Vocoder - Access Virus Rack User Manual

162 CHAPTER 17
The Vocoder

VOCODER

Although vocoder sounds have seen a
comeback in the recent time, many musi-
cians do not specifically know how they
work or even more importantly, how
they sound! Various Vocoder presets are
stored In the last section of SINGLE bank
D (grammalogue "VOC"). These SINGLEs
require an audio signal being fed to the
external inputs. The keyboard also needs
to be played on most of the presets. The
Virus Vocoder is not necessarily easy to
handle. You should always refer to an
existing Vocoder preset for editing.
The Vocoder is one of the most complex
sections of the Virus. For this reason, we
recommend that you use factory sounds
that use the Vocoder as your point of
departure and edit these to create your
own sounds. This means that you don't
have to start from scratch and that the
Vocoder's parameters are set to viable
values, which will facilitate program-
ming considerably.
A Vocoder creates a new sound by com-
bining two signals. The timbre of the so
called analysis signal (or modulator sig-
nal) forms the carrier signal. A typical
example for a modulator signal is the
human voice, another suitable carrier
signal can be a steady tone with rich har-
monics.
The sound characteristic is being ren-
dered into the new sound by two cas-
cades of bandpass filters: The modulator
signal is being send through several par-
allel bandpass filters, which only pass
through a certain part of the whole spec-
trum. Every bandpass filter is followed
by an envelope follower which uses the
level of the signal to create a control sig-
nal. This part of the vocoder is called a
modulator bank.
The carrier signal is treated in a similar
way. It is split into several bands by a
chain of bandbass filters. Different to
the technique used above, the bandpass
filters are not followed by envelope fol-
lowers. The circuit uses amplifiers which
are levelled by the control signal output
of the envelope followers of the modu-
lator banks. This part of the vocoder is
called the carrier bank or synthesis part.
As soon as the modulator detects a sig-
nal in a certain frequency range, its
envelope follower levels the correspond-
ing band in the synthesis part. In other
words: Just the frequency band of the
carrier signal (the steady tone), which is
a part of the modulator signal (the spo-
ken voice) in this second, is being passed
to the output. This is why the steady
tone starts to speak. The pitch of the
output signal depends on the carrier. If a
cord, e.g. a polyphonic pad sound is
being used instead on a single tone, you
can listen to a typical vocoder choir. Of
course, you can use a different Modula-
tor than a spoken voice. Have a try with
a drumloop!