It’s easiest to describe aliasing in terms of a visual sampling system we all know and love—movies. If you’ve ever watched a western and seen the wheel of a rolling wagon appear to be going backwards, you’ve witnessed aliasing. The movie’s frame rate isn’t adequate to describe the rotational frequency of the wheel, and our eyes are deceived by the misinformation!
The Nyquist Theorem tells us that we can successfully sample and play back frequency components up to one-half the sampling frequency. Aliasing is the term used to describe what happens when we try to record and play back frequencies higher than one-half the sampling rate.
Consider a digital audio system with a sample rate of 48 KHz, recording a steadily rising sine wave tone. At lower frequency, the tone is sampled with many points per cycle. As the tone rises in frequency, the cycles get shorter and fewer and fewer points are available to describe it. At a frequency of 24 KHz, only two sample points are available per cycle, and we are at the limit of what Nyquist says we can do. Still, those two points are adequate, in a theoretical world, to recreate the tone after conversion back to analog and low-pass filtering.
But, if the tone continues to rise, the number of samples per cycle is not adequate to describe the waveform, and the inadequate description is equivalent to one describing a lower frequency tone—this is aliasing.
In fact, the tone seems to reflect around the 24 KHz point. A 25 KHz tone becomes indistinguishable from a 23 KHz tone. A 30 KHz tone becomes an 18 KHz tone.
In music, with its many frequencies and harmonics, aliased components mix with the real frequencies to yield a particularly obnoxious form of distortion. And there’s no way to undo the damage. That’s why we take steps to avoid aliasing from the beginning.