Bandwidth of Signals

Bandwidth of Signals

Different message signals need different ranges of frequencies to be represented faithfully, and the communication system required depends on which band is needed.

• Speech needs roughly 300 Hz to 3100 Hz for commercial telephony - a bandwidth of about 2800 Hz.
• Music needs about 20 kHz because musical instruments generate frequencies higher than speech does; the full range of audible sound is about 20 Hz to 20 kHz.
• Picture/video signals need about 4.2 MHz; a complete television signal (video plus its accompanying sound) is usually allotted 6 MHz of bandwidth.
• Digital data is transmitted as rectangular waves. A rectangular wave can be built up mathematically as the sum of a fundamental sine wave (frequency $\nu_0 = 1/T_0$) and its harmonics $2\nu_0, 3\nu_0, 4\nu_0, \ldots$, continuing to infinity. Reproducing the rectangular shape exactly would need infinite bandwidth, but in practice each higher harmonic contributes less than the one before, so keeping only a handful of harmonics still recovers a usable approximation of the signal, with some acceptable distortion - the more bandwidth that is allowed, the closer the reconstructed wave is to the true rectangular shape. …