Model for CD-ROM devices CD-ROM media organization CD Audio error reporting
SCSI-2

14.1.2 CD-ROM physical data format

The physical format of CD-ROM and CD-DA media uses a smaller unit of synchronization than the more familiar magnetic or optical recording systems. The basic data stream synchronization unit is a small frame. This is not the same large frame (sector) as referred to in the MSF unit. Each small frame consists of 588 bits. A sector on CD-ROM media consists of 98 small frames.

A CD-ROM small frame consists of:

  1. 1 synchronization pattern (24+3 bits)
  2. 1 byte of sub-channel data (14+3 bits)
  3. 24 bytes of data (24 x (14+3) bits)
  4. 8 bytes of CIRC code (8 x (14+3) bits) Total: 588 bits.

Data, sub-channel and CIRC bytes are encoded with an eight-to-fourteen bit code; then three merging bits are added. The merging bits are chosen to provide minimum low-frequency signal content and optimize phase lock loop performance.

14.1.2.1 Frame format for audio

Each small frame of an audio track on a two-channel CD-DA or CD-ROM media consists of six digitized 16-bit samples of each audio channel. These 24 bytes of data are combined with a synchronization pattern, CIRC bytes and a sub-channel byte to make a frame. Each frame takes approximately 136.05 uS to play. This gives a sampling rate of 44,1 khz for each channel. The sub-channel information creates the higher level sector grouping for audio tracks.

14.1.2.2 Sector format for data

The data bytes of 98 small frames comprise the physical unit of data referred to as a sector. (98 small frames times 24 bytes per small frame equal 2 352 bytes of data per sector.)

A sector that contains CD-ROM data mode one data has the following format:

  1. 12 bytes Synchronization field
  2. 4 bytes CD-ROM data header Absolute M field in bcd format Absolute S field in bcd format Absolute F field in bcd format CD-ROM data mode field
  3. 2048 bytes User data field
  4. 4 bytes Error detection code
  5. 8 bytes Zero
  6. 276 bytes Layered error correction code

A sector that contains CD-ROM Data Mode two data has the following format:

  1. 12 bytes Synchronization field
  2. 4 bytes CD-ROM data header Absolute M field in bcd format Absolute S field in bcd format Absolute F field in bcd format CD-ROM data mode field
  3. 2 336 bytes User data field (2048 bytes of mode 1 data plus 288 bytes of auxiliary data) ( Many drives are capable of returning CD-ROM data mode one data in a CD-ROM data mode two format. This allows the user to investigate the error detection and error correction codes. However data encoded as CD-ROM data mode two cannot be read as CD-ROM data mode one data. )

14.1.2.3 Sub-channel information formats

The sub-channel byte of each frame is assigned one bit for each of the 8 sub-channels, designated P, Q, R, S, T, U, V, W. Only sub-channels P and Q are defined for CD-ROM media. The other sub-channel bits are zero.

Sub-channel P is a simple flag bit that may be used for audio muting control and track boundary determination.

Sub-channel Q has a higher level of structure. All the sub-channel Q bits of a sector define the sub-channel Q information block. (For audio tracks, decoding the Q sub-channel is the only way to distinguish sector boundaries.)

The sub-channel Q block consists of 98 bits, one bit from each small frame in a sector. Three formats are defined for the sub-channel Q information block. The first format provides location information and is defined as follows:

  1. 2 bits Sub-channel synchronization field
  2. 4 bits ADR field - defines the format
  3. 4 bits Control field - defines the type of information in this sector
  4. 8 bits Track number (in BCD)
  5. 8 bits Index number (in BCD)
  6. 24 bits Track relative MSF address
  7. 8 bits Reserved (0)
  8. 24 bits Absolute MSF address
  9. 16 bits CRC error detection code

This format is required to exist in at least nine out of ten consecutive sectors.

The second and third formats are optional. If used, they must exist in at least one out of 100 consecutive sectors. They include the absolute frame byte of the MSF address to provide location information continuity.

The second format gives the catalogue number of the disc (UPC/EAN bar code number). This information is constant over the whole media.

The third format gives the International Standard Recording Code (ISRC) for each track. The ISRC is defined in ISO 3901. This format is not present on lead-in or lead-out tracks and may change only after the track number changes.