Alias frequency

Suppose the true input signal has frequency f and that the sampling frequency is f_s. The following rules allow the alias frequency f_a to be calculated.

f < ½f_s

As the signal frequency lies below the limit set by the sampling theorem, no aliasing occurs; the sampled signal has the correct frequency.

½f_s < f < f_s

The signal undergoes aliasing, with an alias frequency f_a = f_s - f

f > f_s

Aliasing again occurs. To determine the alias frequency, first calculate the signal frequency f modulo the sampling frequency f_s (i.e. keep subtracting f_s from f until the result is below f_s). Then apply either of the previous two rules, depending on whether the resulting frequency value is below or above ½f_s.

Back to 1.02 version / 1.1 version of aliasing demo applet

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