 |
CD-ROM Specification
This page describes how data is stored on
a CD-ROM in sectors with error correction to support
many applications. |
The CD-ROM specification (Yellow
Book) defines the use of two types of sectors,
mode 1 and mode 2. Mode 2 sectors are used for CD-ROM
XA formats. The main parameters of CD-ROM and CD-ROM
XA discs are given in the table below.
| Parameter |
Value |
Comments |
| Capacity |
680 MB |
Assuming 74 minutes |
| Raw data bitrate |
1.41Mbits/s |
Includes all bytes in sector |
| User data rate |
150 kB/s |
At 1x speed |
| Block (sector) size |
2,352 bytes |
Including header, ECC etc |
| User data per sector |
2,048 bytes |
With full error correction |
| Sector rate |
75 sectors/s |
At 1x speed |
| Sector Modes |
1 or 2 |
See Sectors |
| Sector Forms |
1 or 2 |
Mode 2 only |
CD-ROM Sectors
Data stored on a CD-ROM disc is divided into sectors which are
equivalent to the audio frames for a CD audio disc. At normal (1x)
playback speed, 75 sectors are read every second. For double speed
CD-ROM drives this increases to 150 sectors per second and so on.
Seek times, while the disc rotates to the required starting position,
will also reduce as speeds increase.
Because CDs were designed primarily for audio, their use for computer
data requires the addition of header data and error correction
codes which are included in every sector. There are two different
types of sectors defined in the CD-ROM specification, Mode
1 and Mode 2 (the latter being
used for CD-ROM XA discs).
Video CD discs use Mode 2, Form 2 sectors
for the MPEG video, allowing
faster data rates (172 kbytes/sec instead of 150 kbytes per second).
Mode 1 Sectors
Mode 1 sectors are intended for the storage of computer data and
contain the following fields.
- Sync (12 bytes) which is used to enable the player to
identify the start of each sector.
- Header (4 bytes) consisting of Minutes, Seconds, Sectors
and Mode (= 1).
- ECC (Error Correction Code - 276 bytes), which comprises
an additional level of CIRC error
protection.
- EDC (Error Detection Code - 4 bytes) for detecting errors
to be corrected.
Mode 1 sectors are the simplest type and are used for most CD-ROM
based formats which follow the Yellow
Book.
Mode 2 Sectors
Mode 2 sectors are used for those formats based on CD-ROM
XA and can be either Form 1 or 2.
- Mode 2 Form 1 sectors contain 2048 bytes with the same
ECC as Mode 1 sectors.
- Mode 2 Form 2 sectors contain 2324 bytes of user data
per sector, with no ECC are are suitable only for data where
errors can be concealed (eg audio or video data).
Mode 2 sectors comprise the following fields:
- Sync (12 bytes) which is used to enable the player to
identify the start of each sector.
- Header (4 bytes) consisting of Minutes, Seconds, Sectors
and Mode (= 1).
- Subheader (8 bytes) contains content related parameters
eg data type.
- ECC (Error Correction Code - 276 bytes) which comprises
an additional level of CIRC error
protection for Form 1 only.
- EDC (Error Detection Code) for Forms 1 and 2.
Note that Mode 1 and Mode 2 Form 1 use the same error correction
so can be used interchangeably, but not within the same track and
preferably not on the same disc. Software used to write CD-Rs
can be set for Mode 1 or Mode 2 Form 1. Almost all PCs and
Macs will read Mode 2 Form 1 CD-ROMs as well as Mode 1.
Note that any CD-ROM will contain at least some Mode 1 or Mode
2 Form 1 sectors.
Capacity of a
CD-ROM
The capacity of a CD-ROM depends on whether it is a Mode
1 CD-ROM or Mode 2 CD-ROM XA.
Assuming the maximum size is 76 minutes 30 seconds (as recommended)
this means that there are 336,300 sectors on a CD-ROM. From this
must be subtracted 166 sectors at the start of track 1 plus a
few sectors for the file system, amounting to, say, 200 sectors
leaving 336,100 sectors for user data.
- Mode 1 sectors contain 2048 bytes per sector giving
a total capacity of 688,332,800 bytes or 656MB (where 1 MB =
1024 * 1024).
- Mode 2 sectors contain either 2048 or 2324 bytes per
sector so will have a somewhat higher data capacity depending
on the mix of the two types of sector.
The above assumes a CD-ROM comprising a single track in a single
session. For multiple track/session discs the data capacity will
be reduced.
|
