Digital Signal Processing Reference
In-Depth Information
Multipl.
Ti me domai n
Spectrum
Freq. domai n
Signals
Abs. value
LP filter
LP filter
2,25
1,50
0,75
0,00
2,5
1,5
0,5
-0,5
-1,5
-2,5
2,25
1,50
0,75
0,00
1,50
1,25
1,00
0,75
0,50
0,25
0,00
2,25
1,50
0,75
0,00
2,5
1,5
0,5
-0,5
-1,5
-2,5
2,25
1,50
0,75
0,00
1,50
1,25
1,00
0,75
0,50
0,25
0,00
1,25
0,00
1,25
0,00
0,8
0,6
0,4
0,2
0,0
0,8
0,6
0,4
0,2
0,0
1,25
0,00
1,25
0,00
0,8
0,6
0,4
0,2
0,0
0,8
0,6
0,4
0,2
0,0
025 75 125 175 225 275 325
0
5 0 100
200
300
400
500
ms
Hz
Illustration 157:
Demodulation of two AM signals in the traditional fashion
As examples of source signals (top) a sine (4Hz) and bottom a realistic signal curve with an upper limiting
frequency of 32 Hz have been selected here. The carrier thus lies at 128 Hz. In order that demodulation
can be carried out in the traditional way, both source signals must lie completely in the positive region. As
a result the envelope of the AM signals represents the source signal.
As a result of the absolute value process (rectification) and the subsequent lowpass filtering (R-C-link) the
two source signals are retrieved. These signals are slightly time-displaced compared with the original
signal because all signal processing takes time.
As explained in more detail in the text, the signal demodulation corresponds to a simple multiplication of
the AM signal by a carrier of the same value (here 32 Hz). As precise analog multipliers did not exist,
inventive scientists had to come up with something.
