US2341539A - Loud-speaking telephone system - Google Patents

Description

Feb. 15, 1944. M G|ANN|N1 2,341,539

LOUDSPEAKINQ TELEPHONE SYSTEM Filed Nov. 14, 1941 5 Sheets-Sheet 1 I04 100 Signal (lament flmlz'fz'er Confro/ Bal Hybrid .Q I Nei 10p Neflllk 35.51%? 15 INVENTOR Gabriel M. Giannini BY ATTORNEYS Feb. 15, 1944.

G, M. GIANNINI 2,341,539

LOUDSPEAKING TELEPHONE SYSTEM Filed Nov. 14, 1941. 5 Sheets-Sheet 2 *QQQSLQQQQQQat- F INVENTOR Gabriel M. Giannini ATTORNEYS Feb. 15, 1944. G; M. GIANNINI I LOUDSPEAKING TELEPHONE SYSTEM 5'Sheets-Sheet 5 Filed Nov. 14, 1941 3039'; Balancing NeZLw/"k 3Q4 300 i ybrzcoz Syszem 309 F 3 v INVENTOR.

. Gabriel M Giannini I. I BY I If I l g E 2 flztorneys Feb. 15-, 1944.

s. M. GIANNINI LOUDSPEAKING TELEPHONE SYSTEM Filed Nov. 14. 1941 5 Sheets-Sheet 4 e 1 m 4 efi ..1. WWW

G. M. GlANNlNl 2,341,539

5 Sheets-Sheet 5 Feb. 15, 1944;

LOUDSPEAKING TELEPHONE SYSTEM Filed Nov. 14, 1941 I Patented Feb. 15, 1944 v LOUD-SPEAKING TELEPHONE SYSTEM Gabriel M. Giannini, North Hollywood, calm, as-

signor, by mesne assignments, to Automatic. Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application November 14, 1941, Serial No. 419,165

' 17 Claims. ('Ci. 179-=-l) The present invention relates to loudspeaking telephone systems and, more particularly, to improvements in methods and apparatus for increasing the stability of operation of systems of this character.

Aside from the so-called manually controlled push-to-talk and push-to-listen" arrangements, loud-speaking intercomrnunicating systems of the character mentioned usually. take either of two forms. In one form of the system, as exemplified by Patent No. 2,208,160, Harold J. McCreary, granted July 16, 1940, separate transmitting and receiving channels, each provided with signal current amplifiers, are utilized in the transmission of signal currents between two stations arranged for loudspeaking service. In the other type of system, as exemplified by Patent No. 2,224,477, Roswell H. Herrick, granted December 10,1940, a single two conductor channel is utilized for transmission in either direction, each station connected to the channel usually being provided with transmitting and receiving amplifiers and a hybrid system for obtaining side tone suppression in the station circuit. In either type of system, the problem of preventing the acoustical coupling between the transmitting and receiving elements from setting up an oscillatory condition in the system is difllcult to obviate. Thus. in the two channel arrangement, the acoustical coupling at the two ends of the channels provides a closed signal transmission circuit which includes the two signal amplifiers. This closed circuit tends to oscillate when the gain through the amplifiers exceeds the acoustical and electrical losses of the circuit. Again, in the two-conductor single channel arrangement, each substation network eflectively comprises a closed circult which includes the transmitting and receiving amplifiers, the acoustical coupling between the microphone and loudspeaker, and the electrical coupling through the hybrid system. This closed circuit will likewise oscillate when the gain through the amplifiers exceeds the electrical and acousticaliosses of the closed station circuit. The above-noted common characteristic of the two systems places a definite limitation on the" amount of gain which maybe introduced into the signal transmission channels before singin level.

- extent possible in order to raise the permissible erating both the transmitting and receiving channels at a sub-normal gain setting and for inversely varying the gains of thesignal amplifiers in response to signal current transmission, so that the gain of the channel in useis raised to a normal value and the gain of the inactive channel is decreased .by at least a corresponding amount. While this and like arrangements may be designed satisfactorily to eliminate singing, it gives rise to ancillary problems. For example, with the gain of the inactive channel at an extreme sub-normal value, conversation break ins" can only occur at a very low transmission Also, the problem of preventing speech clipping" is diflicult to obviate.

It is an object of the present invention, therefore, to provide an improved system of the character described wherein the transmitting and receiving elements provided at each station of the system are acoustically decoupled to the greatest level of signal current gain through the transmission channel in use, and wherein the system is so arranged that stability is maintained with an attendant satisfactory gain in the active channel and without an objectionble decrease in the gain of the inactive channel.

or howling will occur, and hence places a'limitation on the volume at which received speech cur-' rents may be transmitted and reproduced. Various so-called voice operated switching arrangements have been devised foreliminating this difliculty. In at least one system of this general character, facilities are provided for normally op-' ated elements, whichspacing isdetermined by the point in the combined frequency response characteristic of the two elements at which maxi- 'mum coupling between the elements tends to occur. 7

According to a further object of the invention, the loudspeaker and microphone elements of the system are of improved construction, characterized by an absolute'minimum of mechanical or conductive coupling between the associated ele ments at each station, and utilize diaphragms which are so proportioned as to size that the acoustical coupling between associated elements is further minimized.

According to a still further object of the invention, sound transmission from the loudspeaker or receiving element at each station is eflected along a substantially vertical path, while sound input to. the associated microphone is eilected along a substantially horizontal path, thercby further to enhance the acoustical impedance between the two elements.

According to still another object of the invention, directional sound wave transmission from the loudspeaker at each station is utilized as a vehicle to minimize sound wave reflection and refraction into the response zone of the associated microphone during operation of the loudspeaker, thereby further acoustically to decouple the two elements.

It is another object of the invention to provide an improved method of operating the system, whereby the two transmission channels are normally operated at the maximum gain consistent with stability of the system, and the inverse changes in the gains of the two channels which occur during signal current transmission are limited to a narrow range just sufficient to preserve the stability of the system.

In accordance with another object of the invention, the operation of the gain control apparatus is prevented until the amplitude of the sig-' nal input to the active channel attains a predetermined value which exceed the normal noise current level.

According to still another object of the invention, the gain control apparatus for the signal amplifiers of the two channels is provided with a frequency discriminating network which favors the frequencies in the lower portion of the operating frequency range, within which the major portion of the sound energy developed by the human voice is generated.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings in which Figs. 1, 2, and 3 illustrate the circuit arrangement of an improved system having incorporated therein the features of the invention briefly referred to above; Fig. 4a illustrates one improved loudspeaker-microphone arrangement which maybe used at each of the stations of the system; Figs. 4b 4c and 4d illustrate a modified. arrangement of the loudspeaker-microphone unit; Fig. 4e illustrates another modification of the loudspeaker-microphone unit; and Figs. 5 to 9, inclusive, illustrate the details of the transmitting and receiving elements provided at each of the substations of the system.

Referring now more particularly to Figs. 1, 2, 3 and 4 of the drawings, the improved loudspeaking intercommunicating system there illustrated comprises, in general, an east station l0 and a west station II which are interconnected by a pair of lines I 3 and I4 having a central repeater l2 connected therebetween. This repeater may be provided at a central point in the system and may be used to stabilize the transmission level over the two connected lines l3 and I4. Briefly described, this repeater comprises an east-west amplifier a west-east amplifier 200, a pair of hybrid systems 202 and 203, balancing networks204 and 205 individual to the hybrid coils and respectively lines l3 and I4, and a signal operated control netthe two amplifiers 200 and 2M in accordance with the direction of signal current transmission over the lines l3 and I4.

