International photographer (Jan-Dec 1934)

Twenty-six T h INTERNATIONAL PHOTOGRAPHER June, 1934 MOTION PICTURE SOUND RECORDING (Continued from Page 24) speech current by an amount determined by the type of tube used, the voltages applied to the tube, and the coupling devices in its input and output circuits. There are several different types of tubes used in sound recording ; but they are all operated from batteries to avoid any possibility of an alternating-current hum being introduced in the sound record, as might well be the case if rectified a-c. supplies were used. In radio receivers the amount of audio amplification is so very much less than in a recording channel that any a-c. hum present in the power supply is not amplified to an extent great enough to be troublesome. The tubes used in the condenser microphone amplifiers and in certain types of pre-amplifiers are of the small "peanut" type that operate on low filament and plate voltages. These tubes are not required to handle a large amount of audio power because the speech energy in this portion of the circuit is at an extremely low electrical level. The other tubes in the recording system are of standard size ; although there is a considerable difference in the electrical characteristics of the different types. The tube used in the low level portion of the main amplifier circuit — the input from the monitor room transmission line — is of the voltage amplified type. This tube provides a large step-up of speech voltage, but is capable of handling very little audio power in its plate circuit. The shape and construction of the elements, their spacing, and the voltages applied to a tube determine its type. The tubes used in the final amplifiers, the ones that supply energy to the recording devices, are of the power amplifier type. They provide very little voltage gain but are constructed to control the flow of a relatively large amount of electrical power in their output circuits. The intermediate tubes are a compromise between these two types. The purpose of voltage amplification is to build up the voltage of the speech energy to the highest possible value without regard to the actual electrical power involved ; and the purpose of power amplification is not so much high amplification of the speech current as it is to increase the energy of the speech current so that it will have sufficient power to do work. Amplifier Circuits The apparatus used in coupling the tubes in an audio amplifier to each other or to other equipment — the coupling device — is an important element in determining the amount of amplification provided by the amplifier. There are three main types of coupling devices and several modifications of these three types. They are : resistance coupling, impedance coupling, and transformer coupling. There is not the space to discuss these circuits exhaustively, so only brief comparisons between them will be made. Resistance coupling employs a resistance in the plate to B battery circuit of one tube, and another resistance (grid leak) in the grid to filament circuit of the following tube, and two circuits being coupled together by a coupling condenser that connects from the plate of the one tube to the grid of the following tube. This is known as a 1-to-l coupling device because it provides no step-up of speech voltage. This coupling arrangement is not economical of plate voltage because of the large voltage drop through the plate resistor ; but on the other hand it does not distort the quality of the speech voltage passing through it, providing when correctly designed absolute fidelity of amplification. Impedance coupling is like resistance coupling with the single variation that an impedance (a choke coil) is substituted for the resistance in the plate circuit of the first tube. This likewise is a 1-to-l coupling device; but it does not require such a high plate battery voltage to supply the same tube plate voltage, because the direct-current voltage drop through the impedance is a great deal less than the voltage drop through the plate resistance. Although the direct-current resistance of the impedance is low, its impedance (resistance) to the alternating speech voltage is as high for most frequencies as that of the plate resistance. The fidelity of amplification of impedance coupling is not as good as that provided by resistance coupling, but it still is excellent. Transformer coupling employs an audio transformer having two windings of wire — a primary and a secondary — on an iron core. The primary winding is connected in the plate circuit of the first tube in place of the plate resistance, and the secondary winding is substituted for the grid leak in the grid circuit of the following tube. No coupling condenser is employed between the tubes, the coupling being furnished by the mutual impedance of the two windings. This form of tube coupling provides a decided voltage step-up between the tubes, the exact amount depending on the ratio of the turns in the two windings. Less stages of amplification are required if transformer coupling is substituted for resistance or impedance coupling; but the output of a transformer-coupled amplifier is a much less faithful copy of the input speech voltage than would be provided by an amplifier employing either of the other types of couplings. In other words, more uniform amplification over the audio frequency range is provided by resistance or impedance coupling. The Recording Channel A sound recording channel is formed of the microphones, the monitoring apparatus, the amplifiers, the recording machines, the motor control system, and the batteries necessary to comprise a complete recording system. A semi-portable (mounted in a motor truck) or portable (trunk-type) recording unit is a single recording channel ; but the permanent installations in studios comprise two or more complete channels. The several recording channels in a studio are separate and independent, with the exception of a common battery room, motor control room, and main amplifier room. The channels each have their own monitor and recording rooms and their own sound stage. Individual intercommunicating telephone systems and signal light systems are provided for each channel. Main Distributing Frame In the permanent sound installation, the amplifier room is located as nearly as possible in the center of the recording studio. In this large room all the wiring for the several channels is brought to a common point, a bulky metal framework supporting dozens of terminal blocks and known as a main distributing frame, or MDF. Every terminal block is equipped with some twenty terminals, each terminal numbered and listed in a catalog. The input and output circuits of practically every piece of speech equipment in the sound recording installation, whether located in the amplifier room or not, are brought to the terminals on these connection blocks. Large panels made up of rows of double jacks are mounted in the main amplifier bays. The terminals of each of these double jacks connect to terminals on the connection blocks. Wire straps link the proper terminals on the connection blocks together, thus placing these double jacks in the input and output circuits of each piece of speech equipment terminating on the connection blocks. By means of special plugs that fit the jacks, connection may be quickly and easily made to the input or output of any piece of speech equipment. Permanent alterations in the circuit of a recording channel can be made by changing the straps between terminals on the connection blocks without the necessity of touching the apparatus itself. All connections at these blocks are carefully soldered. Please mention The International Photographer when corresponding with advertisers.