Hardware Reference
In-Depth Information
spiral track and contain or convey additional information about the disc in general. The subcode bytes
are stored as 1 byte per frame, which results in 98 subcode bytes for each sector. Two of these bytes
are used as start block and end block markers, leaving 96 bytes of subcode information. These are
then divided into eight 12-byte subcode blocks, each of which is assigned a letter designation P-W.
Each subcode channel can hold about 31.97MB of data across the disc, which is about 4% of the
capacity of an audio disc. The interesting thing about the subcodes is that the data is woven
continuously throughout the disc; in other words, subcode data is contained piecemeal in every sector
on the disc.
The P and Q subcode blocks are used on all discs, and the R-W subcodes are used only on CD+G
(graphics) or CD TEXT-type discs.
The P subcode identifies the start of the tracks on the CD. The Q subcode contains a multitude of
information, including the following:
• Whether the sector data is audio or data. This prevents most players from trying to “play” CD
data discs, which might damage speakers due to the resulting noise that would occur.
• Whether the audio data is two or four channel. Four channel is rarely if ever used.
• Whether digital copying is permitted. PC-based CD-R and RW drives ignore this; it was
instituted to prevent copying to DAT (digital audio tape) or home audio optical drives.
• Whether the music is recorded with pre-emphasis. This is a hiss or noise reduction technique.
• The track (song) layout on the disc.
• The track (song) number.
• The minutes, seconds, and frame number from the start of the track (song).
• A countdown during an intertrack (intersong) pause.
• The minutes, seconds, and frames from the start of the first track (song).
• The barcode of the CD.
• The ISRC (International Standard Recording Code). This is unique to each track (song) on the
disc.
The R-W subcodes are used on CD+G (graphics) discs to contain graphics and text. This enables a
limited amount of graphics and text to be displayed while the music is being played. The most
common use for CD+G media is karaoke “sing-along” media. These same subcodes are used on CD
TEXT discs to store disc- and track-related information that is added to standard audio CDs for
playback on compatible CD audio players. The CD TEXT information is stored as ASCII characters
in the R-W channels in the lead-in and program areas of a CD. On a CD TEXT disc, the lead-in area
subcodes contain text information about the entire disc, such as the album, track (song) titles, and
artist names. The program area subcodes, on the other hand, contain text information for the current
track (song), including track title, composer, performers, and so on. The CD TEXT data is repeated
throughout each track to reduce the delay in retrieving the data.
CD TEXT-compatible players typically have a text display to show this information, ranging from a
simple one- or two-line, 20-character display, such as on many newer RBDS (radio broadcast data
system) automobile radio/CD players, to up to 21 lines of 40-color, alphanumeric or graphics
characters on home- or computer-based players. The specification also allows for future additional
data, such as Joint Photographic Experts Group (JPEG) images. Interactive menus also can be used
for the selection of text for display.
 
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