Most people interested in stereophonic reproduction are primarily interested in what is needed to give them this kind of reproduction in their livingroom. But as soon as you get interested in a subject, anything connected with this subject attracts your attention. So most stereophonic sound enthusiasts will also be interested to know "how it is done" in the movie business. This might be regarded as just satisfying idle curiosity, but it also serves a useful purpose. Understanding the different problems associated with sound reproduction in large places, such as movie theaters and auditoriums, helps to understand the opposite problem of reproducing sound in small spaces, such as the livingroom. The problems often encountered are completely reversed.
Techniques
For example a presentation in the movie theater may be intended to convey the impressi9n of a livingroom scene, together with the appropriate sound effects, but the actual area in which the audience is listening is a large auditorium. In the home reproduction systems, the program desired may be a concert from a large auditorium, but the listening place is a small livingroom.
Of the two problems it would seem that presenting an impression of a concert hall in your livingroom is the easier of the two. The reverberation can be recorded along with the program and this is reproduced in the livingroom to give the effect of the larger auditorium. In the opposite direction, the lack of reverberation can be recorded on the sound track corresponding to the small room, but you cannot take reverberation out of the larger auditorium in which you listen. The best, in fact the only approach, is to deaden the auditorium. This does minimize its reverberation, so that after sitting there for a short time you are no longer conscious of it, and the sound you are conscious of is that reproduced over the system without the comparatively small degree of reverberation added by the auditorium.
The important difference between the two kinds of system is the fact that the listener sits at a greater distance from the loud speakers in the movie theater (except possibly those who sit in the front rows). The average listener is at a distance that is several wavelengths, even of the lowest frequency, from the loudspeaker grouping. Consequently some of the things that may be said about stereophonic sound presentation in an idealistic sense, and which do not apply fully in the livingroom because of the limited dimensions, do apply, to a greater extent, in the movie theater.
On the other hand, the spacing between loudspeakers, even the three units generally employed behind the screen, is so great that it is impossible to base the presentation on phase difference effects, because the space in between loudspeakers represents several wave lengths of all except the very lowest frequency sounds. Consequently the most satisfactory method of obtaining a suitable stereophonic sound track presentation in a theater consists of using separate audio channels and synthesizing the individual tracks to represent the different program content.
A multiple-track recording of an orchestra, for example, will be made from microphones close to separate groups of the orchestra, so there is very little time difference between the different sections of the orchestra on the original separate channels recorded. Then, to get the desired positional effects in the auditorium, to correspond with the visual presentation of the orchestra, the different original channels which may be as many as six or more, are mixed into the final stereo tracks to get the required directional effect with each of the different components originally recorded separately.
In some theater systems, "surround" loudspeakers are used in addition to the three loudspeakers behind the screen. These are fed from a fourth sound track and can contain either reverberation material to give the effect of echo, or else can contain completely separate effects such as a choir singing behind the audience. Full utilization of these facilities is not as effective as might be anticipated, because of the big time differences required for sound to travel to different parts of the auditorium. What may be an effective presentation for one seat would not be in another, due to the different distances from the loudspeakers feeding the different positions (Fig. 59). For this reason program material has to be chosen very carefully so the time difference in the receiving of pro gram from different directions is not critically important to the impression conveyed.
For example, a composite choir could be presented over the surround loudspeakers while maybe a solo comes from the stage, thus avoiding any specific time problem. If the arrangement were intended to give an echo effect, the echo might be unduly long from some locations in the theater and almost non-existent in others.
Due to this limitation in the possible application of this kind of effect, what at first appeared to be a wonderful prospect has practically died out. Movie producers hardly ever use surround effects any more.
Systems
The actual systems employed in movie presentations are very numerous. There are so many different wide-screen techniques that it would take a book on its own to describe the picture presentations. Along with the different forms of picture presentation, we have corresponding variations in the sound presentation. But, from the viewpoint of stereophonic recording, we can divide them under three principle headings. Other differences are mainly concerned with where the sound tracks are located on the film relative to the picture, and how many there are.
1. Multiple-Tracie Optical
The earliest attempts at stereophonic presentation in the theater were made before the advent of magnetic recording, or at least before the modern adaptation of it by means of magnetic oxides on an acetate base. Consequently the only way to provide stereo phonic effects was to use extra sound tracks of the type already in use.
