Rca 77-a User manual

I N S T R U C T I O N S

for

UNI-DIRECTIONAL ICROPHONE

TYPE 77-A

( I-4040)

RCA Victor Division

RCA anufacturing Company, Inc.

Camden, N. J., U. S. A.

OPERATING INSTRUCTIONS

FOR

UNI-DIRECTIONAL ICROPHONE

TYPE 77-A

( I-4040)

PART I — DESCRIPTION

1. Int oduction.—e RCA Type 77-A uni-directional micro-

phone is an entirely new type of pick-up device—a microphone

with a directional pick-up pattern wholly diﬀerent from that of

any other microphone. While it resembles the velocity micro-

phone in appearance and construction, and is, in fact, evolved

from research and development work on the latter, the RCA

Type 77-A uni-directional microphone combines the principles

of velocity and pressure operation. For this reason, it possesses

in a surprising degree the best features of each and overcomes

the disadvantages inherent in both. In view of the fact that the

unit is designed to pick up sound arriving from one direction—

or, more accurately, from one side—while almost completely

rejecting sound from the other side, it is admirably adapted to

studio pick-up, public address and sound reenforcement appli-

cations.

Instead of a diaphragm (in the commonly accepted meaning

of the word), the uni-directional microphone contains a thin

metallic ribbon suspended between the poles of a permanent

magnet with its length perpendicular to, and its width in the

plane of, the magnetic lines of force. e ribbon is rigidly

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 innitely 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 nite

length, the desired damping of reection is obtained by ling 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 eld, 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. Desc iption.—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.

Figure 2—Development of Directional Pattern

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 ange to t 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 gures 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 gure

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 reections, and the increased possi-

bilities of obtaining better balance, clarity, naturalness and se-

lectivity in sound pick-up. Extraneous direct or reected sounds

approaching the microphone from side directions and from the

rear will have little or no eﬀect and therefore background noises

and reected sounds in the broadcast are considerably re-

duced, which increases, by comparison, the quality of direct

sounds reproduced. e amount of sound-proong 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.

e transmitter must be used in free space where the ow of

air particles is unimpeded. “Pick-up” from the rear of the mi-

crophone is eliminated by the design and construction of the

unit.

PART II — OPERATION

6. Mic ophone Assembly.—e Type 77-A uni-directional

microphone is shipped with the stand ange attached by means

of three screws to the microphone mounting yoke. e suspen-

sion tting 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 ange) securely to the stand column.

See Section 9. List of Parts and Accessories, for the type of stand

recommended for this purpose.

(b) Sus ension Mounting.—If it is desired to suspend the mi-

crophone overhead, the stand ange must be removed from the

microphone mounting yoke and replaced with the suspension

tting, which contains the eyelets for cord attachment. e t-

ting must be attached securely to the yoke by means of the three

screws formerly used for mounting the stand ange.

NOTE.—When the microphone is suspended, see that its

weight is carried on the suspension tting, with no

strain on the cable.

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 ange 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-ampliers, 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 denitely

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-Di ectional Mic ophone 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—Sc ematic Wiring Diagram of Microp one

there is a tendency toward accentuation of low frequencies,

which may result in making voices sound “boomy.” In this re-

spect the use of the uni-directional microphone diﬀers greatly

from that of the condenser microphone, with which the soloist

usually works at a distance of from 4 to 6 inches. As a point of

useful information, it may be mentioned here that the uni-di-

rectional microphone may be used as a close-talking micro-

phone by talking in the plane of the ribbon. In this position, only

the pressure-operated part of the ribbon is used.

e placement of a speaker or musical instrument oﬀ from the

center line of the microphone will in no way aﬀect the quality

of pick-up, but will merely attenuate the direct sound pick-up,

thereby raising the ratio of reverberation to direct pick-up.

e microphone is uni-directional. Speakers, instruments or

players may be placed on the operating side of the microphone

only. e diagrams (Figures 5, 6, 7, and 9) will serve as examples

which arise from the uni-directional characteristic.

For more satisfactory results, the microphone should not be

placed closer than 3 feet to any solid reecting surface. is

statement is, of course, general and specic conditions may re-

quire otherwise.

e diagrams referred to in the subsequent paragraphs and

the discussion concerning them can only serve to indicate some

of the possible placements under particular conditions. e

nal decision as to what constitutes the proper placement must

rest with someone who is competent to judge the quality of the

results as reproduced by the monitor speaker.

(b) Soloist with Piano.—Interesting eﬀects may be obtained

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.

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 oor. 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 reected pick-up from hard-surfaced

oors. Such reections can be avoided by the use of carpets or

similar material on the oor.

(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 specically, 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.— icrophones used for this purpose

must generally be concealed and may be successfully operated

in the wings, ys, 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 wit Piano Figure 6—Plays Figure 7—Dance Orc estra

VARIOUS MICROPHONE ARRANGEMENTS

Figure 8—Plugs and Receptacles

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 denite 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. Ope ation.—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 oor 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 oor. In order to

raise or lower the stand, the vertical column clamping screw

should rst 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 xed 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 rst

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 ANU-

FACTURING CO PANY, INC., for repair. is may be done by

writing to the RCA anufacturing 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 Pa ts and Accesso ies.—

Descri tion Ty e Stock No.

Uni-Directional icrophone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77-A I-4040

Program Stand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AZ-4090 I-4056

Suspension Fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UP-4212-A I-4071-A

Stand Flange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16857

icrophone Screen Assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16856

Swivel Clamping Nut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16478

Washer (used under Swivel Clamping Nut) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16827

icrophone Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I-62*

icrophone Plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-CG-12 I-4630

Female Cord Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-CG-11 I-4620

Flush Type Wall Receptacle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-13 I-4622

Surface Type Wall Receptacle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-17 I-4621

Flush Type Wall Receptacle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon Type P3-35 I-4625

*Length of cable must be s ecied when ordering.