Compact Disk (CD)[4]
- Introduction
- CD Drive: basic design
- Compact disk (CD): basic design
- Data encoding
- Making CDs
- Spin rates and Data transfer rates
- Interfaces
- Multi-beam technology
- White Papers on compact disk technology
-
Get a Optical Media Data Recovery Quote
Data encoding
In the compact disk, every transition from pit to land and back is interpreted as 1. No transition means 0, and the length of each land segment represents the number of 0s in the data stream. This principle is illustrated in the figure below, which shows a stream of pits and lands and above it is the corresponding digital data stream.
All codes needed to convert bits into their physical representation and back are called channel codes. The channel code for CD-DA and CD-ROM is called EFM: "eight-to-fourteen modulation". It interprets user's data along with the error correction data, address data, synchronization data, and other content into a stream of "channel bits". The "channel bits" are converted into a binary code and, eventually, turned into pits by the mastering machine.
During the play back, the EFM decoder of the CD-ROM works in the opposite direction converting light modulations on the bits into a binary data stream, which is then cleared of any miscellaneous data by the drive's electronics.
Unfortunately, the resolution of the CD drive's optics is not sufficient to read directly a sequence of 1s or 0s following each other too closely, i.e. 111111. Another limitation is the maximum length of a given pit or land, in order to leave room for the clock (syncronization) data. Therefore, it was agreed to keep at least two 0s between two 1s and, that the maximum length and pits was limited to 10 bits in a row. These limitations led to the eight-to-fourteen conversion system which represents 8 user-bit bytes with the (minimally required) 14 channel-bit modulation code.
Another problem can appear if two 14-bit symbols follow each other, i.e. 1 at the end of one symbol could be too close to 1 at the beginning of another symbol. To solve this issue, 3 special "merge" channel bits are placed between the 14 channel-bit symbols. Thus, for each 8-bit user data 17 channel-bits are used.
A basic unit of information stored on a CD is called a frame. The frame equals to 24 17-bit symbols combined with the synchronization pattern, a control and display symbol, and 8 error correction symbols. Frames are grouped together to form blocks (also called sectors). Each block has 2352 bytes of user data in the CD-DA standard or 2048 bytes in the CD-ROM standards (due to tighter error correction technique and more error correction bytes). The next figure shows structure of one CD-ROM block.
| 00 | FF x 10 | 00 | MIN | SEC | SECTOR | MODE | DATA | LAYERED ECC |
| 12 bytes (synch) | 4 bytes (ID) | 2048 bytes | 288 bytes | |||||
| <--------------------------------------- 2352 bytes -----------------------------------------------> | ||||||||
The first CD drives played back 75 blocks per second, which translated into the data transfer rate 1X equal to about 0.15 MB/s.