Session (CD) - Data Encoding

Data Encoding

Before being written to the disc, the LPCM audio data is divided into 12-sample frames (six left and right samples, alternating) and subjected to CIRC encoding, which segments and rearranges the data and expands it with "parity" bits in a way that allows occasional read errors to be detected and corrected. 8 bits of subcode data are added to each frame. The resulting 291-bit frame data is EFM-modulated, where each 8-bit word is replaced with a corresponding 14-bit word designed to reduce the number of transitions between 0 and 1, thus reducing the density of physical pits on the disc and providing an additional degree of error tolerance. 3 "merging" bits are added before each 14-bit word for disambiguation and synchronization. A 24-bit word is added to the beginning of each frame to assist with synchronization, so the reading device can locate frames easily. The EFM, merging bits, and sync words thus expand each frame from 291 to 588 bits of "channel data". The frames of channel data are written to disc physically in the form of pits and lands, with each pit or land representing a series of zeroes, and with the transition points—the edge of each pit—representing 1.

The disc's audio data stream is continuous, but has three parts: the main portion, which is further divided into playable audio tracks, is the program area. This section is preceded by a lead-in track and followed by a lead-out track. The lead-in and lead-out tracks encode only silent audio, but all three sections contain subcode data streams. The lead-in's subcode contains repeated copies of the disc's Table Of Contents (TOC), which provides an index of the start positions of the tracks in the program area and lead-out. The track positions are referenced by absolute timecode, relative to the start of the program area, in MSF format: minutes, seconds, and fractional seconds called frames. Each timecode frame is one seventy-fifth of a second, and corresponds to a block of 98 channel-data frames—ultimately, a block of 588 pairs of left and right audio samples. Timecode contained in the subchannel data allows the reading device to locate the region of the disc that corresponds to the timecode in the TOC.

In the 1990s, CD-ROM and related Digital Audio Extraction (DAE) technology introduced the term sector to refer to each timecode frame, with each sector being identified by a sequential integer number starting at zero, and with tracks aligned on sector boundaries. An audio CD sector corresponds to 2,352 bytes of decoded data. The Red Book does not refer to sectors, nor does it distinguish the corresponding sections of the disc's data stream except as "frames" in the MSF addressing scheme.