Aliasing is a kind of sampling error that can occur during the conversion of an analog audio signal to digital. When an analog signal is recorded into a DAW, the system reconstructs the signal digitally. But the digital reconstruction may not be an exact copy of the analog one, and so we end up with errors. The errors known as aliasing happen when the sample rate is too low so that the system misidentifies a signal frequency, thereby introducing distortion or other artifacts into the recording.

 

The sample rate is the frequency with which the original analog signal is measured (sampled). The shorter the intervals (i.e. the higher the frequency) over which the readings are taken, the closer the digital version will be to the original. In 1927, physicist and engineer Harry Nyquist (1889–1974) determined that, in order to maintain fidelity, the sample rate must be at least twice the frequency of the sound being sampled. For example, in order to accurately sample a sound that occupies a range of frequencies up to 20,000 Hz (the upper limit of human hearing), sample readings must be taken, and digital values stored, at least 40,000 times per second. Failure to do so results in 'aliasing' – the production of undesirable audio artifacts. The upper limit of the frequencies that can be handled accurately by a analog-to-digital converter is the so-called Nyquist frequency. If the sample rate is 48 kHz, for example, the Nyquist frequency is 24 kHz.

 

Standard sample rates are 44.1, 48, 88.2, 96, 176.4, and 192 kHz. All of these should be able to accurately convert signals up to the upper limit of human hearing. However, an audio signal may contain harmonic components of higher frequency than those which can be heard. If the analog signal does contain

any frequencies higher than the Nyquist frequency, these will be interpreted by the converter and assigned to frequencies lower than the Nyquist frequency, in which case aliasing will occur.

 

Aliasing is also sometimes referred to as fold-over.