Digital Signal Processing Reference
In-Depth Information
A second DSK is used to recover/unscramble the original signal (simulating the
receiving end). Use the output of the first DSK as the input to the second DSK.
Run the same program on the second DSK. This produces the reverse procedure,
yielding the original unscrambled signal. If the same 2-kHz original input is con-
sidered, the 1.3 kHz as the scrambled signal becomes the input to the second DSK.
The resulting output is the original signal of 2 kHz (3.3
-
1.3 kHz), the lower side-
band signal.
With a sweeping input sinusoidal signal increasing in frequency, the resulting
output is the sweeping signal “decreasing” in frequency. Use as input the
.wav
file
TheForce.wav
and verify the scrambling/descrambling scheme.
The up-sampling scheme is optional since a 16-kHz sampling rate can be set
directly in the program and commenting the line of code
filtmodfilt(input);
Verify the up-sampling scheme. Are the results the same as before, with an 8-kHz
sampling rate and processing the input twice?
Interception of the speech signal can be made more difficult by changing the
modulation frequency dynamically and including (or omitting) the carrier frequency
according to a predefined sequence: for example, a code for no modulation, another
for modulating at frequency
f
c1
, and a third code for modulating at frequency
f
c2
.
This project was first implemented using the TMS320C25 [51] and also on the
TMS320C31 DSK without the need for up-sampling.
Example 4.9: Illustration of Aliasing Effects with
Down-Sampling (
aliasing
)
Figure 4.29 shows a listing of the program
aliasing.c
, which implements this
project. To illustrate the effects of aliasing, the processing rate is down-sampled by
a factor of 2 to an equivalent 4-kHz rate. Note that the antialiasing and recon-
struction filters on the AIC23 codec are fixed and cannot be bypassed or altered.
Up-sampling and lowpass filtering are then needed to output the 4-kHz rate samples
to the AIC23 codec sampling at 8 kHz.
Build this project as
aliasing
. Load the slider file
aliasing.gel
(on the
CD). With
antialiasing
initially set to zero in the program, aliasing will occur.
1.
Input a sinusoidal signal and verify that for an input signal frequency up to
2 kHz, the output is essentially a loop program (delayed input). Increase the
input signal frequency to 2.5 kHz and verify that the output is an aliased 1.5-kHz
signal. Similarly, a 3- and a 3.5-kHz input signal yield an aliased output signal of
1 and 0.5 kHz, respectively. Input signals with frequencies beyond 3.9 kHz are
supressed due to the AIC23 codec's antialiasing filter.
2.
Change the slider position to 1, so that antialiasing at the down-sampled rate
of 4 kHz is desired. For an input signal frequency up to about 1.9 kHz, the output is
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