A significant technical development is this lowcost stereophonic/optical recording system, used without changing any basic monophonie standards
The subject of this article is an optical recording and reproduction system which can be used for standard 35mm motion pictures and will record and reproduce two-channel stereo sound.
With this process we can record on one optical track such a compatible modulation combination which contains stereo information. This can be done without changing any basic standard for monophonie optical sound recordings.
This article will introduce the basic requirements for transferring stereo recordings to the optical track, and its reproduction, the underlying theory, associated techniques and the up-todate experience.
Audio-recording techniques have reached a point today where stereo records and broadcasts are everyday matters. In the past few years disc manufacturers have been making exclusively stereo recordings.
What is the situation like in our field, of the motion picture business?
With the development of the widescreen technique, multiple-channel systems came into being. In the case of the 35mm film, regardless of its format, the only feasible method remained the monophonie optical recording and reproduction.
History shows that the use of magnetic soundtrack became workable only with 70mm films and with 35mm films the mono optical soundtrack remained in use. In Hungary, motion picture theaters equipped with fourchannel magnetic systems are only partly utilized.
Where money is not available for the production of 70mm films, production companies are forced to use the less expensive 35mm ones, the CinemaScope method, and - despite the prevailing artistic requirements - naturally use the single channel monophonie optical sound system. In the meantime, the demand for high-quality stereo sound would be just as great for these 35mm productions.
In the Sound Department of MAFILM/Hungarian Company for Motion Picture Production, we have worked out a stereo optical recording system, as well as a reproducing system, which are based on combined modulation and are compatible with the traditional optical systems.
Let us take a look at this optical stereo system.
1. The optical track in its size and place on the film is identical with the one on its "mono sister". It will not require any special perforation or change in present standards of the optical track.
2. The light valve of the optical recording camera is so designed that if a simple monophonie signal is fed into it, mono information will be exposed/recorded, but if we feed in a combined stereo signal, the exposed signal will hold stereo information.
3. Copies of the film can be made the usual way, as in case of films with the normal optical track.
4. This stereo optical track can be reproduced on any projector and will give normal monophonie sound in this case. In motion picture theaters equipped with projectors using the optical stereo heads recommended by us and multiple channel loudspeaker systems, a full stereo sound will be reproduced.
5. The sound quality of the system is better than that of any optical recording system known to us to date. We have several results to prove this statement regarding the dynamics and frequency range of the recording.
6. An additional but rather important advantage of our stereo system is that in motion picture theaters equipped with the recommended lightadapter the conventional optical tracks will be reproduced with better quality than through the traditional system.
How does this stereo optical system work?
The system is based on the Sumand-Difference recording method which is well-known in the disc recording business. In this recording system the sum channel is produced as the sum of the right and left signal and is the actual carrier of the sound. The signal of the difference channel is obtained by subtracting the signals of the right and left channels from each other; this carries the information regarding the direction of the sound. …