Doepfer A-188-1 BBD Module - Manual (Page 6)

DOEPFER DOEPFER DOEPFER DOEPFER System A-100 BBD Module A-188-1
6
The following sketch shows the effect of normal/inverted
mixing by means of a simple sawtooth signal as audio input.
Fig. 4: positive/negative mixing of original and BBD signal
The center terminal of the feedback polarity switch is
connected to the BBD output socket. Pay attention that the
polarity of this output is affected by the position of the
feedback polarity switch (especially there is no signal at the
output socket if the switch is in the center position) ! The
feedback input is normalled to the output socket. The
combination of these two sockets allows to process the
feedback loop with external modules (e.g. a VCA or a VC
polarizer for voltage controlled feedback, or other modules
like filter, phaser, frequency shifter, waveshaper, wave
multiplier, ring modulator or another BBD module for special
voltage controlled feedback effects)
The polarity of the feedback signal leads to clearly audible
different sounds as different frequencies are emphasized or
attenuated for positive or negative feedback.
The feedback can be increased up to self-oscillation. In
contrast to other feedbacks (e.g. filters, phasers) the result
in the self-oscillation state depends upon the "audio history"
(i.e. the contents of the BBD when the self-oscillation is
triggered). The reason is that there is not only one possible
stable self-resonant state for the BBD. Any cyclic waveform
"stored" in the BBD is able to resonate provided that the
feedback maintains the waveform. One can try this out e.g.
with different audio signals (e.g. digital noise and VCO
sawtooth) as audio input before self-oscillation is triggered
(e.g. by switching the feedback polarity switch from center
position to positive or negative position).
Different BBD circuits (128/256/512 ... 4096 stages)
influence a lot of sound parameters. Of course the delay
time range and consequently the basic sound, but even the
feedback behaviour (both the self-oscillation and the
"smoothness" of the feedback), the distortion behaviour and
the output level. It is hard to say which is the "best" solution.
It depends upon the desired sound "bending". For typical
analog delay sound BBDs with more stages are the better
solution. But for "oppressive" flanging sounds caused by
short delays or for Karplus-Strong synthesis shorter BBDs
are recommended.