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 digital techniques, storing samples as numbers. The process of converting analogue to digital is known as digitising or sampling. The analogue waveform is chopped into a number of slices per second. At each slice, the amplitude is measured and rounded to the nearest available value. Clearly the more chops per second (sampling rate) and the finer the values assignable to the amplitude (dynamic range), the better the representation of the original.
CD digital employs a sampling rate of 44.1kHz and a 16-bit dynamic range. That is, 44,100 chops every second, each one describing the waveform amplitude at that moment in time with a 16-bit number; 16-bit itself offering 65,536 steps from which to choose. This sampling rate provides a frequency response adequate for sounds up to 20kHz in pitch – although some audiophiles argue that it isn’t sufficient to capture essential psychoacoustic effects that occur beyond the range of human hearing. Conventional wisdom holds that 16-bits provides ample dynamic range for loud and soft musical passages. The audio is recorded on two tracks, for stereo sound.
A 44.1kHz rate means there are 44,100 chops every second, each one describing the waveform amplitude at that moment in time with a 16-bit number; 16-bit itself offering 65,536 steps from which to choose. With samples occupying two bytes on each of two channels – each sample yields a data transfer rate of just over 176 KBps. A single-speed CD-ROM transfers data at the same rate, but a portion of the data stream is taken by error-correcting information – reducing the effective transfer rate to 150 KBps. A CD can hold up to 74 minutes of encoded stereo audio data – which, when the ECC overhead is taken account of, equates to the standard CD capacity of 680MB.
The table below summarises the relevant parameters:
| Parameter | Value |
|---|---|
| Sample rate | 44.1kHz |
| Channels | 2 (stereo) |
| Bits per sample, per channel | 16 |
| Levels per sample | 65,536 |
| Total data rate | 176 KBps |
| Effective data rate | 150 KBps |
| Effective CD capacity | 680MB |
| CD playing time | 74 minutes |
- CD-ROM Red Book
- CD-ROM Yellow Book
- CD-ROM XA
- CD-ROM Green Book
- CD-ROM Orange Book
- CD-ROM White Book
- CD-ROM Blue Book
- CD-ROM Purple Book
- CD-ROM CD-I Bridge
- CD-ROM Photo CD
- CD-ROM File Systems
- CD-ROM Manufacturing
- CD-ROM The Disc
- CD-ROM Operations
- CD-ROM Digital Audio
- CD-ROM CLV
- CD-ROM CAV
- CD-ROM Applications
- CD-ROM Interfaces
- CD-ROM DMA vs. PIO Mode
- CD-ROM TrueX Technology