More specifically considered, the west-east amplifier 200 comprises a coupling transformer! having its input winding connected to thesignal transmitting terminals of the hybrid system 203,

and its output or secondary winding coupled to the input electrodes 201!) and 2010 of the threetrodes 201a and 2010 of this tube are coupled to the signal input terminals of the hybrid system 202 through a coupling transformer 208. The opposite or east-west amplifier 20l similarly comprises a coupling transformer 2 having its primary winding connected to the signal transmitvting terminals of the hybrid system 202, and its secondary winding coupled to the input electrodes 2i2b and 2l2c of the three-electrode amplifier tube M2. The output electrodes H211 and 2 We of this tube are coupled to the signal input terminals of the hybrid system 203 through a coupling transformer 2 l3. Preferably, the amplifier tubes 201 and H2 are of the well-known 605 type, utilizing indirectly heated cathodes and characterized by a comparatively high amplification factor.

The control network 220, as indicated above, is provided for the purpose of increasing the gain of the sign-a1 amplifier 200 or 20! which is in use during signal current transmission over the two lines l3 and I4, and for concurrently effecting a corresponding decrease in the gain through the signal amplifier included in the inactive channel. Briefiy described, this network comprises a three-electrode amplifier tube 223 having its input electrodes 223b'and'223c coupled to the input circuit of the west-east channel amplifier tube 201 through an a'dustable Voltage dividing resistor 222 and a coupling transformer 22!. The output electrodes of the tube 223 are coupled to one rectifying section 23l a, 23Ic of a duplex diode 23l, through a coupling transformer 224. The other section 23ld, 23le of the vacuum tube 23! is coupled to the input circuit of the east-west channel amplifier tube 20l over a signal current path which includes the coupling transformer 228, the three-electrode amplifier tube 221, the adjustable voltage dividing resistor 226 and the coupling transformer 225. The duplex diodie 23l functions to rectify signal currents appearing across the secondary windings of the coupling transformers 224 and 228, re-

rent amplifier tubes 232 and 233 in opposite senses. Thus, it will be noted that the diode section 2311:, 23lc is included in a closed direct current circuit which includes the space current path between these electrodes, a resistor 230, the

secondary winding of the transformer 224, and a resistor 234. The other two electrodes 23ld, 23Ie of the duplex diode 23l are similarly included in a circuit which includes the resistor 234 the secondary winding of the transformer 228 and a resistor 229. A smoothing condenser 236 is connected in shunt with the resistor 234 in order to smooth out alternating components of the voltage developed across this resistor and to determine the time constant of the control circuit. The voltages developed across the two halves 234a and 234b of the resistor 234 are impressed in opposite senses across the'input electrodes of two three-electrode control tubes 232 and 233:1!- spectively, through the adjustable cathode biaswork 220 which is operative to control the gain of ing resistors 235a. and 235b, respectively. The

output circuit .of the tube 232 includes a load resistor 236a which is shunted by a smoothing condenser 231 and is so connected that the voltage developed thereacross is impressed as a bias voltage between the input electrodes of the westeast amplifier tube 201. More particularly, the

- resistor 236a is connected to the control grid 201b'of the tube201 over a path which includes the secondary windingof the transformer 206.

electrode amplifier tube 201. The. output elec- 16 This winding is bridged by a-terminating network which comprises the shunt-connected resistor 209 and condenser 2I0. Similarly, the output circuit of the other tube '233'includes a biasing resistor 23Gb which is shunted by asmoothing condenser 230 and is connected through the secondary winding of the coupling transformer H I to the control grid 2I2b of the east-west amplifier tube 2I2. This secondary winding is also bridged by a terminating network which includes theshunt-connected resistor 2I3 and condenser 2 I4. Preferably, the'control and amplifier tubes 223, 221, 232 and 233 are of the commercial type 605, and the duplex diode is of the 6H6 type.

Anode voltages are impressed upon anodes 201a and 2I2a of the two signal amplifier tubes 201 a control network 3I6 is'provided. The various anode and biasing voltages required for operation of the amplifier tubes and the tubes of the control network 3I5 are derived from a. power pack indicated generally at 335.

More specifically considered, the transmitting I amplifier 300 comprises a thermionic tube 306 of and 2I2 from the voltage source 2, over paths which respectively include the signal current choke coils 245 and 2I5. source 24I are by-passed for signal currents by the condensers 244 and 2I6. In this regard it will be noted that the cathodes M and 2010 of the two amplifier tubes are connected to the tapped point along the voltage dividing resistor 242 in order to provide the required potential on these cathodes. The voltage of the source MI is also positively applied to the anodes of the These coils and the the well-known pentode type, having its input electrodes 306b'and 306a coupled to one of the three microphones 4M, 4 and 42I, over a chan- -nel which includes an adjustable voltage dividing resistor 305, a condenser 363, a coupling transformer 304 and the cord conductors 430. The output electrodes of this tube are directly coupled to the input winding of the hybrid system 302, which is shunted by a resistor 366. The other signal amplifier I similarly comprises a thermionic tube 308 of the well-known pentode type, having its input electrodes 30811 and 3080 coupled to the receiving terminals of the hybrid system 302 through an adjustable voltage dividing resistor 30?.

420 in use,'over a channel which includes the coupling transformer 309 and the cord conductors 43I. The two tubes 306 and 308 are provided with cathodes 3000 and 3080 which are of the filamentary type, such that rapid changes in the divider 239, which divider is bridged across the.

section 24%. of the resistor 242.

Referring now more particularly to the equipment provided at the east station l0; this equipment is illustrated in Fig. l of the drawings as comprisin a transmitting element in'the form of a microphone I00, a receiving element in the form of a loudspeaker'IOI, a hybrid system I02, and a balancing network I03 which is provided to balance the impedance of the line I3 so that signal energy transfer between the transmitting and receiving channels of the station through the hybrid system is reduced to a minimum. The station equipment further comprises a transmitting amplifier I04, a receiving amplifier I06, and a control network I06, which functions to control the signal 'current gain through ;the' two amplifiers in the manner more fully pointed out hereinafter.

In general, the arrangement of the equipment provided at the west st'ation II is identical with that provided at the east station I0 and, accordingly', only the details of the station II have been illustrated in the drawings. Briefly described, the equipment provided at the west station I.I

includes one of the three loudspeaker-microphone units illustrated in Fig. 4 of the drawings, and

' the circuit equipment illustrated in Fig. 3 of the electron emission of-the cathodes is obtained in response to changes in the current traversing the cathodes.

The input and output terminals of the control network 3I5 are coupled to the receiving channel of the station circuit at a point just following the signal amplifier tube 308. Briefly described,

this network comprises a signal amplifier tube' 322 having its input electrodes 322D and 322c transformer 324 to therelectrodes of. a triode 326 which is connected to o erate as a diode rectifier.

A load resistor 328 sh ted by a signal current by-pass condenser 329 is connected in circuit with the space current path of the tube 326, and

the voltage'developed thereacross is utilized to control the potential difference between the input electrodes 33% and 330001? the control tube 330. In order to'prevent the control circuit 3|! from effecting a change in the gain of either of the two channel amplifier tubes 306 and 300 when the signal input to the station II is below a predetermined value, a biasing battery 325 is providedin circuit with the tube 326.

functions to prevent the tube 326' from passing, current until the signal input to the station exceeds the selected predetermined value.

transmitting and receiving terminals of a hybrid system 302. 'A balancing network 303-01 the adjustable resistive type is provided for balancing the impedance of the line I4 to minimize energy.

transfer between th transmitting and receiving channels of the-station II. controlling the gain of the two amplifiers 300 and 30I to preserve stability of the station circuit,

The control tube 330 is provided-with a space current path between its output electrodes 330m and'330c which may be connected in parallel with the space current path of the signal amplifier tube 308, and in series with a cathode biasing resistor 352. It will be apparent that with the For the purpose of space current path of the tube 330shunting the output circuit of the amplifier tube 308, the signal current gain through the receiving amplifier 30I This battery is varied directly in accordance with the resistance of the space current path through the tube 330. It will also be noted that the cathode biasing resistor 352 is shunted by the cathode 3060 of the transmitting amplifier tube 306, whereby the electron emission of the tube 306 is controlled in accordance with the current traversing the space current path of the tube 330. This cathode is shunted by a large condenser 365 which functions to by-pass the cathode for signal frequency currents.