Fig. 59. The problem with the use of surround speakers, listeners in different
position, (A and B) get sounds from the screen loudspeakers (heavy dashed
lines --- ) at different interval, from the sound from surround loudspeakers
(light dashed lines). The listener at A will hear the surround speakers
considerably ahead of the screen units.
An advantage of the optical sound track is its good dynamic range, because of the noise reduction facility. Background noise had always been a problem with movie sound tracks and this resulted in the development of noise-reduction systems. This is an arrangement which cuts down the amount of light passed through the film in proportion to the sound level. When there is only a very feeble sound track it is also a very narrow one, and the rest of the track is completely opaque. As the sound becomes louder, the width of the track is opened up allowing a greater amount of light to penetrate, which is controlled in turn by the variations in density or width of the clear section of the film (Fig. 60). The exhibitor's problems with multiple-track optical presentation arc very considerable. The movie projectors installed in the average theater have the sound heads located in the correct position for standard optical track. To present stereophonic optical, which has additional sound tracks located somewhere else on the film, requires the addition of extra sound heads in the projector. Usually this means the complete installation of a new section of the projector to replace the old sound head section, a very expensive proposition.
Fig. 60. How noise reduction works on optical sound track ("variable
area"). Quiet sections are narrowed to limit light transmission and
reduce background noise.
2. Multiple-Tracie Magnetic
The advent of magnetic tape-recording made possible the application of the magnetic oxide to the actual movie film. This is done after the printing of the picture on to the film by a process called "striping". Then the sound is "printed" on the stripe, not by a regular printing process, but by a magnetic-recording process.
By analogy with the process of producing the picture and the older optical tracks, the word "printing" has stuck, although it is difficult to sec how it applies in any literal sense.
Magnetic striping has its problems. It is very difficult to get one reliable magnetic stripe on to the film, let alone three or four.
They are apt to be insecurely cemented to the film so that pieces flake off. If they don't flake off, they wear off relatively easily, because they arc so narrow compared to the regular ΒΌ inch tape used for proper tape-recording.
Due to the narrowness of the sound track the dynamic range is not very good at best. It can sound deceptively good in a stereo phonic system by keeping the volume level pretty constant. No passages are allowed to drop down to a very low "intimate" level of presentation. This, of course, is not too much of a burden on many movie producers, because they rather like to have the intimate "whispers" on the screen loud enough to almost deafen the audience! The compatibility problem here is solved more readily, because the magnetic section of the projector clamps on the top and is often called a "penthouse" for this reason. In this position it will not interfere with the regular functioning of the projector on optical sound films, but means a changeover is necessary, by means of switching, to transfer from single-channel optical, to multichannel magnetic.
When the film is new and the oxide tracks are in good shape, and the playback heads in the penthouse are brand new, the quality is very good. Unfortunately this quality does not maintain too well.
The heads wear down, as well as the oxide wearing off the film, so that quality deteriorates both as a system and also with usage of a film. This is a defect that occurs to a much lesser extent with optical track.
The manufacturers recommend replacement of the penthouse heads at regular intervals, but this is an extra chore and one that is apt to get overlooked. All the while the system still plays, no change is made, consequently many of the magnetic stereophonic systems continue operating in very inferior conditions.
3. Perspecta Sound
This uses a single-track coded presentation, which we have discussed earlier in the guide. It could easily use either magnetic or optical sound track as a base, but it was developed in optical.
This made the system completely compatible with single-channel presentation.
The system even includes a relay that automatically switches from single-channel presentation over the center-channel loud speaker to three-channel presentation over the three separate loud speaker channels when the Perspecta coding signal comes on. This relieves the projectionists of all responsibility for changing over connections according to what kind of film is running. This is not meant to suggest that projectionists are lazy. But when time comes to change from one reel to the next, he may have so many things to do at once that one can easily get overlooked. It is even possible to splice sections of film having single-channel and Perspecta sound tracks into the same reel, and the system will automatically switch over from one presentation to the other as soon as the sound track changes.
Of course, the Perspecta system has the same limitations mentioned with the coded system earlier in the guide. It cannot give simultaneous direction identity. But it is surprising how realistic this presentation is, especially in view of the fact that in theater presentation all sense of direction is more dependent upon relative intensity from the different channels than it is upon phase differences. As the coded system depends entirely upon intensity variations, this enables it to exploit this difference to at least as good advantage as the regular multichannel stereo.
