CHAPTER 13

What Is a CD-ROM? Chapter 13
697
Subcodes
The subcode bytes enable the drive to find songs (which are confusingly also called tracks) along the
spiral track and convey additional information about the disc. The subcode bytes are stored as 1 byte
per frame, which gives 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 is used to identify the start of the tracks on the CD. The Q subcode contains a multitude
of information, including
■ Whether the sector data is audio (CD-DA) or data (CD-ROM). This prevents most players from trying
to “play” CD-ROM 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. CD-R and RW drives ignore this; it was instituted to prevent
copying to DAT (digital audio tape) 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 frame 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. 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 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 Photographics Expert Group (JPEG) images. Interactive menus also can be used for the selection
of text for display.