US2313867A - Sound recording system - Google Patents

Description

March 3- w. E. GARITY EI'AL 2,313,367 I SOUND RECORDING SYSTEM Filed July 31, 1940 P/ F Pa M44 MM 5 GAR/7') JOHN N/i. HAWK/N6 INVENTORS ATTORNEY.

Patented Mar. 16, 1943 uma STATES PATENT OFFICE j 2.31am

adgnorItoWaltDisaeyl'ro-' (XIII, mmfillfsaotcalifcraia 'ihisinventionisdirectedtowardtbeprovision ofmeansandmethods'ofaoundrecordingsandcontrolrecordingswhereby the volume or amplitude and location of the reproducingsourcemayberegulatuiandmodifledatwill orautomatically throughout the theatre or other enclosureinwhichthesoundprogramisbeing rewoduced.

Heretofore'attempts have been made to reproduce a recorded program and to shift the pointotreproductiondmingtheprosramscas tomatetheeifectofamovingaourceotsormd, such movement being limited to points behind the projection screen-mwhichthephotoplayis beingshown. llechanlcalmeansforproducing suchshiftaswcllascertainrelaymeanshave beenemployed-forthlspurpose. The present invention 's" 1""*- therefrominthatitis withtheproductionofacontrolrecord whereby the output of any desired sound recordingmaybedividedbetweenapluralityof loud speakprs or the entire output reproduced throughasingleloudspeakensaidloudspeakem being podtioned at mulflplaner pointsinatheatresothattheysuhstantiallyench-cletheaudience. By the provision of a iflurality of loud speakers positioned at multiplaner points, it is pomibletoproduceagreatvarietyofnoveleffecilsothatthesound ordcsiredportions of .the sound programmaybesimultaneouslyreproducedinvariousportionsofthethcatreor in variousthree dimensional relations with respect to members of the audience. 7

More particularly, the present application is directed to means and methods whereby a control intelligence or control film may be prepared in an expeditious manner so that the various effects may be automatically produced without the necessity of careful manual control during the presentation of the photoplay and its accompanying sound effects.

Generally stated, the control means employed in the present invention comprise separate tones which are recorded upon a common control track, eachotthetonesbeingadaptedtocontrol the output of a given loud speaker means. (Strictly speaking, frequency is a parameter of'a wave or tone. and should not be considered synonymous with tone.) The invention described hereinafter the formation of such control track in precise with a. photoplay or other program tion also provides for means whereby the reproduction'of a sound program may be gradually shifted fromlone or more loud speakers to others while maintaining a substantially uniform total sound level.

f'lhe method of recording herein disclosed also permits sound to be reproduced from a series of loud speaking means arranged longitudinally of the theatre so that a fore and aft movement of the mud may be attained. In addition, the sound may be caused to move around the theatre or other enclosure, or particular components of.

a sound program such as, for example, reproduction of an aria, solo, rendition'or sound from a specific section of an orchestra may be repro duced' selectively from a particular location. Many novel eflects may be produced.

In addition to effects which are dependent upon the shifting of sound from one loud speaker means to another, the method of recording herein described is also of value in controlling the amplitude or expansion of sound during reproduc tion. In the reproduction of concert music and other accompaniments for photoplays and the like, it is often desirable that the reproduced sound cover a greater range of amplitude than can be ordinarily recorded on the sound track itself. To accomplish this desired expansion, the present method provides independent automatic means which may be associated with the posi- V A further object is to provide means and methods whereby numerous novel sound effects may be produced and controlled.

A still further object is to provide means and methods for recording a control intelligence governing three dimensional sound reproduction or the range ofexpansion of program tracks.

An object of the invention is to provide means and methods for controlling the reproduction of one or more program tracks through a plurality of loud speaker means by a single control intelligence.

or form of entertainment, is The inven- 56 These and other objects will be apparent from an exemplary form of the invention described hereinafter, and shown on the appended drawing.

Generally stated, the method of this invention toward methods and means for reproducing sound programs. The plurality of control frequencies generated in any suitable manner is then variably attenuated in accordance with the efiect which it is desired to produce during the reproduction of a program. Variations in the amplitude of each of the control frequencies may be caused to control the gain of an amplifier associated with a loud speaking means correlated to such control frequency. During the formation of the control track, the outputs of the various tone sources are therefore manually controlled and modified in accordance with the ultimate effect desired during reproduction and the various control frequencies are then recorded in any suitable 'manner, preferably upon film. Since these control frequencies are preferably recorded in superimposed relation, it is desirable to carefully select the character of the control frequencies in the manner hereinafter described. Furthermore, during such recording, means should be provided whereby proper synchronization between the control track and the program track or tracks is assured.

