Optical tone method

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Film image with two optical sound tracks for stereo sound in jagged writing (two double jagged tracks)

The optical sound process is the oldest sound film process still in use today , in which image and sound information are applied to the same carrier. The sound of a cinema film is photographically stored on a strip called a soundtrack that is a maximum of one tenth of an inch (i.e. a maximum of 2.54 mm) between the individual images and the perforation holes in the film. Since the images are conveyed on step by step while an analog sound signal has to be scanned from the constantly running film strip, the image and sound are stored on the carrier with a time offset, see time offset .

As an alternative to the optical sound method, the magnetic sound method is used. Compared to the magnetic sound process, the optical sound process has several advantages. On the one hand, the soundtrack is copied when the film is made, so no additional steps are required. On the other hand, the audio track is more stable over time and cannot be accidentally deleted. The disadvantage (as with the actual film image) is the susceptibility to scratches, which can lead to sound disturbances.

history

The optical toning process was the first to record the sound on the same medium as the image. It was first presented to the public by Sven Berglund in 1921 and demonstrated in the United States on June 9, 1922 by the Polish engineer Józef Tykociński-Tykociner . A few months later, the German engineers Hans Vogt , Joseph Massolle and Joseph Benedict Engl showed their first films. The first film with an integrated optical soundtrack, Der Brandstifter by producer Erwin Baron , was shown in Germany in 1922 in the Berlin Alhambra-Lichtspiele . The rights to the process were sold to William Fox in 1928 . The technology was not limited to the film alone. For example, the world's first time announcement from 1933 in Paris also worked according to this method, after the mechanical scanning in test operation did not meet the high requirements for continuous operation. The inventor Lee de Forest had his first commercial success with the optical sound process in film, and received an honorary Oscar for it in 1960 . It is reported, however, that in the early twenties he stole the idea of ​​the sound film from his classmate Theodore Willard Case at Yale.

Optical tones with the Dolby A noise reduction system have been around since 1976 . This system greatly improved the sound quality. It was also possible to accommodate two optical sound tracks in the space that previously required a track, and in these two tracks to accommodate the information for a surround channel and a center channel. That was the beginning of Dolby Stereo . The Dolby Spectral Recording noise reduction system (Dolby SR) has been in use since 1987 . Accordingly, the light tone was called Dolby Stereo SR or simply Dolby SR .

Recording principle

Sprout script and serrated script (double zigzag)

There are two methods of production: the intensity method (sprout writing) and the amplitude method (serrated writing).

Intensity method (rung)

The sound is recorded on the film strip with a constant width, but the blackening or density is variable, which is caused during film production by the light incidence of different amplitudes. The period of the recorded wave-like blackening corresponds to the quotient of the film speed and the audio frequency . The resulting recording on the film strip is called sprout writing. Stereo sound can be recorded by using two tracks .

The method has the disadvantage that the density curve has a great influence on the sound quality and increasing distortions can also occur during copying.

One of the first inventors to devise sound recording based on this principle was Heinrich Stefan Peschka .

Amplitude method (zigzag)

In the case of the amplitude method, the tone coding, surrounded by a high level of blackness, is recorded on the film strip in a jagged manner. The amplitude determines the width fluctuations of the spikes, the frequency their period. The resulting image is called serrated script. Two sound tracks are not absolutely necessary to generate stereo sound - the deflection of the spikes on each side then determines one channel of the stereo sound. When deflected on two sides, the font is also called double-pointed font. Two double-pointed tracks are usually used for analog stereo recording.

With older copies, there can be a lot of noise, especially at the beginning and end of each roll. The intensity of the interference (dirt) depends on the proportion of light areas in the sound track. This is why a light waveform on a dark background is normally used. This means that there is less interference in quiet areas (only narrow white waveforms). In loud passages with predominantly low tones, however, the scratchy noise can be annoyingly audible (as it is not covered by high frequency components); its intensity fluctuates with the volume of the useful signal. This is why dark double-pointed fonts are used on a light background. In order to keep the white component in the sound image as low as possible, a black envelope is placed around the actual waveform, which runs as close as possible to the peaks of the waveform. However, the natural frequency of this envelope curve must be below the lowest playback frequency of conventional cinema sound systems in order to remain inaudible itself. Are used for low frequency reproduction, e.g. B. used for disaster films, modified systems, the envelope u. U. can be reproduced as infrasound, which is not consciously audible for the viewer, but can lead to feelings of fear through unconscious perception. That is why such cinema systems should be equipped with switchable low-frequency filters that filter out the envelope curve modulation for "normal" feature films.

