Frequency division multiplex divides a broad band of frequencies into slots, each wide enough to accommodate an information channel. This is achieved by amplitude modulating a higher frequency subcarrier with each speech signal to form groups of channels.
Each speech channel contains frequencies between 300 and 3400 Hz plus, in some systems, an out-of-band signalling tone of 3825 Hz and a guard band. Each channel modulates a base band subcarrier spaced at 4 kHz intervals upwards from 64 kHz. This produces an upper and lower side band from each channel (Figure 13.1). The carrier and upper side band are removed by filters and the lower (inverted) side band is transmitted. At the receiver, the base band frequencies are again mixed with the same subcarrier frequency to restore the original speech. The subcarrier frequencies are maintained to an accuracy of ±1 Hz which creates the ±2 Hz frequency translation error quoted in some telephone line specifications.
Twelve such channels form a ITU-T basic group B occupying the band between 60 and 108 kHz. This basic group B may now be173
Lower side band retained
60 kHz
Upper side band removed by filters
60.6 kHz
(3400 Hz audio) 63.7 kHz
(300 Hz audio)
12 kHz 60 kHz 108 kHz Group A Group B 120 kHz (originally Group B and (inverted)
now erect)
(c) Production of groups A and B
mixed with 120 kHz to produce a lower side band of 12 to 60 kHz, now basic group A. Filters leave 60 to 108 kHz free for a new basic group B.
The process may be repeated by using five basic groups to modulate still higher-frequency carriers, to produce super- and hypergroups.
For FDM data communications, the bearer circuit bandwidth of 3000 Hz is divided into 12 channels each of 240 Hz bandwidth. Data is transmitted at 110 bits per second allowing a send-and-receive channel in each block of 240 Hz.