During reproduction, as more fully described in the co-pending application above identified, the output of the control track pick-up is supplied to a plurality of band pass filters capable of separating th original control frequencies and each of the frequencies or control tones is then caused to automatically regulate and control the output of one or more loud speaker means.

In the specific embodiment shown on the appended drawing, tone sources are generally indicated at S, band filters at F, manually controllable variable attenuators at T, sound reproducing pick-up means by P, tone rectifiers and filters by R, variable gain amplifiers by V, power amplifiers by A, and loud speaking means by H. The various tone sources S may comprise oscillators, each being capable of generating a single and different frequency or tone in comparison with the output of other oscillators. In choosin the frequency for a control tone system of the character here described, it is desirable that the second and third harmonics of any tone shall fall outside of the passing band of all high fre quency tone chanels. The pilot ton-e frequencies which have been successfully used are those whereby each frequency represents an approximate constant percentage increase above the next lower frequency, this resulting in simplification of the band pass filter design and construction. Symmetrical, constant impedance type band pass filters have attenuation and phase characteristics which extend at .equal percentages on each side of the pass frequency, this characteristic being often refered to as logarithmic symmetry. For this reason, logarithmic spacing of the pilot or control tones allows the use of relatively simple filters and allows filter and oscillator frequency tolerances to be expressed in logarithmic increments combined with the preferred number series also allows the number of chanels to be increased or decreased easily with out making any of the older oscillators or filters obsolete. In an illustrative example the tone sources comprise oscillators SI to S8 covering the following frequencies: 160, 250, 400, 630, 1,000, 1,600, 2,500, 4,000 and 6,300 cycles. It is to be noted that each frequency is the next lower multiplied by about 1.6. If a smaller number of channels is needed, larger separation between control tone frequencies may be well employed as, for example, in a series of frequencies as follows: 100,250, 630, 1,600, 4,000, 10,000 cycles.

It is to be noted that the lower harmonics of 250 cycles fall at 500 and 750 cycles and thereby miss the'400, 630, and 1,000 cycle channels. The magnitude of th harmonics above the third is usually so low that they need not be considered.

The outputs of the various oscillators or tone sources S are then sent through the clean up" band pass filters F, the output of each oscillator being sent to its associated band pass filter, such band pass filters being identified as Fl to Fl inclusive. These cleanup filters serve to eliminate harmonics from the oscillators which might represent a spurious signal in a higher frequency tone channel.' It has been found advantageous to use constant impedance filters to eliminate the necessity of incorporating isolating network and expensively designed filters in the system. Proper selection of the control tones, as stated hereinbefore, permits the use of relatively simple filters of similar design characteristics.

In many instances it is desirable to also pass the output of all or a part of the tone sources percentage regardless of frequency. The use of ,means H2 to H1 inclusive.

into a combining amplifier H. In the appended diagram the outputs of tone sources or oscillators SI to S1 inclusive, are shown connected to the combining amplifier H. One of the advantages of using a combining amplifier lies in that cross talk is prevented and a volume range may be attained during reproduction as well as during recording which can not be attained by the use of individual attenuators on the outputs of the corresponding band pass filters Fl to F1 inclusive.

Again referring to the appended diagram, it will be noted that the outputs of the band pass filters F2 to F1 are associated with an attenuation network including three pads TI, T2 and T3. These three pads may be either individually controllable or preferably TI and T3 are mechanically interlocked or coupled and T2 is separately operable. The details of construction, operation, and many uses of variable attenuators of the type herein referred to are shown in a co-pending application Serial No. 267,416 filed by William E. Garity andJ'ohn N. A. Hawkins.

At this point it is to be noted that the tone sources S2 to 81 inclusive which are modulated by the variable attenuators Tl, T2 and T3, correspond to and control outputof loud speaker These loud speaker means may be positioned at the left front, left center, left rear, right rear, right center and right front zones in a theatre or other enclosure. It is to be noted, therefore, that the tone sources or control frequencies S2 to S1 are capable of controlling the reprduction ofa program from the front, center and rear of a theatre and therefore control all fore and aft movements of sound reproduction in the theatre.

