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RCA 77-A Instructions Manual

Uni-directional?microphone.
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I N S T R U C T I O N S
for
UNI-DIRECTIONAL MICROPHONE
TYPE 77-A
(MI-4040)
RCA Victor Division
RCA Manufacturing Company, Inc.
Camden, N. J., U. S. A.
Copyright 1936 by the RCA Manufacturing Company, Inc.
Instructions IB-25838

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  • Page 1 I N S T R U C T I O N S UNI-DIRECTIONAL MICROPHONE TYPE 77-A (MI-4040) RCA Victor Division RCA Manufacturing Company, Inc. Camden, N. J., U. S. A. Copyright 1936 by the RCA Manufacturing Company, Inc. Instructions IB-25838...
  • Page 2 OPERATING INSTRUCTIONS UNI-DIRECTIONAL MICROPHONE TYPE 77-A (MI-4040) PART I — DESCRIPTION 1. Introduction.—e RCA Type 77-A uni-directional micro- 2. Description.—e uni-directional microphone shown in phone is an entirely new type of pick-up device—a microphone Figure 1 consists of a microphone unit mounted in a horizontal with a directional pick-up pattern wholly different from that of swivel on the top of a program stand. “Aiming” is accomplished any other microphone. While it resembles the velocity micro- partially by means of this swivel and partially by rotating the phone in appearance and construction, and is, in fact, evolved vertical column of the program stand. e transmitter is en- from research and development work on the latter, the RCA closed within a circular, perforated metal casing, so designed Type 77-A uni-directional microphone combines the principles as  to  conform  to  the  circular  construction  of  the  labyrinth, of velocity and pressure operation. For this reason, it possesses which occupies the lower part of the unit. in a surprising degree the best features of each and overcomes e labyrinth consists of a series of circular sections, the inte- the disadvantages inherent in both. In view of the fact that the rior of each section having a spiral partition, an opening at the unit is designed to pick up sound arriving from one direction— beginning or the end of which communicates with the begin- or, more accurately, from one side—while almost completely ning or the end, respectively, of the section of the labyrinth that rejecting sound from the other side, it is admirably adapted to...
  • Page 3 Figure  2.  In  this illustration (a) is the directional pattern of a velocity micro- phone, (b) is the directional pattern of a pressure microphone. While these figures are the theoretical or idealized patterns, they correspond, for ribbon microphones, quite closely to ac- tual measured characteristics. When these patterns are added, the forward lobe of the figure 8 pattern adds to the circular pattern, while the rear lobe, which is 180 degrees out of phase, opposes. the result is the same as that obtained when the signals of a vertical antenna and a loop antenna are added; viz., a cardioid of revolution, as shown at (c). In practice, the actual measured response of the Type 77-A Figure 2—Development of Directional Pattern uni-directional microphone, as shown in Figure 3, approaches this cardioid very closely. For all frequencies up to 6,000 cycles e Type 77-A microphone unit is supported in a mounting the  cancellation  is  very  good.  At  higher  frequencies  a  small yoke (containing the aforementioned horizontal swivel), which “tail” occurs because of the slight phase displacement that be- permits it to be tilted as desired. is mounting yoke is supplied gins to become noticeable in this range. with a threaded stand flange to fit a standard Type AZ-4090 pro- It is at once apparent that the uni-directional characteristic is...
  • Page 4 Figure 4—Schematic Wiring Diagram of Microphone e transmitter must be used in free space where the flow of microphones. If two microphones are placed close together, the air particles is unimpeded. “Pick-up” from the rear of the mi- result will be practically zero output if their circuits are out of crophone is eliminated by the design and construction of the phase at the overall mixer. unit. To check the phasing of two or more microphones connected in a single pick-up, place the units close together, two at a time, PART II — OPERATION with the attenuators (mixers) turned to the off position. Turn on the attenuator of one microphone to some arbitrary position 6. Microphone Assembly.—e Type 77-A uni-directional where the output will be distinctly audible or register definitely microphone is shipped with the stand flange attached by means on the volume indicator meter, if such a device is used. Talk into of three screws to the microphone mounting yoke. e suspen- the microphone and note the output volume. Now, without dis- sion fitting is shipped in an envelope in the box with the micro- turbing  the  setting  of  the  attenuator  of  the  microphone  just phone unit. used, turn on the attenuator of the second microphone to the (a) Stand Mounting.—If it is desired to mount the microphone same setting. Talk into the two microphones and note the result. unit on a program stand, it is necessary merely to screw the mi- If there is an increase in volume, the microphones are in phase.
  • Page 5 there is a tendency toward accentuation of low frequencies, its dead side toward the audience and close to the footlights, or which may result in making voices sound “boomy.” In this re- in an equivalent position, the 20-dB discrimination will provide spect the use of the uni-directional microphone differs greatly the desired attenuation of audience noise, while the broad pick- from that of the condenser microphone, with which the soloist up angle—useful through nearly 150 degrees—will afford pick- usually works at a distance of from 4 to 6 inches. As a point of up of the whole stage, or that part of the studio where the artists useful information, it may be mentioned here that the uni-di- are located. See Figure 6. rectional microphone may be used as a close-talking micro- (d) Dance Orchestra.—e set-up for dance orchestra is sim- phone by talking in the plane of the ribbon. In this position, only ilar to that just outlined for stage plays, the dead side of the uni- the pressure-operated part of the ribbon is used. directional  microphone  being  toward  the  dance  floor.  e e placement of a speaker or musical instrument off from the diagram (Figure 7) is self-explanatory, the only precaution nec- center line of the microphone will in no way affect the quality essary being to keep the soloist at least 2 feet, and preferably 3 of pick-up, but will merely attenuate the direct sound pick-up, feet, from the microphone. thereby raising the ratio of reverberation to direct pick-up. In locating the microphone with respect to an orchestra, care e microphone is uni-directional. Speakers, instruments or should be taken to avoid reflected pick-up from hard-surfaced players may be placed on the operating side of the microphone floors. Such reflections can be avoided by the use of carpets or only. e diagrams (Figures 5, 6, 7, and 9) will serve as examples similar material on the floor.
  • Page 6 TURNED  APPARATUS”  tag  and  “REPORT  BLANK.”  Before all that is necessary is to lift it to the desired point and there it doing this, however, make absolutely certain that the trouble is will lock itself automatically. Usually, it will remain fixed at this in the microphone and not elsewhere in the circuit. Figure 8—Plugs and Receptacles 9. List of Parts and Accessories.— Description Type Stock No. Uni-Directional Microphone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77-A MI-4040 Program Stand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AZ-4090 MI-4056 Suspension Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UP-4212-A MI-4071-A Stand Flange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16857 Microphone Screen Assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16856 Swivel Clamping Nut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...
  • Page 8 Figure 9—Microphone and Orchestra Arrangement for Symphony Orchestra LEGEND D Director 4 Flutes T2 2 Tympani and Traps V3 8 Violas M Microphone H1 2 Harps T3 4 Trombones V4 6 Cellos H2 8 French Horns T4 1 Tuba V5 4 String Bass B 4 Bassoons Ob 3 Oboes V1 12 First Violins C 4 Clarinets T1 3 Trumpets V2 10 Second Violins Total: 75 Musicians IB-25838 Photos of 77-A courtesy of Scott Henderson SO-867079-4 450...

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