Digital Signal Processing Reference
In-Depth Information
Finally, we note that the signal is labeled as speech if and only if all three
criteria are satisfied.
4.1.2
Speech Adaptive Beamformer (SA-BF)
The function of SA-BF is to form an appropriate beam pattern to enhance
the speech signal. Since adaptive filters are used to perform the beam
steering, we can change beam pattern with a movement of the source. The
degree of adaptation steering speed is decided by the convergence behavior of
the adaptive filters. In our implementation, we select microphone 1 as the
primary microphone, and build an adaptive filter between it and each of the
other four microphones. These filters compensate for the different transfer
functions between the speaker and the microphone array. A normalized LMS
algorithm updates the filter coefficients only when the current signal is
detected as speech. There are two kinds of output from the SA-BF: namely
the enhanced speech
d
(
n
) and noise signal
which are given as follows,
for channels
i
=2,3,4,5, where
is a fixed filter.
4.1.3
Noise Adaptive Beamformer (NA-BF)
The NA-BF processor operates in a scheme like a multiple noise canceller,
in which both the reference speech signal of the noise canceller and the
speech free noise references are provided by the output of the SA-BF. Since
the filter coefficients are updated only when the current signal is
detected as noise, they form a beam that is directed towards the noise, thus the
reason to name it a noise adaptive beamformer (NA-BF). The output response
is given as,