These reproducers also enter into any operation involving the movement of sound completethe output of their associated tone sources and maintain such attenuations in a complementary relationship, the attenuators being so coupled as to produce a complement 01' attenuations when expressed as power ratios which is constant throughout the attenuation devices operating range. Other attenuation relationships may be iound desirable and independent manual control may be substituted whenever the particular adaptation of the present invention does not require the specific form herein described.

At this point it may be well to note that by reference to "left front" and right front loud speaking means, reference is made'to loud speaking means that are positioned in the front portion of the theatre but which are distinguishable from the screen sides or speakers at the sides of the screen or proscenium since the speakers Hi and B3 are ordinarily located at the screen.

Independent attenuation devices may be used to control the output of control tones generated at Bi, 8! and SI as well as the output of the combining amplifier H. For example, T4 may modulate the output 0118i; Tl may be a master attenuator controlling the output of the bank of attenuators Ti, T2, T3; T8 may effect the output of 8|; '1! may be associated with S8: and Tilt may modulate the output of the combining amplifier ii. All of these various attenuators may include impedance matching devices.

The outputs of the various tone sources and variable attenuators may then be combined in a bus bar. i2 leading to an attenuator, amplifier or other master control device II. a ter control device I! the combined modulated control tones may be supplied by lead M. to a synchronizing panel generally indicated at I5.

During the recording of the control intelligence which is eventually going to automatically control the reproduction of a program in a theatre or other enclosure so as to produce sound eii'ects suitable for use in accompanying photoplays or any other purpose, it is desirable that the program tracks be reproduced at the time the control track is recorded so as to permit the recording operator to personally hear the eilect which is being obtained by the control track at the time it is being recorded. Generally stated, the synchronizing panel II contains appropriate relays and attenuators and is used to transfer from one or more program tracks to the tone recorder, at the beginning of each recording, suitable marks, capable oi being used in synchronizing the program track with the tone track when subsequently suchtohe control track is used in the control of the program track or tracks.

The program sound recordings may be carried on one or more individual tracks. In the creation oi certain sound efiects it may be highly desirable to have one sound or type oi program circulate or move in a three dimensional manner around the theatre while another portion of the program is reproduced from the screen or some other fixed portion of the theatre. In order to attain this result, it is highly desirable that the various sound eflects be carried by separate programtracks.

From such mas-.

As shown in the accompanying drawing, three program pick-ups PI, PI and P3 may be employed, each one of these pick-ups being adapted to act as a reproducing pick-up for a separate sound recording. For example, one of said recordings may carry dialogue, another may carry sound eilects, while the third may carry musical accompaniment. These three program sound recordings are preferably synchronized with one another in any suitable manner as, for example,

by the use of bloop marks. Any one or all of these pick-ups may be connected to an amplifier i6 supplying its output to the synchronizing panel I. A dividing network is shown as embodied in the synchronizing panel I! for the purpose of dividing the incoming program sound between a monitory speaker supplied by line ll and a re-' cording device supplied by line i8, the recording device being not shown and generally indicated only by II. The output lines l1 and it are connected to a double pole, double throw switch indicated at 20. In the position shown, the program from P3 and amplifier I8 is being supplied by line H to a bridging coil 2| leading to the power amplifier A! and loud speaker means H8. It is to be understood, however, that the output line i! may lead directly to a separate loud speaker means located at any suitable place within audible distance oi. the recording operator or monitor. As soon as the recording operator or monitor hears the synchronizing mark or bloop of the program being reproduced by P3, he may actuate the switch 20 so as to connect the line it (supplying control tones) to the amplifier 22, said amplifier being inserted in line It leading to the recording device 19. The amplifier 22 may be of the variable gain type and provided with suitable control means as well as an indicating device shown at 23 adapted to indicate the overall level of the tones being recorded.

