state var allpass output

[Michael B. Irwin]

Hi Martin,
Regarding using a state-variable filter to obtain an allpass response,
last year I designed a 3-notch phaser using a cascade of state-variable
allpass ( not notch ) sections  with summation of the output with the
input in standard phaser fashion. Voltage-controlled Q was  included, as
well as a resonance control. Variation of the Q does not affect the
amplitude response of the allpass output, but varies the phase/freq
slope, which in turn varies the width and spacing of the  notches. A
wide variety of phasing sounds are possible (very closely spaced notches
a few semitones apart to notches spaced several octaves apart). By
turning up the resonance and dynamically varying the Q with an LFO or
env generator, vocal-like sounds with 2 formants can be produced. The
National Semiconductor app notes for the MF10 filter show different SVF
configurations including one which has the allpass response, which can
be implemented using op amps and OTA's instead of switched cap
circuitry.