Doepfer A-100(~ 40 MB) - Manual (Page 318)

A-129 /1/2 Modular Vocoder System A - 100 doepfer
4
3. Basic principles
The fundamental modules in this vocoder are the
analysis section A-129 /1 and the synthesis section
A-129 /2 (see Fig. 1).
The speech signal is analysed in the A-129 /1, by
being passed through a set of steeply sloping band
pass filters, with a low- and high- pass filter mop-
ping up the bottom and top frequencies respectively.
Attached to each of these filters is an envelope follo-
wer, which analyses the audio signal level passing
through, and sends a proportional voltage out of its
dedicated CV output (see below for further details).
The instrument signal is likewise sent through
another set of steeply sloping band pass filters, and a
low- and high-pass filter in the A-129 /2 synthesis
section, and is split into individual frequency bands.
This time, each filter has an associated VCA (voltage
controlled amplifier), which is governed by the vol-
tage present at its CV input.
In this way, each frequency band in the instrument
signal has the dynamics of the corresponding band
from the speech signal superimposed onto it. The
pattern of the speech signal is thus re-constructed
from the tonal raw material of the instrument signal.
The closer the audio spectra of the speech and carrier
signals are, the more speech-like the resulting recon-
struction.
Fig. 1: Block diagram of the A-129 analysis and
synthesis sections
LPF VCA
BPF 1
BPF 13
LPF
BPF 1
BPF 13
EF
EF
EF
Mix
HPF
HPF EF
Analysis A-129 /1 Synthesis A-129 /2
Speech In Instrument In
Voc.
Out
High
Pass
Out
VCA
VCA
VCA