For CTM Festival 2015, Berlin DE, with support from the Edith-Russ-Haus für Medienkunst, Oldenburg DE

Derek Holzer

The Vocoder: a Short History

The Problem of Privacy

The desire to speak privately in a public space has spurred technological developments since antiquity. The Echo Hall in Ancient Greek Olympia was a massive echo chamber of acoustic confusion, preventing eavesdroppers from hearing others’ conversations. Similarly, the 17th Century Jesuit scholar Athanasius Kircher imagined elaborate architectural tubes and passages to convey sounds secretly between public and private spaces.

The Wireless, Global Agora of Radio

20th Century inventions such as Guglielmo Marconi’s Wireless Telephone (1915), for example, made it possible for voices to be heard in a wireless “public space” around the world. This created a need to encipher the spoken word so that it could only be understood by certain listeners. By the 1940’s, these efforts centralized around the technology of the Vocoder.

The SIGSALY Speech Encipherment System

The 50-ton Allied military vocoder SIGSALY (1943) employed matched pairs of one-time-use vinyl records of random thermal noise, played back simultaneously on precision-timed turntables at both ends of encrypted military radio communications during World War II.

Alan Turing and the Delilah Machine

British mathematician, cryptanalyst, and one of the father-figures of modern computing Alan Turing, proposed the Delilah machine in 1943 as a portable voice-encryption system. Prototypes were completed in 1944, however it was never used in the field.

Aleksandr Solzhenitsyn & The First Circle

While Turing was officially persecuted for his homosexuality, Russian dissident Aleksandr Solzhenitsyn was imprisoned for his political views and forced to work on a Soviet version of speech encipherment (approx. 1947-50) -- an experience upon which he based his later novel The First Circle. Both men would have intuitively understood the need to converse in public without their intentions (political, social, sexual or otherwise) falling on the wrong ears.

North American Time Capsule

Alvin Lucier’s North American Time Capsule (1967) was one of the first musical uses of the vocoder. He instructed performers of the Brandeis University Chamber Chorus to try and communicate Earth’s present situation to beings from a faraway space or time.

Unblocked link to video:

The Robot Voice of Kraftwerk

However, most people will be more familiar with tracks like Kraftwerk’s Trans Europe Express (1977), which used a Sennheiser VSM 201 Vocoder for the synthetic sounding yet strangely harmonized vocal sections.

The Vocoder: How It Works

Vocoder as Bandwidth Reducer

A vocoder runs the input speech through a series of bandpass filters, which measures the amount of the energy in each band and sends that information to the receiver. This reduces the overall amount of information needed to be sent to a limited number of channels. The receiver reconstructs an approximation of the original signal by applying this information to a complex signal (such as noise) run through another set of bandpass filters.

Vocoder as Voice Scrambling Encryption Device

If the information channels of the vocoder are not sent in parallel, but rather swapped around in different orders, the result may still resemble human speech in its cadence, but in a language and from the throat of a speaker far from human. In this way, an “encrypted” version of human speech can be transmitted as sound.

Vocoder as Receiver and Decoder

By transposing the swapped channels on the receiving end, some part of the spoken sound information can be recovered. The key to decrypting this signal depends on knowing how the channels have been swapped, and adjusting the receiver accordingly.

Through the Vocoder and Back

In this short demonstration sketch, we first hear the original voice reading Fernando Pessoa's "I’ve gone to bed with every feeling", then the scrambled signal, and finally the descrambled signal.


DELILAH TOO Installation

The installation is comprised of two Private Spaces, communicating via an enciphered acoustic channel through a Public Space.

DELILAH TOO Private Spaces

The Private Spaces are two isolated booths, similar to the translators’ cabins shown. Inside each is a matching vocoder based enciphering and deciphering matrix, a microphone and a pair of headphones, allowing private communication through the Public Space (i.e. the exhibition room in Kunstraum Bethanien).

DELILAH TOO Vocoder System

The vocoders in each Private Space will be constructed using as much analog technology as possible, in reference to size and character of the original devices used in the 1940’s-70’s.

DELILAH TOO Matrix Control Console

In the Public Space, participants in the installation can adjust several parameters of the encipherment process. While this will affect the overall character of the sound they hear, they will have no way to understand the speech being transmitted.


Production of the DELILAH TOO installation would not have been possible without the support and assistance of the Edith Russ House for Media Art, the Grants for Media Art 2014 of the Foundation of Lower Saxony, Edit Molnar, Marcel Schwierin, CTM Festival, Carsten Stabenow, Jaanus Kalde/Tech-thing OU, Martin Kuentz, Juan Duarte/Aalto University Media Lab, Kris Barnett, Ray Wilson/Music From Outer Space, raumlabor, Wolfgang Spahn, Timo Toots and Montse Torredà Martí.


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DELILAH TOO CTM Installation Info - Google Slides