The control tones modulated by means of the variable attenuators T are then supplied by line 2| to a tone divider panel 25 containing appropriate apparatus to set output levels, and if desired, a visual volume indicator. From this panel 25 the modulated control tones are sent by line 26 to a bus bar 21, such bus bar being connected to the various tone rectifiers and filters R. An additional amplifier 28 may be inserted into the line 26 (when desired) to further amplify the control tones before they are selected by the filters R from the bus bar 21. Each of the filters and tone rectifiers R includes band pass filter particularly adapted to select and isolate a frequency from the range of frequencies passed by the filters F, and means for rectifying the selected control tone. An amplifier of any desired type may also be included in these units. Variable gain amplifiers are shown at V, the gain of each of these being controlled and regulated by the bias from the unit R. Generally stated, the units It and V taken jointly may be termed v'ariolossers and the term as used hereinafter and in the claims shall refer to any variable gain type amplifier or other device capable of introducing a variable transmission loss into an audio channel, which device is not manually operated. It

circuit of one or more stages of the amplifier. Either a positive or a negative grid bias may be used, depending upon whether expansion or contraction in the transmission characteristic of the amplifier is desired. Furthermore, attention is called to the fact that such system is applicable to any multi-grid tube. Instead of varying the grid bias, similar effects may be obtained by varying the plate resistance, varying one leg of a bridge attenuation circuit, varying the regeneration or degeneration of the amplifier, etc. All these general methods are hereinafter referred to as variolossers.

The sound records which comprise the program may be picked up from any or all of the units Pl, P2 and P3 and pass into a program divider 30 containing component networks and other devices adapted to permit the program to be supplied to any one or more of the amplifiers V. In the specific form of arrangement shown in the drawing it will be noted that the pick-up Pl supplies program to V2 to V1 inclusive whereas P2 supplies its sound recording to V9, and P3 supplies program sound to VI and V8. By the use of several pick-ups and a program divider 30, the program sound may be originally recorded in such manner that special eilfects are carried by one strip of film, dialogue by another strip and still further sound eiiects by the third strip. By associating these various pick-ups and their programs with suitable variable gain amplifier and loud speaker units, desired effects can be obtained most readily.

Although separate program pick-ups have been indicated in the drawing and reference has been made to the fact that these program elements may be recorded upon separate strips of fihn, this invention also contemplates a method in which a single strip of film carries a plurality of parallel program tracks or program elements. The use of a multiplicity of program tracks on the same strip of film has advantages in that film cost is reduced and a more positive and accurate synchronization of the various program elements is attained. Ordinarily when two or more sound recordings carried on two or more separate strips of film are being reproduced through two or more separate reproducing heads and pick-ups, it is virtually impossible to obtain a synchronization between such program tracks which is more accurate than the distance between sprocket holes on the film advancing sprockets of the various machines. It is very possible, therefore, to have the recordings out of phase so that cancellation takes place during reproduction in various parts of the theatre or other enclosure in which the sound is being reproduced. Such cancellation is particularly noticeable and detrimental when socalled steriophonic or binaural reproduction is attempted. In the event these various program tracks are carried upon a single strip of film, however, the scanning points may be synchronized to a thousandth of an inch, and out-ofphase relationships thus eliminated and perfect synchronization attained.

It may be noted at this point that in the event the gain control characteristics of the variolossers R and V are not straight line functions, the R units may be designed to give an inverse characteristic to that of the amplifiers in response to the tone signal volume input. In this manner, the combined influence of R and V may then produce a change in program level which is equivalent, in decibels, to the change-in control tone level. In other forms of these units, particularly when it is desired-to expand the volume beyond the volume ordinarily carried by the program sound record, the units B may be so designed as to cause a larger expansion or amplification of the program sound than the variation in level of the control tone, as by including a variable gain amplification stage in the R unit before its output is caused to control V.

The outputs of the various amplifiers V may then be fed to power amplifiers A before the program is reproduced through the loud speaker means H.

The operation of the recording system will be understood from the description of the apparatus given hereinabove but to review the operation, attention is called to the fact that a multiplicity 01' separate tones or control frequencies is generated in any suitable manner as by means 01' the oscillators SI to SI. These tones are filtered by the band pass filters P, such control tones being then supplied to the synchronizing panel l5. During the production oi the control record which will carry the various control tones, the program sounds which are to be controlled are picked up by the reproducing pick-ups P and sent through the program divider II to the varsuitable multiplaner location within the theatre I or other enclosure in which the sound efiects are to be reproduced. The synchronizing panel I! is used for the purpose of insuring suitable synchronization between the control record which is being formed and the program which is being modulated and controlled during the recording operation. The various tone-sources are manually adjusted and attenuated either by a network employing variable attenuators such as TI, T2, T3, or by independent attenuators such as T4, T8 and T9. The recording operator may listen to the reproduction of the program sound in the theatre through the various loud speaker means HI to H9 and in addition he may listen to the program sound by introducing a monitor speaker and its amplifier 38 into the line leading from the pick-up such as P3, to the synchronizing panel l5.

