## audiofilter

The audiofilter application is used to "tune" the sound files in such a way that they sound good when being used while in a compressed format. The values in the scripts for creating the sound files supplied in repotools is essentially a high-pass filter that drops out audio below 100Hz (or so).

(There is an ITU specification that states for 8KHz audio that is being compressed frequencies below a certain threshold should be removed because they make the resulting compressed audio sound worse than it should.)

The audiofilter application is used by the 'converter' script located in the scripts subdirectory of repotools/sound_tools. The values being passed to the audiofilter application is as follows:

-     audiofilter -n 0.86916 -1.73829 0.86916 -d 1.00000 -1.74152 0.77536

The two options -n and -d are 'numerator' and 'denominator'. Per the author, Jean-Marc Valin, "These values are filter coefficients (-n means numerator, -d is denominator) expressed in the z-transform domain. There represent an elliptic filter that I designed with Octave such that 'the result sounds good'."