' 349 through a filter network which comprises a series choke 346, a pair of shunt connected filter condensers 347 and 348, and a bleeder resistor 350. The mid-point of the high voltage winding 339 is connected to the low voltage terminal of the load resistor 349 and to ground through a load resistor 343 which is tapped at two points to provide bias voltagesfor the two tubes 306 and 330. Thus, the voltage of approximately 5.5 volts which is developed across the section 343a of-the resistor 343, during operation of the apparatus,

7 is negatively applied to the control electrode 30% of the tube 306 over a path which includes the filter resistor 364 and the lower sectionof the voltage dividing or volume control resistor 305. This section of the resistor 343 is shunted by an alternating current by-pass condenser 344 to prevent noise from being introduced into the signal amplifier'300 from the power pack. The voltageof approximately 18.5 volts which is developed across the section 34311 of the resistor 343 during operation of the power pack 335, is negatively applied to the control electrode 33011 of the tube 330 over a path which includes the resistor 328. This section of the resistor 343 and the resistor 328 are by-passed for alternating currents by the condensers 33l and 345. The entire voltage of approximately 2'70 volts developed across the load resistor 349, during operation of the power'pack 335, is positively applied to the anode 308a of the receiving amplifier tube 308 over a path which includes the resistor 362 and the primary winding of the coupling transformer 389. This, voltage is also positively applied to the screen electrode 308d of the tube 308 over a path including the resistor 362. Depending upon the position of an on-off switch 36I,- which is provided for rendering the control network 3l5 active or inactive as desired, this'voltage is also either directly applied to the anode 330a of the tube 330 or is applied to this anode through the resistor 362 and the primary winding of the transformer 309. A portion of the voltage developed across the load resistor 349, i. e., approximately 110 volts, is positively applied to the anode of the signal amplifier tube 322 through the primary winding of the coupling transformer 324, and to the anode of the transmitting amplifier tube 306 over a path which includes the resistor 366 and the signal input winding of the hybrid system 302 in parallel. This voltage is also directly applied to the screen electrode 306d of the transmitting amplifier tube 306.

A by-pass condenser 321 is connected in shunt with the portion of the resistor 349 from which 'the cathode 3030 is normally maintained at a potential which is substantially positive with respect to its associated control grid This biasing path is by-passed for alternating currents by a shunt connected condenser 353. Cathode heating currents are supplied to the cathodes of the three tubes 322, 326 and 330 from the three low voltage windings 351, 356 and 358 of a voltage step-down transformer 354, this transformer be- 7 ing provided with a primary winding 355 which acoustical damping material, such, for example, as

shunted by a signal current by-pass condenser 360.

The third low voltage winding 358 is connected to supply cathode heating current to the cathode 3300 of the tube 330 and is provided with a tapped center point. which is connected to ground'through the cathode biasing resistor 352.

' As indicated above, any one of the threelcudspeaker-microphone arrangements illustrated in Figs. 4a, 4b, 4c, 4d and 4e of the drawings may be used in conjunction with the circuit equipment shown in Fig. 3 of the drawings. The common purpose of the three arrangements is to minimize acoustical coupling-between the loudspeaker and the microphone and thus reduce the tendency of the station circuit to oscillate and produce singing or howling. Briefly considered, the unit illustrated in Fig. 4a of the drawings comprises a microphone 40! which is arranged to be primarily responsive to sound waves I transmitted thereto along a horizontal path, and a loudspeaker 402 which points upward and is arranged to transmit sound waves in a substantially vertical direction. These two translating devices are housed in a casing 400a which is provided with curved outer surfaces, and in order to minimize conductive or mechanical coupling between the two-translating elements 40l and 402, the casing 400a is filled with sets of elongated sound transmission passages 4 and M3 of tubular form, which respectively communicate with the chambers adjacent the diaphragms of the two translating elements. -In this arrangement, also, the loudspeaker M2 and the microphone 4 are housed in a casing 011 and r are surrounded by felt or other acoustical insulating. material to minimize the conductive coupling therebetween. trated in Figs. db, 4c and 4d, the sound chamber i adjacent the diaphragm of the loudspeaker 122 communicates with: two sets of sound passages lilo and izlb which extend to the air surrounding the unit. These passages, at their outer ends, are flared away from each other so that the sound emitted therefrom is directed away from the zone intermediate the two sets of sound transmission 1 openings. The transmitter element or microphone ii2i is disposed between the two sets of sound passages 424a and 424b and is provided with a diaphragm which communicates with the air surrounding the unit through the elongated sound passages 423. As'best shown in Fig. 4d of the drawings, these passages are directed slightly downward to prevent sound reflected from the walls and ceiling of the room in; which the unit is .located from being transmittedto the diaphragm of the microphone MI. The loudspeaker M2 and the microphone d2! are-housed within a alternative to this arrangement, the two ele- U verge at the openings of these passages and are transmitted away from the zone in which sound is transmitted to the sound passages 423 which communicate with the diaphragm of the microphone Mi. By virtue of this arrangement, a

minimum of the reproduced sound energy is fed back through the passages 423 to the'air chamber adjacent the microphone diaphragm.

Further to consider the three'embodiments of the microphone-loudspeaker units, it has been found that station transmitter receiv'er combinations, in general, have a number of peaks in the combined frequency response characteristic @thereof, which peaks represent particular frequencies at which maximum coupling between the receiver and transmitter obtains. In other words,

' signal currents having frequencies corresponding to the response characteristic peaksand reproduced by the receiver element, cause the maximum response of the associated transmitter ele- 'ment. It has also been observed that the acoustical coupling between the station transmitter and receiver elements is affected by the spacing between the sound transmitting and sound receiving openings which connect the movable diaphragms of the coupled elements with the surrounding air. More specifically, the coupling is greatest when the spacing between the sound transmitting and sound receiving openings of the In the arransemcniiaillustwo elements is of the order of one-half of the wave length representing the sound frequency under observation. Accordingly, when the openings of the sound transmitting and receiving elements are spaced apart by a distance equalto an odd number of quarter wave lengths of a particularly objectionable frequency, a minimum amount of, acoustical coupling between the two elements and, hence, a maximum of system stability, are obtained. Usually the most objectionable peak in. the combined frequency response characteristic of the transmitting and receiving elements occurs in the lower portion of the voice frequency range. For example, tests on a-specific combination of elements have indicated a maximum combined response of the two elements at a frequency of-the order of 600 cycles persecond,

which corresponds to -a wave length, measured at ordinary room temperature, of approximately 22.3 inches. In this case 'a mean spacing between the sound transmitting and receiving openings of the loudspeakerand microphone, respectively, of approximately 5.6 inches may be used to obtain the maximum of acoustical attenuation between the two elements. Another factor which in part determines the acoustical coupling between the two translating elements 40 l' and 402, for example, is the size of the diaphragm used in the loudspeaker 102. In general, it may be stated that the smaller the loudspeaker diaphragm, the less the acoustical coupling between the loudspeaker and its associated microphone. In other words, a louddiameter is capable of producing a greater output per unit of diaphragm area without producing instability of the station network. More specifically, it has been found that if the efiective diameter of the loudspeaker cone is made more than one-half the minimum wave length of the operating frequency range, the acoustical coupling between the loudspeaker and the microphone elements becomes dimcult to control. In the system under consideration, a transmission band ranging from about 350 cycles per second to 5 approximately 3300 cycles persecond is utilizedfor voice current transmission. Accordingly, a

