DSP APPLICATIONS AND ARM TECHNOLOGY2
Introduction:
This particular project is divided in two parts. The first part is the implementation of a speech
Vocoder through MATLAB. The Vocoder is basically a software that can synthesize a sound
by speech analysis and then produce the speech in a form which is known as the ‘Shannon
Speech’. Basically at first in the Vocoder the sound is segmented into different channels and
then the sound is rectified by half wave rectifiers. Then it is passed through various low pass
filters and then additive Gaussian white noise is applied to each signal. Then the noise is
filtered through some previously designed filter and finally the output signal is produced by
summing all the filtered signals (Ahammed et al. 2015). This output signal gives clearer
audio than the original signal. Now, the sampling rate that is used in the 3rd order Butterworth
filter is 22.5 KHz as obtained while reading the sound wave in MATLAB. Although the
different kinds of filters can be used to extract the cut off frequencies at 160 Hz, like 3rd and
6th order elliptical filter, Butterworth filter but investigating the different possibilities it is
found that Butterworth produces low pass filtered signals better than any other method
(Sandhu, Kaur and Kaur 2016). Now, the 3rd order elliptical filter is used in the initial
analysis filter of different components of the sound signal (Podder et al. 2014). The filtered
signals are compared based on their frequency spectrum also. Finally, the synthesised and
summed up all the component sound wave is needed to be played with the sound function in
MATLAB to find the difference between the original signal and the synthesized signal.
In the second part a C/C++ code is needed to be designed which acts as a digital filter bank
on the ARM STM32F407 Discovery board. As specified in the question the input signal to
the A/D converter of the digital filter bank of the board is a superimposed three sinusoids of
frequency 500 Hz, 1500 Hz and 2500 Hz respectively. This signal is generated in MATLAB
first and the equivalent code in C is developed by MATLAB Coder. After that, among all the
possibilities of different filters the 10th order Butterworth filter is used for filtering the
individual frequency components as the stop-band Butterworth filter suppresses the stop-band
frequencies into a lower level than other filters. The generalized equation of a Butterworth
filter is given by (Podder et al. 2014),
|H(jw)| =
1
√1+€2
(w
℘)2n
(1)
n= filter order, w = 2*π*f , wp = 2*π*fp, fp = pass band frequency.
€ = maximum pass band gain.
Hence, the 10th order filter is given by putting n= 10 in equation (1).
Now, for removing the individual frequency component 500, 1500 and 2500 Hz the stop band
defined by [499 501], [1499 1501] and [2499 2501] respectively.
Now, the after filtering all the signal components the final signal is displayed in MATLAB.
Methodology and obtained results:

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