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CIRCUIT DESCRIPTION
—General-
Sound from Musical
Instrument
Voice
from
XIC
High Sound
3yr:as3 Filter
Com
pressor
1st
Filter
Amplitude
Detector
Sound
Detector
2nd Filter
-*• VCA •*
Sxcander
Holi
In the Vocoder, the voice signal from a Mic is frequency-analysed
through
a group of filters to
slice up a voice signal frequency
3pectruna featuring huaar. voice.
Then the spectrum is duplicated
to another group of musical sound signal filters to obtain func
tions equivalent to human acuth and throat and thus to simulate
human voice with musical sound signals.
Fundamental Vocoder functions are discribed below according to
t.ie Blocsk Diagram shown above.
1. Analyzing (first) Filter and Amplitude Detectors
A Mic iixput signal is resolved by a group of filters into freqen-
cy band
components which are amplitude-detected and supplied tc
the
VCAs of the Synthesiser Filter (second filter).
Signals
passing through second filters are. controlled in volume
at VCA b.y the control signal coming from corresponding frequency
band of
tne first filter.
Z. Synthesiser ,'3e;or.a; liter and "CAs
Like the first filter, aausicai 3cund signal
being supplied is resolvd into
frequency
spectrum components.
Sise a musical
3ound
passing through the 3ecori group of filters
is
proportional
to
the
f*st
filter output
amplitude,
the spectrum £ the second filter
output
is analogous
to
trit of
the vcice
signal.
In ether words, :he second filter
output
is mixture of trie
input musical
3cund signal and
the firs: signal output.
Thus, uniform 3ound signa. spectrum would be
ideal for reproduction of human voice,
but
it
is no longer of a musi:al instrument.
3• Compander
The
compander
is
a
combination
3f
a
compressor
and
an
excajier.
The compressor reduces a a ic input signals
range in amplitude and su: plies smaller out-
cut signals range than inj ut signals'
to the
first
filter.
On the contrary, the expan der, for a given
range of amplitude input v oltages,
produces
a larger amplitude range o f output voltage.
Thus restores the ordinal
volume range.
i.
Kigir. Frequency Voice Signjil Bypass
Filter
\Reiionant Filter)
3mce musical
sounds rarely include high frequency
r.oise
components
such as
"fricative"
may
be
in
V.ioe,
the
second
filter has no spectrum
to respen:
to.
Furthermore,
such a sound,
hardly relating t:
musical
intervals,
is
separated
from
a
mic
inpu"
signal,
passes
through this circuit and
is
re-
ccmbined with
the
second
filter outputs.
5.
Musical Sound Signal Detector
This
circuit
-obstructs
tne
second filter
output
a;
long as a musical sound
is not supplied to
the
Vocoder and
tells the
circuits 3 and i whether
a
musical scur.d signal
is being fed cr net.
Thi3 circ .it enables Vocoder to hold its
output
luring an interruption in mic signal,e.g. when a
singer inspires,
-he function can also be used for
some
special
effect applications.
During holding, this circuit retains spec trams and
volume by holding amplitude ietectors output
volt
ages and expander control voltage.
Tne compressor gain is minimised and the vcice gate
is turned off so as to 1-ceep vcice
unchanged
ever.
though ,Xic
input singal is changing.
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