User Guide

Chapter 15 EVOC 20 PolySynth 227
Feel free to do what you like when setting the Formant parameters. The intelligibility of
speech is surprisingly little affected by shifting, stretching, or compressing the
formants. Even the number of frequency bands used has a minimal impact on the
quality of 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 which make up their voices.
Our perception (recognition) of speech is based on an analysis of the relationships
between these formants. In the EVOC 20 plug-ins, these stay intact, even when extreme
formant settings are used.
Vocoder History
You may be surprised you to learn that the voder and vocoder date back to 1939 and
1940, respectively.
Homer Dudley, a research physicist at Bell Laboratories, New Jersey (USA) 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.
These were the:
 Parallel bandpass vocoder: A speech analyzer and resynthesizer, invented in 1940.
 Vocoder speech synthesizer: A voice model played by a human operator, invented in
1939. This valve-driven machine had two keyboards, buttons to recreate consonants,
a pedal for oscillator frequency control, and a wrist-bar to switch vowel sounds on
and off.
The analyzer detected the energy levels of successive sound samples, measured over
the entire audio frequency spectrum via a series of narrow band filters. The results of
this analysis could be viewed graphically as functions of frequency against time.
The synthesizer reversed the process by scanning the data from the analyzer and
supplying the results to a number of analytical filters, hooked up to a noise generator.
This combination produced sounds.
The Voder was demonstrated at the 1939 World Fair, where it caused quite a stir. In
World War II, the vocoder (now called VOice enCODER) proved to be of crucial
importance, scrambling the transoceanic conversations between Winston Churchill and
Franklin Delanore Roosevelt.