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CD Physical Specification
The CD comprises a 12cm disc with the data
stored in pits on its upper surface. Now find out more. |
The main Compact Disc parameters are given in the table below.
| Parameter |
Value |
Comments |
| Diameter: |
12 (or 8) cm |
Few 8 cm CDs are made |
| Thickness: |
1.2 mm |
Tolerance +0.3, -0.1 mm |
| Width of pits: |
0.5 microns |
Approximate |
| Length of pits: |
0.8 to 3 microns |
Depends on data stored |
| Depth of pits: |
0.15 microns |
Approximate |
| Scan velocity: |
1.3 m/s |
Tolerance +/- 0.1 m/s |
| Track pitch: |
1.6 microns |
Tolerance +/- 0.1 microns |
| Laser wavelength: |
770 to 830 nm |
Typically 780 nm |
| Playing time: |
76m 30s |
Playing times can be longer |
| Number of tracks: |
99 max |
Can use indexes to subdivide tracks |
| Modulation |
EFM |
8 to 14 bits + 3 merge bits |
| Channel bit rate: |
4.3218 Mb/s |
Actual raw data rate |
Compact Discs are normally 12 cm in diameter and 1.2 mm thick.
The pits containing the audio or other data are about 0.15 microns deep and 0.8 to
3 microns in length depending on the data stored. This raw data is read at 4.32
Mb/s, but after demodulation (17 bits become 8) and error correction the data rate is 1.4
Mb/s. The total length of the helical 'track' of pits is about 5,700
metres for a 12 cm disc with maximum content.
CD
Construction
The compact disc comprises a sandwich as shown in the diagram. A 1.2 mm thick
polycarbonate substrate containing pits moulded into the upper surface is coated with
aluminium, which is then protected by a lacquer on which the disc label is printed.
An infra red laser beam is focused on the pits through the
clear optical grade polycarbonate plastic. This reduces the affect of any
scratches or dirt on the underside of the disc. Pits are embossed into the
upper polycarbonate
surface by an injection moulding process.
The aluminium layer provides a reflective surface which is
protected from corrosion and damage by a lacquer on which a disc label can be printed.
CD
Layout
The
layout of the CD is shown in the diagram.
CDs measure 12cm in diameter with a 15mm diameter centre
hole. The audio or computer data is stored from radius 25mm (after the lead-in) to radius
58mm maximum where the lead-out starts.
The part of the disc containing data
is divided into three areas:
- Lead-in (from radius 23 mm to 25 mm) which
contains digital silence in the main channel plus the Table
of Contents in the subcode Q-channel.
It also allows the laser pickup head to follow the pits and synchronise to the audio or
computer data before the start of the program area. The length of the lead-in is
determined by the need to store the Table of Contents for up to 99 tracks.
- Program area
(from radius 25mm to 58 mm maximum) which contains up to about 80 minutes of data divided into 99 tracks maximum.
- Lead-out which
contains digital silence or zero data. This defines the end of the CD program area.
All audio CDs are played at a constant linear velocity
(CLV) of 1.3 m/s. The angular velocity (rpm) will reduce from the lead-in to the lead-out
by a factor of 58/23 = 2.52. This means that pits retain the same geometry wherever they
are on the disc and there will be no change in performance across the disc.
The disc layout is identical for all types of CD whether for
audio or computer data, although the data itself will vary.
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