Doepfer A-188 User Manual 2

DOEPFER System A-100 BBD Module A-188-2

1. Introduction

Module A-188-2 based on a so-called Bucket Brigade

evice (abbr. BBD). In contrast to the A-188-1 the A-188-

D

circuit, i.e. the circuit has available

polarities of

Fig. 1: A-188-2 Controls and In/Outputs

2 uses a tapped BBD
six different outputs at the BBD stages 396, 662, 1194,
1726, 2790 and 3328. Each of the taps is available as a
single output but the module features also two mixing
sub-units. For both mixers the levels and
each BBD output can be adjusted to obtain two different
mixtures. Each mixer has a wet/dry control that allows
choosing between the original signal, the BBD mix or
anything in between. The outputs can be treated as
stereo outputs but that's not . Due to the six taps even six
different feedback types are possible. The feedback input
socket is normalled to the "396" output. But by patching
the feedback socket to any of the single output sockets
another BBD tap can be used for positive or negative
feedback to obtain different sounds. Even external
feedback processing is possible. The A-188-2 uses a
high speed VCO to generate the BBD clock. The clock
and consequently the delay time can be controlled
manually and by the two control voltage inputs CV1 and
CV2. CV2 is equipped with a polarizer. The module has a
clock output available that is normalled to the clock input
socket. These sockets can be used to link different BBD
modules and to run them with the same clock. A voltage
composed of the manual delay control, CV1 and CV2 is
available at the CV out socket, e.g. to control an external
clock suppression filter as the A-188-2 is not equipped
with such a filter.

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DOEPFER System A-100 BBD Module A-188-2

2. Basic Principles

A BBD circuit can be regarded as a chain of Sample &
Hold units (S&H) which pass on their voltages to the next
S&H in the chain at each clock pulse. From this also the
name Bucket Brigade Device is derived as each stage of
the BBD can be treated as a bucket. At each clock pulse
the content of each bucket is forwarded to the next bucket
in the chain and the current bucket is filled with the
contents of the preceeding bucket.

Fig.2: Bucket Brigade

Remark
stage as the content of a bucket has to be stored
temporarily in a "slave bucket" before it can be filled with
the contents of the preceeding bucket, in contrast to a
"real" bucket brigade not the buckets are passed on but
only the contents.
In the BBD the water is replaced by analog voltages
which again represent audio signals. The first bucket (1)
is the audio input, the last bucket (n) is the output of the
BBD. As in reality there are losses while the water (resp.

: In reality two "buckets" are required for each

voltage) is passed on, because some drops of water go
wrong and at the end of the chain not the same amount of
water (resp. not exactly the same voltage) appears. In a
BBD the buckets are replaced by capacitors and analog
switches. As the capacitors of a BBD are very small
(some pF only) even the time required to pass on the
input to the output is crucial as the capacitors loose their
charges if it takes too long. This is why a minimum clock
frequency is specified for each BBC circuit. Below this
frequency the flawless operation of a BBD is not
guaranteed. In the A-188-2 intentionally frequencies can
be used that go below this value to obtain special "dirty"
and "crunchy" effects.
Standard BBD circuits as described above are available
(or rather have been available) with different numbers of
stages. Usual numbers are 128, 256, 512, 1024, 2048 or
4096 stages. In the A-188-2 a very special BBD circuit
with 3328 stages is used (MN3011) that is equipped with
additional outputs (taps) at the stages 396, 662, 1194,
1726 and 2790.
The number of stages defines the delay time that
corresponds to a certain clock frequency. The higher the
number of stages, the longer is the delay. The higher the
clock frequency, the shorter is the delay.

Example

10.24 ms for a BBD with 1024 stages and 20.48 ms for a
BBD with 2048 stages.

: At 100kHz clock frequency the delay time is

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DOEPFER System A-100 BBD Module A-188-2

The following table shows the relation between clock frequency, used output stage (tap) and delay time for the BBD circuit
used in the A-188-2 (MN3011).

Table 1: Relation between clock frequency [kHz], used output (tap) and delay time [ms]

clock frequency

(clock input socket)

[kHz] [kHz] 396 662 1194 1726 2790 3328

1 0,5 396,00 662,00 1194,00 1726,00 2790,00 3328,00
2 1 198,00 331,00 597,00 863,00 1395,00 1664,00
5 2,5 79,20 132,40 238,80 345,20 558,00 665,60

10 5 39,60 66,20 119,40 172,60 279,00 332,80

20 10 19,80 33,10 59,70 86,30 139,50 166,40

50 25 7,92 13,24 23,88 34,52 55,80 66,56
100 50 3,96 6,62 11,94 17,26 27,90 33,28
200 100 1,98 3,31 5,97 8,63 13,95 16,64

