US2365187A - Transmission system - Google Patents

Description

IOO

Dec. 19, 1944.

G. M. GIANNINI ET AL 2,365,187

TRANSMISSION SYSTEM Filed Nov. 14, 1941 5 Sheets-Sheet 1 FIG. I

SIGNAL CURRENT AMPLIFIER CONTROL BALI HYBRID NETWORK I NETWK SYSTEM SIGNAL CURRENT AMPLIFIER EAST STATION I O GABRIEL M IZITIKII II BY BEN EISENBERG MAMI W m ATTORHEL 1944 e. M. GlANNlNl ET AL TRANSMISSION SYSTEM 5 Sheets-Sheet 2 Filed Nov. 14, 1941 CENTRAL REPE'ATER I2 II "II- moakmz 4/ A INVE M. 6!

NTORS GABRIEL ANNINI BY BEN EISENBERG Mxw, M xim ATTORNEYS G. M. GIANNINI ET AL 2,365,187

TRANSMISSION SYSTEM 5 sheets Sheet 5 Filed NOV. 14 1941 WEST STATION H uov mm N mN% w V B mM 6 EE M A M GB Yf BM 3 Cu F ATTORNEYS 19,1944. G. M. GIANNINI Em 2 365,187

TRANSMI SS IC JN SYSTEM Filed Nov. 14, 1941 5 Sheets-Sheet 4 INVENTORS GABRIEL M. GIANNINI BY BEN EISENBERG ATTORNEYS 19 e. M. GlANNlNi ETAL 2,365,137

TRANSMISSION SYSTEM Filed Nov. 14, 1941 s Sheets-Sheet 5 lun 5 .2.

' INVENTORS GABRIEL M. GIANNINI BY BENEISENBERG ATTORNEYS Patented Dec. 19, 1944 m orncE -TRANSMISSION SYSTEMY Gabriel M.'Giannini, North Hollywood, Calif., and

Ben Eisenberg, Brooklyn, N.'Y., assignors, by mesne assignments, to Automatic Electric Laboratories, Inc a corporation of Delaware Application November 14, 1941, Serial No. 419,164 14 clai s. (01. 1715170) I The present invention relates to signal current transmission systems and, more particularly,- to improvements in voice operated control circuits of the character utilized in conjunction with two-- channel transmission systems to prevent the combined signal current gains of the two coupled channels from exceeding the combined losses through the channels and through the couplings between the channels.

It is an object of the present invention to pro; ,vide a signal operated control circuit of the characterdescribed which is simple in arrangement and is positive and reliable in'its operation to increase the signal current gain through the channel in use and concurrently to decrease the gain through the inactive channel.

According to another object of the invention, an improved arrangement is provided for preventing the control circuit from altering the gain of either of the two channels until the signal input to the channel in use exc'eedsa predeter- -mined value.

It is another and more specific object of the invention to provide an improved signal operated control circuit of the character described where in the space current path of a control tube, which is controlled in accordance with the signal input to one of the channels, is connected in circuit with the cathode of one of the amplifier tubes in the other channel, so that as the space current re-- sistan'ce of the control tube is varied under the influence of signal currents traversing the one channel, the heating current of the amplifier tube cathode is changed to decrease the signal current 'gain through the other channel.

According to another object of theinvention,

.the space current path of the control tube is also connected to controlthe shunt impedance ,includes the features of the invention briefly referred to above; Fig. 4a illustrates one improved loudspeaker-microphone arrangement which may be usedat each of the stations of the system;

Figs. 4b, 4c and 4d illustrate amodified arrangement of the loudspeaker-microphone unit; Fig. 4e illustrates another modification of the loudspeaker-microphone unit; and Figs. 5 to 9, inclusive, illustratethe details of the transmitting and receiving elements provided at'each of the suband a'west station l l interconnected by a pair oi lines l3 and HI having a central-repeater I2 connected therebetween. This repeater may be pro- ,vided at a central point in the system and may be used to stabilize, the transmission level over the two connected lines l3 and 14. Briefly described, this repeater comprises. an veast-west amplifier 201, a, west-east amplifier 200, a pair of hybrid systems 202 and;203, balancing networks 204 and 20-5 individual to the hybrid coils and respectively operative to balance the line impedances of the lines l3 and J4, and a signal operated control network 220which is operative to control the gain .of the twqamplifiers 200 and MI in accordance with the direction of signal current transmission over the lines 13 and I4.

