Digital Sound & Music: Concepts, Applications, & Science, Chapter 6, last updated 6/25/2013
same frequency regardless of the note played on the MIDI controller. The Phase, FM, Mix, and
Mode controls determine the way these two oscillators interact with each other.
Figure 6.21 Example of a sound generator in a synthesizer
A filter is another object that is often found in the audio path. A filter is an object that modifies
the amplitude of specified frequencies in the audio signal. There are several types of filters. In
this section, we describe the basic features of filters most commonly found in synthesizers. For
more detailed information on filters, see Chapter 7.
Low-pass filters attempt to remove all frequencies above a certain point defined by the
filter cutoff frequency. There is always a slope to the filter that defines the rate at which the
frequencies are attenuated above the cutoff frequency. This is often called the filter order. A
first order filter attenuates frequencies above the cutoff frequency at the rate of 6 dB per octave.
If your cutoff frequency is 1 kHz, a first order filter attenuates 2 kHz by 6dB below the cutoff
frequency, 4 kHz by 12 dB, 8 kHz by 18 dB, etc. A second order filter attenuates 12 dB per
octave, a third order filter is 18 dB per octave, and a fourth order is 24 dB per octave. In some
cases, the filter order is fixed, but more sophisticated filters allow you to choose the filter order
that is the best fit for the sound you’re looking for. The cutoff frequency is typically the
frequency that has been attenuated 6 dB from the level of the frequencies that are unaffected by
the filter. The space between the cutoff frequency and frequencies that are not affected by the
filter is called the filter typography. The typography can be shaped by the filter’s resonance
control. Increasing the filter resonance creates a boost in the frequencies near the cutoff
High-pass filters are the opposite of low-pass. Instead of removing all the frequencies
above a certain point, a high-pass filter removes all the frequencies below a certain point. A
high-pass filter has a cutoff frequency, filter order, and resonance control just like the low-pass
Bandpass filters are a combination of a high-pass and low-pass filter. A bandpass filter
has a low cutoff frequency and a high cutoff frequency with filter order and resonance controls
for each. In some cases, a bandpass filter is implemented with a fixed bandwidth or range of
frequencies between the two cutoff frequencies. This simplifies the number of controls needed
because you simply need to define a center frequency that positions the bandpass at the desired
location in the frequency spectrum.