International photographer (Jan-Dec 1935)

Ten The INTERNATIONAL PHOTOGRAPHER April, 1935 Motion Picture Sound Recording Chapter XVI HE preceding chapter served to introduce this discussion ol the arrangement and manner of connection employed between the pieces of equipment in the standard sound recording installation of the Western Electric system. The mounting of the equipment on metal panels supported by "relayracks, " which are grouped in recording units known as "channels," as well as the methods of circuit connection and fusing were described in that introductory chapter. We now conclude with this chapter that discussion of the physical construction and arrangement of the amplifying equipment used by most sound receiving studios. Grounding Arrangement A solid copper grounding bar running the whole length of the channels is fastened along the bottom of the bays (a bay is the unit of a single relay rack and its mounted panels) in permanent installations; and all of the ground wires from the pieces of equipment in the bays are soldered to it. A number of thick copper wires connect this ground bar to copper plates buried in the earth. Portable sound recording installations are usually grounded solidly to the metal frame of the truck on which they are mounted. In every amplifier in the permanent recording equipment, each grounding point is connected individually to the copper bus bar by a separate wire, the purpose of this seemingly unnecessary labor being the elimination of "ground loops." This grounding scheme is illustrated at A in Figure 1. o! to OUTPUT Ground loop FIG. 1. Correct and Incorrect Methods of Grounding Amplifiers. A ground loop in an amplifier, it must be explained, is a portion of a ground circuit that carries current. An example of the ground loops that would occur if only a single ground wire were used on the amplifier shown at A is depicted at B in Figure 1. This is an important point that must be watched carefully in the design of amplifiers, whether they are for sound recording, public address systems, radio transmission or reception, or other purpose. Connection Between Amplifiers Another important factor to be considered in the design of the most efficient type of sound recording system is the type of wire employed for carrying speech current between the amplifiers and associated equipment, and from the connection blocks on the channels to the M.D.F. This is a most important matter, particularly in elaborate recording installations. Even the placing of each wire with relation to other wires and equipment is a vital matter where highest quality is desired. The wire most commonly employed is No. 18 gauge; the wires are rubber covered for insulation and placed in pairs, each pair of wires being enclosed in a lead sheath. With the older type of wire, the two conductors were laid parallel ; but in the wire that is now employed, the conductors are twisted together within their lead covering. This twisting causes the electromagnetic fields (magnetic fields produced about the wires by the current flowing through them ) of the two conductors to cancel out each other, with the result that there remains practically no external electromagnetic field about the pair of wires to couple with adjacent wiring. The lead covering is grounded to the heavy grounding bar and serves as an electrostatic shield for the conductors. This tends to reduce the possibility of outside electrical disturbances, such as induction hums from A.C. wiring, being picked up by the wires and fed to the amplifiers. The twisting of the wires and the grounded lead shielding, together with careful division of the wires into "high-level" and "low-level" groups and judicious distribution of them in the racks and gutters, helps to eliminate all sources of undesirable coupling between portions of the recording system. Among the troublesome forms of coupling are cross-talk (the transference of speech energy from one circuit to an adjacent circuit by induction) and: the feed-back of energy from the output of an amplifier to its input (the presence of which causes audio-frequency oscillation, or "howling" in the amplifier). Since the impedance of the transmission lines connecting amplifiers is low, being usually 50, 200, or 500 ohms, and the voltage handled bv them is also low, the small amount of electrical capacity that exists between the two conductors forming each line is not harmful. The Jack Circuits The jacks employed in the standard motion picture sound recording installation are of the two-hole type and require a special double plug. The jacks are mounted Please mention The International Photographer when corresponding with advertisers.