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Tracks and
Indexes
The audio data on all CDs can be subdivided
into tracks and these can optionally be subdivided into indexes |
The audio on a CD is divided into a maximum of 99 tracks, used to
separate different items, eg songs, on the disc. Each track must be at least 4 seconds in
length and a pause of 2 seconds may be inserted between tracks. Any track may be accessed rapidly and tracks may be played in random order.
For each track an ISRC must be included to
identify that track uniquely.
Usually a track will contain two indexes, 0 and 1. Index 0 marks the pause (normally 2 seconds) at
the beginning of each track, while index 1 is for the main part of the track
containing the music. Additional
index values (up to 99 in all) may be used where the 99 track limit is inadequate
(as in the second example in the diagram above). For
example a CD of short audio clips may comprise more than 99 'tracks' by subdividing some
tracks into different index values.
Table of Contents
Track start times (but not indexes) are defined in the Table of Contents (TOC)
which is contained in the Q-channel during the lead-in area.
The TOC comprises the time-code for each track
(as minutes, seconds and frames) stored three times and is used by CD-players
for fast random access to tracks and features such as shuffle.
The last time-code (defined
as hexadecimal AA) gives the start of the lead-out.
The lead-in area must be long enough
to store the TOC for 99 tracks. The TOC also defines the track type, which can be audio or
data.
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"Track start times ... are defined in the Table of Contents"
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