By properly manipulating the various variable attenuators, not only will the reproduction of the program sound through loud speaker means HI to H9 be caused to include the desired eflects of stereoscopic sound reproduction, movement fore and aft, movement from left to right, movement around the theatre, reproduction from the ceiling, etc., but at the same time the modulated control tones which create such effects will be recorded by the recording instrument I9.

After a recording of the control tones has been made, the apparatus shown on the appended drawing may be used for reproduction of the same program sound by the aid of the previously made control record. It will be noted that the tone divider panel 25 may be supplied with current from a pick-up P4. The previously made tone control film or record may be passed through the pickup P4, the original tone sources S being either rendered inactive or the entire control tone circuit disconnected from the rest 01' the system by adjustment of the master attenuator Hi. The program sound will then be reproduced and fed through the reproducing program divider 30 to the various amplifiers whereas the previously recorded control track will supply suitable modulated control tones from P4 through line 26 to the bus. 21 leading to the various tone rectifiers I amass? and filters R. Reproduction of the program through the loud speaker means HI to II! will be eifected automatically.

The system described hereinabove is capable of many uses and modifications. For example, instead of attemptingto produce a control track capable of controlling all of the program sound simultaneously, it may be desirable to make separate control tracks for various effects. For example, a separate control'track may be made for the purpose of controlling only the sounds which are to emanate from the left and right screen horns, such as loud speaker means Hi andHi.

In order to accomplish this result, it is only necessary to energize tone sources Si and SB and to use program pick-up P3 alone. The attenuation of the two tone sources can then be accomplished either by attenuators T4 and T8 or by combining such attenuators into a variable attenuator of the type TI (or of the type described in the co-pending application hereinbefore identiiied).

Similarly, a separate tone control record may be made of certain effects which may be recorded on say a program record being reproduced through Pl so that all of the shifting of sources or reproduction in this particular record are controlled by a separate track.

Durlng the recording of any one of the separate tracks, a previously made tone control for one of the other program sound tracksmay be reproduced by P4 so that the combined effect is made evident to the recording operator.

The production of separate control tone tracks or separate channels is sometimes desirable inasmuch as the various efiects may be separately edited and subsequently dubbed together onto one track. It also permits cutting and editing for the purpose of modifying levels, correcting placement of efi'ects, etc. When a tone track has more than one frequency on it, it can be cut by dubbing such track through band pass filters to two or more new and separate tracks, cutting one or both of the new tracks and then dubbing the cut tracks together again.

, It is to be understood that those skilled in the art will readilyconceive of numerous uses and adaptations of the methods and arrangements herein discussed, in addition to those specifically referred to hereinabove. All changes, modifications and adaptations of the invention areembraced hereby. 1

We claim:

1. In a method of producing a control recording for controlling the reproduction of sound through a plurality of loud speaker means, the steps of: allocating a separate control frequency to control the response of each of a plurality of loud speaker means; each of said separate control tones being substantially pure and free from harmonics, said control tones being members of a logarithmic progression, no member of said progression being close to a second or third order harmonic of a lower; desirably modulating the amplitude of said sustained control tones while reproducing a sound program through loud speaker means controlled by said control tones.

2. In a method of producing a control recording for controlling the reproduction of sound through a plurality of loud speaker means, the steps of: allocating a separate control frequency to control the response of each of a plural ty of lour speaker means; each of said separate con- 7 trol tones being substantially pure and free from harmonics, said control tones being members of a logarithmic'progression, no member of said prcgressionbeingclosetoaseoondorthirdorder harmonic of a lower; reproducing a sound program through loud speaker means controllable by such control tones; desirably modulating the amplitude of such sustained control tones while observing the sheet of such modulation upon the program being reproduced, mixing said desirably modulated control tones, and making a single record of said tones.

3. In a method of producing a control recording for controlling the reproducidon of sound through a plurality of loud speaker means, the steps of: allocating a separate control frequency to control the response of each of a plurality of loud speaker means; each of said separate control tones being substantially pure and free from harmonics, said control tones being members of a logarithmic progression, no member of said progression being close to a second or third order harmonic of a lower; reproducing a sound program through a plurality of loud speaker means arranged in multiplanar relation to each other, the output of each of said loud speakers being controlled by a direct current resultant of said control tones, desirably modulating the amplitude of said sustained control frequencies while reproducing a sound program through said loud speaker means; mixing said desirably modulated control tones, and "making a single record oi said tones.

