Digital Sound & Music: Concepts, Applications, & Science, Chapter 6, last updated 6/25/2013
Bandstop filters (also called notch filters) creates a boost or cut of a defined range of
frequencies. In this case the filter frequency defines the center of the notch. You might also have
a bandwidth control that adjusts the range of frequencies to be boosted or cut. Finally, you have a
control that adjusts the amount of change applied to the center frequency.
Figure 6.21 shows the filter controls in our example synthesizer. In this case we have two
filters. Filter 1 has a frequency and resonance control and allows you to select the type of filter.
The filter type selected in the example is a low-pass second order (12 dB per octave) filter. This
filter also has a keyboard tracking knob where you can define the extent to which the filter cutoff
frequency is changed relative to different frequencies. When you set the filter cutoff frequency
using a specific key on the keyboard, the filter is affecting harmonic frequencies relative to the
fundamental frequency of the key you pressed. If you play a key one octave higher, the new
fundamental frequency generated by the oscillator is the same as the first harmonic of the key
you were pressing when you set the filter. Consequently, the timbre of the sound changes as you
move to higher and lower frequencies because the filter frequency is not changing when the
oscillator frequency changes. The filter keyboard tracking allows you to change the cutoff
frequency of the filter relative to the key being pressed on the keyboard. As you move to lower
notes, the cutoff frequency also lowers. The knob allows you to decide how dramatically the
cutoff frequency gets shifted relative to the note being pressed. The second filter is a fixed filter
type (second order low-pass) with its own frequency and resonance controls and has no keyboard
tracking option.
Figure 6.22 Example of filter settings in a synthesizer
We'll discuss the mathematics of filters in Chapter 7.
Signal Amplifier
The last object in the audio path of a synthesizer is a signal amplifier. The amplifier typically has
a master volume control that sets the final output level for the sound. In the analog days this was
a VCA (Voltage Controlled Amplifier) that allowed the amplitude of the synthesized sound to
be controlled externally over time. This is still possible in the digital world, and it is common to
have the amplifier controlled by several external modulators to help shape the amplitude of the
sound as it is played. For example, you could control the amplifier in a way that lets the sound
fade in slowly instead of cutting in quickly.
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