Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

36 EXHIBITORS HERALD-WORLD May 10, 1930 REPRODUCTION IN THE THEATRE By RCA Photophone System JOHN O. AALBERG* [Concluded from last insue] I he voltage amplifier consists of three stages push-pull UX 210 amplification which are battery operated. The voltage amplifier feeds an A.C. operated power amplifier capable of delivering 10 watts undistorted power. Its push-pull output consists of 2 UX 250 tubes. Such a power amplifier feeds four dynamic cones. Each power amplifier also has a rectox unit for supplying direct current to the fields of the cones connected to it, eliminating the use of horn batteries. For larger theatres, similar power units are paralleled, all being connected to the same voltage amplifier. On such larger installations, the voltage amplifier is duplicated for emergency7 use and is readily placed in service by throwing a switch. As each power amplifier is independent even to having its own speakers, on all installations of theatres seating over 750 all equipment is duplicated. A rack having two voltage amplifiers and two power amplifiers is shown in Fig. 4 A A A I he loudspeaker used is the electro-dynamic cone. It consists of a parchment cone with a small coil affixed to its apex, which is slipped loosely around the core of a cylindrical electro-magnet excited by a direct current from its own power amplifier. When the signal current passes through the small coil, its magnetic reaction with the electro-magnet vibrates the parchment in synchronism with the signal current. This vibratory motion acts on a column of air and becomes sound. The cones are mounted on baffles aiding the reproduction of the lower frequencies. In reverberant houses these baffles are made directional. AAA Given a good commercial reproducing equipment, perfectly adjusted, and a theatre with good acoustics, we still have the variables of film condition and projection. The crackling noises we hear from a film record are known as ground noise. Some of it is recorded on the film, having been picked up when the record was taken or some added by faulty amplifiers. Much of it, however, is caused by dirt on the sound track in the form of small specks. This can be eliminated by cleaning the film. Additional noises mav be added by improper patching. Whenever a patch is made it should be painted as in Fig. 5, (next page) so the change in light entering the photoelectric cell is gradual, thereby causing little or no sound. With sound the changeover from reel to reel becomes very important so that no dialog is lost. It is becoming practice in release prints to have a scene at the end and beginning of each reel in which no dialog occurs so changeovers can easily be made without danger of losing dialog. No sound feature should be shown without being rehearsed so that it may be checked for dirt, splices, changeovers, and volume. Many people differ on what volume should be, but in general it is agreed that the volume should be such that the persons on the screen speak at a level which gives the audience the illusion that the sound is coming from the action on the screen. For instance, we see a closeup of an actor speaking and, if the sound is too ^Reproduction Supervisor, RKO Studios. Ele.enth paper in Technical Digest. FIGURE 4 weak, there is no illusion because the sound seems to be coming from a point far behind the screen. Conversely, if the voice of a person back in a long shot is played too loudly, the illusion is also spoiled. Volume should be raised for a crowded theatre over what it is for a half filled one. The sound track on film is about .100 inches wide and replaces that amount of picture. The old ratio of picture height to width was 3 to 4, a frame being approximately in. x 1 in. Removing .100 inches in width leaves the picture nearly square. Theatres seem to prefer the 3x4 picture for artistic reasons and to secure interchangeability with films of the old standard size by merely changing lenses. It has become the practice in many theatres, therefore, to use an aperture which restores the 3x4 proportion by cutting ten per cent from the height of the picture. To meet this condition nearly all cameramen are now composing their pictures with extra head room. As theatres using the smaller aperture also use a shorter focal length lens to make the picture as large as it was with a standard aperture the film grain, dirt, scratches and photographic defects become slightly more apparent with the greater magnification. Pictures produced to be accompanied by sound on disc only are generally photographed and projected the same size as silent pictures. a m e A reproducing system which is not properly adjusted will, of course, spoil any record. It is essential that all the vacuum tubes operate at their proper voltages. The adjustment of the light beam which falls on the photoelectric cell is important. Should this beam be wider than .001 inches, a loss of high frequencies results. In case the beam falls to one side of the track it will pass through the sprocket holes and give a 96 cycle hum, or, if the other way, it will reproduce a click for each picture frame line passing it. On variable area sound track a light beam off position will cause distortion because it will only be covering part of the sound modulation. In variable density no similar distortion occurs from this source but the volume falls off. The presure pad which holds the film taut as it passes the reproducing light beam must apply just the right amount of pressure. Too much is likely to produce flutter, which reveals itself by making voices gurgle. Too little pressure will allow the film to move in and out of focus, causing loss of high frequency response and articulation. AAA Anything that causes the film to pass the light beam jerkily produces flutter. Prominent among the causes are projectors driven through unevenly cut gears or having poorly adjusted intermittent movements. The degree of film shrinkage and condition of sprockets and sprocket holes also affects flutter. The industry’s problem is to get natural and intelligible sound in theatres. Each craft must do its best for the record as it evolves from sound to input to sound output and only as each craft realizes the problems of the others can perfect reproduction be hoped for. REPRODUCTION IN THE THEATRE BY RCA PHOTOPHONE SYSTEM, by John O. Aalberg. Paper based upon lecture-demonstration before School in Fundamentals of Sound Recording and Reproduction conducted by the Academy of Motion Picture Arts and Sciences, Hollywood, 1929. ( P-2-Aa.)