International projectionist (Jan 1941-Dec 1942)

dling capacity. In addition, three channels allow more loudness to be used before the sound becomes offensive, because the multiple source and multiple standing-wave pattern prevents sharp peaks of loudness of long duration. Three tracks and program channels have other advantages over a singlechannel system. Cross-modulation between different sounds can be greatly minimized. Dialog, music, and effects could conceivably be placed upon separate tracks. It should be pointed out that single-frequency, steady-state measurements of amplitude distortion do not necessarily give an indication of the amount of cross-modulation that may be present in a single channel. It has been found that low-frequency transients, caused by' even-order overtones, can cause objectionable cross-modulation at levels somewhat below the nominal peak overload point of the amplifier. For economic reasons, it is almost impossible to eliminate this source of cross-modulation from single-channel reproducers. It is a simple matter to isolate conflicting program material on a three-channel system. The use of three program channels allows phase differentiation to supplement amplitude differentiation in obtainlg directional perspective. The phase lifferentiation also minimizes trouble rith acoustic interference in the theatre, vhich often accompanies attempts to lse a multiplicity of horns on a singleDrogram channel. Mfferential Junction Network The first step toward Fantasound occurred when we were asked to make a sound move back and forth across the screen. It was found that by fading between two speakers, located about 20 feet apart, we could simulate a moving sound-source, provided that the total level in the room remained constant. It became obvious at once that simple mechanical ganging of the volume conrols feeding the two speaker circuits was not capable of producing the desired effect. A special two-gang volume-control was then designed with complementary attenuations in the two circuits such that the sum of the attenuations, expressed as power ratios, equalled a constant. Many uses have been found for this type of network. It is extensively used in our Fantasound re-recording system to make possible constant output fades possible. A special 3-circuit differen FIG. 4. Program rack with front cover removed tial junction network, nicknamed "The Panpot," is used to dub one original track onto one, any two, or all three of our Fantasound program tracks with smooth transitions and any desired level difference. Thus we simulate a moving sound-source by starting on either sidetrack and progressively moving the program material through the center-track to the other side-trade. This move through three tracks, and thus three horns, is made smoothly by maintaining constant the total output of the three tracks and horns, regardless of the distribution among the three program circuits. The simple 2-circuit differential junction network has been used to make smooth, constant-level fades between two sound-sources. It also has been used to vary the ratio of close to reverberant microphone pick-up without affecting the output level. It was found to be a convenient means of controlling reverberation. Fantasound Reproducing System A simplified block diagram of the reproducing equipment is shown in Fig. 1. On the left are shown the four photocells which scan three program tracks and a pilot control-track. Each program photocell feeds a variable-gain amplifier, then, through power amplifiers, the three-stage horns. Associated with each variable-gain amplifier is a tone rectifier, which selects one of the three pilot tones on the control-track, rectifies it, and applies the resulting D.C. control bias to the grids of the variable-gain stage. Thus the output from each loud speaker varies with the amplitude of its associated control tone. The heart, or perhaps we should call it the brain, of the Fantasound reproducer is the tone-operated, gain-adjusting device, abbreviated Togad. The Togad equipment is composed of two units — the variable-gain amplifier and the tone rectifier. A sine-wave control-tone is applied to the input of the tone rectifier, where it is transformed into a D.C. bias voltage, which is then applied to the variable-gain amplifier to vary its transmission. The equipment is arranged so that a 1-db change in tone level causes a 1-db change in program transmission through the variable-gain amplifier. Variable-Gain Amplifier. — The variable-gain amplifier, abbreviated, VGA, is a single stage of transformer-coupled push-pull pentode voltage amplification (Fig. 2). Its transmission is a function of the D.C. bias applied to its grid circuit. A variation of 5 db in the UGUST 194 1 17