Vocoders
Vocoder effects are familiar to us in the form of robot voices or musical effects such as Peter Frampton's "talk box":
They work by determining a base frequency of a voice and then measure the spoken words in terms of variation from that frequency. Then, the synthesized playback is done by generating the base frequency and varying it according to the measurements. For effects like Frampton's, the playback varies a musical tone (such as guitar chords) instead of generating & using the base frequency.
Development of the Vocoder began surprisingly early--1928. Bell Lab engineer Homer Dudley created the vocoder as a way of encrypting speech for secure radio transmission and compressing speech for transmission over telephone lines.
More precisely, a Vocoder is the component that analyzes speech and a Voder (Voice Operating Demonstrator) is the component that recreates it. The early voders were manual filters (requiring a trained operator) consisting of consoles with fifteen touch-sensitive keys and a foot pedal.
Voder operator in 1939 and as demonstrated at the 1939 World's Fair:
A sound sample from Dudley's 1939 Voder, with introduction (170k au file)
Obsolete.com writes:
They work by determining a base frequency of a voice and then measure the spoken words in terms of variation from that frequency. Then, the synthesized playback is done by generating the base frequency and varying it according to the measurements. For effects like Frampton's, the playback varies a musical tone (such as guitar chords) instead of generating & using the base frequency.
Development of the Vocoder began surprisingly early--1928. Bell Lab engineer Homer Dudley created the vocoder as a way of encrypting speech for secure radio transmission and compressing speech for transmission over telephone lines.
More precisely, a Vocoder is the component that analyzes speech and a Voder (Voice Operating Demonstrator) is the component that recreates it. The early voders were manual filters (requiring a trained operator) consisting of consoles with fifteen touch-sensitive keys and a foot pedal.
Voder operator in 1939 and as demonstrated at the 1939 World's Fair:
Obsolete.com writes:
Werner Meyer-Eppler, then the director of Phonetics at Bonn University, recognised the relevance of the machines to electronic music after Dudley visited the University in 1948, and used the vocoder as a basis for his future writings which in turn became the inspiration for the German "Electronische Musik" movement.
posted by Ethan at
8:00 AM
1 Comments:
It looks like your description of how a vocoder works was taken from the Wikipedia article. Sadly, that article appears to be more-than-usually broken. (Why would one *ever* filter a given frequency with bandpass filters at other frequencies? This is just called "attenuation", and there's easier ways to get it.) Fortunately, their first reference link is to http://www.paia.com/ProdArticles/vocodwrk.htm, which provides a clear explanation of how a particular analog vocoder works, and confirms my recollections of the process.
The traditional analog vocoder is indeed a two stage process. In the encoding stage, human speech is sampled by a bank of fairly narrow bandpass filters at frequencies chosen to capture important speech features, and then the amplitudes of the filter bank outputs are measured.
The compression that was the target of the original algorithm comes from the fact that the amplitudes of the signals coming out of the filter bank carry most of the speech information, but are smooth in such a way that they can be compactly represented and transmitted infrequently.
In the decoding stage, of a traditional vocoder, the band amplitudes coming from the encoder are used to modulate sinusoids at the bank center frequencies, recovering enough of the original signal that the speech is understandable. This also gives the vocoder its characteristic "choir" sound.
In electronic music, one typically uses a second filter bank to filter frequencies out of a musical source such as a guitar chord, and then modulates the output of the second bank with the amplitudes from the first bank. This is how the sort of "talking guitar" effects one often hears are produced.
While LPC etc are technically vocoding, they bear so little resemblance to the original technology that they are usually referred to by other names.
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