Aliasing - a basic introduction
Channel: David Dorran
Date: 04/09/2012

Demonstrates how sampling a sinusoidal signal at an inappropriate rate results in a discrete signal which is interpreted as being a sinusoid of a different frequency.
Aliasing occurs if you do not sample at a suitably high rate. Your sampling rate has to be at least twice the highest frequency present in a signal, otherwise aliasing will occur. For example, a speech signal has loads of different frequencies present but (for most speech) the highest frequency will be about 8000 Hz, so to avoid aliasing you would need to sample at 16 kHz. 
If you don't sample at a high enough rate, then the signal you measure won't 'seem' correct. For example, imagine you had an audio signal that contained a 1000 Hz beep sound. Let's say you sampled at 1200 Hz (which is less than twice the highest frequency), If you played that sampled signal back it would sound like a 200 Hz beep rather than a 1000 Hz beep (this apparent change in frequency after sampling is aliasing). 
If you sampled the 1000 Hz beep sound at a rate above 2000 Hz then the sampled beep would sound like a 1000 Hz beep.