Digital Signal Processing Reference
In-Depth Information
L P filter
Time domain
Sp e ctr u m
Freq. domain
Sig n als
Mu l
tipl.
Mu ltipl.
L P filter
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
-1,25
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
1,00
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,00
-1,25
0,50
0,25
0,00
-0,25
-0,50
-0,75
0,275
0,250
0,225
0,200
0,175
0,150
0,125
0,100
0,075
0,050
0,025
0,000
0,150
0,125
0,100
0,075
0,050
0,025
0,000
0,150
0,125
0,100
0,075
0,050
0,025
0,000
0,150
0,125
0,100
0,075
0,050
0,025
0,000
0 100
250
400
550
700
850
050 150 250 350 450 550
Hz
ms
Illustration 159:
AM signals without a carrier: modulation and demodulation using DASYLab
The starting point here is a source signal which was generated by means of lowpass filtered noise (it con-
tains all the information about the lowpass). In the second series you see the corresponding double-side-
band AM signal without a carrier. The first step in demodulation consists in the multiplication of the latter
signal by the “mid frequency” or the carrier of 256 Hz. In the third row there is in the frequency domain,
on the left the source signal, and on the right the AM signal with a double mid or carrier frequency.
Finally, after filtering by the lowpass we get the original source signal back (bottom series).
For this reason - just as in the “wireless” sphere - it is extremely important to use frequen-
cy bands as effectively as possible.
Double sideband AM modulation is therefore uneconomical because all the information
is contained in each of the two side bands. For a long time it has been technically possible
to load thousands of telephone single sideband voice channels close together on a coaxial
cable to exploit them better.
This process will now be checked and precisely analyzed using DASY
Lab
. After a double
sideband AM signal without a carrier was generated in Illustration 160 the upper sideband
(regular position) is filtered out by means of a high precision bandpass filter. In the third
series we thus obtain a single sideband signal.