An advantage of the coded stereo for movie presentation is the fact that the system itself enables further noise reduction to be effective by means of the coding frequencies. The three channels can be turned down to lower level, or one channel can be turned down and the other channel turned right off, or any combination desired. But the effect is that background noise is turned down along with the program sound.
This amounts to electronic noise reduction, which is additional to the noise reduction that can already be incorporated on the optical sound track to minimize noise that may occur due to dust or other particles on the surface of the film. Applied to magnetic recording (which at the date of writing has not been done) this process could give magnetic recording the advantage of noise reduction and greatly improved dynamic range.
Making Movie Sound Tracks
So much for the systems used for film and theater stereophonic presentation. Regardless of which system is used to carry the stereophonic program material, whether it is a magnetic system or optical and whether the system uses separate tracks or a coded arrangement, the method of work for producing the ultimate pro gram that will be heard from the film is very similar at the studio, and quite different from that generally used in producing program material for home presentation. This again reflects the difference between the requirements for home and theater presentation of stereophonic sound.
Fig. 61. Multiple channel recordings are used to synthesize motion picture
sound tracks, using "panning" mixers to get the right mixtures
into the three mixtures channels for the final "print".
Film studios usually make at least six separate master sound tracks from which the ultimate composite is made, to be presented on the film (Fig. 61) . This is true even for the older non-stereo phonic presentation. The background music is usually made in about three separate tracks that contain different groupings of the composite orchestra. This enables the complete program to be recorded, and different parts of the orchestra can be emphasized or toned down in producing the ultimate composite, according to the "feeling" the director desires to be associated with the pictorial presentation it happens to accompany.
If the pictorial presentation shows a person singing a vocal solo, this will require another track. This is recorded completely on a separate track so as to allow of adjustment being made in producing the final composite, to match up with the changes in viewpoint presented by the camera.
For example, when the camera shows the soloist front view close-up, the sound track could emphasize the solo and put all the other music into the background. If the shot changes to a picture of a section of the "audience" listening to the soloist, then the sound needs to change to give this effect at the same time. By having the complete program recorded on separate tracks this change can be produced in making the final composite to go with the picture.
Another track is required for dialogue. Dialogue may or may not be accompanied by background music (or perhaps the sound of a soloist continuing with a performance, all going on at once) so we need another track for the dialogue that runs right through the picture.
Finally, all kinds of effects are needed and these require at least one more track. This is for sounds like people walking around, knife and fork clatter on plates when people are eating, and one hundred and one sounds to represent other every day happenings. Sounds used for this purpose are seldom what you actually see.
In the days of dramatic radio presentation sound effects were produced by coconut shells, gravel, pieces of wood, sheets of metal, and all kinds of props. Tricks of this kind are still used for producing sound tracks for movies. Sound produced in this way always seems to be more realistic than picking up the actual sound with a microphone. For this reason it is necessary to make a complete sound track for the required effects, separately, and then dub it in, with dialogue, music, solo and other parts of the film.
In making a final composite for producing a stereophonic picture the same source tracks are used, but the final three- or four-channel tracks, or the coded arrangement, are produced by a process called "panning". This consists of mixing the different individual tracks in different proportions onto the separate final tracks.
This means it is possible to make the sound of people using knives and forks come from the right side of the screen, while the sound of the orchestra or cabaret on stage can come from the left, or any other desired combination that may be required by the picture presentation.
In conclusion then, stereophonic sound for the movies has the same basic problems as for the home. But the best solutions, both in making the sound tracks and in presenting them, can differ considerably. All sound, intentionally or not, is stereophonic. But sound recorded with this intention can produce a much better illusion, provided we know what we are trying to do, and the best approach to use, according to the many things that influence it.
So stereophonic sound is quite a story. That 3-D picture analogy is quite inadequate to explain the tremendous variety of different subtle effects that sound waves can produce. Our surroundings, particularly the listening room (livingroom or auditorium) affect our impressions far more than we often realize. For this reason it is impossible to generalize about the best way to produce stereo phonic sound.
The experiment that started in 1881 still has some way to go before we can truthfully say we know all about it. Systems so far developed represent a tremendous advance, but this form of listening pleasure in the home may still be expected to show further improvement. The advent of stereophonic sound is just one more phase in the quest that is the pursuit of high fidelity.
(Adapted from: Stereophonic Sound (1957) by Norman H. Crowhurst)
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