Optical Disc Formats for Audio Reproduction and Recording 1135subcode data. Both DDP 1.0 and DDP 2.0 are used; the
2.0 specification writes the TOC to the end of the tape.
It is generally recommended to supply a replication
plant with an Exabyte tape with DDP files (including
PQ and ISRC data). With Exabyte tapes, glass masters
may be created at faster than real time speeds. In some
cases, audio data is written to a master CD-ROM (CD
read-only memory) disc as 24-bit WAV or AIFF files.
DAT tapes and CD-R discs can be used as masters, but
their relatively higher error rates and susceptibility to
damage make them nonideal. An analog tape can also
be used as the master. Digital recordings made at a dif-
ferent sampling rate must be passed through a sample
rate converter.
CD encoding is the process of placing audio data in a
format suitable for storage on the disc. A frame struc-
ture provides a means to distinguish the data types. The
information contained in a CD frame (prior to modula-
tion) contains a 27-bit sync word, 8-bit subcode, 192
data bits, and 64 parity bits.
Encoding begins with the audio data. Six 32-bit
PCM audio sampling periods (alternating from 16-bit
left and right channels) are grouped in a frame, left
channel preceding right. Each 32-bit sampling period is
divided to yield four 8-bit audio symbols. Subsequent
signal processing prepares the audio data for storage on
the disc surface. In particular, error correction encoding
must be accomplished.
The raw error rate from a CD is around 10^5 to 10^6 ,
or about one error for every 0.1 to 1.0 million channel
(stored) bits. This is impressive storage capability, but
considering that a disc outputs 4.3218 million channel
bits per second, the need for error correction is obvious.
With error correction, 220 errors per second can be
completed corrected; interleaving distributes errors, and
parity corrects them.
The Cross Interleave Reed-Solomon Code (CIRC)
algorithm is used for error correction in the CD system.
The CIRC algorithm uses two correction codes for
correcting capability, and three interleaving stages to
encode data before it is placed on a disc and to decode
the data during playback. Because of cross interleaving,
the separation of two error correction codes by an inter-
leaving stage, one Reed-Solomon code can check the
validity of the other code. The Reed-Solomon code used
in CIRC is well suited for the CD system because its
decoding requirements are relatively simple. The
complete CIRC encoding scheme is shown in Fig. 30-3.
With this encoding algorithm, data (twenty-four 8-bit
symbols) from the audio signal are cross-interleaved,
and two encoding stages generate 8-bits of parity.
30.2.3 SubcodeFollowing CIRC encoding, an 8-bit CD subcode symbol
is added to each frame. The eight subcode bits are desig-
nated as P, Q, R, S, T, U, V, and W. Only the P or Q bits
are required in the audio format. The CD player collects
subcode symbols from 98 consecutive frames to form a
subcode block, with eight 98-bit words. Thus the eight
subcode bits (P through W) are used as eight different
channels with each CD frame containing 1 P bit, 1 Q bit,
etc. A subcode block is complete with a synchronization
word, instruction and data, commands, and parity. The
start of each subcode block is denoted by sync patterns
in the first symbol positions of two successive blocks.
The P channel contains a flag bit originally designed
for use by simple players to access disc information. In
practice, players ignore the P bit and use information in
the more comprehensive Q channel. The Q subcode
channel is vital for reading audio data on the disc. The
Q channel contains four kinds of information: control,
address, Q data, and cyclic redundancy check code
(CRCC). Each subcode block contains 72-bits of Q data
and 16-bits for CRCC, used for error detection on the
control, address, and Q data information. The control
information flag bits handle several player functions:- The number of audio channels (two or four) is indi-
cated; this distinguishes between a two- and four-
channel CD recording (the latter not implemented). - Preemphasis (on/off) is indicated; a CD track may
be encoded with preemphasis, a noise suppression
method (this is rarely employed). - Digital copy prohibited (yes/no) is indicated.
- Audio or data content is indicated.
The address information consists of four bits desig-
nating the three modes for the Q data bits. Primarily,
Mode 1 contains the number and start times of tracks,
Mode 2 contains a catalog number, and Mode 3 contains
other product codes. Mode 1 stores information in the
disc lead-in area, program area, and lead-out area; the
data format in the lead-in area differs from that in the
other areas. Mode 1 lead-in information is contained in
the CD table of contents (TOC). The TOC stores data
indicating the number of music selections (up to 99) as
a track number and the starting points of the tracks in
disc running time. The TOC is read during disc initial-
ization, before the disc begins playing audio data.
In the program and lead-out areas, Mode 1 contains
track numbers, indices (subdivision numbers) within a
track, time within a track, and absolute time. A time
count is set to zero at the beginning of each track and
increases to the end of the track. At the beginning of a