DOEPFER A-188-2 Посібник - Сторінка 2

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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: In reality two "buckets" are required for each

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.

System A-100

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: At 100kHz clock frequency the delay time is

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

BBD with 2048 stages.

BBD Module A-188-2