This Lasionycteris noctivagans call exhibits a strong series of harmonic components, both real and aliased. The recording was made using a Pettersson D140 detector that has a sampling frequency of 300 kHz. According to digital sampling theory, this detector can only resolve real, i.e. non-aliased, sounds up to one half of its sampling frequency, or 150 kHz. This limiting frequency is called the Nyquist frequency. The frequency scale for this sonogram was selected to end at the Nyquist frequency of 150 kHz, because any information displayed above that frequency would be nonsensical. However, unless removed with analog filters before digital recording, call components above the Nyquist frequency do render effects. These effects are known as aliases, because they are higher frequency signals aliasing as lower frequency signals. Aliasing is a direct effect of digital sampling. With most audio work it is desirable to pevent aliasing becasue it corrupts and adds noise to recordings. However, because of the discrete nature of bat calls, and because aliases occur in a predictable way, the alias information may be exploited to extend the effective frequency range of the analysis beyond the Nyquist limit. |
|
The following pages explain how aliasing occurs, and demonstrate how to interpret aliased components. |
 |
|
|
|
