An example of aliasing

I probably understand aliasing better than any person alive. When I was at Credence I developed a formula to calculate where in a spectrum aliases would appear. While at Nextest I invented a technique for finding the true frequency of any aliasing waveform which allowed me to use a 33MHz mini-tester to measure frequencies into the hundreds of Megahertz. In fact there is limit to the applicability of this technique, only the tester comparator bandwidth was the limiting factor.

Aliasing occurs when frequency components of a sound go higher than HALF THE SAMPLING RATE, also known as the Nyquist limit.

In this experiment I recorded the top 6 keys of April's piano at an 11KHz sampling rate.

Hit the play button on the controller to hear it.

6 notes sampled at 11KHz

Next I downsampled the sound to a 5KHz rate. That means that any sound over 2.5KHz will alias, making a lower sound. You won't believe your ears!

6 notes re-sampled at 5KHz

Remember, this is the same sound as the first one, just resampled.

Another example

In this experiment I recorded a whistle rising in pitch with a sample rate of 11KHz. All of the components of the sound are lower than half of the sample rate (5.5KHz).

Hit the play button on the controller to hear it.

Whistle sampled at 11KHz

The next sound is the same whistle as above but resampled at 7KHz. This means that the Nyquist frequency is around 3.5KHz, any frequencies higher than that will alias down as lower tones. In this sound you will hear not only the whistle rising in pitch, but you will also hear another component decrease in pitch. That component was in the original whistle, but is now aliasing, so as the whistle rises, the pitch of this component falls.

Hit the play button to hear it.

Whistle resampled at 7KHz

Movies can alias too, the most obvious example is on the old western movies when the wagon wheels appear to be going backwards. The wheels are turning faster than half the shutter speed of the movie camera, so the wheels appear to go backwards. The wheels will go forward again if they speed up to above the shutter speed.

Here is a nice apps note that I wrote for Nextest fully explaining aliasing.

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