International photographer (Jan-Dec 1934)

Cwelve The INTERNATIONAL PHOTOGRAPHER January, 1934 The Mixer Controls The control portion of the monitoring equipment is quite simple, being made up entirely of potentiometers, with key switches for disconnecting unused controls. Three potentiometers and the associated three key switches are mounted as a unit on a metal panel ; and two mixer control panels of this type are employed in the normal installation. The two panels are built into a cabinet with a slanting front, which stands on the desk in such a position that it is convenient for the monitor man to reach the controls. An idea of the functioning of a potentiometer and its key switch may be obtained from Figure 1. Actually, a more complicated impedance-matching potentiometer is employed, in order that the output impedance of the microphone will be exactly matched to the input impedance of the monitoring amplifier at all adjustments of the potentiometer ; but the simple potentiometer shown illustrates the theory much more clearly. X is a circular winding of resistance wire, which has a resistance, in this case, of 200 ohms ; while Y is a metal blade attached to the control knob. By turning the control knob, the blade can be made to connect with different points on the circular resistance element. When the blade is at the point A , the full output of the microphone is fed to the monitoring amplifier, and the resistance element is simply shunted across the line. But when the blade is at the point B, none of the output of the microphone is supplied to the monitoring amplifier. At intermediate points on the resistance element, different proportions of the microphone output energy are supplied to the monitoring amplifier ; the nearer the blade is to the point A the greater the amount of energy transmitted by the potentiometer. The switch is provided to permit the potentiometer and the microphone with which it is associated to be disconnected from the monitoring amplifier. One potentiometer and its key switch are required for each microphone ; so a six-position mixing panel provides controls for a maximum of six microphones. The switches turn off the microphones that are not in use ; and the potentiometers regulate the volume of sound (in the form of electrical energy) that is supplied to the recording system by each microphone. The potentiometers are carefully constructed and will not cause noise in the system if they are kept clean. The key switches are designed not to produce clicks when operated, so that microphones can be turned on or off during the shooting of scenes without interfering with the recording. The Monitoring Amplifier The electrical output level of a condenser or dynamic microphone is so low that it is necessary to amplify it by a monitoring amplifier after the energy has passed through the mixer potentiometer. The normal circuit arrangement is shown in Figure 2. Squares represent the microphones and the amplifier, and pairs of wires are represented by solid lines. The conventional symbol is used for the potentiometers. The monitoring amplifier is also called a pre-amplifier, or "booster," amplifier. There are two types of audiofrequency amplifiers in general use for this purpose. One type of amplifier employs small "peanut" tubes in a resistance-coupled circuit of three stages ; while the other type uses larger tubes in a two-stage impedance-coupled circuit. The three-stage amplifier is equipped with only a single switch for regulating the amount of gain in the amplifier. But gain control on the two-stage amplifier is provided by two tap switches, one of which regulates the gain in steps of five decibels each and the other provides fine adjustment in one decibel steps. Meanings of Technical Terms Terms which are not clear will be explained in a later chapter ; but it will be mentioned here that the term gain is merely another name for amplification. The word itself is almost self-explanatory. Just as a current that has passed through a resistance is said to have suffered a loss, or diminution in value, current that has been increased by an amplifier is said to have received a gain in value. Electrical level is a measure of the current at any point in a circuit with reference to some other point in the circuit, and is not necessarily defined in any unit of measurement. The level of an electric current at the output of an amplifier is higher than the level at the input because there has been a gain in the amplifier. Likewise, the electrical level at the output of a resistive device, such as a potentiometer or attenuation network, is lower than at the input because there has been a loss in the resistance. All of the terms just defined are concerned with speech current, which is the current generated by the sounds picked up by the microphone. It is called speech current because of the convenience of that term ; although the sounds that produce the current might be noises or musical notes as well as speech sounds. It is this speech current that is dealt with in sound recording; for it is this current that, after suitable amplification, causes the variation in the recording device that produces the sound record on film. Comparison of Amplifiers The three-stage booster amplifier is equipped with meters for reading the plate and filament current of all tubes ; but the two-stage amplifier is without meters. Both amplifiers have rheostats for adjusting the filament current. These rheostats must be kept particularly free of dust and corrosion, because any noise produced in this low-level part of the circuit will be enormously amplified. Just as for the microphone amplifiers, a separate set of plate-supply batteries, as well as filament batteries, are often used with these booster amplifiers to prevent electrical coupling through the batteries with the main amplifiers. These are known as "local" batteries. Sometimes, however, it is possible to operate this amplifier directly from the main system batteries without feed-back troubles. The two-stage amplifier is to be preferred for this work, despite the lower gain it provides, because the "peanut" tubes of the three-stage amplifier are inclined to be more microphonic than the larger tubes. This means that if the tubes are set into mechanical vibration by a jar or blow, the electrical "singing" produced in the amplifier by the vibration of the tube elements will be much more troublesome and objectionable for the "peanut" tubes than for the larger tubes. Necessity For Pre-A mplification It is necessary that the electrical level of the speech current be raised at this point in the recording circuit instead of waiting until it has reached the main recording amplifiers in the amplifier room. If it were not boosted in the monitoring equipment, the speech current would be at such a low electrical level when it reached the main amplifiers that it might be below the noise level of the system. By noise level is meant the level of the annoying electrical currents generated in the recording system by tube hiss, by minute amounts of electrical leakage, and by a multitude of other tiny sources of disturbances in the system. If the speech current is permitted to fall to the noise level, it becomes necessary to amplify the noise as much as the speech current when the speech current is raised to recording level. The monitoring amplifier serves to increase the speech current to a level that is high enough to over-ride any extraneous electrical disturbances that are picked up in the monitoring circuit and by the transmission line carrying the speech current down to the main amplifiers. The next chapter will conclude the description of the permanent monitoring equipment and will explain the usual procedure in coordinating the activities of the sound department with those of the picture companv on the sound stage. 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