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Create sounds using analogue electronics (Part 6)

Posted: 04 Apr 2012     Print Version  Bookmark and Share

Keywords:AM  FM  formant  ring modulation  ringing circuits 

In contrast, by using the optical soundtracks that are often used in film projectors, it is possible to directly input the raw sound itself. Modern film projectors can use magnetic or digital techniques as well, but the basic method uses a light source and an optical sensor on either side of the film. When the soundtrack is clear, all the light passes through the film to the sensor, and conversely, when the film is dark, then no light passes to the sensor. By varying the amount of light that can pass through the film to the light sensor, the output of the sensor can be controlled. If the film soundtrack varies at a fast enough rate, then audio signals can be produced at the output of the sensor.

Film soundtracks usually control the amount of light by altering the width of the clear part of the film – the wider the gap, the more light passes through to the sensor. The part of the film used to record this 'sound' track is by the side of the picture, and looks much like an oscilloscopic view of an audio signal, except that it is mirrored around the long axis (figure 10).

Figure 10: A film soundtrack uses the amount of light passing through the film to represent the audio waveform.

By taking film that has no sound recorded onto it, and then drawing onto the film soundtrack with an opaque ink, it is possible to create sounds that will only be heard when the film is played. Sounds can thus be drawn or painted directly onto film. Although this sounds like an effective marriage of art and science, it turns out that the process of drawing sounds by hand is a slow and tedious one, and the precision required to obtain consistent timbres is very high.

The rough '30-dB rule of thumb' that says that a drawn audio waveform represents only the most significant 30 dB of the harmonics is very relevant here. Combining the drawing skills of optical sound creation with the tape manipulation processes of music concrete can offer a much more versatile technique. In this case, only short segments of film soundtrack need to be drawn, since the resulting short sounds can be recorded onto tape, copied many times to provide longer sounds and then manipulated using tape techniques.

Sound effects
Perhaps the ultimate 'analogue' method of synthesising sounds is the work of the 'sound effects' team in a film or television studio. Using a floor covered with squares containing various surfaces, and a large selection of props, 'Foley' artistes produce many of the everyday sounds that accompany film and television programs. For more unusual 'spot' effects, specialised props or prerecorded sound effects may be used. Choreographing the sound effects for a detailed scene can be a very complex and time-consuming task, very similar to controlling an orchestra!

Disc techniques
Using a turntable, slipmat and a robust cartridge can also be a flexible and versatile analogue method of sound generation. Since the 1980s, the use of the vinyl disc as a source of complex sound effects, rhythms and musical phrases has become increasingly significant, and this has happened alongside the use of samplers.

- Martin Russ

Printed with permission from Focal Press, a division of Elsevier. Copyright 2009. "Sound Synthesis and Sampling" by Martin Russ.

To download the PDF version of this article, click here.


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