300 150 1,32 2,21 3,98 5,75 9,30 11,09
400 200 0,99 1,66 2,99 4,32 6,98 8,32
500 250 0,79 1,32 2,39 3,45 5,58 6,66

BBD clock frequency

(= 1/2 clock input)

MN3011

tap/stage output

Remarks:
The grey italic characters indicate parameters out of the data sheet specifications as the MN3011 circuit is specified only for
clock frequencies between 10kHz and 100kHz. But the A-188-2 is able to generate even clock frequencies below 10kHz
and above 100kHz. From our experience the MN3011 cannot be damaged if it is operated with frequencies out of spec. This
is why we decided to include out-of-spec frequencies to obtain special audio effects – especially for clock frequencies below
10kHz. But the regular circuit behaviour is no longer guaranteed (especially increasing voltage losses at lower frequencies).
As the clock frequency falls below ~ 15 khz the clock signal itself will become audible. This clock noise can be used as
unusual audio source or it can be filtered out with an external low pass filter.

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DOEPFER System A-100 BBD Module A-188-2

Fig. 3: A-188-2 module scheme

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DOEPFER System A-100 BBD Module A-188-2

Fig. 3 shows the internal circuit details of the module: the
upper part is the high speed VCO (HSVCO), the lower part
the actual BBD section.

The HSVCO generates the clock signal that is required to
drive the BBD. It has available a manual control and two CV
inputs. CV1 is without attenuator and follows approximately
the 1V/octave standard (for precision 1V/octave applications
an external precision HSVCO is required). CV2 is equipped
with a polarizer.

Polarizer function
control the effect of the control voltage applied to the socket
CV2 is zero. Turning the knob clockwise from the center
position the control voltage effect becomes positive with
increasing amount. Turning the knob counterclockwise from
the center position the control voltage effect becomes
negative with increasing amount.

The HSVCO has a CV output available that corresponds to
the sum of all CVs that define the HSVCO's frequency, i.e.
the manual control, CV1 and CV2. It's main purpose is to
control the CV input of external low pass filter(s) that can be
used as anti-aliasing filter and clock filter. If desired a low
pass filter can be used behind the audio output(s) to
suppress the clock noise when the clock frequency falls
below ~15 kHz. Another filter can be used at the audio input
to reduce the max. frequency of the inc oming 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

: In the center position of the polarizer

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 also a clock output
that can be used e.g. to synchronize two A-188-2 or an A­188-2 and A-188-1 (i.e. both BBD modules 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-188-2 or A-188-1 or any other clock signal in the required
frequency range). For all clock signals from and to the A­188-1/2 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.

Each tap of the BBD circuit is available as a single output:
396, 662, 1194, 1726, 2790 and 3328. These outputs can
be used for example in an additional external (e.g. voltage
controlled) mixer or for multi-channel applications (e.g. 5.1).

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DOEPFER System A-100 BBD Module A-188-2

Each tap is connected to two polarizers (the polarizer
function is described on the last page). One of the polarizers
is assigned to the sub-mixer 1, the second to sub-mixer 2.

The sub-mixers are labelled adder with the Σ sign (sum) in
the module scheme. The reason for the polarizers instead of
simple attenuators is that the polarity of each tap is crucial
for the output mixing (for both tap output + original signal
and even for the taps among each other).
The following sketch shows the difference between adding
and subtracting one tap output to/from the original signal by
means of a simple sawtooth signal as audio input.

Fig. 4: positive/negative mixing of original and BBD signal

The two sub-mixers are used to mix the outputs of the six
BBD taps with adjustable levels and polarities. Each sub­mixer has available wet/dry control to adjust the relation
between the original and the effect signal. The effect signal
is the sum of the tap outputs.

The A-188-2 features a feedback section that is very similar
to the one in the A-188-1. But as now six different BBD
outputs are available even six different feedback types are
possible. The feedback input socket is normalled to the 396
output. But by patching the feedback socket to any of the
other single output sockets another BBD tap can be used for
the feedback to a obtain different sound. Even external
feedback processing is possible (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 feedback control
has polarizer function, i.e. the neutral position is at the
center. Left from the center the feedback signal is
subtracted from the input signal, right from the center it's
added (negative/positive feedback). The polarity of the
feedback signal leads to clearly audible different sounds as
different frequencies are emphasized or attenuated for
positive or negative feedback.

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DOEPFER System A-100 BBD Module A-188-2

The feedback can be increased up to self-oscillation. In
contrast to other feedbacks (e.g. filters or 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.

A special feedback can be obtained if one of the sub-mixer
outputs is used for the feedback function. In this case for
each tap the positive or negative feedback share can be
adjusted. This leads to multiple peaks and/or notches in the
frequency response of the module.

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DOEPFER System A-100 BBD Module A-188-2

3. Overview

1a

§
4

3

$

"
!

