CD ROM and CD R tracks and sectors

written by: Terry McLean; article published: year 2006, month 12;


In: Categories » Computers and technology » Storage devices » CD ROM and CD R tracks and sectors

The pits are stamped into a single spiral track with a spacing of 1.6 microns between turns, corresponding to a track density of 625 turns per millimeter, or 15,875 turns per inch. This equates to a total of 22,188 turns for a typical 74-minute (650MiB) disc. The disc is divided into six main areas:

  • Hub clamping area. The hub clamp area is just that: a part of the disc where the hub mechanism in the drive can grip the disc. No data or information is stored in that area.

  • Power calibration area (PCA). This is found only on writable (CD-R/RW) discs and is used only by recordable drives to determine the laser power necessary to perform an optimum burn. A single CD-R or CD-RW disc can be tested this way up to 99 times.

  • Program memory area (PMA). This is found only on writable (CD-R/RW) discs and is the area where the TOC (table of contents) is temporarily written until a recording session is closed. After the session is closed, the TOC information is written to the lead-in area.

  • Lead-in. The lead-in area contains the disc (or session) TOC in the Q subcode channel. The TOC contains the start addresses and lengths of all tracks (songs or data), the total length of the program (data) area, and information about the individual recorded sessions. A single lead-in area exists on a disc recorded all at once (Disc At Once or DAO mode), or a lead-in area starts each session on a multisession disc. The lead-in takes up 4,500 sectors on the disc (1 minute if measured in time, or about 9.2MB worth of data). The lead-in also indicates whether the disc is multisession and what the next writable address on the disc is (if the disc isn't closed).

  • Program (data) area. This area of the disc starts at a radius of 25mm from the center.

  • Lead-out. The lead-out marks the end of the program (data) area or the end of the recording session on a multisession disc. No actual data is written in the lead-out; it is simply a marker. The first lead-out on a disc (or the only one if it is a single session or Disk At Once recording) is 6,750 sectors long (1.5 minutes if measured in time, or about 13.8MB worth of data). If the disc is a multisession disc, any subsequent lead-outs are 2,250 sectors long (0.5 minutes in time, or about 4.6MB worth of data).

The hub clamp, lead-in, program, and lead-out areas are found on all CDs, whereas only recordable CDs (such as CD-Rs and CD-RWs) have the additional power calibration area and program memory area at the start of the disc.

Officially, the spiral track of a standard CD-DA or CD-ROM disc starts with the lead-in area and ends at the finish of the lead-out area, which is 58.5mm from the center of the disc, or 1.5mm from the outer edge. This single spiral track is about 5.77 kilometers, or 3.59 miles, long. An interesting fact is that in a 56x CAV (constant angular velocity) drive, when reading the outer part of the track, the data moves at an actual speed of 162.8 miles per hour (262km/h) past the laser. What is more amazing is that even when the data is traveling at that speed, the laser pickup can accurately read bits (pit/land transitions) spaced as little as only 0.9 microns (or 35.4 millionths of an inch) apart!

The table below shows some of the basic information about the two main CD capacities, which are 74- and 80-minute. The CD standard originally was created around the 74-minute disc; the 80-minute versions were added later and basically stretch the standard by tightening up the track spacing within the limitations of the original specification. A poorly performing or worn out drive can have trouble reading the 80-minute discs.

CD-ROM Technical Parameters
Advertised CD length (minutes) 74 80
Advertised CD capacity (MiB) 650 700
----------------------------------------------------------------------------------
1x read speed (m/sec) 1.3 1.3
Laser wavelength (nm) 780 780
Numerical aperture (lens) 0.45 0.45
Media refractive index 1.55 1.55
----------------------------------------------------------------------------------
Track (turn) spacing (um) 1.6 1.48
Turns per mm 625 676
Turns per inch 15,875 17,162
Total track length (m) 5,772 6,240
Total track length (feet) 18,937 20,472
Total track length (miles) 3.59 3.88
----------------------------------------------------------------------------------
Pit width (um) 0.6 0.6
Pit depth (um) 0.125 0.125
Min. nominal pit length (um) 0.90 0.90
Max. nominal pit length (um) 3.31 3.31
----------------------------------------------------------------------------------
Lead-in inner radius (mm) 23 23
Data zone inner radius (mm) 25 25
Data zone outer radius (mm) 58 58
Lead-out outer radius (mm) 58.5 58.5
Data zone width (mm) 33 33
Total track area width (mm) 35.5 35.5
----------------------------------------------------------------------------------
Max. rotating speed 1x CLV (rpm) 540 540
Min. rotating speed 1x CLV (rpm) 212 212
Track revolutions (data zone) 20,625 22,297
Track revolutions (total) 22,188 23,986
B = Byte (8 bits)
KB = Kilobyte (1,000 bytes)
KiB = Kibibyte (1,024 bytes)
MB = Megabyte (1,000,000 bytes)
MiB = Mebibyte (1,048,576 bytes)
m = Meters
mm = Millimeters (thousandths of a meter)
um = Micrometers = microns (millionths of a meter)
CLV = Constant linear velocity
rpm = Revolutions per minute


The spiral track is divided into sectors that are stored at the rate of 75 sectors per second. On a disc that can hold a total of 74 minutes of information, that results in a maximum of 333,000 sectors. Each sector is then divided into 98 individual frames of information. Each frame contains 33 bytes: 24 bytes are audio data, 1 byte contains subcode information, and 8 bytes are used for parity/ECC (error correction code) information.The table below shows the sector, frame, and audio data calculations.

CD-ROM Sector, Frame, and Audio Data Information
Advertised CD length (minutes) 74 80
---------------------------------------------------------------------------------------
Sectors/second 75 75
Frames/sector 98 98
---------------------------------------------------------------------------------------
Number of sectors 333,000 360,000
Sector length (mm) 17.33 17.33
Byte length (um) 5.36 5.36
Bit length (um) 0.67 0.67
Each Frame:
---------------------------------------------------------------------------------------
Subcode bytes 1 1
Data bytes 24 24
Q+P parity bytes 8 8
---------------------------------------------------------------------------------------
Total bytes/frame 33 33
Audio Data:
---------------------------------------------------------------------------------------
Audio sampling rate (Hz) 44,100 44,100
Samples per Hz (stereo) 2 2
Sample size (bytes) 2 2
Audio bytes per second 176,400 176,400
Sectors per second 75 75
---------------------------------------------------------------------------------------
Audio bytes per sector 2,352 2,352
Each Audio Sector (98 Frames):
---------------------------------------------------------------------------------------
Q+P parity bytes 784 784
Subcode bytes 98 98
Audio data bytes 2,352 2,352
---------------------------------------------------------------------------------------
Bytes/sector RAW (unencoded) 3,234 3,234
Hz = Hertz (cycles per second)
mm = Millimeters (thousandths of a meter)
um = Micrometers = microns (millionths of a meter)

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