Specifications

84 Pure Data essentials
delwrite~ $0-a1 100
delwrite~ $0-a2 100
vd~ $0-a2vd~ $0-a1
* 2
*~
+~
*~
+~
+ 1
* 5
s~ $0-d
r~ $0-d
*~ 0.3
*~ 1
r~ $0-fb
s~ $0-fb
*~
+~
* 0.4
osc~osc~
inlet~ inlet fb
clip -1 1
inlet rate inlet depth
outlet~ l outlet~ r
fig 7.26: A chorus type effect
start
pd chorus-flanger
dac~
feedback
pd sample_loop_player
rate
depth
fig 7.27: Testing the
chorus
A small part, scaled by the feedback value on the sec -
ond inlet, is sent back to be mixed in with the input
signal, while another copy is sent to the left stereo out-
let. So there is a dry copy of the signal on one side
of the stereo image and two time shifted copies on the
other. By slowly varying the delay times with a couple
of sig nal rate LFOs a swirling chorus e ffect is achieved.
The low frequency oscillators are always 1Hz apart and
vary between 1Hz and 5Hz. It is necessary to limit the
feedback control to be sure the effect cannot become
unstable. Notice that feedback can be applied in positive or negative phase to
create a notching effect (phaser/flanger) and a reinforcing effect (chorus). Test-
ing out the effect is best with a sample loop player. Try lo ading a few drum
loops or music loop clips.
Simple reverberation
A reverb simulates dense reflections as a sound bounces around inside some
space. There are several ways of achieving this effect, such as convolving a sound
with the impulse re sponse of a room or using allpass filters to do a similar thing.
In Fig. 7.28 you can see a design for a recirculating reverb type that uses only
delay lines. There are four delays which mutually feed back into one another,
so once a signal is introduced into the patch it will circulate through a complex
path. So that reinforcement doesn’t make the signa l level keep growing some
feedback paths are negative. The recirculating design is known a s a Schro e der
reverb (this example by Claude Heiland-Allen) and mimics four walls of a room.
As you can see the number of feedback paths gets hard to patch if we move to
6 walls (with floor and ceiling) or to more complex room shapes. Reverb design
is a fine art. Choosing the exact feedback and delay va lues is not easy. If they
are wrong then a feedback path may exist for certain frequencies producing an
unstable effect. This can be hard to detect in practice and complex to predict in