%

)

Fig.5: A-188-2 front panel

Note: The labelling of the output sockets is wrong. The left pair of sockets is Output 2, the right pair is Output 1

2a

2b1b

5
6
(

/

&
=

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DOEPFER System A-100 BBD Module A-188-2

Controls:
1a, 1b : mix levels/polarities of the BBD taps

3328, 2790, 1726, 1194, 662 and 396
(polarizers)

2a, 2b Mix: wet/dry controls for outputs 1 and 2

(
(

1/2a are assigned to sub-mixer 1)

1/2b are assigned to sub-mixer 2)
3 Delay : manual delay/clock control
4 CV2: attenuator for CV2
5 Input Level: audio input attenuator
6 Feedback: feedback level and polarity control

(polarizer)

In- / Outputs:

! CV1: CV1 input HSVCO
" CV2: CV2 input HSVCO

§ Clk Out: clock output HSVCO
$ Clk In: BBD clock input

(normalled to Clock Out

§)
% Audio In: audio input
& Ext.FB In: external feedback input

(normalled to BBD Out

) 396)
/ Output 1: mixed audio output 1
( Output 2: mixed audio output 2
) Single outputs: single audio outputs of the taps

3328, 2790, 1726, 1194, 662 and 396

= CV Out: CV output HSVCO

The labelling of the output sockets is wrong. The left

Note:
pair of sockets is Output 2, the right pair is Output 1

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DOEPFER System A-100 BBD Module A-188-2

4. Controls and In- / Outputs

4.1. High Speed VCO Section

3 Delay : manual delay/clock control
! CV1: CV1 input HSVCO
" CV2: CV2 input HSVCO
4 CV2: attenuator for CV2

§ Clk Out: clock output HSVCO
= CV Out: CV output HSVCO

This group of elements is responsible for the clock
frequency generated by the high speed VCO (HSVCO).
knob 3 Delay Clock is used to adjust the clock manually.
Two CV inputs (! CV1, " CV2) are available to control the
clock by external control voltages (e.g. LFO, envelope
follower, random, ADSR, keyboard CV, sequencer,
theremin, ribbon controller, foot controller, Midi-to-CV,
shepard generator ...). The sensitivity of CV1 is
approximately 1V/octave. The diagram on the right side
shows the connection between CV1 and clock frequency.
The straight line represents the perfect 1V/oct graph. The
slightly bended curve is the real behaviour of the HSVCO. If
an absolutely "perfect" 1V/octave control is required an
external precision HSVCO or a VCO with PLL has to be
used.
The second control voltage input " CV2 is equipped with a
polarizer. The function of a polarizer is described on page 5.

Fig 6: relation between CV1 and Clock Frequency

§ Clk Out: clock output HSVCO

This is the clock output of the HSVCO. It is internally
connected to the clock input of the BBD section. The

waveform is rectangle with about ± 3V level. The rectangle
slopes flatten with increasing frequency and the waveform
turns more and more into triangle. Even the load on the
output has influence to the waveform and level.
Consequently for all clock patches from and to the A-188-2
only short patch cables (~ 30 cm) should be used because
long cables function as low pass filters for signals above
20kHz. The max. frequency at this output is about 250 kHz.

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DOEPFER System A-100 BBD Module A-188-2

= CV Out: CV output HSVCO

This CV output indicates the clock frequency at output
and is nothing but the sum of all CV inputs (manual, CV1
and CV2). The main purpose of this output is to control the
CV input of 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, especially when the clock frequency falls below
~15 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. But the CV output can be used for other
applications as well, e.g. controlling parameters of the
feedback loop like feedback amount/polarity (CV of a VCA
or VC polarizer used in the feedback loop), filter frequency
(CV of a VCF used in the feedback loop), phase shift (CV of
a VC phaser used in the feedback loop).

§

4.2. BBD and Sub-Mixer Section

$ Ext. Clk In: BBD clock input

This is the clock input of the BBD section and is internally
connected to the clock output § of the HSVCO section (i.e.
it is normalled to the Clock Out socket §). If an external
clock source is used the clock output of this source is
patched to socket $. In this case the internal connection to
the HSVCO is interrupted. The clock frequency table can be
found on page 3. The required level for the clock signal is
about +3V (levels up to +12V cause no problems). This
socket can be used e.g. to synchronize two A-188-2
modules or an A-188-2 and A-188-1 (i.e. using one HSVCO
for both modules).

% Audio In: audio input
5 Level: audio input attenuator

Sockets % is the audio input with the assigned attenuator 5.
Feed the audio signal that has to be processed by the A­188-2 into socket %. Adjust the Level control 5 so that the
output signal does not distort - unless you want to obtain
distortion. For normal A-100 levels (e.g. VCO A-110)
distortion appears at about three o'clock position of control

5.

