EMS Electronic Music Studios EMS Vocoder 2000 Mixer
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EMS Vocoder 2000
Signal Processing Device
The Vocoder 2000 is a signal processing device, which takes
two input signals and produces one output. These two signals
are known as the 'speech' and the 'excitation'. The output
from the Vocoder is a signal which contains a proportion of
the harmonic structure of the excitation and the formant
structure (the articulation) of the speech. Thus it is
possible to make normally inarticulate sounds speak. For
instance, if you were to take speech and an organ, it would
be possible to make a 'talking organ'.
The heart of the Vocoder is the analysing and synthesizing
The speech is analysed into 16 frequency bands which cover
the audio spectrum. The time varying energy levels in each
channel is extracted by an envelope follower. This is in
fact a real time spectrum analysis of the speech.
Another signal, the excitation is introduced into the Vocoder.
This is the signal that we will make talk. The excitation
signal is also analysed into 16 frequency bands throughout
the audio spectrum. However, the signal that is presented to
each band is multiplied by a control voltage, which is the
envelope from the speech channels. Thus the time varying
spectrum of the speech is imposed upon the excitation
signal; that is the excitation is filtered in a way entirely
prescribed by the speech signal.
The speech and the excitation signals are connected via mono
jack sockets to the input amplifiers. The signal level is
adjusted and given a pre-emphasis, a top lift. A switch
connects a VU meter to either the speech or the excitation
signal. A device called a voiced/unvoiced detector looks at
the speech signal and decides whether or not th^ speech is
unvoiced ('s' sounds) or voiced (sounds derived from vocal
chord vibrations). These voiced/unvoiced decisions can be
used to turn on or off the machine's internal oscillator and
noise source. Also, the unvoiced decision can be used to
alter the external excitation signal so that it becomes more
'S' like. This is done by operating the 'S' generator
switch. The Vocoder 2000 has two internal excitation
sources, the noise source and the oscillator. The noise
source is generated by zener diode and can be used
continuously, or gated on by the unvoiced decisions or it
can be switched off so that it has no effect. The oscillator
produces a pulse waveform which can also be used
continuously or gated ON by the voiced decision or it can be
switched off. Also the pitch of the oscillator can be
manually set as well as the option of being varied by a
control voltage generated by the filter bank analysis. The
slew freeze section controls the synthesis section of the
filter bank. It is possible to freeze or to slew limit the
control voltages inside the filter and this will of course
have an effect on the Vocoder's output. When frozen, the
formant structure remains fixed. When heavily slewed, the
output sounds very reverberent. This is because information
in the filter bank is time smeared. There are two possible
types of time smearing that are available, symmetrical and
. The symmetrical option takes as long for a sound to build
up as it does to decay, but the asymmetrical gets loaded
with a sound structure immediately and then it slowly decays.
When you are making a piece of orchestral music talk, the
amplitude dynamics of the speech will be imposed upon the
output. Thus there will be lots of big gaps in the Vocoder
output which might not be very desirable. To overcome this
problem we use a device called a pause stuffer. This fills
in the silences by fading up the music signal, immediately
fading it down when the Vocoder output reappears. Speech or
excitation can be used to 'stuff' the pauses and there is a
choice of fade in times. The output Mixer
is used to select
either the Vocoder output or the speech or external
excitation signal. It is also possible to control any of
these signals with an external swell pedal.
11.80 Mbytes (12370217 Bytes)
01 May 1977
Scanned document, reading partly badly, partly not readable.
16 August 2017