RCA 77-A Instructions Manual

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

clamped at the center, as well as at the top and the bottom. e lower half is open front and back and operated as a regular ve- locity microphone. In order to make the upper half of the ribbon operate as a pressure microphone, it is, of course, necessary that the rear of this section of the ribbon be enclosed. At the same time it is not possible just to block this section off, as such a con- trivance would result in a response increasing with the fre- quency.  Rather,  it  is  necessary  to  present  an  acoustic impedance to the back part of the ribbon. An infinitely long tube would be the ideal impedance; but this, of course, is im- possible. Instead, an ingenious labyrinth, which gives practi- cally the same effect, is used. While this labyrinth has a finite length, the desired damping of reflection is obtained by filing it very loosely with sound-absorbing material. e result is that the upper half of the ribbon becomes an efficient pressure-op- erated microphone. e vibration of each part of the ribbon is in exact accordance with the sound vibrations and, occurring as it does within the magnetic field, sets up corresponding alternating electric po- tentials across the primary of its associated transformer. Since the two microphones (i.e., the velocity-operated section and the pressure-operated section of the Type 77-A microphone) are a part of the same ribbon, the voltages developed in the two sec- tions are, of course, in series, and the output level is obtained from the ends of the ribbon in essentially the same manner as in the case of the velocity microphone. 2. Description.—e uni-directional microphone shown in Figure 1 consists of a microphone unit mounted in a horizontal swivel on the top of a program stand. “Aiming” is accomplished partially by means of this swivel and partially by rotating the vertical column of the program stand. e transmitter is en- closed within a circular, perforated metal casing, so designed as to conform to the circular construction of the labyrinth, which occupies the lower part of the unit. e labyrinth consists of a series of circular sections, the inte- rior of each section having a spiral partition, an opening at the beginning or the end of which communicates with the begin- ning or the end, respectively, of the section of the labyrinth that immediately precedes or immediately follows it. e sections occupying the upper part of the labyrinth are so designed as to provide a cavity to accommodate the line coupling transformer, which thus forms a part of the microphone unit.

e Type 77-A microphone unit is supported in a mounting yoke (containing the aforementioned horizontal swivel), which permits it to be tilted as desired. is mounting yoke is supplied with a threaded stand flange to fit a standard Type AZ-4090 pro- gram type microphone stand. A suspension mounting (Type UP-4212-A) is supplied to permit the suspension of the unit overhead when desired. e microphone program stand (Type AZ-4090) is of the ad- justable single vertical column type with a three-point base. e height of the transmitter may be adjusted to maximum and minimum heights of 84 inches and 59 inches respectively. 3. Sensitivity.—With an input sound pressure of 10 dynes per square centimeter perpendicular in the plane of the ribbon, the Type 77-A uni-directional microphone will deliver 317 micro- volts across a 250-ohm load, which is equivalent to an output level of –75 dB, as compared with a zero level of 12.5 milliwatts, or –72 dB as compared with a zero level of 6 milliwatts. On an open circuit basis of measurement, i. e., with an input of 1 dyne per square centimeter (1 bar) perpendicular to the ribbon, the output of the microphone on open circuit is the equivalent of –89 dB with reference to a zero level of 12.5 milli- watts. 4. Quality of Response.—e operating range of the micro- phone extends from 60 cycles to 10,000 cycles. When the mi- crophone is located less than 2 feet from the source of sound, the low-frequency response is increased somewhat, and when operated at a greater distance (up to 4 feet), the low-frequency response is slightly attenuated. Beyond the 4-foot operating dis- tance the response characteristic is unchanged by changes in the operating distance. e frequency response is essentially unchanged by the direction of the incident sound over an angle of 150 degrees at the front of the microphone. 5. Directional Characteristics.—One of the most important characteristics of the Type 77-A microphone is its uni-direc- tional property. On the front, or operating side, of the micro- phone the response is very uniform, while at the rear of the microphone sounds are attenuated an average of 20 dB, thus giving a 10-to-1 ratio of desired to undesired pick-up. Sound waves originating in front and along an axis perpendicular to the plane of the ribbon will have the maximum effect. e outstanding advantage of the Type 77-A microphone is derived from the fact that the unit combines the action of a ve- locity-operated and a pressure-operated microphone, and re- sults from the manner in which the velocity-operated and the pressure-operated parts of the ribbon add together. Without going into mathematical expressions for these volt- ages, it is possible to obtain a picture of the action from a con- sideration  if  the  three  patterns  shown  in  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 uni-directional microphone, as shown in Figure 3, approaches this cardioid very closely. For all frequencies up to 6,000 cycles the cancellation is very good. At higher frequencies a small “tail” occurs because of the slight phase displacement that be- gins to become noticeable in this range. It is at once apparent that the uni-directional characteristic is of considerable value in the solution of some of the difficulties encountered in reverberant locations by the reduction of the effect of undesired sound reflections, and the increased possi- bilities of obtaining better balance, clarity, naturalness and se- lectivity in sound pick-up. Extraneous direct or reflected sounds approaching the microphone from side directions and from the rear will have little or no effect and therefore background noises and  reflected  sounds  in  the  broadcast  are  considerably  re- duced, which increases, by comparison, the quality of direct sounds reproduced. e amount of sound-proofing necessary for sound originating in the “dead zone” can be greatly re- duced—and, in many cases, “dead end” construction can be entirely eliminated. For the same allowable reverberation pick-up, the operating range of the uni-directional microphone is approximately 1.73 times greater than a non-directional microphone having the same sensitivity. When used for public address and sound reenforcement pur- poses, the directional characteristic is of considerable value in reducing feedback effects between the microphone and the loudspeaker. Sound concentrators and baffles used with condenser micro- phones are unnecessary with and inapplicable to the uni-direc- tional microphone because of the fundamental difference in the principle of its operation.

