DOEPFER A-188-2 Manual - Página 5

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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: In the center position of the polarizer

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 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

System A-100

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 A-

188-2 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 A-

188-1/2 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).

BBD Module A-188-2