' loudspeaker is provided which is equipped with a diaphragm having an effective diameter of less than 6 centimeters.

assemblies are of the electro-dynamic type and. preferably, each thereof is of the form illustrated in Figs. 5, 6, 7, 8 and 9 of the drawings. These figures illustrate a working embodiment of the assembly, and in Fig. 5 all parts are drawn to full size, scaled dimensions. In brief, the assembly there shown comprises a self-contained magnetic circuit which includes a permanently magnetized annular ring 500 formed of Alnico, a bottom plate tuba center pole piece 502 and a top plate 603. r, The parts 50!, 502 and 503 are preferably formed of Allegheny Electric metal or other v highly permeable magnetic material. The center pole piece 502 is provided with a lower portion 05 502a of reduced diameter, which extends within a. centrally disposed opening 'provided in the lower. plate Bill. -A'sweat connection between the side walls of this opening and the sides of the extended portion 502a is used rigidly to secure a the center pole piece ililto the bottom plate 50!.

a At its upper end the center pole piece 502 is provided with a portion 50% of reduced diameter, which extends within a centrally disposed opening 503a formed in the top plate 503. As best 7 shown in Fig. 50f the drawings, the diameter speaker unit having a small effective diaphragm The individual microphone and loudspeaker of the opening 503a is slightly larger than the diameter of the upper portion 50% of the pole piece 502. The upper end of the center pole piece is concentrically disposed within the opening 5031a, whereby an annular air gap is formed between the adjacent side wallsof the opening and the center pole piece. The magnetic circuit assembly is completed by means of three clamping screws 5041 which extend through openings provided in the bottom plate 58! and are threaded into tapped holes drilled in the top plate 503. These screws serve rigidly to clamp the annular permanent magnet 500 between the top and bottom plates 583 and 5M.

The moving system of the translating device comprises a cone-shaped diaphragm 505 which is formed of pressed fiber and is carried by a supporting structure which comprises a bottom ring 506, six spacing collars 501, a pair of clamping rings 508 and 509, two paper washers 5m and 5i i, and six assembly screws 5l2. Thecharacter of the diaphragm depends upon whether the device is to be used as a transmitting or receiving element. If the device is to be used as a transmitter, the diaphragm is of one-piece construction and is provided with an outer fiat annular ring portion 565a which extends between and is cemented. to the paper washers 5| and H and is uniformly clamped around its periphery between these washers by the two clamping rings 5% and 509. If the device is to be used as a receiving element, the diaphragm construction illustrated in Fig. 8 of the drawings is used. In this construction only the bulbous portion of the diaphragm 505 is constructed of molded or. pressed fiber, the periphery of this element being secured to an annular ring 5051; formed of treated silk which is disposed'between and cemented to the paper washers 5H] and 5! l. The purpose of this arrangement is to permit substantially free vibration of the diaphragm element 505, particularly at the low frequencies of the operating frequency range, whereby the response of the device at these frequencies is enhanced. With the onepiece diaphragm arrangement, on the other hand, the diaphragm is considerably stiffer and, ac-

cordingly, the response thereof, particularly atlow frequencies, is substantially less than when the Fig. 8 arrangement is used. By virtue of this increased stillness, low frequency noise components' of sound are prevented from producing any substantial response of the device.

A moving coil 5| 3 is cemented to the lower bulbous portion of the diaphragm 505 and is concentrically disposed within the air gap defined by the upper portion 50212 of the pole'piece 592 and the opening 503a in the top plate 503. The terminal ends of this coil may be electrically connected to lead-in wires in any-desired manner although, preferably, the connections are made atv anchor posts'which extend through and are secured to the lower bottom portion of the diaphragm 585. In order to preserve the correct lateral and axial spacing of the moving c'oil M3 in the. air gap of the magnetic circuit,-and to provide additional support for the diaphragm. 505, a string supporting arrangement is provided.

This arrangement comprises three connected strings Etta, 5 Nb and 5He which extend through equi-angularly spaced openings cut through the.

clamping the same to the lugs 5l5a, 5 l5b and 5i5c which extend radially inward and are turned upward from the supporting ring 506. Small clamping plates 5|6a, 5i6b and 5l6c disposed beneath shortened spacing collars 501 are used to clamp the free ends of the strings 5l4a, Sllb and'5l4c against the lugs 5l5a, 5i5b and 5|5c. Preferably the three strings 5l4a, 5ND and 5l4c are formed of silk fishline or the like, and it will 'be understood that by appropriately tensionlng the three strings the required support for the moving coil 5 i 3 may be obtained.

In order to seal the annular cavity, formed between the spaced apart annular permanent magnet 500 and the pole piece 592, from the acoustical cavity 5H adjacent the lower side of the diaphragm 505, thereby to enhance the damping of the diaphragm and thus produce a more uniform response thereof over the entire operating frequency range, a sealing ring 5 I 8 is provided which snugly encloses the upper end portion 502?: of the center pole piece 502, and is clamped to the underside of the top plate 583 by means of as-' sembly screws 5 i 9. This ring is preferably formed of rubber, molded Bakelite or other insulating material having non-magnetic properties.

In the assembly of the translating device the magnetic circuit structure is built by first securing'the sealing ring bit to the underside of the top plate 593 and then setting the assembly screws 505 to clamp the annular permanent magnet 50. between the top and bottom plates 503 and 5M. After the magnetic circuit structure is thus assembled the portion of the sealing ring 5! facing outward through the air gap of the structure may be painted with shellac or other sealing compound, to provide an acoustical seal which blocks oil'the cavity oi'the magnetic circuit structure from the cavitylii'l adjacent the underside of the diaphragm 505. The diaphragm assembly is prepared by cementing the two paper washers III and 5| l to the top and bottom surfaces of the flat annular portion 505a of the diaphragm 505, assuming that a transmitting element is being constructed, and by cementing the moving coil III to the lower central portion of the diaphragm member. Ifthe device is to be used as a loudspeaking receiver element, the'diaphragm structure illustrated in Fig. 8 of the drawings is constructed by first cementing the paper washers Ill] and III to the opposite peripheral surfaces of the silk ring 505b, after which the inner upper edge surfaces of the silk ring are cemented to the underside of the diaphragm 505. Following the construction of the diaphragm structure, the three strings Ella, 5-l4b and 5H0 are drawn through the angularly spaced openings provided in the base of the diaphragm 505. After this operation is performed, the supporting ring 506, the clampins rings 508 and 509 and the assembled diaphragm structure may be stacked on the top plate 503, and the assembly screws 5|! may successively be inserted through the registering openings provided in the stacked clamping rings and diaphragm structura'the spacing collars "L gand the supporting ring 506. Initially the screws which extend through the clamping elements III are not tightened, but the alternate assembly screws which extend only through alternate spacing collars 501 are tightened sumciently to provide a semi-rigid assembly. Following this opera.- tion the ends or the strings Ella, Bill: and lllc may be threaded between the clamping elements 5i6a, Mao and 5150 and their associated lugs 855a, blob and biiia and properly tensioned to provide the required lateral and axial support for device is completed.

As pointed out above, each transmitting and receiving element, as constructed. in the manner just described, is preferably embedded in a block of acoustical insulating material to prevent the transmission of sound waves and mechanical vibrations to themoving system of the element. In the arrangement illustrated in Fig. of the drawings, the translating device is disposed in an opening formed in a block of acoustical insulating material 520. This block of insulating material, which may be of the desired configuration, and morespecificallymay be of any configuration illustrated in Figs. 4a, 4b, 4c, 4d and 4e of the drawings, is preferably preformed to the desired shape in accordance with the method de scribed and claimed in co-pending application Serial No. 467,695, filed Dec. 3, 1942, Giannini. In brief, the method there disclosed comprises mixing a loose insulating material of the rock wool type with a cement binder, and molding the mixture into a solid block of the desired shape before drying of the binder starts. The molded block obtained after the binder is fully set, is somewhat porous and will still transmit air through some of its surfaces.

