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Chapter 5: Oscillators and Filters
120 ANDROMEDA A6 REFERENCE MANUAL
Slope
A filter’s slope refers to the rate at which the frequencies are filtered out. Harmonics
are not chopped off abruptly by the filter but are “rolled-off” or reduced in
amplitude gradually. This gradual reduction in volume occurs at a rate expressed in
“decibels per octave”. Abbreviated “dB”, the decibel is a standard of measuring
amplitude. And since harmonics have frequency, they can be identified by musical
intervals over a range of several octaves above the pitch of the fundamental.
The illustrations used so far in this section depict
FILTER 1, which is a 2-pole filter that
gradually reduces the harmonics’ amplitude at a rate of 12dB per octave.
FILTER 2 is a
4-pole filter that rolls off the high end at a faster rate of 24dB per octave.
This graph illustrates how a 4-pole low pass filter reduces high-frequency harmonics
faster than the 2-pole low pass. Notice the difference in harmonic filtering: at the
same
FREQ setting: the 4-pole filter removes more of the highs than the 2-pole.
Resonance
A characteristic of many acoustically-produced sounds is resonance. This occurs when
a harmonic, whose amplitude is normally lower than that of the fundamental, rises in
amplitude. In some cases, one or more harmonics are as loud as the fundamental. In
fact, several instruments in the woodwind family – flute, oboe, bassoon for example –
rely heavily on resonance for their sound.
In an analog synthesizer, the RESONANCE control is used to raise the amplitude of the
harmonic that is nearest the cutoff frequency. In the illustration above, a low pass