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DOEPFER System A-100 BBD Module A-188-1

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

1. Introduction

Module A-188-1 is a so-called Bucket Brigade Device

module (abbr. BBD). BBDs have been used to delay

audio signals before digital delays dethroned the BBD

based effect units. But BBDs have some very unique

advantages (or disadvantages dependent on the point of

view) over the digital counterpart which result from the

special properties of the BBDs. BBD circuits can be

treated 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. A more detailed explanation – including

the different types of BBDs – can be found in the

following chapter.

In any case the sounds generated by module A-188-1 are

very special. Typical applications are: Flanger, Chorus,

Analog Delay or Karplus/Strong synthesis. But as the A-

188-1 has a lot of very unique features (voltage controlled

clock rate / delay time with extreme range, polarity

switches for the CV inputs, feedback and BBD out/mix,

clock and CV output of the high speed VCO, BBD clock

input, feedback insert, feedback up to self-oscillation) a

lot of unusual applications can be realized with the

module (e.g. delay controlled by ADSR, random,

envelope follower or sequencer with positive or negative

effect). The A-188-1 also has no built-in anti-aliasing filter

in order not to limit the possibilities of the module. For this

the CV out is intended.

Summary of content (15 pages)

PAGE 1
DOEPFER System A-100 BBD Module A-188-1 1. Introduction Module A-188-1 is a so-called Bucket Brigade Device module (abbr. BBD). BBDs have been used to delay audio signals before digital delays dethroned the BBD based effect units. But BBDs have some very unique advantages (or disadvantages dependent on the point of view) over the digital counterpart which result from the special properties of the BBDs.
PAGE 2
System A-100 DOEPFER 2. Basic Principles As mentioned in the introduction 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.
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System A-100 DOEPFER BBD Module A-188-1 The following table shows the relation between clock frequency, delay time and number of stages for some typical BBD circuits.
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DOEPFER System A-100 BBD Module A-188-1 Fig.
PAGE 5
DOEPFER System A-100 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.
PAGE 6
DOEPFER System A-100 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.
PAGE 7
System A-100 DOEPFER 3. Overview % 6 1 ! & 7 2 / 3 " BBD Module A-188-1 Controls: 1 Delay Clock : 2 CV2: 3 Level: 4 Feedback: 5 Mix: 6 Polarity: 7 Polarity: 8 Polarity: 9 Polarity: In- / Outputs: clock output HSVCO ! Clk Out: BBD clock input " Ext. Clk In: ( 8 4 §b ) 9 5 §a Audio In: §b Audio In: $ Mix Out: % CV1: & CV2: / CV Out: ( Ext.FB In: $ ) BBD Out: §a Fig.
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System A-100 DOEPFER BBD Module A-188-1 4. Controls and In- / Outputs 4.1. High Speed VCO Section 1 % 6 & 7 2 Delay Clock : CV1: Polarity: CV2: Polarity: CV: manual delay control CV1 input HSVCO CV1 polarity CV2 input HSVCO CV2 polarity attenuator for CV2 This group of elements is responsible for the clock frequency generated by the high speed VCO (HSVCO). knob 1 Delay Clock is used to adjust the clock manually.
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System A-100 DOEPFER Consequently for all clock patches from and to the A-188-1 only short patch cables (~ 30 cm) should be used as long cables function as low pass filters for signals above 20kHz. The max. frequency at this output depends upon the BBD used in the module and is related to the max. clock frequency of the BBD in question (pls. refer to the table on page 3). It is about 250 kHz for BBDs with 2048 and 4096 stages and about 450kHz for BBDs with 1024 stages and less (i.e.
PAGE 10
System A-100 DOEPFER 4.2. BBD 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 suitable clock frequencies depend upon the BBD used in the module (pls. refer to the table on page 3).
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DOEPFER System A-100 The position of the polarity switch 9 defines if the normal or the inverted BBD signal is mixed to the original signal (please refer to page 6 concerning this function). Especially for short delay times the position of this switch leads to clearly audible different sounds. In the middle position of the switch the BBD share of the mixed signal is off and only the original signal is heard. Consequently the switch can be used to turn on/off the BBD effect at the mix output $. ( Ext.
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System A-100 DOEPFER BBD Module A-188-1 5. User Examples Standard Flanger Patch Voltage Controlled Feedback 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), envelope (A-140, A-141, A-142, A-143-1, A143-2), S&H (A-148, A-152), sequencer (A-155), theremin (A-178), ribbon controller (A-198). Feedback is processed by an external voltage controlled polarizer (A-133) to obtain voltage controlled feedback.
PAGE 13
System A-100 DOEPFER BBD Module A-188-1 "Enveloped" BBD Filtered Feedback Control voltage for the A-188-1 is generated by the envelope of the processed audio signal. Try positive and negative setting of the CV2 polarity switch and different CV2 levels ! The feedback loop is processed by an external filter. The example shows an A-124 Wasp filter in the feedback loop.
PAGE 14
System A-100 DOEPFER Clock Filter The BBD audio output is filtered with a low pass (e.g. A108). The frequency of the low pass filter follows the BBD clock frequency as the CV output of the module A-188-1 is used to control the frequency of the low pass filter. A second filter can be used at the audio input of the A-188-1 to limit the frequency range of the processed audio input signal. BBD Module A-188-1 Basic Karplus/Strong Synthesis Patch CV and gate are delivered e.g.
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DOEPFER System A-100 BBD Module A-188-1 Karplus/Strong Random melody patch (A149/1) This patch shows another example for the Karplus/Strong synthesis. The LFO A-145 is used as clock oscillator but any other clock source could be used as well. The rectangle output is connected to the clock input of the Quantized / Stored Random Voltages module A-149-1 (upper or lower section may be used) and to the gate input of the envelope generator A-140.