What are Adaptive Filters?

The principal use of filters in a forensic setting is to separate the wanted part of the signal (generally speech) from the undesired. If the characteristics of the noise are statistically constant (for example hiss from a tape or a constant hum or whistle) it is possible to design a static filter that, in some mathematical sense, optimally separates the speech from the noise. This is what we instinctively do when setting up a notch filter to remove a simple tone. There are, however, circumstances when this job is difficult or impossible to do manually. For example:

  • when the noise has a complicated spectrum, such that the filter needs a large number of parameters to be adjusted
  • when the noise characteristics are varying rapidly, such that manual setting and adjustment of the parameters is impossible
  • when the noise exhibits both of these characteristics (e.g. the sound of a TV broadcast mixed in with the signal)

In these cases we require a filter which has a large number of internal parameters that it can automatically adjust on our behalf (perhaps with some minimal degree of guidance), in accordance with the varying signal characteristics. Such filters are called adaptive filters and CEDAR Cambridge offers two types, both implemented in single-channel and cross-channel forms.

Forensic overview