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Compact Disc Basics
 | Laser Technology
| Compact Disc Introduction
| CD Physical Parameters
| CD Construction
| CD Layout
| Lead-in, program and lead-out areas
| Data Modulation & Error Correction
| Subcode Channel | |
Laser Technology
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The word LASER stands for Light Amplification by the Stimulated Emission of Radiation. Lasers generate coherent light, ie light comprising photons with the same wavelength and in-phase. This allows the light beam to be focused to a very small spot size similar to the actual wavelength of the light itself. The advent of lasers and in particular low cost laser emitting diodes has allowed the compact disc technology to become one of the most successful consumer electronic technologies of all time. |
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In the late 60s, Philips developed the laser video disc, the first such application of the laser for a consumer electronics product. The 30cm disc was capable of storing up to 60mins of analogue video per side. A low power laser was used to read the video information stored in pits in the disc surface. The video and audio signals were represented in analogue form by these pits which were arranged in a spiral pattern, like vinyl records. |
Compact Disc Introduction
The Compact Disc was
originally developed from the larger Laser disc for music recordings and uses a
similar 'pit' structure. The main difference (apart from the size of disc) is
that the CD uses a digital technique where the pits indicate whether a data bit
is '0' or '1'.
Whereas Laserdiscs can be either CAV (Constant Angular Velocity) or CLV (Constant Linear Velocity), all CDs use CLV. This means that the pit sizes do not vary from inside to outside of the disc but the angular velocity does vary.
The read head is a small laser diode emitting infra red 'light' which is focused to a beam small enough to read the sub-micron pits. The laser diode is mounted on a swivel arm which can be moved radially to follow the pits up and down to keep them in focus.
A semi-reflective mirror allows the reflected light to pass back to a photo detector. When the laser beam falls on a pit very little is reflected. The changing light pattern detected is then converted into a series of zeros and ones which are then decoded into the original audio or computer data signal.
Parameters
The main Compact Disc parameters are given in the table below.
|
Parameter |
Value |
Comments |
| Diameter: | 12 (or 8) cm | 8 cm CDs have not been successful |
| Thickness: | 1.2 mm | Tolerance of +0.3, -0.1 mm |
| Width of pits: | 0.5 microns | |
| Length of pits: | 0.8 to 3 microns | Depends on data stored |
| Depth of pits: | 0.15 microns | |
| 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: | 74 mins 44 secs | Playing times can be longer |
| Number of tracks: | 99 max | Can use indexes to subdivide tracks |
| Modulation | EFM | 8 to 14 bits plus 3 merging bits |
| Channel bit rate: | 4.3218 Mb/s | Actual raw data rate |
Compact Discs are 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 5,800m.
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. Pits are embossed into the 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, including the Lead-in,
Program &
Lead-out areas with their start and end
radii and other dimensions.
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.
Lead-in/out and Program Area
The area of the disc which contains data is divided into three areas:
Lead-in 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 which contains up to about 76 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.
This structure is identical for all types of CD whether for audio or computer data, although the data itself will vary.
Modulation & Error Correction
It is not possible to manufacture CDs where every pit is intact. Small defects in manufacture are permissible and even minor scratches which can occur with use do not usually affect the disc's playability. Therefore the CD specification includes two levels of error correction to compensate for these defects. These are CIRC error correction and EFM modulation.
| CIRC | A CIRC (Cross Interleaved Read-Solomon Code) encoder adds two dimensional parity information, to correct errors, and also interleaves the data on the disc to protect from burst errors. CIRC corrects error bursts up to 3,500 bits (2.4 mm in length) and compensates for error bursts up to 12,000 bits (8.5 mm) such as caused by minor scratches. |
| EFM Modulation | The EFM (Eight to Fourteen) modulation scheme encodes each 8-bit symbol as 14 bits plus 3 merging bits. The EFM data is then used to define the pits on the disc. The merging bits ensure that pit & land lengths are not less than 3 and no more than 11 channel bits. This reduces the effect of jitter and other distortions on the error rate. |
CD-ROM discs generally include a third level of protection
Subcode Channels
In addition to the main data channel (which contains audio or other data) there are 8 subcode channels labelled P to W interleaved with the main channel on the disc and available for use by CD audio and CD-ROM players.
P-channel indicates the start and end of each track and was intended for simple audio players which did not have full Q-channel decoding.
Q-channel contains the timecodes (minutes, seconds and frames), the TOC (in the lead-in), track type and catalogue number.
Channels R to W are for subcode graphics (known as CD-G) and
CD Text which accompany the main audio data.
When the CD was first developed, the subcode was included as a means of placing control data on the disc. The main channel was intended entirely for audio not any other form of data. Since then the main channel has been used for other types of data and the new DVD specification omits the subcode channels used for CDs.
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