Color of the sound track

With color films, the color of the soundtrack depends on the process. While a black and white soundtrack is mostly used, the early color film process Cinecolor used a blue soundtrack. This was not ideally suited for playback with conventional projectors. The black-and-white soundtrack has become generally accepted in positive films (cinema copies), the contrast of which has been enhanced by the application ("slime") of a silver-containing solution ("clay slime"). This was necessary in order to obtain optimum sound quality when scanning the light tone on the film projector with white light. Since each cinema copy had to be "glued" individually, the process was very time-consuming and therefore expensive. In addition, it was argued that the silver coating was toxic. Therefore, alternatives were sought and in the early 1990s it was decided to convert the analog optical sound track to the color cyan (Cyan Dye Track) and to scan it with red laser light or red light-emitting diodes . To do this, the cinemas had to convert their film projectors from white light to red light. Since the conversion of the film projectors could not take place at the same time and a cyan light tone with white light scanning produces unusable sound quality, the magenta "Hi-Magenta" light tone track was introduced for the transition period. This brought acceptable sound quality with both white light and red light. After the conversion of the cinema projectors to red light was largely completed in the early 2000s, the cyan optical sound track was introduced across the board.

Digital procedures

35 mm film with analog and digital soundtracks.

Meanwhile, there are digital optical sound tracks , such as Dolby Stereo SR-Digital, today mostly simply called Dolby Digital (the most widely used digital sound method) or SDDS , a system from Sony .

With this method, in contrast to analog optical sound methods, the sound is not copied onto the film in analog form, but digital information that is captured by a photo receiver and then converted into sound signals in a decoder.

These methods allow a higher dynamic, less susceptibility to damage and more channels, which allows a better spatial representation and more creative freedom in the sound. Furthermore, with Dolby Digital and SDDS, the sound tracks are applied redundantly, i.e. This means that even if individual images are damaged, the entire audio signal can be reconstructed.

In the DTS system , the sound is not stored on the film, but only a time signal (time code) that is used to synchronize the image and the sound stored on an external CD-ROM. The spread of DTS is declining, as many film distributors shy away from the increased logistical effort involved.

There are extensions of these digital systems by more channels, for example DSRDEX as an extension of DSR-D (Dolby Digital).

THX is not a separate sound system, but a certification process for optimized sound reproduction. For this purpose, elements of the sound system as well as the acoustics and optics in cinema halls are checked and certified. The quality improvement proclaimed by THX is, however, controversial among experts.

Playback principle

Movie projector mechanics

A small lamp shines on the sound strip which, depending on the amplitude and frequency of the recorded sound signal, allows more or less light to pass through (analog sound storage). Thus, light of varying intensity falls on a photodiode (or earlier on a photocell ), which is attached to the other side of the film. The photodiode converts the light into an alternating voltage , which is amplified and then fed to the loudspeakers in the cinema. A slit screen ensures that only a very short piece of the sound track is illuminated in order to be able to reproduce high frequencies.

Offset between picture and sound

In the cinema projection, the images come to a standstill for 1/24 of a second, are shone through by the projector lamp and projected onto the so-called screen. To do this, a piece of film is moved step by step / image by image between freely swinging loops. The soundtrack, on the other hand, has to be read from a constantly running piece of film. In the light sound device, the so-called sound wave, which is connected to a flywheel, ensures that the film runs smoothly. This happens - seen in the direction of the film - behind the projection lens of the projector. Because of these technically different playback conditions and to take account of the structural conditions, the image and soundtrack on the screening copies are offset from one another. A sound event is therefore always 20 images in front of the associated image - and not directly next to it. The picture-sound distance is standardized for all film formats and sound processes, but not always the same. This applies equally to analog and digital processes.

brand names

In the United States, optical tone technology was popular under the following brand names in the 1920s:

literature

  • Joachim Polzer (Hrsg.): Weltwunder der Cinematographie - Contributions to a cultural history of film technology - Rise and fall of the sound film - with historical presentations on optical sound and magnetic sound. 6th edition 2002, Polzer Media Group, Potsdam 2002, ISBN 3-934535-20-8
  • Hans Vogt: The Invention of the Sound Film. A look back at the work of the Engl – Massolle – Vogt inventors' association. Erlau near Passau 1954
  • Karl Röwer: The technology for projectionists . VEB Wilhelm Knapp Verlag, Halle (Saale) 1953, No. 380/49/51

See also

Individual evidence

  1. ^ The arsonist in the IMDb
  2. ^ Website of the Paris observatory: L'horloge parlante officielle française de l'Observatoire de Paris. La première horloge parlante au monde. (The official French speaking clock of the Paris Observatory. The world's first speaking clock). Retrieved February 22, 2013

Web links