Sampling

The conversion from a continuously changing measure to a series of measured values at discrete time instances is called sampling. The rate at which the sampling is done, is along with the quantization depth the most important quality factor of digital recording equipment. You will not be able to record the highest audible frequencies if it is set too low. A CD quality recording is measured at a rate of 44 100 samples per second. We say that the sampling rate (or sampling frequency) is 44 100 Hertz (or short Hz).

In fact, all frequencies above half the sampling frequency, which is known as the Nyquist frequency, are substituted by frequencies below the Nyquist frequency unless the audio input is filtered. This effect is called aliasing. To avoid aliasing a sampling system contains of a low pass filter which ideally filters out all frequencies above the Nyquist frequency and leaves all frequencies below unaffected. In the case of CD audio, the highest frequency that can theoretically be recorded is 22 050 Hz.