How to interpret cdrecord output?

Prev		Next

Newsgroups: comp.os.linux.misc
Date: Thu, 20 Mar 2003 21:15:48 -0800

> How much data I can I write to a CD-ROM?

Go to http://www.google.com . Search for

CD-ROM sector size

The first result is http://www.disctronics.co.uk/technology/cd-rom/cdrom_spec.htm which contains the exact information you desire.

John Reiser

CD-ROM Specification

http://www.disctronics.co.uk/technology/cd-rom/cdrom_spec.htm

This page describes how data is stored on a CD-ROM in sectors with error correction to support many applications.

The CD-ROM specification ([24]Yellow Book) defines the use of two types of [25]sectors, mode 1 and mode 2. Mode 2 sectors are used for [26]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 [27]Sectors
Sector Forms	1 or 2	[28]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, [29]Mode 1 and [30]Mode 2 (the latter being used for [31]CD-ROM XA discs).

[32]Video CD discs use Mode 2, Form 2 sectors for the [33]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 [34]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 [35]Yellow Book.

Mode 2 Sectors

Mode 2 sectors are used for those formats based on [36]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 [37]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 [38]Mode 1 CD-ROM or [39]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.

Disctronics Manufacturing (UK) Ltd.
       Copyright (c) 1999 - 2003