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10.2.3 The "inverse allpass"-function of VCF 1
Now we want to explore the effect of the "inverse allpass" function. Move the [balance]-knob (41) to
center position, both [resonance]-knobs (8) and (29) to fully counter clockwise position. Select "inverse
allpass" with the [curve]-rotary-switch (16) and the "LP 24db" position with the
[curve]-rotary-switch (37). Move the [frequency]-knob (7) to fully counter clockwise position and the
[frequency]-knob (28) to fully clockwise position.
10.
Move the [frequency]-knob (7) slowly to fully clockwise position. The lower frequencies of the
signal are attenuated, sonically equal to a highpss, because of phase cancellation of all frequen-
cies below the cutoff frequency. Turning clockwise, the effect increases. To get a most complete
erasure of frequency portions, search the exact position (near 12 o'clock) with the [balance]-
knob (41).
11.
Move the [frequency]-knob (28) slowly back and all signal-portions return, but this time inverted
(inaudible). Since you are closing VCF 2, the signal-portions that interfer with these of VCF 1, drift
more and more to lower frequency ranges. The high frequency ranges are attenuated by VCF 2
and thus cannot be erased by the inverted signals from VCF 1.
12.
Move both [frequency]-knobs (7) and (28) back counter clockwise. Now move the [resonance]-
knob (8) slowly clockwise to the 1 o'clock position. The sawtooth becomes immediately audible.
The reason is not the resonance effect, but the fact that signals below the cutoff frequency are
attenuated up to 14 db as soon as the resonance increases (see fig. 8.5.13 in chapter 8.5). This is
because of technical reasons. When you now move the [balance]-knob (41) between 9 o'clock
and 10 o'clock, the original ratio up to signal-erasure returns.
13.
Turn the [resonance]-knob (8) back counter clockwise and the [resonance]-knob (29) to
1 o'clock position. The sawtooth becomes audible again, because all the lowpasses show an at-
tenuation of approx. 3 db in level, when resonance is altered. Re-adjusting [balance]-knob (41) to
approx. 1 o'clock position restores the original condition.
14.
Move both [frequency]-knobs (7) and (28) back clockwise, the [resonance]-knob (8) counter
clockwise, [resonance]-knob (29) to 1 o'clock position and the [balance]-knob (41) to approx.
1 o'clock, in order to maximise the effect of phase cancellation. Step through all the lowpasses
by turning the [curve]-rotary switch (37) clockwise and play with [frequency]-knobs (7) and (28).
15.
Turn the [frequency]-knob (7) clockwise and the [frequency]-knob (28) counter clockwise. Leave
both [resonance]-knobs in their current position. Now select with the [curve]-rotary-switch (37)
highpass "HP 6db". Adjust the [balance]-knob (41) to 12 o'clock again, until the signal erasure
has reached its maximum. When you now move the [resonance]-knob (29), the effect of the fre-
quency erasure does not change. Play around with the [frequency]-knob (28). At counter clock-
wise position the low frequencies are erased. Select highpass "HP 12db" and play on. Since the
level of high- and bandpasses is not affected by the resonance, the erasure is still audible. With
bandpasses, the effect is not as clearly, because just partial. Change the setting of
[resonance]-knob (29).
16.
Select "notch" with the [curve]-rotary-switch (37). Move the [frequency]-knob (7) clockwise and
the [frequency]-knob (28) counter clockwise, both [resonance]-knobs (8) and (29) counter
clockwise. Set the [balance]-knob (41) to 12 o'clock, until the erasure effect reaches its maxi-
mum. Play with the [frequency]-knob (28); at the extreme positions is the effect most clearly,
inbetween signals are audible. Now move the [resonance]-knob (29) to 1 o'clock position. Play
again with the [frequency]-knob (28); at fully counter clockwise position, the erasure effect re-
l
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