CD Audio Facts

The Compact Disc Digital Audio (CD-DA) standard was developed by Philips and Sony and introduced into the market in 1982. Compact Discs are superior to vinyl discs and cassettes in a number of ways:

Superior sound quality without skips, hiss or other defects
Fast random access to any track on the disc
Long-life; compact discs virtualy last forever
Compact size: only 12cm in diameter so they take up little storage space
The superior quality of CDs and their compact size is made possible by the use of digital technology.

Digital Audio

On vinyl and audio cassettes, the audio waveform is recorded as an analogue signal. Therefore any imperfections will be heard as noise (hiss) or other defects. To reduce these defects, CDs use Pulse Code Modulation (PCM), the simplest of digital coding technologies.

Using PCM technology samples of the analogue waveform are taken at intervals and stores each sample as a number. The example above shows the conversion of an analogue waveform (which could be part of an audio signal) to digital by representing each sample by a number (from 0 to 100 in this simple example).

In practice the range of values and sampling rate must be high enough to ensure accurate reproduction of the original analogue waveform. The upper limit for the human ear is about 20kHz therefore the audio must be sampled at 40,000 times per second or higher (since two samples are required for both halves of a sine wave). To reduce distortion and quantisation noise each sample must be represented by at least a 16-bit number giving 65,536 values or levels (0 to 65,535) per sample.

Compact Discs sample the audio 44,100 times per second. The total information needed for 1 second of audio is therefore 44,100 x 2 x 16 = 1,411,200 bits. A bit is a binary digit and has the value 0 or 1. Although longer times are possible, the maximum playing time for all CDs is about 74 minutes to ensure compatibility with earlier CD players. This means that one CD must store 1,411,200 x 74 x 60 = 6,265.728 million bits. This can also be given as 783.216 million bytes, where one byte = 8 bits.

Digital coding allows the use of error correction codes, which are necessary to correct errors resulting from the manufacturing process and minor damage or marks which may occur from handling and use. The result is that the amount of data stored on a CD is nearly four times the data needed to represent the audio only.

CD digital audio should provide the quality needed for all audio applications, but for the purist this is not always enough. For this reason an enhanced format (HDCD) has been introduced and the new DVD-Audio format incorporates new features including higher sampling rate, more bits per sample and multi-channel surround sound.

CD Audio Parameters

Compact Discs have the following parameters for audio stored on them:

Sample rate: 44.1 kHz
Channels: 2 (stereo)
Bits per sample, per channel: 16
Levels per sample: 65,536
Total data rate (Mb/s): 1.4112

Digital encoding can produce virtually noise-free results but an error correction scheme is needed to protect against minor damage to the CD which can produce incorrect data.

CD digital audio should provide the quality needed for all audio applications, but for the purist this is not always enough. For this reason an enhanced format (HDCD) has been introduced and the new DVD-Audio format will incorporate new features including higher sampling rate, more bits per sample and multi-channel surround sound.

Playing Time

The playing time of a CD can be calculated from the area of the program area, the linear velocity and track pitch. The results are shown in the table beloe for different values of track pitch (1.5 to 1.7 microns) and linear velocity (1.2 to 1.4 m/s). The start radius is assumed to be 25 mm and the end radius 58 mm for a 120 mm disc. The longest playing time of nearly 80 minutes is achieved by reducing the track pitch to 1.5 microns and the linear velocity to 1.2 m/s, which gives no allowance for tolerances in the manufacturing processes.