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All signals that are transmitted consist of multiple frequencies. The range of frequencies a signal occupies is called the bandwidth of the signal. The bandwidth is measured in terms of Hertz (Hz).

The bandwidth of a signal depends on the amount of information contained in it and the quality of it. The range of frequencies necessary for an analogue voice signal, with a fixed telephone line quality (recognizable speaker), is 300 - 3400 Hz. This means that the bandwidth of the signal is 3,100 Hz. A human voice contains much higher frequencies, but this bandwith gives a good compromise between the quality of the signal and the bandwidth. To transmit audio, a much wider bandwidth of about 20 kHz is needed. The bandwidth of a television signal is in the order of 5,000,000 Hz or 5 MHz.

Bandwidth, together with noise, is the major factor that determines the information-carrying capacity of a telecommunications channel. The term bandwidth is often used instead of data rate or bit rate to express the capacity of a digital channel. Although they are closely related, they are not the same.

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