Image Processing Reference
In-Depth Information
Aliasing Artifacts
Chapter 6 discussed digital-imaging systems and the aliasing artifacts that happen when
you draw diagonal lines. These exhibit themselves as jaggies. The same effect happens on
audio except that we cannot see it of course. We can hear it though. It is a horrible scratchy
buzzing sound.
In the example showing severe quantization errors, the wave shape is reconstructed
reasonably well if a sine curve-fitting algorithm is used. It is not such a good solution with
frequencies that are slightly detuned from the sample rate. If that is not dealt with cor-
rectly, unpleasant sub-harmonics are introduced. The effects of that are far worse. At least
with a square or sawtooth wave the added overtones are inaudible. An aliasing artifact
generates frequencies that are lower than the sample rate and these are in the audible
range. Figure 7-10 illustrates the effect.
Samples taken from a frequency nearly at the sample rate
Reconstructed waveform is at a beat frequency
Figure 7-10 Aliasing waveform reconstruction.
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