International photographer (Jan-Dec 1934)

Record Details:

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Twenty-four The INTERNATIONAL PHOTOGRAPHER June, 1934 Motion Picture Sound Recording CHAPTER IX HIS chapter introduces the discussion of audiofrequency amplification, particularly as it is employed in motion picture sound recording to increase the weak speech current to a value great enough to operate a recording device. Many of the modern miracles of science owe their existence to the vacuum tube and its application as either rectifier, oscillator, detector, or amplifier. Sound motion pictures, long-distance telephony, radio broadcasting, television, and a host of other twentieth-century marvels are outgrowths of the application of the vacuum tube in one or more of its various capacities. The Transmission Line When the monitored speech current leaves the main volume control in the monitor room it is carried by a transmission line to the amplifier room, which in permanent installations is usually located fifty or more feet from the monitor room. Like all the transmission lines in the sound recording studio, this line is formed of two rubber GRIP-j {-PLATE PLATE. BATTERV FIGURE \. ONE STAGE TRANSFORMER C OU PL E D AMPLIFIER covered wires encased in a lead shield. In the older type of transmission line the two wires were run parallel within the shielding; but in the newer type of line the wires are twisted about each other inside the shield. This may seem like a trifling detail, but that is not the case. The lead covering over the wires serves as an electrostatic shield, preventing stray electrostatic currents being induced in the wires, but it does not serve so effectually to block the influence of any electromagnetic fields to which the transmission line may be exposed. The result was that when a number of these shielded transmission lines of the older type were bound together in a cable, the electromagnetic fields about each pair of wires had an effect on the adjacent lines. Twisting the wires about each other inside the lead shielding, as is now done, causes the electromagnetic fields about the two wires to cancel each other out, resulting in the almost total elimination of any external electromagnetic field about the transmission line. The Vacuum Tube The type of vacuum tube used in sound recording has three elements: a filament, a grid, and a plate. The filament is a hairpin of wire, usually formed of a platinumnickel alloy coated with a mixture of certain oxides that give out electrons freely when heated. The grid is a spiral of fine wire around the filament ; and the solid metal plate is supported outside of and surrounding the grid. These three elements are enclosed in an evacuated glass bulb. The two ends of the filament, the grid and the plate leads are connected to the four prong terminals on the base. By Charles Felstead Associate Editor An electric current from the filament battery (the A battery) flowing through the metal of the filament heats the oxide coating and causes it to emit electrons. The plate is maintained positive and at a much higher potential (voltage) than the filament by means of a plate, or B, battery. Operating on the electrical principle that like charges repel and unlike charges attract, the positivelycharged plate attracts the electrons (which are negative charges of electricity) released by the filament and draws them through the grid mesh. This flow of electrons from the filament to the plate sets up a plate current that is steady in value as long as the plate voltage and filament temperature are not changed and there is no potential applied to the grid of the tube. Functioning of the Grid When a vacuum tube is used as an amplifier, the grid is kept at small negative potential with respect to the filament bv means of a grid-bias (C) battery, or by means of a voltage obtained from the filament or plate battery. This negative charge on the grid tends to repel many of the electrons emitted by the heated filament and prevent their passage to the plate, in that way limiting the plate current flow to a value that is determined by the amount of the negative charge on the grid. The higher the negative grid potential the lower the value of the plate current, filament temperature and place voltage remaining the same. The alternating voltage that is to be amplified (the speech voltage, or speech current, in sound recording) is applied to the grid circuit of the tube in series with the grid-bias battery, as shown in Figure 1, the amplified diagram of a one-stage transformer-coupled amplifier. The variation in the speech voltage causes the potential of the grid to vary proportionately. When the speech voltage is on the negative half of its cycle it increases the value of the negative potential on the grid, and when it is on the positive half of its cycle it decreases the negative grid bias potential. The fixed grid bias voltage is chosen so that at no time will the grid be permitted to attain a positive value, because that would cause electrons to be drawn to the grid, creating an appreciable grid current flow and resulting in distortion of the amplified speech current. The variation in grid potential under influence of the applied speech voltage produces a corresponding but much greater variation in the number of electrons that are permitted to pass through the grid mesh and reach the plate. This results in a variation of plate current which, after it is fed through the output transformer OT to the next tube in the amplifier or to a transmission line, becomes a variation in speech voltage. This output speech voltage is — in a correctly designed amplifier circuit — an exact but considerably amplified copy of the speech voltage supplied to the input transformer IT. Tubes Used In Sound Recording The audio amplifiers used for sound recording have from one to four stages of amplification — that is, one to four tubes — each stage serving to raise the level of the (Turn to Page 26) Please mention The International Photographer when corresponding with advertisers.