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DOEPFER System A-100 BBD Module A-188-2

) Single Outputs

The six sockets ) are the "raw" outputs (taps) of the BBD
stages 3328, 2790, 1726, 1194, 662 and 396. These
outputs can be used e.g. for for feedback patching, for
external mixing (e.g. with a voltage controlled mixer) or for
multi-channel applications (e.g. 5.1 sounds).

1a, 1b : mix levels/polarities of the BBD taps

3328, 2790, 1726, 1194, 662 and 396
(polarizers)

2a, 2b Mix: wet/dry controls for outputs 1 and 2

(
(

1/2a are assigned to sub-mixer 1)

1/2b are assigned to sub-mixer 2)
/ Output 1: mixed audio output 1
( Output 2: mixed audio output 2

This group of elements is responsible for the two mixed
outputs appearing at the sockets / Output 1 and ( Output

2. In the left/ccw position of the knobs 2a/b Mix the original
signal appears at the sockets //(. In the right/cw position of
the knobs 2a/b Mix the internal pre-mix signal composed
from the the six BBD taps with the polarizers
at the sockets //(. In the intermediate positions of the
knobs 2a/b Mix a mix of these two signals appears at the
corresponding output socket.

The labelling of the output sockets is wrong. The left

Note:
pair of sockets is Output 2, the right pair is Output 1

1a/b appears

& Ext.FB In: external feedback input
6 Feedback: feedback level and polarity control

(polarizer)
This group of elements is responsible for the feedback

functions of the module. Socket & Ext.FB In is the input of
the feedback loop and is normalled to socket ) BBD Out

396. Patching the feedback socket to any of the other single
output sockets another BBD tap can be used for the
feedback function to a obtain different sound. If an external
module is used to control the feedback loop (e.g. a VCA or
VC polarizer) the module has to be inserted between one of
the six sockets ) BBD Out and socket & Ext.FB In. The
feedback control
position at the center. Left from the center the feedback
signal is subtracted from the input signal, right from the
center it's added (negative/positive feedback). The polarity
of the feedback signal leads to clearly audible different
sounds especially at short delay times as different
frequencies are emphasized or attenuated for positive or
negative feedback. For longer delay times ("analog delay"
application) the sound differences are much smaller
In the fully right/cw or fully left/ccw position of the feedback
control
mentioned in chapter 2 the result in the self-oscillation state
depends upon the "audio history" (for details please refer to
page 7).

6 the module goes into self-oscillation. As already

6Feedback is a polarizer with the neutral

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DOEPFER System A-100 BBD Module A-188-2

5. User Examples

Standard Modulation Patch

Suitable control voltage sources are LFO (A-145 as shown
in the example, A-146, A-147, A-143-3), random voltage (A­118, A-149-1), envelopes (A-140, A-141, A-142, A-143-1, A­143-2), S&H (A-148, A-152), sequencer (A-155), theremin
(A-178) or ribbon controller (A-198).

Even for the "enveloped BBD" patch with the envelope
follower of the A-119 as control voltage the A-188-2 can be
used (described in the user's manual of the A-188-1 on
page 13).

Voltage Controlled Feedback

The feedback is processed by an external voltage controlled
polarizer (A-133) to obtain voltage controlled feedback.
Instead of the polarizer even a VCA can be used. But with a
VCA only positive or
the single outputs of the A-188-2 can be used for the
feedback loop. Even other sound processing modules (e.g.
filters, frequency shifter, wave multiplier, spring reverb,
phaser) can be used for voltage controlled feedback
processing.

negative feedback is possible. Any of

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DOEPFER System A-100 BBD Module A-188-2

Clock Filter

The BBD audio output is filtered by a low pass (e.g. A-108).
The frequency of the low pass filter follows the BBD clock
frequency as the CV output of the module A-188-2 is used
to control the frequency of the low pass filter. If both outputs
have to be filtered another low pass is required for the
second output.

An additional filter can be used at the audio input of the A­188-2 to limit the frequency range of the processed audio
input signal.

The single outputs of the A-188-2 are patched to the inputs
of the addressed switch A-152. The common output of the
A-152 is patched to the feedback input of the A-188-2. The
CV or the clock input of the A-152 is used to switch between
different feedback types. If a sequencer is used for each
step of the sequence another feedback type can be chosen.
The digital output #7 of the A-152 is connected to the reset
input to limit the number of used steps to six.

Clocked or voltage controlled feedback type

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DOEPFER System A-100 BBD Module A-188-2

Voltage controlled output mixing

Four of the single outputs of the A-188-2 are patched to the
audio inputs of a voltage controlled mixer A-135-1. The four
control voltages of the A-135-1 are used to define the levels
of the BBD taps.

Even the Karplus/Strong patches described in the A-188-1
user's manual can be realized with the A-188-2. The mixed
outputs of the A-188-2 lead to more complex sounds
compared to the A-188-1.

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DOEPFER System A-100 BBD Module A-188-2

16