Q encodes a table of contents for the disk giving track number and
starting location.  Standards have been proposed that would use the
remaining channels for line graphics and ASCII character strings, but
these are seldom used.

Track Types

Tracks can have two types as specified in the control bit field of
subchannel Q.  The first type is CD digital audio (CD-DA) tracks.  The
two-channel audio is sampled at 44.1 Khz with sixteen bit linear sampling
encoded as twos complement numbers.  The sixteen bit samples are separated
into two eight-bit bytes; the bytes from each channel alternate on the
disc.   Variations for audio tracks include pre-emphasis and four track
recording.

The other type of track specified by the subchannel Q control bit field is
the data track.  These must conform to the CD-ROM standard described
below.  In general, a disc can have a mix of CD digital audio tracks and a
CD-ROM track, but the CD-ROM track must come first.

Editorial:  This first level error correction (the only type used for CD
Audio data) is extremely powerful.  The CD specification allows for discs
to have up to 220 raw errors per second.  Every one of these errors is
(almost always) perfectly corrected by the CIRC scheme for a net error
rate of zero.  For example, our tests using Apple's CD-ROM drive (which
also plays audio) show that raw error rates are around 50-100 per second
these days.  Of course, these are perfectly corrected, meaning that the
original data is perfectly recovered.  We have tested flawed discs with
raw rates up to 300 per second.  Net errors on all of these discs?  Zero!
I would expect a typical audio CD player to perform similarly.  Thus I
expect this raw error rate to have no audible consequences.

So why did I say "almost always" corrected above?  Because a sufficiently
bad flaw may produce uncorrectable errors.  These very unusual errors are
"concealed" by the player rather than corrected.  Note that this
concealment is likely to be less noticeable than even a single scratch on
an LP.  Such a flaw might be a really opaque finger smudge; CDs do merit
careful handling.  On the two (and only two) occasions I have found these,
I simply sprayed on a little Windex glass cleaner and wiped it off using
radial strokes.  This restored the CDs to zero net errors.

One can argue about the quality of the process of conversion of analog
music to and from digital representation, but in the digital domain CDs
are really very, very good.