In any sampling process, it is necessary to obtain
enough samples to be representative of the whole. The Nyquist
theorem states that the minimum number is 2 samples per Hertz.
In actual practice it is often necessary to take more than this
In a telephony codec, the ratio is .
Aliasing occurs if less than 2 samples/Hz are
obtained. Notice what happens in the time domain, if a sine wave
is sampled at less than the Nyquist rate:
Aliasing also occurs in computer simulations
if the System Clock is not set to a high enough rate for the model.
In most cases, choose a value of about 5 – 10 times the highest
frequency in the model.
Don't forget that pulses consist
of many high frequency components, therefor the System Clock might
have to be set 10 times higher yet again!
There are two types of sampling: natural and
flat-topped. Of these two, natural sampling is the easier to analyze.
However, in practice, flat-topped sampling is used in an ADC.