Bandwidth of Signals

What bandwidth means. The bandwidth of a signal is the width, on the frequency axis, of the band of frequencies that the signal actually contains (or that must be preserved for the signal to be reproduced acceptably at the receiving end). A signal built from a single, pure sine wave would in principle need zero bandwidth; every real signal -- built from many superimposed frequency components -- needs a finite band, and different kinds of signal need very different widths of band.

Speech. An ordinary telephone-quality speech signal is generally taken to need only the band from about 300 Hz to about 3100 Hz -- a bandwidth of roughly 2800 Hz -- since this narrow band already carries enough of speech's frequency content for the words to remain clearly intelligible, even though the full range of frequencies the human voice and ear can produce and detect is considerably wider. Music, by contrast, needs a much wider band (commonly taken as the full audible range, about 20 Hz to 20 kHz) to be reproduced with good fidelity, because musical instruments generate strong overtones at much higher frequencies than the voice does.

Television. A picture (video) signal needs a very much larger bandwidth again -- of the order of a few megahertz (commonly around 4.2 MHz for the picture information by itself) -- because a moving picture, scanned line by line many times a second, changes far more rapidly and carries far more independent detail per second than an audio signal does. Once the accompanying sound and, for colour transmissions, the colour information are added on as well, a complete television channel is typically allotted a still wider slice of spectrum (of the order of 6 MHz in total) to carry picture, sound and colour together without any of them interfering with the others. …