Sample Rates and Bit Depth for Digital Audio

48kHz, 44.1kHz

AES recommended practice for professional digital audio, preferred sampling frequencies for applications employing pulse-code modulation: 'A sampling frequency of 48 kHz is recommended for the origination, processing, and interchange of audio programs employing pulse-code modulation. Recognition is also given to the use of a 44,1-kHz sampling frequency related to certain consumer digital audio applications, the use of a 32-kHz sampling frequency for transmission-related applications, and the use of a 96-kHz sampling frequency for applications requiring a higher bandwidth or more relaxed anti-alias filtering.'

Why 44.1kHz ?

For digital consumer audio the sample rate of 44.1 kHz was chosen because early digital recording systems using PCM adapters recorded on 3/4" video tape (e.g. the Sony PCM 1610 and PCM 1630 machines). The sample frequency had to be related to a frequency common to both 625 and 525 line video systems. In 625 line / 25 Hz systems (PAL) the line frequency is 15.625 kHz and 588 of the 625 lines are "active" for carrying video information. If three samples are recorded per line the sample rate is 15.625 x 588/625 x 3 = 44.1 kHz.

In 525 line / 30 Hz systems (NTSC) the line frequency is 15.75 kHz and 490 of the 525 lines are "active" for carrying video information. If three samples are recorded per line the sample rate is 15.75 x 490/525 x 3 = 44.1 kHz.

Because of the fixed standard for the regular audio CD, the standard of 44.1 kHz will be around for a long time.

48kHz, 96kHz, 192kHz

A sample rate of 48 kHz is the current standard which most manufacturers follow, even when the internal processing rate is 96k or 192k in order to make the internal process faster. The conversion from 48 kHz to 96 kHz or 192 kHz is much easier than the conversion from 44.1 kHz to 48 kHz or vice versa.

The sample rate of 48 kHz is adopted from the entire film, video and broadcast industry and also DVD is based on the standard of 48/96 kHz.

Bit Depth - Dynamic Range

The dynamic range is the difference between the highest and the lowest possible signal level, the level difference between the maximum level and the noise floor. It is measured in dB. In digital audio it is equivalent to the bit depth. Bit depth is the maximum possible number of bits in an audio sample. It is also called 'resolution'. The higher the bit depth or resolution the more precise is the reproduction of an analog audio signal.

 dynamic range 24-bit must be lowered by 48dB to equal a 16-bit recording

 Bits per sample Number of possible values Dynamic Range 2 4 12dB 3 8 18dB 4 16 24dB 8 256 48dB 12 4096 72dB 16 65536 96dB 24 1677216 145dB 32 4294967296 193dB

(Not to confuse bit rate and bit depth: bit rate is the amount of crried bits per second, bit depth is the resolution per sample.)