microphone is shipped with the stand flange attached by means of three screws to the microphone mounting yoke. e suspen- sion fitting is shipped in an envelope in the box with the micro- phone unit. (a) Stand Mounting.—If it is desired to mount the microphone unit on a program stand, it is necessary merely to screw the mi- crophone (using the stand flange) securely to the stand column. See Section 9. List of Parts and Accessories, for the type of stand recommended for this purpose. (b) Suspension Mounting.—If it is desired to suspend the mi- crophone overhead, the stand flange must be removed from the microphone mounting yoke and replaced with the suspension fitting, which contains the eyelets for cord attachment. e fit- ting must be attached securely to the yoke by means of the three screws formerly used for mounting the stand flange. NOTE.—When  the  microphone  is  suspended,  see  that  its weight is carried on the suspension fitting, with no strain on the cable. (c) Cable Connections.—e microphone is shipped with the microphone cable already connected at the microphone termi- nal board. is terminal board is rendered accessible for inspec- tion or service by taking out the three screws located about the microphone screen mounting flange and removing the screens. (d) Phasing.—When more than one microphone is used in a single pick-up, it is possible that the output of the of the various microphone circuits may not be in phase when fed into a com- mon circuit. e microphone circuits include the microphones themselves, microphone pre-amplifiers, microphone attenua- tors (mixers) and the necessary connecting lines. e output of the microphone attenuators (mixers) when fed into the overall attenuator (mixer) must be in phase, or varying degrees of dis- tortion will result, depending on the relative placement of the microphones. If two microphones are placed close together, the result will be practically zero output if their circuits are out of phase at the overall mixer. To check the phasing of two or more microphones connected in a single pick-up, place the units close together, two at a time, with the attenuators (mixers) turned to the off position. Turn on the attenuator of one microphone to some arbitrary position where the output will be distinctly audible or register definitely on the volume indicator meter, if such a device is used. Talk into the microphone and note the output volume. Now, without dis- turbing the setting of the attenuator of the microphone just used, turn on the attenuator of the second microphone to the same setting. Talk into the two microphones and note the result. If there is an increase in volume, the microphones are in phase. If there is a decrease in volume, remove the screen of one mi- crophone and reverse the connections at the microphone cable terminal board. If more than two microphones are employed, using one microphone as a reference, check the other units against it, one at a time, in the manner outlined above. If any are found to be out of phase, reverse the cable connections, at the microphone cable terminal board, of the lesser number of microphones  necessary  to  bring  all  the  units  into  phase.  A thirty-foot cable is furnished as part of the microphone equip- ment. e microphone plug must be furnished by the customer. For  microphone  connections  refer  to  Figure  4,  Schematic Wiring Diagram. 7. Technique of Uni-Directional Microphone Placement.— e proper placement of the microphone is essential in order to realize fully its inherent advantages. For this reason, the fol- lowing instructions should be carefully studied, and close at- tention should be given to the results of any special placement, with a view toward future improvement of technique. ese in- structions can, of course, serve only as a guide, and a study should be made to determine the best microphone placement for each condition. (a) General.—e Type 77-A uni-directional microphone has a pick-up angle of approximately 150 degrees. e source of sound, speaker, announcer, actor or musical instrument, should not be placed closer to the microphone than 2 feet, and a dis- tance of from 3 to 4 feet is to be preferred. At shorter distances Figure 4—Schematic Wiring Diagram of Microphone

