The Eltro

In the world of audio signal processing, before the digital era, there was a variety of analog samplers that could pitch-shift and time-compress audio signals on magnetic tape. In all cases, the essence of these devices depended not on one, but typically four tape heads, uniformly placed along a circular drum. By varying the speed of the tape and the rotational speed of the drum, the relative head-to-tape speed could be manipulated, in ways not previously possible, in order to achieve analog pitch-shifting and time-compression of the signal. The idea of a rotating drum having multiple tape heads dates back to as early as the 1920s. But it wasn’t until 1954 that the ball actually started rolling, when G. Fairbanks, W. Everitt, and R. Jaeger built a working machine, with a rotating drum at its core, for pitch-shifting and time-compressing tape signals, and perhaps more importantly, formally explained its inner workings. There were others, thinking in parallel on similar devices, but it was the work of Fairbanks, Everitt and Jaeger that really stood out. Some years later, in the 1960s, along came the Eltro Information Rate Changer, perhaps the most celebrated of these largely forgotten analog samplers. The Eltro was primarily used in the advertising industry to adjust the time duration of an audio signal whenever the accompanying film needed to run at a different rate so that a commercial could fit into the standard 30 or 60 second time slots. If, for example, a commercial film needed to run faster, and the audio signal were simply sped up by the same amount, we’d get the familiar “chipmunk” effect, where all of the signal frequencies have been shifted up by an amount corresponding to the increased tape speed. The Eltro got around this problem by preserving the signal’s frequency information while compressing or expanding its time duration. The most famous usage of the Eltro, however, was not for time compression, but for pitch-shifting the dying voice of HAL9000 in Stanley Kubrik’s “2001: A Space Odyssey”. In the late 1960s and early 1970s, when computers were finally catching up with DSP theory, naturally, pitch-shifting and time-compression of audio, particularly speech signals, was of great interest. Foulke, in 1970, and Lee, in 1972, were the first to digitally implement the rotating head tape recorder, primarily motivated by reducing the number of bits required to convey the same amount of information, or compression, in the modern sense of the word. Sadly, along with the others of its kind, this is when the Eltro got away. As a form of DSP archaeology, I would like to bring it back, in digital form, if for no other purpose than the pure enjoyment of hearing it once more. For my research, I will formalize the signal processing theory necessary to create an algorithm, which, in the digital domain, will faithfully emulate the Eltro Information Rate Changer. I will first find a continuous, closed-form equation that governs the positions of the four tape heads along the tape, as measured from the tape’s beginning, as a function of one independent variable, time. I will then discretize the equation, and in the process develop the algorithm necessary for a faithful emulation of the original, analog Eltro. In addition to developing a theoretical framework and efficient algorithm, I intend to implement the system physically in the form of a stand-alone, embedded DSP device.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Electrical & Computer Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2019

Author dc:creator

• Sconza, Justin

Contributors dc:contributor

• Haken, Lippold
• Jones, Douglas L.

• pitch shifting
• time compression

Statement dc:rights

• Copyright 2018 Justin Sconza

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/102897

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/102897