Accordingly the outer surfaces of the molded block are painted with one or more coats of lacquer to minimize sound transmission therethrough. It has been found that the number of coats or the thickness of the layer'oflacquer on the outside surfaces of r the molded block determine, to some extent, the acoustical permeability of the structure. Hence, by appropriately controlling the depth of the lacquer layer, the acoustical characteristics of the ings. More specifically, the Fig. 9 arrangement comprises a block'of acoustical insulating material 52! which is provided with a number of sound transmitting passages 522 molded or cut therethrough along the axis of sound-transmis- "sion to and from the diaphragm of the associated transmitting or receiving element. These passages are ofthe correct dimensions, 1. e., diam-j eters and lengths, to provide'for maximum sound energy transfer between the atmosphere and the cavity immediately adjacent the outer surface of the diaphragm. The block of acoustical insulating material 52l maybe secured to the block of insulating material 62!! to clamp the'aluminum sheet 524 and the silk screen 523 'to the top surface of the block 520 by means of assembly screws passed through registering openings in the two insulating blocks.

From the foregoing explanation it will be understood that a transmitting and a receiving element, each constructed in the improved manner illustrated in Figs. 5 to 8, inclusive, of the drawings, may be mounted in one and the same block of acoustical insulating material. This block may be preformed to provide the desired mean spacing between the sound transmitting and sound receiving openings adjacent the diaphragms of the loudspeaker and microphone elements, respectively. It will also be understood that in the unit as thus formed the conductive coupling between the two elements is reduced to a minimum by virtue of the vibration absorbing characteristics of the acoustical block. Moreover, the small effective diameter of the loudspeaker element, as shown in Fig. 5 ofjthedrawings, contributes materially to the acoustical attenuation between the transmitting and receiving elements. This attenuation may also be enhanced by utilizing pedance of air, over which is placed a thin sheet of aluminum 524. having a number of holes of appropriate size cut therethrough for the purpose of transmitting sound to or from the diaphragm of the enclosed element. A clamping ring and assembly screws, which are passed through registering openings in the clamping ring, the aluminum plate .524 and the molded block of insulating material, may be used to 'hold the aluminum plate 524 and the silk screen 523, in assembled relationship on the block of insulating material.

' If directional characteristics are to be imparted to the device, an additional block of molded acoustical insulating material, constructed in accordance with the method briefly outlined above, and having sound transmitting passages of apprcpriate lengths and dimensions preformed therein, may be provided. A simple embodiment of an acoustical insulating block provided with such passages is illustrated in Fig. 9 of the drawings as being adapted for use in conjunction with the unit illustrated in Fig. 5 of the draw-v .the sound directing arrangements illustrated in Figs. 4b, 4c, 4d and 4e of the drawings in the amplifiers at each of the two connected stations I0 and H, as well as the channel amplifiers in the central repeater l2, may be operated with normal gain settings which are substantially greater than the permissible gain settings when conventional microphone and loudspeaking ar.- rangements are used.

In considering the operation of the system, it

may be assumed that, the signal amplifiers and the control network, as provided at each of the two stations in and H, are active, and that the central repeater I2 is conditioned for operation."

In this regard it will be noted that to condition the circuit equipment provided atthewest stae tion H for operation, the switch 838 ls'manually operated to its closed-circuit position wherein alternating current is delivered to the parallelconnected windings 355 and 331 of the power transformers 35 4 and 336. With these transformers energized, low voltage'alternating current is deliveredto the cathodes of each" of the tubes342, 322, 326, 830 and 308 over obvious circuits. With the rectifier tube 342 in operation, current alternately traverses the two anodes of the tube 342 during alternate half-cycles of ode acsc.

the voltage developed across the high voltage secondary winding 338, and is passed through each of the two resistors 3 53 and 3 39 in the same direction. The alternating components of the voltage thus developed across the resistor 349 are minimized through operation of the filter network comprising the choke coil 346 and the shunt-connected condensers t ll and 368. The

direct voltage developed across the section 343a of the resistor 343 is negatively applied to the control electrode Nth of the transmitting amplifier tube '306 to determine the operating point on the characteristic of this tube. Similarly, the voltage drop across the resistor section 3532) is negatively applied to the control electrode 3301) of the tube 330 to determine the normal space current flow through this tube.

Initially, the voltage dividing resistor 385 is so adjusted that the maximum signal output from the amplifier 3M consistent with stability of the station circuit is obtained when heating current of rated'value traverses the cathode 3850 of the tube 39%. Similarly, the voltage dividing resistor dill is initially adjusted so that the maximum signal output from the amplifier till consistent with stability of the station circuit is obtained when the amplifier 3M is operating with normal signal current gain therethroughl In this regard it will be apparent that the stability of the system is determined by the combined signal current gains through the two amplifiers 30B and 8M and, hence, the gain setting of each amplifier is dependent upon the setting of the other amplifier. Assuming that the on-ofi" switch 386 occupies the position illustrated in the drawings, the signal current output from the receiving amplifier $68 is in part determined by the setting of the adjustable voltage dividing resistor 39! and in part by'the resistance of' the shunt connected space current path of the control tube 330. The resistance of the space current path of the tube 33!} depends, in turn, upon the negative bias voltage impressed across the input electrodes 83012 and 8300 thereof. Normally this voltage is equal to the sum of the voltage drop across the'resistor section 3431) and the voltage drop across the cathode biasing resistor 352. The total magnitude of this bias voltage is normally of the order of 23 volts.- With the two tubes 308 and 330 thus conditioned for operation, the current traversing the parallel-connected space current paths there- 308. receiving terminals of the hybrid system 203 is transmitted through the coupling transformer assume the setting of the adjustable cathode biasin resistors 235a and 235b. Each of these resistors is initially adjusted so that the space current flow through the associated tube develops 9. voltage of approximately 12 volts across the associated resistor 236a. or 23617; The voltageacross the resistor 236a is negatively applied to the control electrode 20'") of the west-east amplifier tube 201. Similarly the voltage developed across the resistor 23Gb is negatively applied to the control electrode 2 l 2b of the east-west amplifier tube 2 I 2.

Assuming that the microphone-loudspeaker unit illustrated in Fig. 4a of the drawings is utilized in conjunction with the circuit equipment illustrated in Fig. 3 of the drawings and that a user of the station equipment speaks into the microphone 40 l the signal voltage developed in the moving coil of this microphone is transmitted through the coupling transformer 304 and the portion of this voltage appearing across the lower portion of the voltage dividing resistor 365 is impressed upon the input electrodes 30% and 3960 of the transmitting amplifier tube 308. The signal currents as amplified by the tube 306 are transmitted through the coupled windings of the hybrid system 302 and over the conductors of the line H to the windings of the hybrid system 203. By virtue of the arrangement of the coupled windings of the hybrid system 302 and the action of the balancing network.303, only a small portion of the signal current: energy is transmitted through the hybrid system to the input electrodes of the receiving amplifier tube The signal voltage appearing across the 206. and is impressed across the input electrodes 20'": and 2010 of the west-east amplifier tube 201 in parallel with the primary winding of the coupling transformer 22L The signal currents as amplified by the tube 201 are transmitted through the coupling transformer 203 and the coupled windings of the hybrid system 202 to the line l3. From this point the signal currents are transmitted over theline. l3 and through the hybrid system I02 to the input circuit of the signal current amplifier I05. These currents as amplified by the amplifier I05 are transmitted to the moving coil of the loudspeaker [0| for reproduction. The signal voltage as impressed between the input electrodes 22312 and 2230 of the amplifier of divides between the resistor 852 and the cathode 3060 of the transmitting amplifier tube 300, The value of the resistor 352 may be'so chosen that rated current normally traverses the oathtube 223 through the coupling transformer) and the voltage divider 222, is amplified through With the gain settings of the two amplifiers $00 and $0! determined inthe above-described manner, the circuit constants of the station cir cuit are so chosen that the signal current gains- .through the two amplifiers 309 and 3M are suboperation, but inactive, no signal currents are transmitted through either of the two amplifier tubes 223 and 22? to either of the two rectifying sections of the duplex diode tube 23L and hence no bias voltage is developed across the resistor 286. Accordingly the space current flow through the tubes 232 and 238 and their respective assoelated resistors 23% and 28th is determined by by way of the risistor 230, the anode am, the

1 space current path between this anodeland the cathode 23lc and the resistor 234 to the lower terminal of the indicated transformer winding.

