Mystic

The synthesis method used by Mystic is based on three parallel comb filters with feedback. A comb filter is a filter with a number of notches in its frequency response, with the notch frequencies harmonically related to the frequency of the fundamental (lowest) notch.

A typical example of comb filtering occurs if you are using a flanger effect or a delay effect with very short delay time. Raising the feedback (the amount of signal sent back into the delay or flanger) causes a resonating tone – this tone is basically what the Mystic produces. This synthesis method is capable of generating a wide range of sounds, from gentle plucked-string tones to weird, non-harmonic timbres.

The basic principle is the following:

• You start with an impulse sound, typically with a very short decay.

  The spectrum of the impulse sound largely affects the tonal quality of the final sound.

• The impulse sound is fed into the three comb filters, in parallel. Each of these has a feedback loop.

  This means the output of each comb filter is fed back into the filter. This results in a resonating feedback tone.

• When the signal is fed back into the comb filter, it goes via a separate, variable low-pass filter.

  This filter corresponds to the damping of high frequencies in a physical instrument – if this is set to a low cutoff frequency it causes high harmonics to decay faster than the lower harmonics (as when plucking a string on a guitar, for example).

• The level of the feedback signal is governed by a feedback control.

  This determines the decay of the feedback tone. Setting this to a negative value simulates the traveling wave in a tube with one open end and one closed end. The result is a more hollow, square wave-like sound, pitched one octave lower.

• A detune control offsets the fundamental frequencies of the three comb filters, for chorus-like sounds or drastic special effects.

Finally you have access to the common synth parameters – two LFOs, four envelopes and an effect section.

• By default, envelope 2 controls the level of the impulse sound – this is where you set up the short impulse decay when emulating string sounds, etc.