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To summarize this section, the following pointers can be offered:
- Verify that aliasing can be heard and sounds bad before working
to get rid of it.
- Aliasing (bandwidth expansion) is reduced by smoothing
``corners'' in the nonlinearity.
- Consider using an oversampling factor for nonlinear
subsystems that is sufficient to accommodate the bandwidth expansion
caused by the nonlinearity.
- Make sure there is adequate lowpass filtering in a feedback
loop containing a nonlinearity.
As a specific example, consider the cubic nonlinearity used in a
feedback loop (as in §4.12). This can be done with
no aliasing at low levels (i.e., at levels below hard
clipping) provided we use
- oversampling, and
- a lowpass filter to
after the nonlinearity.
To avoid oversampling in the entire feedback loop, we may
downsample by 3 after lowpass filter and upsample by 3 just before
nonlinearity. If the lowpass filter is good, the downsampling by 3 is
trivially accomplished by throwing away every 2 out of 3 samples. For
upsampling, however, an additional third-band lowpass-filter is needed
for the interpolation (see §I.3).
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