Signals must be filtered prior to sampling. Theortically the maximum frequency that can be represented is half the sampling frequency. In practice a higher sample rate is used to allow for non-ideal filters.
The signal is now represented at multiples of the sampling period, T,
as s(nT) which is also written 
.
Telephone speech is sampled at 8 kHz. 16 kHz is generally regarded as sufficient for speech recognition and synthesis. The audio standard is a sample rate of 44.1 kHz (Compact Disc) or 48 kHz (Digital Audio Tape) to represent frequencies up to 20 kHz.