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There are many ways to reshape a waveform. The most obvious would be altering the
pulse width of a square wave, as discussed in Common Synthesizer Waveforms. Other
waveform-altering options include changing the phase angle, moving the start point of
a waveform cycle, or simply combining multiple waveforms in multioscillator synthesizers.
When reshaped in these and other ways, the relationships between the fundamental
tone and other harmonics change, thus altering the frequency spectrum and the basic
sound being produced.
Filters
The purpose of the filter in a subtractive synthesizer is to remove portions of the
signal—the frequency spectrum—that is sent from the oscillators. After being filtered, a
brilliant-sounding sawtooth wave can become a smooth, warm sound without sharp
treble.
The filter sections of most subtractive synthesizers contain two primary controls known
as cutoff frequency—often simply called cutoff—and resonance. Other filter parameters
can include drive and slope. The filter section of most synthesizers can be modulated by
envelopes, LFOs, the keyboard, or other controls such as the modulation wheel.
Types of Filters
There are several filter types. Each has a different effect on various portions of the
frequency spectrum:
Highpass Band Reject Lowpass
• Lowpass filter: Low frequencies are passed; high frequencies are attenuated.
• Highpass filter: High frequencies are passed; low frequencies are attenuated.
• Bandpass filter: Only frequencies within a frequency band are passed.
• Band Reject filter: Only frequencies within a frequency band are attenuated.
• Allpass filter: All frequencies in the spectrum are passed, but the phase of the output
is modified.
358 Appendix Synthesizer Basics