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

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

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-

2 uses a tapped BBD

circuit, i.e. the circuit has available

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

polarities of

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.

Summary of content (16 pages)

PAGE 1
DOEPFER System A-100 BBD Module A-188-2 1. Introduction Fig. 1: A-188-2 Controls and In/Outputs Module A-188-2 based on a so-called Bucket Brigade Device (abbr. BBD). In contrast to the A-188-1 the A-1882 uses a tapped BBD circuit, i.e. the circuit has available 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.
PAGE 2
DOEPFER System A-100 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.
PAGE 3
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).
PAGE 4
DOEPFER System A-100 BBD Module A-188-2 Fig.
PAGE 5
DOEPFER System A-100 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.
PAGE 6
DOEPFER System A-100 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).
PAGE 7
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.
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DOEPFER System A-100 BBD Module A-188-2 3. Overview § $ 1a 2a 1b 2b 4 3 5 6 ( " ! / % & = ) Fig.5: A-188-2 front panel Note: The labelling of the output sockets is wrong.
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DOEPFER System A-100 Controls: 1a, 1b : 2a, 2b Mix: 3 4 5 6 Delay : CV2: Input Level: Feedback: BBD Module A-188-2 In- / Outputs: mix levels/polarities of the BBD taps 3328, 2790, 1726, 1194, 662 and 396 (polarizers) wet/dry controls for outputs 1 and 2 (1/2a are assigned to sub-mixer 1) (1/2b are assigned to sub-mixer 2) manual delay/clock control attenuator for CV2 audio input attenuator feedback level and polarity control (polarizer) ! " § $ CV1: CV2: Clk Out: Clk In: CV1 input HSVCO CV2 input HS
PAGE 10
DOEPFER System A-100 BBD Module A-188-2 4. Controls and In- / Outputs 4.1. High Speed VCO Section 3 ! " 4 § = Delay : CV1: CV2: CV2: Clk Out: CV Out: manual delay/clock control CV1 input HSVCO CV2 input HSVCO attenuator for CV2 clock output HSVCO 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.
PAGE 11
DOEPFER = CV Out: System A-100 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.
PAGE 12
DOEPFER System A-100 ) 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).
PAGE 13
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).
PAGE 14
DOEPFER System A-100 BBD Module A-188-2 Clocked or voltage controlled feedback type 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. The single outputs of the A-188-2 are patched to the inputs of the addressed switch A-152.
PAGE 15
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