In: Categories » Computers and technology » Storage devices » Subcodes
| Subcode bytes enable the drive to find songs (which are confusingly also called tracks) along the spiral track and also 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 PW. 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 RW subcodes are used only on CD+G (graphics) or CD TEXTtype 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
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 RW 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 TEXTcompatible 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 Photographics Expert Group (JPEG) images. Interactive menus also can be used for the selection of text for display.
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