Signal preprocessing is a packaging for to function calls. The first is
silence removal, the second is a high pass filter.
If there was silence removal, part of the voiceprint would come from the
silence and an imposter could easily pass by just recording silence. So,
it is imperative that the silent parts at the beginning and the end of
the recording be deleted. The first 0.05 seconds of the recording is treated
as silence. A silence threshold is generated by finding the maximum amplitude
in the first 0.05s. The program then finds the first time in the recording
where the amplitude exceeds the threshold and uses that as the starting
index to the recording. Silence at the end of the file is removed using
the same threshold and is identified by the last time the recording drops
below the threshold.
High Pass Filter
A high pass filter was used to filter out the low frequency background
noise. The cutoff frequency of the filter is ~1900Hz. A frequency response
plot is in figure 2 below.
Figure 2: Preprocessing Filter Frequency Response