Loudspeaker (Jan-Aug 1931)

the limits of this characteristic previously utilized, there is no change in film technique. The processing which produced the best quality of reproduction with the former method gives the best quality with this new method. Referring to Figure 1, which is an approximate characteristic of the ground noise obtained from film of various densities, the point A indicates the approximate density employed in normal recording. By shifting the ribbons to have something less than their normal spacing, we can increase the density during the quiet portions of the sound track to point B. This results, then, in a reduction of the noise in the quiet intervals. Automatic Spacing Then, as the sound currents are applied to the valve, its spacing automatically varies, so that it at a'l times has sufficient carrying capacitity, as represented by the spacing between the ribbons to carry the applied sound currents. A slight amount of margin is always established as a factor of safety, in order that a sound which builds up suddenly will not clash the ribbons. The manner in which the carrying capacitv of the light-valve or, in other words, the spacing of the ribbons varies with the applied round currents is illustrated in Figure 2. It will be seen from this that for weak sound currents below certain minimum amplitude the ribbon spacing is always the minimum, and the averaging spacing is unvarying. As the sound currents build up to near their maximum amplitude, it is seen that the average spacing of the ribbons (or their carrying capacity) is gradually increased up to a maximum which corresponds to that of the normal light-valve. As the input is further increased, there is no further increase in the ribbon spacing, and clash occurs as in the normal light-valve. It is entirely possible to continue the carrying capacity of the ribbons upward by allowing their spacing to exceed the normal spacing. No useful purpose is served by this, however. since the carrying capacity of the photographic emulsion would be exceeded by so doing and the effect equivalent to clashing of the lightvalve would be maintained. Therefore, the device has been purposely arranged so that photographic overload and light-valve overload occur simultaneously, if the recording 'amp has been set for normal recording. The general principles of noise reduction for sound records on film may be applied to other than the present form of light-valve recorders by making circuit changes as required by the particular type of equipments involved. Relatively Simple Operation of the apparatus employed in this recording system is relatively simple, although considerable time and effort have been spent or its development. The equipment is divided into two units — an amplifier situated usually at the location of the main amplifiers and a control unit fed by the amplifier and situated at the film recorder. Adjustment of these units is simple and means are provided for checking the adjustments quickly and at fairly frequent intervals. At the present time commercial recordings are being made with a reduction of ten decibels in the ground noise. It is expected that as more experience is gained in the use of this equipment the noise reduction may be increased. Even ten decibels is a very noticeable reduction and permits an extension of the volume range to a point where sound previously completely obscured in ground noise became definitely a part of the reproduction with a consequent considerable enhancing of the dramatic effect and naturalness. The practical elimination of this ground noise in the theatre gives to the audience a feeling of being present at the action and a removal of the mechanical from the sound. Fifte e n