3.5.2 Gabor's machines for time distortion

        Fig 3.2: sound film projector converted into an experimental frequency converter. (Gabor 1946: 445)
 
 

Gabor built machines using a compression-expansion idea for frequency conversion. His machines operated with mechanically moving parts. He called them the kinematical method of frequency conversion.¹ The second type of system he proposed did not require any mechanically moving parts. He called it the electrical method. (Gabor 1946: 445)
 
 

Gabor never actually made an electrical method frequency converter, but he suggested the mathematics behind the idea. His main reasoning being that the waves can be added together without using any mechanical means. (Gabor 1946: 454)
 
 

The kinematical method was developed using a film projector (see figure 3.2) (Gabor 1946: 452). Gabor ran a sound film through the projector. The film would move at a constant velocity past a slit through which light would be projected to a photocell. The photocell would convert the light to a sound signal. Gabor decided to replace the light slit with a rotating drum that contained many slits. He added a window between the sound film and the photocell. He decided to add more slits to the drum so that the next slit would start going past the window just as the previous slit left the window. Gabor realised that this would produce a loud crack at the start of every cycle over the window, so he graded the window so that it was clear in the middle but darker to opaque on the sides so that the frequency would fade in and out without any clicking. (Gabor 1946: 446) See figure 3.3.
 
 
 
 

        Fig 3.3 kinematical frequency converter with sound film.(Gabor 1946: 446)
 
 

The film would pass over the spinning drum at different speeds. Using this method Gabor was able to adjust the pitch of the sound. Each time the slit passed over the window it produced just a small segment of sound. The segments were then added together to reproduce the source signal at a different frequency. Gabor noted that for some frequencies he got a nearly faithful reproduction, yet for others he got strong beats. (Gabor 1946: 447) This becomes very clear with a graphic analysis. See figure 3.4.

        Fig 3.4: A diagram of the sound segments separate and added(Gabor 1946: 447)
 
 

Gabor discusses many things that could be changed to improve the quality of the pitch being reproduced. These deal with things such as the speed the drum is spinning, the number of slits and the rate of the film that goes through the projector. Gabor discovered that the number of slits makes the biggest change. If there are too many slits the reproduction will be of lower quality and very noisy. If there are too few slits the reproduction will be very good in some places, yet non-existent in other places. Gabor stated it works best if the next slit starts when the previous slit is half way across the window. (Gabor 1946: 448)
 
 

Gabor made another similar machine in which he adapted a 16mm camera to rotate the drum at different speeds. This produced different sized grains. He determined that the grains need to be between 20 and 50 ms.
 
 

Gabor also made another such machine this time using a tape recorder. He designed it so that the tape was in a big loop. After reading each section of the tape it would be erased and something new could be recorded before that section of the tape got read again. This way it could be used infinitely without running out of film, and it could be constantly updated. The tape recorder read the tape using a pick-up coil that Gabor inserted inside a drum. The drum consisted of a non-magnetic material with iron rods embedded into it. The pick-up coil could only read the magnetic tape when the iron rod was between the tape and the pick-up. The drum was covered with an oil film so that there would be reduced friction and scraping from contact with the tape. (Gabor 1946: 453) See figure below.
 
 

3.6 Springer
 
 

The German company Springer made a device called the Tempophon. The device was similar to that designed by Gabor (Figure 3.5). This time/pitch changing device was installed in a number of analogue electronic music studios around Germany (Roads 1996a: 441). It was the first time granular methods were used in a commercial environment. In 1963 Herbert Eimert used the Tempophon for an electronic music composition called Epitaph für Aikichi Kuboyama (Wergo 60014). There were a number of similar machines built after the Springer Tempophon (Roads 1996a: 441).