Classical modulation procedures - Signals, Processes, and Systems - page 241

Time domain FFT Freq . dom ai n

Signals LP filter Multipl. S FFT B P filter IFFT Multipl. R

Addition

LP filter

Carrier

Time domain Frequency domain

0,4
0,3
0,2
0,1
0,0
-0,1
-0,2
-0,3
-0,4
-0,5
0,4
0,3
0,2
0,1
0,0
-0,1
-0,2
-0,3
0,3
0,2
0,1
0,0
-0,1
-0,2
-0,3
0,20
0,15
0,10
0,05
0,00
-0,05
-0,10
-0,15
-0,20
-0,25
0,20
0,15
0,10
0,05
0,00
-0,05
-0,10
-0,15
-0,20
0,15
0,10
0,05
0,00
-0,05
-0,10
-0,15
-0,20

0, 035
0, 030
0, 025
0, 020
0, 015
0, 010
0, 005
0, 000
0, 045
0, 040
0, 035
0, 030
0, 025
0, 020
0, 015
0, 010
0, 005
0, 000
0, 045
0, 040
0, 035
0, 030
0, 025
0, 020
0, 015
0, 010
0, 005
0, 000
0, 030
0, 025
0, 020
0, 015
0, 010
0, 005
0, 000
0, 040
0, 035
0, 030
0, 025
0, 020
0, 015
0, 010
0, 005
0, 000
0, 045
0, 040
0, 035
0, 030
0, 025
0, 020
0, 015
0, 010
0, 005
0, 000

12 kHz

16 kHz

20 kHz

5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

0.5 2.0 3.5 5.0 6.5 8.0 9.5

Hz

ms

Illustration 167: Simulation of the re-conversion of a pre-group in three telephone channels

This realistic simulation using DASYLab shows the signal processing in the lower series of the block
diagram. Apart from confirmation of the representation in Illustration 164 you also see (bottom) roughly
the curve of the source signal in the time domain of the two upper groups of three.

In order to analyse and explain exactly the frequency multiplex principle for modulation
and demodulation in single sideband modulation the pre-group formation shown in
Illustration 164 will be first formed and then “demodulated” into three channels or
converted by means of a DASY Lab simulation (Illustration 166 and Illustration 167).