Double Density CD - Page 7

Physical Sector

The physical format defined by the DDCD media standard provides 2,352 bytes per sector. For computer data applications, 2,048 bytes is used for user data, 12 bytes for a synchronization field, 4 bytes for a sector address, 8 bytes for an additional information and 276 bytes – the auxiliary field –for Layered ECC.

Sector Structure

A Sector, called Mode2 Sector, shall consist of 2,352 bytes arranged with 12 Sync bytes, 4 Header bytes, 8 SubHeader bytes, 2,048 User Data bytes, and 4 Error Detection Code (EDC) bytes, 172 P-Parity bytes, 104 Q-Parity bytes. The User Data bytes are identified from 24th to 2071st. The Header shall consist of 3 bytes of Sector Address and 1 Mode byte. The Mode byte shall be 02h to indicate Mode 2 Disc Type.

The physical format of DDCD media uses smaller unit of synchronization than the other magnetic or optical recording systems. The basic unit of the data stream synchronization is a small frame. This is different from large frame (sector) as referred to in the HMSF unit. Each small frame consists of 588 bits. A sector on DDCD media consists of 98 small frames.

A DDCD 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 to 14-bit codes according to the EFM table; then three merging bits are added. The merging bits are chosen to minimize DSV (Digital Sum Value) and provide DC free characteristics. 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 equals 2,352 bytes of data per sector.

Sub-Channel Information Formats

The sub-channel 1 byte of each frame is assigned one bit for each of the 8 sub-channels, designated P, Q, R, S, T, U, V, W. Sub-channel P and R to W are all reserved and set to zero. All the sub-channel Q bits of a sector define the sub-channel Q information block. The sub-channel Q block consists of 98 bits, one bit from each small frame in a sector.

The format provides the information of the location 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
5. 8-bits index number
6. 28-bits reserved
7. 28-bits Absolute HMSF address ( 4-bits Hour, 8-bits Minutes, 8-bits Seconds, 8-bits Frames)
8. 16-bits CRC error detection code

TNO = 01 to 99 is the track number in BCD
INDEX = 00.
AHOUR, AMIN, ASEC, AFRAME
= the absolute time address expressed in 7 BCD digits.