. This pulsating current is smoothed through the action of the condenser 233 which shunts the resistor 234. The voltage thus developed across the resistor section 234a is negatively applied to trade 20'": of the active west-east signal amplifier transmitted through the coupling transformer 2|! and thehybrid system 203 to the line I4.

' From this point they are transmitted over the 1y decreased. Thus it wmihe noted that the voltage drop across the resistor section 2341) is positively applied to the control electrode of thetube 233, whereby the space current flow through this tubeis increased. As a result the voltage drop across the biasing resistor 23Gb is increased.

Since this voltage is'negatively applied to the control'electrode 2I2b oi the tube 2l2, it will 'be understood that a corresponding decrease in'the signal current gain through the amplifier tube 2l2 occurs.

From the above explanation it will be understood that the increase in signal current gain through the active west-east transmission channel 200 is accompanied by a1 corresponding decrease in the gain through the inactive east-west transmission channel 20! of the central repeaterv l 2.- Thus the stability of the repeater network is preserved. In this regard it will be noted that the network includes a substantially closed signal current circuit which comprises the two channels 200 and 20! and the coupling paths through the two hybrid systems 202 and 203, and that if the two amplifier tubes 201 and M2 are both operated at the high gain settings required during signal current transmission, the total gain around the closed circuit may substantially exceed the electrical losses of the circuit. When this condition prevails the circuit network is. highly unstable and an oscillatory condition maybe set up therein even when signal currents of relatively small amplitudes are transmitted therethrough. Due to the action of the control network.220, however, the total gain around the closed circuit is maintained below the predetermined value at'wh'ich.

the circuit is stable by concurrently increasing the gain of the channel in use and decreasing the gain of the inactive channel. Accordingly, the stability of the repeater is preserved even though signal currents of substantial magnitude are transmitted through the repeater. Each time signal current transmission over the line I4 is terminated to arrest the signal input to the control network 220, the bias voltages across the two resistor sections 234a and 23% are reduced to zero, whereby the normal bias voltages across the two biasing resistors 230a and 23612 are restored. The signal current gain through the amplifier tube 201 is thus decreased to normal and the signal current gain through the amplifier tube 2| 2 is increased to itsnormal value. The rate at which the bias voltages across the resistors 236a and 23Gb are restored to normal is primarily determined by thetime constant of the network comprising the-resistor 234 and the condenser 236. The resistance and capacitance values of this network are proportioned to provide the necessary hangover period at each signal current break-off point, in order to prevent syllable clipping which would otherwise occur.

When sound waves are transmitted to the microphone I00 provided at the east station l0, corresponding signal currents are developed in the output circuit of the signal current amplifier I04 and are transmitted through the hybrid system I02, over the line l3 and through. the hybrid system 202 to the parallel-connected input circuits'of the channel amplifier tube 2l2 and the control amplifier tube 221. These signal currents as amplified by the channel amplifier 20! are line H and through the hybrid system 302 to the input circuit of the receiving amplifier 30! provided at the West station ll. After being amplified by the receiving amplifier tube 308, the

signal currents are transmitted through the.

' coupling transformer 309 and over the cord conductors 43! to the moving coil of the loudspeaker 402 for reproduction.

The signal voltage as impressed between the input electrodes of the amplifier tube 221 through the coupling transformer 225 and the voltage divider226 is amplified'by this tube and impressed through the coupling transformer 220 I across the anode 23ld and the cathode 23Ie of the duplex diode 23L More specifically, the'voltage appearing across the secondary winding of the transformer 228 causes a pulsating direct current to traverse the. resistor-234 in a direction which may be traced as extending from the upper terminal of this winding by way of the resistor 229, the anode 23ld, the space current path between this anode and the cathode 231e, and the resistor 234 to the lower terminal of the indicated t'ransformer'winding. This pulsating current is smoothed through the action of-the condenser 236, so that a substantially steady direct voltage is developed across the resistor 234. The portion of this voltage appearing across the resistor section 234a is positively applied to the control electrode of the tube 232 to increase the space current flow through this tube and thus increase the voltage drop across the biasing resistor 236a. When this voltage drop is increased the negative bias on the control electrode 20111 of the inactive west-east signal amplifier tube 201 a is increased to decrease the amplification factor of this tube. At the same time that the gain throughthe inactive west-east channel 200 of the repeater I2 is thus decreased, the gain of the active east-west channel 20l is correspondingly increased. Thus it will be noted that the voltage drop across the resistor section 2341) is negatively applied to the control electrode'of the tube 233 so that the space current flow through this tube is decreased. As a result, the voltage drop across the biasing resistor 23Gb is decreased. Since this voltage is negatively applied to the control electrode 2l2d of the tube 2 l2, it will be understood that a decrease in the magnitude of this voltage produces a corresponding increase in the signal current gain through the amplifier tube 2l2.

Thus it will be seen that when signal currents are transmitted in the east-west direction from the station I 0 to the station I I, the signal current gain through the active channel 20l is increased the required amount for satisfactory reproduc- 'tion of the signal currents by the loudspeaker 402 in use, and this signal current gain is accompanied by a correponding decrease in the gain of the inactive west-east channel 200, whereby the stability of the central repea er I2 is preserved.

It will also be understood from the above explanation that each time signal current transmission over the line l3 from the east station I0 is terminated to arrest the signal input to the con-' trol network 220, the bias voltages across the two resistor sections 23% and 2341) are reduced to zero after a short time interval which is determined by the time constant of the shunt circuit including the'resistor 234 and the condenser 230.

- Accordingly, the bias voltages across the two biasing resistors 236a and-236b are again equalized at their respective normal values, whereby the signal current gain through the amplifier tube;

201 is increased to normal and the signal current gain through the amplifier tube 2i2 is decreased to its normal value. I

