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The spectra of the analysis and synthesis signals should almost completely overlap.
Coupling low male voices with synthesis signals in the treble range doesn’t work well.
The synthesis signal must be constantly sustained, without breaks. The incoming side
chain signal should be played or sung legato, as breaks in the synthesis signal will stop
the vocoder’s output. Alternatively, the Release parameter of the synthesis signal—not
the Release time of the analysis section—can be set to a longer time. You can also achieve
nice effects by using a reverberation signal as a synthesis signal. Note that the two latter
methods can lead to harmonic overlaps.
Do not overdrive the vocoder. This can happen easily, and distortion will occur.
Enunciate your speech clearly if the recording is to be used as an analysis signal. Spoken
words, with a relatively low pitch, work better than sung vocals—even if the creation of
vocoder choirs is your goal! Pronounce consonants well, as exemplified in the rolled “R”
of “We are the Robots,” by Kraftwerk, a classic vocoder track. This exaggerated
pronunciation was specifically made to cater to the vocoder.
Feel free to do as you wish when setting the Formant parameters. Shifting, stretching, or
compressing the formants has a surprisingly minimal effect on the intelligibility of speech.
Even the number of frequency bands has a minimal impact on the quality of speech
intelligibility.
The reason for this is our ability to intuitively differentiate the voices of children, women,
and men, whose skulls and throats vary vastly by nature. Such physical differences cause
variations in the formants that make up their voices. Our perception, or recognition, of
speech is based on an analysis of the relationships between these formants. In the EVOC 20
plug-ins, these relationships are maintained even when extreme formant settings are
used.
A Brief Vocoder History
You may be surprised to learn that the voder and vocoder date back to 1939 and 1940,
respectively.
Homer Dudley, a research physicist at Bell Laboratories in New Jersey, developed the
Voice Operated reCOrDER as a research machine. It was originally designed to test
compression schemes for the secure transmission of voice signals over copper phone
lines.
It was a composite device consisting of an analyzer and an artificial voice synthesizer, as
follows:
• Parallel bandpass vocoder: A speech analyzer and resynthesizer, invented in 1940.
158 Chapter 7 EVOC 20 PolySynth