LibXtract is a simple, portable, lightweight library of audio feature
extraction functions. The purpose of the library is to provide a
relatively exhaustive set of feature extraction primatives that are
designed to be 'cascaded' to create a extraction hierarchies.

For example, 'variance', 'average deviation', 'skewness' and
'kurtosis', all require the 'mean' of the input vector to be
precomputed. However, rather than compute the 'mean' 'inside' each
function, it is expected that the 'mean' will be passed in as an
argument. This means that if the user wishes to use all of these
features, the mean is calculated only once, and then passed to any
functions that require it.

This philosophy of 'cascading' features is followed throughout the
library, for example with features that operate on the magnitude
spectrum of a signal vector (e.g. 'irregularity'), the magnitude
spectrum is not calculated 'inside' the respective function, instead,
a pointer to the first element in an array containing the magnitude
spectrum is passed in as an argument.

Hopefully this not only makes the library more efficient when
computing large numbers of features, but also makes it more flexible
because extraction functions can be combined arbitrarily (one can
take the irregularility of the Mel Frequency Cepstral Coefficients
for example).

Author: Jamie Bullock