Biomedical Engineering Reference
In-Depth Information
FIGURE 6-35
Spectrograms of the
word
elephant
spoken by two
different people.
FIGURE 6-36
Schematic diagram
showing the CIS
strategy for
electrode
stimulation.
the band-pass filters controlled the amplitude of the pulses applied to produce a channel
vocoder (voice encoder).
As shown in Figure 6-36, the CIS strategy, as implemented, starts with a preemphasis
filter to attenuate strong components in speech below 1.2 kHz. This is followed by the
standard band-pass filter bank after which the envelope from each filter output is obtained
by rectification and low-pass filtering. These envelope signals must then be compressed
by a nonlinear function (typical logarithmic) to map the wide dynamic range (typically
>
90 dB) into the narrow dynamic range for electrically induced hearing, which is typically
about 10 dB or a little more. These envelope outputs amplitude modulate the pulse trains
applied to the individual electrodes. The biphasic pulse trains for the individual channels
are interleaved in time so that the pulses across all of the channels are nonsimultaneous.
This eliminates a principal component of electrode interaction of each area on the basilar
membrane that would otherwise receive a signal proportional to the weighted vector sum
of all of the outputs. The corner frequency of each of the low-pass filters is set to 200 Hz
or higher so that the fundamental frequencies of the speech sounds are represented by the
modulation waveform.
This algorithm can be implemented in a digital signal processor using the fast Fourier
transform (FFT), and the envelope extraction can use the Hilbert transform. Alternatively, it
can use a more conventional band-pass filter and envelope detector followed by a low-pass
filter.
