Doepfer A-188-1 BBD Module - Manual (Page 5)

DOEPFER DOEPFER DOEPFER DOEPFER System A-100 BBD Module A-188-1
5
Fig. 3 shows the internal details of the module A-188-1: the
upper part is the actual BBD section, the lower part the high
speed VCO (HSVCO).
The HSVCO generates the clock signal that is required to
drive the BBD. It has available a manual control and two CV
inputs (CV1 without attenuator, CV2 with attenuator). For
both CV inputs three-position polarity switches (negative /
off / positive) are available. The position of these switches
defines if a positive going CV has positive, none or negative
effect on the clock frequency. CV1 has a sensitivity of
approximately 1V/octave. The HSVCO has a CV out
available that corresponds to the sum of all CVs (manual,
CV1 and CV2). It's main purpose is to control the CV input
of one or two external low pass filters that can be used as
anti-aliasing filter and clock filter. If desired one low pass
filter can be used behind the audio output to suppress the
clock noise when the clock frequency falls below ~20 khz.
Another filter can be used at the audio input to reduce the
max. frequency of the incoming audio signal, consequently
reducing aliasing artefacts. As the CV output reflects the
clock frequency (affected by the manual control, CV1 and
CV2) the external filters automatically follow the clock
frequency of the BBD module. The higher the slope of the
external filter (e.g. 12/24/48 dB/octave) the better is the
clock suppression. The HSVCO features a clock output that
can be used e.g. to synchronize two A-188-1 (i.e. both A-
188-1 use the same clock source) or as high speed clock for
other applications (e.g. graphic VCO, switched capacitor
filter).
The clock output of the HSVCO is normalled to the clock
input of the BBD section. The clock input makes it possible
to control the BBD by an external clock source (e.g. another
A-188-1 or any other clock signal in the required frequency
range). For all clock signals from and to the A-188-1 only
short patch cables should be used, as long cables function
as low pass filters for signals above 20kHz.
A two-phase converter generates the two opposite clock
signals that are required to drive the BBD circuits.
The audio input of the BBD module is equipped with an
attenuator that enables to reduce the input level to avoid
distortion. The audio input signal behind the attenuator is
mixed with the feedback signal (details below) and fed to the
audio input of the BBD circuit. The audio output of the BBD
is processed by an inverter to have both the normal and the
inverted BBD output available. The reason for this feature is
that the polarity is crucial for both the output mixing (BBD +
original) and the feedback behaviour of the module. The
normal output of the BBD and the inverted output are fed to
the terminals of two three-position polarity switches
(negative / off / positive) for mixing polarity and feedback
polarity.
The output mixer is used to mix the original signal with the
normal or inverted BBD signal. The position of the mix
polarity switch defines if the normal, none or the inverted
BBD output is mixed with the original audio signal.