Referring now more particularly to the method of controlling the network 3i5 provided 'at the west station Ii, it will be notedthat a portion of the signal voltage appearing across the output circuit of the receiving amplifier tube 308 is imthat the ratio of the signal voltages across the two resistors 3iil and 32i is determined by the reactance of the condenser 320. At relatively low signal voltage frequencies the reactance of this condenser is relatively high so that a relatively large proportion of the available voltage drop across the two series-connected resistors 3i8 and 32i appears across the resistor 32i. On the other hand, as the signal current frequency increases, a corresponding decrease occurs in the reactance of the condenser 320, so that an increasing percentage of the available signal voltage appears across the resistor 3i8. Thus by properly proportioning the constants of the network 3i8, this network may be operated to favor signal currents of the frequencies within the band indicated. The reason for providing a discriminating network 3i6 which favors signal current frequencies within the low end of the operating range, is to .make the control network 3i5 primarily responsive to the band of frequencies at which the major portion of the voice current energy is produced. Thus the predominant portion of the frequencies used in ordinary speech lies within the band ranging from 300 to approximately 1200 cycles per second. The signal voltage as developed across the resistor 32l is amplified by the,tube 322, and the amplified signal currents cause corresponding induced voltages in the secondary winding of the. coupling transformer 32, which voltages are applied across the anode and cathode of the rectifying tube 326. This tube is biased by means of the battery 32! to a point on its operating characteristic such that no current will traverse the space current path thereof until the signal voltage appearing across the secondary winding of the transformer 320 exceeds a predetermined value. Thus, so long as the signal voltage across the output circuit of the receiving amplifier tube 308 does not exceed a predetermined value, the biasing battery 328 prevents the control network 3i! from changing the gain of either of the two amplifiers 30i or 302. When, however, the sig-- nal voltage appearing across the output circuit across the resistor 328 is additive with respect to the bias voltage across the section 33% of the resistor 343. Accordingly the net bias yoltag across the inputelectrodes 330D and 338c of the control tube 330 is increased to increase the resistance of the space current path of this tube. Incident to this increase in space current resistance of the tube 330, the shunting effect of the tube on the output circuit of thereceiving amplifier tube 308 is correspondingly decreased to produce an increase in the signal current gain through the amplifier 30i. This decrease in the shunting effect of the tube 330 is supplemented by an increase in the amplification factor of thespondingly decreased, so that a larger portion of the available voltage across the resistor 349 is applied to the anode 308a of the tube 308. The

- increase in gain through the amplifier 30l is accompanied by a corresponding or even greater decrease in the gain of the transmitting amplifier 300. Thus when the resistance of the space current path through the tube 330 is increased, the direct current traversing thi path and the shunt-connected resistor 352 and cathode 306c is correspondingly reduced. When thecurrent traversing the cathode 3080 is reduced, the electron emission of this tube is decreased to lower the amplification factor of this tube. From the above explanation it will be understood that the signal current amplitude at which the control circuit 3i5 starts inversely to change the gains of the two amplifiers 300 and 3!, may be adjusted, through suitable adjustment of the voltage divider 3l8, to vary the proportion of the available signal voltage which is impressed between the input electrodes of the tube 322. This adjustment is determined to a large extent by the normal gain settings of the two amplifiers 300 and 301. Hence, after the required signal current gain through the two tubes is once established, the voltage divider 3i8 may be adjusted to provide the minimum signal voltage input to the amplifier tube 322 consistent with the required control of the station circuit.

It will also be understood from the above explanation that after the control circuit 3i5 starts to operate, the extent to which the signal current gains of the two amplifiers 300 and 30i are inversely changed depends, within limits, upon the magnitude of the signal currents incoming to the west station Ii over the line It. Preferably, the circuit constants of the network 3i5 are so adjusted that the amount of the decrease gain of the signal amplifier 300 which occurs in response to any given change in the input signal intensity is just sufilcient to maintain the stability of the station circuit. As indicated by the above explanation, the gain. control action realized through operation of the network 3i! is not effective until the signal input to the loudspeaker 402 exceeds a predetermined value. By virtue ofthis arrangement, the control network "3i! is rendered substantially non-responsive to noise currents resulting from'background noises which may be acoustically imposed on the systemor to noise currents resulting from electrical,

transients imposed on the system, which currents are usually of a low order .of magnitude. Since such noise currents are usually of low frecuency, the coupling condenser 3" may be chosen of such value as to prevent any substan- .tial portion of the resulting voltage across the accuse Thus, the response of the network 3l5 is limited to speech or other actual signal current transmission over the two connected lines of the sys-: tem in an east-west direction. Further, the constants of the station circuit are so chosen that thevariation in gain of the amplifier 300,. cffected through operation of the control network 3l5, are limited to a definite range which will include all cases of normal signal current intensities. I

In the event it is desired to render the control .network 3|5 inactive, the on-off switch 36! is operated to its 011 position wherein theparallel connection between the space current paths of the two tubes 308 and 3301s broken and anode potential is supplied to the control tube 330 directly from the high voltage terminal of the load and an. has been illustrated in the drawings, it

will be understood that a second control network of like arrangement and responsive :to*signal currents transmitted from the microphone l to the amplifier 300,-may be provided ifnecessary. In such case, the input and output cir-. cults of the second control network will be bridged across the output circuit of the amplifier tube 306 and the space current traversing the control tube of the second network will determine the cathode heating current of the receiving amplifier tube. With such an arrangement the second control circuit will function to increase the gain I or the transmittingamplifier tube 306 and to resistor 303 included in the power pack 335. With the switch 36l in its off position, it will be apparcut that amplified signalvoltages appearing across the output circuit of the tube 306 are not' While it will be understood that the specifications or the circuit provided at the station H may vary according to the designof a particular installation, the following specification of circuit constants for the station circuit of Fig. 3 is included by way of example as being satisfactory.

Tube 306 Commercialtype 4'7 Tube 306---: Commercial type 1A5G Tube 322 Commercial type 26 Tube 326 Commercial type 26 Tube 330... Commercial type 10 Tube 302 Commercial type Condenser 363 0.25 microfarad Condenser 365 50.0 microfarads Condenser 3H 0.001 microfarad Condenser 320 0.002 microfarad Condenser 323. 0.05 microfarad Condenser321 8.0 microfarads Condenser 331 0.25 microfarad Condenser 323-; 0.01 mlcrofarad Condenser 305 8.0 microtarads Condenser 300-1 8.0 microfarads Condenser 308 16.0 microi'arads Condenser 301 8.0 microfarads Condenser 353 5.0 microiarads Condenser 360 -4 5.0 microfarads Resistor 305 l 250,000 ohms Resistor 360 i 500,000 'ohms Resistor 366 500,000 ohms Resistor-30L 250,000 ohms Resistor 362 1250 ohms Resistor 3l8 250,000 ohms Resistor 3J9 100,000 ohms- Resistor 32l 500,000 ohms Resistor 326 500,000 ohms Resistor 303 250 ohms Resistor 303 25,000 ohms Resistor 350 15,000 ohms Resistor 351 420 ohms Resistor 352 ohms Resisto 350 1000 ohms Battery 325 4.5 volts Although only one control network3l5, which responds to signal currents incoming to the station H to control the gains of the amplifiers 300 decrease the gain of the receiving amplifier tube when sound waves exceeding a predetermined value are transmitted to the microphone 00I.

The control circuit shown in Fig. 2 of the drawings of the present application is claimed in the copending application of Giannini and Eisenberg.

Serial No. 472,722, filed January 18, 1943, and the control circuit shown in Fig. 3 of the drawings of the present application is claimed in the copending application of Giannini and Eisenberg. Serial No. 419,164, filed November 14, 1941.

. While one embodiment of the invention has k been described, it will be understood that various modifications may be made therein without departing from the true spirit and scope of the invention.

What is claimed is: 1. In a loudspeaking intercommunicating system comprising a station network having acoustically coupled transmitter and receiver elements 5 which are electrically coupled to a two-way transmission line over transmitting and receiving channels respectively, and wherein a signal amplifier is'provided which'tends to impart instability to said network, the method of operation which comprises directionally transmitting sound waves produced by said receiver element away from the response zone of ,said transmitter along a substantially vertical path; altering the gain of said amplifier in the correct sense and only by the required amount to maintain the stability of said .network'when signal currents exceeding a predetermined value traverse said receiving channel, and maintaining the gain of said amplifier at the maximum level consistent with stability of said network when signal currents not exceed-* i118 said predetermined value traverse said receiving channel. Y r 2. In a loudspeaking intercommunicating system. a network comprising a transmitting element disposed to respond to sound waves trans- I mitted theretoalong a substantially horizontal plane, a receiving element disposed to transmit sound waves produced thereby in a vertical direction, variable gain transmitting and receiving channels respectively coupled to said transmitting and receiving elements, means for automatically decreasingthe gain of. one of said channels only when the signal input to the other of said channels exceeds a predetermined value, and means for limiting the. decrease inthe gain of said one channel to an amount just suificient to maintain the stabllityof said network.

