Kurzweil K2661 - Harmonicsuppress; Harmonicsuppress Filtering at Full Expansion

KDFX Reference
KDFX Algorithm Specifications
10-245
975 HarmonicSuppress
Stereo algorithm to expand a single frequency band or harmonic bands.
PAUs: 2
HarmonicSuppress is a special expander algorithm. In most respects it is identical to Expander. However,
HarmonicSuppress expands on only harmonically related bands of frequencies. Why would we do this?
Imagine you are working with a sampled recording which contains an obnoxious 60 Hz hum. You can
suppress just the 60 Hz component witha parametric filter. Set the threshold above the level of the 60 Hz
tone so that real sounds in the 60 Hz range above threshold pass through the expander while 60 Hz hum
below threshold get squashed. If the 60 Hz is more of a hum or buzz than a tone, then distortion harmonics
are present. In this case, you might have more success using HarmonicSuppress.
HarmonicSuppress is based on comb filtering—a simple filter which removes harmonically related
frequency bands with a spectrum which looks like a comb. With the Harmonics parameter, you can choose
to expand the odd harmonics (including the fundamental) or even harmonics (not including the
fundamental) or all harmonics. (Choosing all harmonics is the same as choosing even harmonics at half the
frequency.
Figure 128 HarmonicSuppress filtering at full expansion
F marks fundamental
Harmonics are Even (i), Odd (ii), All (iii)
The algorithms expand the signal in the specified band(s) (reduce the signal’s gain) when the signal falls
below the expansion threshold in the specified band(s). You can select which channel, left (L), right (R) or
the larger of the two (L & R) is used to control the expansion (side chain processing) with the SC Input
parameter. You can also select which channel is actually expanded, again left (L), right (R) or both (L & R)
using the ExpandChan parameter. The amount of expansion is expressed as an expansion ratio.
Expanding a signal reduces its level below the threshold. The expansion ratio is the inverse of the slope of
the expander input/output characteristic. An expansion ratio of 1:1 will have no effect on the signal.
A zero ratio (1:∞), will expand all signal levels below the threshold level to the null or zero level. (This
expander expands to 1:17 at most, but that’s a lot.) Thresholds are expressed as a decibel level relative to
digital full-scale (dBFS) where 0 dBFS is digital full-scale and all other available values are negative.
F F linear Freq
(iii)
F
(i) (ii)