Q. Discuss FDM and illustrate how CCITT stndards help in building the base band?
Ans:
Frequency Division Multiplexing: It is the process of combining numerous information channels by shifting their signals to different frequency groups within spectrum so that they can be transmitted simultaneously over common transmission facility.
Bandwidth of a telephone speech signal is rather less than 4 kHz. While the available bandwidth on unloaded cables pair, is well above 100 kHz. It is consequently possible, using modulation techniques, to divide up cable bandwidth so that anumber of telephone speech paths can be carried simultaneously along a single cable pair. Normal arrangement comprises 24 telephone channels per cable pair, modulation into 24 channels being carried out in two stages. In first stage, 12 channels are multiplexed together to form what is commonly termed as a basic group. Basic group arrangement is demonstrated in figure. Each of the 12 telephone signals are single side-band amplitude modulated on to carriers spaced at 4 kHz intervals from 64 kHz to 108 kHz. Lower side-band (LSB) is used in each case. 12 base-band signals are therefore translated into frequency band from 60 kHz 108 kHz as shown. Block diagram for channel translating equipment is given in figure. To form a 24-channel system, to basic groups are taken together. One basic group (B) is transmitted directly as it stands. Other basic group (A) is amplitude modulated on to a carrier are 120 kHz and lower side-band is taken so as to occupy frequency range from 12 kHz to 60 kHz as demonstrated in figure by using two stages of modulation for basic group A, it is probable to reduce the physical size of component required for LSB filters of the figureas the lowest cut- off frequency required is at 64 kHz rather than 12 kHz if the whole block of 24 channels were assembled together in one stage of modulation.