What is Sample Rate in Audio ?

« Sample rate » is omnipresent in the fields of digital audio. It is mentioned in any of the audio interface’s technical sheets. Let’s see what does it mean.

Sampling is a process that samples at pre-defined intervals several measurements of a determined signal. This principle is the base of digital concerns such as « Photography, Audio, Video ». In the field of digital audio, it allows the conversion of analog signal generated through a microphone for example (or from a hardware device such as a synthesizer) into a digital signal, this format being required for being treated and analyzed by computers.

The « sampling rate » (expressed in Hertz) defines how many values has been sampled by time’s unit. A sampling rate of 44,1 kHz (44.100 Hertz) means that the audio signal has been measured 44,100 times by second.

Here-after is a simplified graph represented by the sinusoidal wave (in black) while the red lines represent measurements that have been done to sample that audio signal.

The precision of the waveform is obviously linked with the number of measurements performed : precision will increase together with their number.

SAMPLING RATE’S CHOICE

The Nyquist-Shannon’s theorem mentions that the maximal frequency to be generated requires a sampling rate being equal at twice that frequency. The highest frequency human ear can hear is 20 kHz, so 40 kHz seemed like a great value for an audio sample rate, right ?

WHY HAVING SELECTED 44.1 kHz ?

Why has this « decimal value » been selected instead of a more « natural » value like 40 kHz for example ?
First of all, technically, it’s not possible to perform a sampling without a little margin of error. This explains why a sampling rate being exactly twice the value of 20 kHz could not have been technically feasible.

Moreover, the reference value of 44.1 kHz has not been created from scratch for digital audio and CD, it was already existing for the Video tape recorders. Therefore, it has been selected too for audio.

These days, 44.1 kHz remains the « standard value ». Every converter available on the market are using this sample rate and, in most of the cases, some additional sample rates are also available (88,2 kHz, 176,4 kHz). The Audio CD (while being declining those days) uses this 44.1 kHz value.

If such reference value of 44.1 kHz is used for most digital audio purposes, this is not the case for the « TV channels » and for all gears related to the field of Video. The « standard » used in such area are based on a 48 kHz value and its multiples (96 kHz, 192 kHz).