3. In a loudspeaking intercommunicating system, a network comprising a receiving element for directionally transmitting sound waves produced thereby along two spaced apart and diverging paths, a transmitting element disposed substantially equidistant between said paths, whereby sound energy transfer from said receiving element to said transmitting element is mini- 12 mized, variable gain transmitting and receiving 3 channels respectively coupled to said transmitting and receiving elements, means for automatically I decreasing the gain of one of said channels only when the signal input to the other of said channels exceeds a predetermined value, and means for-limiting the decrease in the gain of said amplifier to an amount just suflicient to maintain the stability of said network. v I

4. In a loudspeaking intercommunicating system, a network comprising transmitter and re- ,ceiver elements having a combined frequency re- .sponse characteristic which peaks at a predetermined frequency within the voicefrequency range and having their sound receiving and discharge.

openings spaced apart a distance equal to substantially an odd number of times one quarter of the wave length which corresponds to said fre quency, variable gain transmitting and receiving channels respectively coupled to said transmitting and receiving elements, means for automatically decreasing the gain of one of said channels only when the signal input to the other of said channels exceeds a predetermined value, and means for limiting the decrease in the gain of said one said lower portion of the voice frequency range for automatically decreasing the gain of one of said channels only when the signal input to the other of said channels exceeds a predetermined value.

6. In a loudspeaking intercommunicating system, a network comprising transmitter and receiver elements having a combined frequency response characteristic which peaks at a predetermined frequency within the lower portion of the voice frequency range, variable gain transmitting and receiving channels. respectively-coupled to said transmitting and receiving elements, means comprising a frequency discriminating circuit which favors the frequencies within said ,lower portion of the voice frequency range for automatically decreasing the gain of one of said channels only when thesignal input to the other of said channels exceeds a predetermined value,

.and means for limiting the decrease in the gain of said one channel to an amount just sufficient to maintain the stability of said network.

7. In a loudspeaking intercommunicating system, a network comprising transmitter and receiver elements having a combined frequency response characteristic which peaks at a predetermined frequency within the lower portion of the voice frequency range and having their sound receiving and discharge openings spaced apart a distance substantially equal to an odd number of times one-quarter of the wave length which corresponds to said frequency, variable gain transmitting and receiving channels respectively coupled to said transmitting and receiving elements, and means comprising a frequency discriminating circuit which favors the frequencies within said lower portion of the voice frequency range for automatically decreasing the gain of one of said channels only when the signal input to the other of said channels exceeds a predetermined value.

8. In a loudspeaking intercommunicating sysdetermined frequency within the lower'portion of the voice frequency range and having their Y sound receiving and discharge openings spaced apart a distance substantially equal to an odd number of times one-quarter of the wave length which corresponds to said frequency, variable gain transmitting and receiving channels .respectively coupled to said transmitting and receiving elements, means comprising a frequency discriminating circuit which favors the frequencies within said lower portion of the voice frequency range for automatically decreasing the gain of one of said channels only when the signal input to the other of said channels exceeds a predetermined value, and means for limiting the decrease in the gain of said one channel to an amount just sufficient to maintain tern, a network comprising a transmitter elev ment, a loudspe ment, a loudspeaking receiver element including a vibrating diaphragm having an effective diameter less than one-half the wave length which corresponds to the highest frequency of the operating frequency range of the system, variable gain transmitting and receiving channels respectively coupled to said transmitting and receiving elements, and means for automatically decreasing the gain of one of said channels only when the signal inputto the other of said channels exceeds a predetermined value.

10. Ina loudspeaking intercommunicating system, a network comprising a transmitter element, a loudspeaking receiver element including a vibratory diaphragm having an effective diameter less than one-half the wavelength which corresponds to the highest frequency of the operating frequency range of the system, said elements having a combined'frequency response characteristic which peaks at a predetermined frequency within said'range and being disposed so that their sound receiving and discharge openings are spaced apart by a-distance which is substantially equal to an odd multiple of onequarter of the wave length which corresponds to said predetermined frequency, variable gain transmitting and-receiving channels respectively. coupled to said transmitting and receiving elements, and means for automatically decreasing the gain of one of said channels only when the signal input to the other of said channels exceeds a predetermined value.

11. In a loudspeaking intercommunicating system, a network comprising a transmitter eleaking receiver element including a vibrating diaphragm having an effective diameter less than one-half the wave length which corresponds to the highest frequency of the operating frequency range of the system, said elementshaving a combined frequency response characteristic which peaks at a predetermined frequency within the lower portion of said range, variable gain transmitting and receiving channels respectively coupled to said transmitting and receiving elements, and means comprising a frequency discriminating circuit which favors the frequencies within said lower porton of said range for automatically decreasing the gain of one of said channels only when the signal input to the other of said channels exceeds a predetermined value.

12. In a loudspeaking intercommunicating system, transmitter and receiver elements havin a combined frequency response characteristic said elements having their sound receiving and discharge openingsspaced apart by a distance substantially equal toan odd multiple of onequarter of the wave length which corresponds to said predetermined frequency.'varlable gain communication channels respectively coupled'to said elements, and means comprising a frequency discriminating network which favors si nal frequencies within said lower portion of said operating frequency range for controlling 'the'gain of at least one of said channels to prevent said system from oscillating due to undesired coupling between said channels.

14. In a loudspeaking intercommunicating system, a transmitter element, a loudspeakin'g receiving element which includes a diaphragm having an effective diameter less than one-half of the wave length corresponding to the highest frequency of the operating frequency range of the system, said elements having a combined frequency response characteristic which peaks at a predetermined frequency within said range and having their sound receiving and discharge openings spaced apart by a distance substantially equal to an odd multiple of one-quarter .of the wave length which. corresponds to said predetermined frequency, and transmitting and receiving channels respectively coupled to said transmitting and receiving elements.

15. In a loudspeaking intercommunicating systen, a transmitter element, a loudspeaking receiving' element which includes a diaphragm having an effective diameter less than one-half of the wave length corresponding to the highest frequency of the operating frequency range of the system, said elements having a combinedfrequency response characteristic which peaks at a predetermined frequency within-the lower.

portion of said range and having their sound receiving and discharge openings spaced apart by a distance substantially equal to an odd multiple of one-quarter of the wave length which corresponds to said predetermined frequency, variable gain communication channels respectively coupled to said elements, and means comprising a. frequency discriminating network which favors signal frequencies within said lower portion of said operating frequency range for controlling the gain of at least one of sale. channels to prevent said system from oscillatin due to undesired coupling between said channels.

16. In a loudspeaking intercommunicating system, a transmitter element and a lcudspeaking' receiver element which includes a diaphragm having an elective diameter less than one-half the wave length corresponding to the highest frequency of the operating frequency rangeof the system, and transmitting and receiving channels respectively coupled to said transmitting and receiving elements.

1'1. In a loudspeaking inter-communicating sys.

tem, a transmitter element and a loudspeakin. receiver element which includes a diaphragm having an effective diameter less than. one-half the wave length corresponding to the highest frequency of the operating frequency range of the system, said elements having a combined frequency response characteristic which peaks at a predetermined frequency within the lower portion of said operating frequency range, yariabla gain communication channels respectively coupled to said elements, and means comprising a frequency discriminating network which favors signal frequencies within said lower portion of said operating frequency range for controlling the gain of at least one of said channels to preventsaid system from oscillating due to undesired coupling between said channels.

GABRIEL M. GIANNIN'L

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