30
RESONANCE
Higher
levels
of
delay taps A
and
B in the
INPUT
MIXER
cause longer durations
of
recirculated
delay.
At
some
point the
levels
become
high
enough
to cause
resonances, the frequencies
of
which
are determined
by
the delay times used.
Turn
up
just
one
of
the delay taps
until
the resonance
effect
begins.
At
this
point resonances will
be
activated
by
the audio signal information, be-
ing heard
at
peaks but otherwise decaying.
It
is
possible to increase the
amplitudes
until
the
additional stimulus.
runaway
tendency but
resonance
11
runs
away
11
, increasing in level without
any
The
ROLLOFF
frequencies
can
be
lowered to minimize
this
it
can
still
happen
and
should
be
avoided.
Resonances
can
be
used in conjunction with
other
effects
such
as doppler pitch
shifting,
double
and
triple
tracking,
and
flanging to
create
more
exotic sounds.
FLANGING
Flanging and,
more
generally,
comb
filtering
is
effected
by
the
PHASE
INVERT
·-,
function. Set
up
a flange with
DELAY
SELECT
A
and
B
at
0
and
1
msec
respect-
~
ively:
monitor equal
levels
of
both delay taps through the
MASTER
OUTPUT
and
adjust
the
VCO
for
obvious
effect.
Broadband
noise
can
be
used
as a source
to
make
the
effect
even
more
obvious.
Now
switch the phase
of
one
of
the
delay taps using the
INVERT
function. Notice the change in tone quality of
the
effect
resulting
from
this
phase inversion. This can
be
explained in
terms
of
the
comb
filter
(which
is
implemented
like
the flanging in section
3.2 except without varying the delay with the
VCO).
In
the
first
case with
no
phase inversion, the delayed signal simply
adds
to
the undelayed
signal.
This
reinforces
low
frequency components, since the
short
delay time represents only a small phase
shift.
The
phase
shift
in-
creases with frequency
until
at
500Hz
it
is
180
degrees causing a null
which
is
repeated
at
1500Hz
and
at
1kHz
intervals
thereafter.
See
FIG.
3.9.
In
the second case, the
phase
inverted
delayed signal
adds
to the undelayed
signal.
This cancels out
low
frequency components, but there
is
less
can-
cellation
as frequency (and phase
shift)
increase
until
a
peak
at
500Hz
is
reached
after
which
nulls
occur
at
1000Hz
and
every
1000Hz
thereafter.
The
inverted
comb
filter
is
in
effect
a
low
freq.
filter
which
has
a higher
cutoff
frequency as the delay time gets smaller. This
is
the reason for
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