4. A sound recording system comprising a plurality of control tone generating means, each thereof including a "clean up filter adapted to attenuate harmonics, each of the plurality of the frequencies so passed being a member or a logarithmic type progression, no frequency of such plurality being close to a. second or third harmonic of another; a variable attenuating device associated with the output of each of said band pass filters; a common output for said attenuating devices, a recording machine connected to said common output, a progrmn pick-up device and a loud speaker means; and a synchronizing control means in said common output for selectively connecting and disconnecting said recording machine, said synchronizing control mamas including means for connecting program pick-up means with said loud speaker.

5. In an electrical system for making a recording for the control of program sound through a plurality of loud speaker means, the combination of: a plurality of sources of control tones of diifering frequencies; an attenuation system connected to said control tone sources and having a common output; a plurality of tone rectifiers connected to said common output, each of said rectifiers including filtering means receptive only to one of said control tone frequencies, a plurality of program amplifiers and associated loud speaker means and including a program channel, a program pick-up associated with the program channels of said amplifiers and loud speaker means, and means associating one of said rectiflers with a separate program amplifier whereby the amplification characteristics of said amplifier are varied by the output of said rectifier.

6. In a recording system for producing a control record, a plurality of control tone frequency sources, a constant impedance symmetrical band pass filter in the output of each of said sources,

said sources and filters being adapted to produce a series of control frequencies, no one of which isclosetoaseoondorthirdharmonicofanother;

' trol record, a plurality of control tone frequency sources, a band pass filter in the output of each' of said sources, said sources and filters being adapted to produce a series of control frequencies, no one of which is close to a second or third harmonic of another; a manually variable attenuating system connected to the output of said filters, said attenuating system including a differential attenuation network associating at least a part of said filter outputs; a common output for said attenuating system, a combining amplifier connected to a part at least of said band pass filter outputs and an attenuator in the output of said combining amplifier, said attenuator being connected to said common output.

8. In a recording system for producing a control record, a plurality of control tone frequency sources,'a band pass filter in the output of each of said sources, said sources and filters being adapted to produce a series of control frequencies, no one of which is close to a second or third harmonic of another; a manually variable attenuating system connected to the output of said filters, said attenuating system including a differential attenuation network associating at least a part of said filter outputs; a common output for said attenuating system, and a recording device connected to said common output, a plurality of program variolossers, each thereof having a loud speaker means associated therewith, each of said program variolossers being responsive to a single control tone frequency, all oi. said program variolossers being connected to the common output from said attenuating system, and program pick-up means connected to the program variolossers.

9. In a recording system for producing a control record, a plurality of control tone frequency sources, a band pass filter in the output of each of said sources, said sources and filters being adapted to produce a series of control frequencies, no one of which is close to a second or third harmonic of another; a manually variable attenuating system connected to the output of said filters, said attenuating system including a differential attenuation network associating at least a part of said filter outputs; a common output for said attenuating system, and a recording device connected to said common output, a plurality of program variolossers, each thereof having a loud speaker means associated therewith, each of said program variolossers being'responsive to a single control tone frequency, all of said program variolossers being connected to the common output from said attenuating system, and program pick-up means connected to the program variolossers, a synchronizing control means in the common output from said attenuators for selectively connecting and disconnecting said recording machine, a loud speaker associated with said control means, and a program pick-up means connected to said synchronizing control means. 7

10. In a recording system for producing a control record, a plurality of control tone frequency sources, a band pass filter in the output of each of said sources, said sources and filters being adapted to produce a series of control frequencies, no one of which is close to a second or third harmonic of another; a manually variable attenuating system connected to the output of said filters, said attenuating system including a'diiferential attenuation network associating at least a part of said filter outputs; a common output for said attenuating system, and a recording device connected to said common output, a plurality of program variolossers, each thereof having a loud speaker means associated therewith, each of said program variolossers being responsive to a single control tone frequency, all of said program variolossers being connected to the common output from said attenuating system, a plurality of program pick-up means connected to said variolossers and a selectively adjustable attenuation and distribution means between said pick-up means and said variolossers.

WILLIAM E. GARI'IY. JOHN N. A. HAWKINS.

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