by changing the angle of the microphone with respect to the piano, thus changing the ratio of reverberation to direct pick- up.  e  distance  between  the  soloist  and  the  microphone should be determined by the strength of his (or her) voice, and the piano should be placed accordingly. e general arrange- ment  is  shown  in  Figure  5.  Under  no  condition  should  the soloist be less than 2 feet from the microphone. (c) Stage Plays.—In the case of stage plays and those pick-ups of the type that occur in the case of auditorium-type studios, where a sizeable audience is present—and in remote pick-ups at theatres, night clubs and the like, where audience noise is a serious problem, the use of the uni-directional microphone possesses a distinct advantage. By placing the microphone with its dead side toward the audience and close to the footlights, or in an equivalent position, the 20-dB discrimination will provide the desired attenuation of audience noise, while the broad pick- up angle—useful through nearly 150 degrees—will afford pick- up of the whole stage, or that part of the studio where the artists are located. See Figure 6. (d) Dance Orchestra.—e set-up for dance orchestra is sim- ilar to that just outlined for stage plays, the dead side of the uni- directional  microphone  being  toward  the  dance  floor.  e diagram (Figure 7) is self-explanatory, the only precaution nec- essary being to keep the soloist at least 2 feet, and preferably 3 feet, from the microphone. In locating the microphone with respect to an orchestra, care should be taken to avoid reflected pick-up from hard-surfaced floors. Such reflections can be avoided by the use of carpets or similar material on the floor. (e) Large Orchestra.—An arrangement for a large symphony orchestra is shown in Figure 9. It is to be noted that the wide angle of coverage (150 degrees) of the uni-directional micro- phone will permit a satisfactory pick-up in many cases, such as that shown, with but one microphone. It must be borne in mind,  however,  that  the  physical  proportions  and  acoustic properties of the studio have a direct bearing on the arrange- ment of the orchestra and placement of the microphone. Where space considerations do not govern, changes from the arrange- ment shown should not necessarily be very extensive in order to give excellent results under the usual acoustic conditions. (f) Public Address.—For public address use, the microphone can usually be placed near the loudspeakers (within 3 or 4 feet). It is important to see that the dead side of the microphone is to- ward the loudspeaker system—more specifically, the micro- phone should not be placed in front or directly behind the loudspeakers to prevent acoustic feedback. If the speaker must have latitude of movement on the stage, it may be necessary to have a microphone installed at each side to obtain satisfactory pick-up. (g) Sound Reenforcing.—Microphones used for this purpose must generally be concealed and may be successfully operated in the wings, flys, etc., or at the front of the stage, where some simple method may be devised for their concealment. Such a system usually requires the use of a number of microphones Figure 5—Soloist with Piano Figure 6—Plays Figure 7—Dance Orchestra VARIOUS MICROPHONE ARRANGEMENTS

and their detailed location is largely determined by their exact use, the constructional details of the stage and other conditions so numerous as to preclude any definite statement of rules or methods of applications. e uni-directional feature of the mi- crophones may be utilized to great advantage in eliminating un- desirable noise emanating from the audience area. It is also to be noted that, because of the wide pick-up angle of the uni-di- rectional microphone, fewer units of this type than of any other will be required for proper coverage. 8. Operation.—In general, the microphone will operate sat- isfactorily and require very little attention. It should give the normal output listed in Section 3. e microphone may be suspended or it may be mounted on a program or floor stand. is stand is adjustable as to height. e center of the uni-directional microphone may be located at any height from 59 to 84 inches above the floor. In order to raise or lower the stand, the vertical column clamping screw should first be loosened. If it is desired to raise the microphone, all that is necessary is to lift it to the desired point and there it will lock itself automatically. Usually, it will remain fixed at this position unless there is vibration or the microphone and stand are moved around. is moving may cause the stand to slide slowly downward. the clamping screw is provided in order to prevent this. However, if the microphone does not tend to creep, it is not necessary to use the clamping screw. When it is desired to lower the microphone stand, the clamping screw should first be  loosened,  then  the  inner  tube  of  the  microphone  stand should be raised slightly while pressing the sliding column latch which projects at the side of the locking device. is will release the lock and allow the microphone to be lowered to the desired position, at which point the latch should be released and the stand will automatically lock itself. en the clamping screw may be tightened if desired. It is not recommended that the customer attempt to repair the microphone, but, rather, that it be returned to the RCA MANU- FACTURING COMPANY, INC., for repair. is may be done by writing to the RCA Manufacturing Company, Inc., for a “RE- TURNED  APPARATUS”  tag  and  “REPORT  BLANK.”  Before doing this, however, make absolutely certain that the trouble is in the microphone and not elsewhere in the circuit. 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16478 Washer (used under Swivel Clamping Nut). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16827 Microphone Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MI-62* Microphone Plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-CG-12 MI-4630 Female Cord Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-CG-11 MI-4620 Flush Type Wall Receptacle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-13 MI-4622 Surface Type Wall Receptacle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-17 MI-4621 Flush Type Wall Receptacle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-35 MI-4625 *Length of cable must be specified when ordering.

LEGEND D Director F 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