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Old 02 December 2013, 23:05   #38
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Originally Posted by commodorejohn View Post
Also, samples are fed in at varying rates and played back without interpolation, which means that any aliasing noise is going to come out at a different frequency, likely well within the range of human hearing, which is also going to color the sound.
Even with perfect interpolation aliasing will occur if the original signal is sampled below twice the Nyquist limit.

Aliasing of this sort has absolutely nothing to do with Amiga audio hardware in particular. It happens at the moment the original signal is sampled below twice the Nyquist limit without proper filtering and is stored. Before the first byte of the stored sample is output, aliasing has already been introduced into the stored waveform. The Amiga doesn't "resample" this stored waveform. It outputs each and every point of the waveform at a desired rate. No additional aliasing is created.

Reconstruction filters, OTOH, prevent the creation of harmonics that are multiples of the output frequency. These aren't the aliases created by poor sampling of the original signal. Sometime these harmonics are even desirable. A square wave when output contains all the odd harmonics of the fundamental. This gives it a particular, hollow, timbre. A perfect reconstruction filter tuned to the half the output rate would turn an interesting square wave into a boring, but perfect sine wave. We don't always want that.
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