Ell/lore specifically considered, the west-east amplifier 200 comprises a coupling transformer 206 having its input Winding connected to the signal transmitting terminals of the hybrid system 203, and its output or secondary winding coupled to the input electrodes 20112 and 2010 of the threeelectr d amplifier tube 201. The output electrodesflola and20lc of this tube are coupled to the, signal input terminals of the hybrid system 202 through a coupling transformer 208. The

1 opposite or east-west amplifier 20] similarly comprises a coupling transformer 2 having its prii ting terminals 'of thehybrid system 202, and its secondary windingcoupled to the input electrodes mary-windingjconnected to the signal transmit- 2l2b and 2l2c.,,of-the'three-electrode amplifier tube 2l2.- The output electrodes HM and 2I2c ofthis tube are coupled to the'signal input terminals of thehybrid system 203 through a cow pling transformerZ l3. Preferably, the amplifier tubes 20'! and 2l2 are of the well-known 6C5 type, utilizing indirectly heated cathodes and characterized by a comparatively high amplifi- -.cation' factor.

The. control network 220, asindicated above, is

I provided for the purpose" of increasing the gain ofthesignal amplifier200or 20Lwhich. is in use: during signal current transmission overthe tubes 232 and'233 in opposite senses.

two lines I3 and I4, and for concurrently efiecting a corresponding decrease in the gain through of the east-west channel amplifier tube 201 over a signal current path which includes the couplin transformer 228, the three-electrode amplifier tube,

221, the adjustable voltage dividing resistor 226 and the coupling transformer 225. The. duplex diode 23I functions to rectify signal currents appearing across the secondary windings of the coupling transformers 224 and 228, respectively, and to impress the rectified voltagesjupon the input electrodes of the two direct current amplifier Thus, it will be noted that the diode section23Ia, 23Ic 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 23Id, 23I e of the duplex diode 23I 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 thevoltage developed across this resistor and to determine the time constant of the control circuit. The voltages developed across the two halves 234a and 2341) of winding of the transformer 200. This winding is bridged by a terminating network which comprises the shunt-connected resistor-209 and condenser 2I0. Similarly, the output circuit ofthe other tube 233 includes a biasing resistor 23Gb which is shunted by a smoothing condenser 238 and is connected through the secondary winding of the coupling transformer 2 to the control grid 2l2b of the east-west amplifier tube 2I2. This secondary winding is also bridged by a terminating network which includes the shunt-connected resistor 2I3 and condenser 2l4. Preferably, the control and amplifier tubes 223, 221, 232 and 233 are of the commercial type 6G5, and the duplex diode tube 23I is of the 61-16 type.

Anode voltages are impressed upon anodes 201a and 2I2a of the two signal amplifier tubes 20'! and 2I2 from the voltage source 2, over paths which respectively include the signal current choke coils 245 and 2I5. These" coils and the source 24I are by-pa-ssed forsignal currents by .the condensers 244 and 2I6. In this regard it will be noted that the cathodes H20 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 24I is also positively applied to the anodes of the two tubes 223 and 221 through the primary windings of the coupling transformers 2-24 and 22,8, respectively. The direct voltage across the section 242a of the resistor 242 is also positively applied to the anodes of the two control tubes 232 and 233 through the resistors 235a and 23612, respectively. The cathodes of the two tubes 223 and 221 are positively biased with respect to their associated control electrodes by the voltage drop across the variable section 239a of the voltage divider 239, which divider is bridged across the section 242a-of the resistor 242.

Referring now more particularly to the equipment provided at the east station I0, this equipment is illustrated in Fig. l of the drawings as comprising 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. 0 The station equipment further comprises a transmitting amplifier I04, areceiving amplifier I05, 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 station II is identical with that provided at the east station 10 and, accordingly, only the details of the station II have been illustrated in the drawings. Briefly described, the equipment provided at the west station II includes one of the three loudspeaker- .microphone units illustrated in Fig. 4 oi the drawings, and the circuit equipment illustrated in Fig. 3 of the'drawings. This circuit equipment comprises a transmitting amplifier 300 and a receiving amplifier -30l which are respectively coupled to the "transmitting and receiving terminals of a hybrid system 302. A balancing network 303 of the adjustable resistive type is provided for balancing the impedance of the line I4 to minimizeenergy transfer between the transmitting and receiving channels of the station II. For the purpose of controlling the gain of the two amplifiers 300 and 323i to preserve stability of the station circuit, a control network 3I5 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 amplifier 300 comprises a thermionic tube 306 of the well-known pentode type,' having its input electrodes 30Gb and 306c cou-pled to one of the three microphones 40I, M I and 42I over a channel which includes an adjustable voltage dividing resistor 305, a condenser 363, a coupling transformer304 and the cord conductors 430. The output electrodes of this tube are directly coupled to the input win-ding of the hybrid system 302, which is shunted by a resistor 338. The other signal amplifier 30I similarly comprises a thermionic tube 308 of the well-known pentode type, having 'itsinputelectrodes 300p and 300c'coupled 2,365,187 to the receiving terminals of the hybrid system.

302 through an adjustable voltage dividing resistor 301. The output electrodes of this tube may be coupled to the loudspeaker of any one of the three translating units 400, 4 I and 420 in use, over a channel which includes the coupling transformer 309 and the cord conductors 43!. The two tubes 306 and 308 are provided with cathodes 3060 and 3030 which are of the filamentary type, such that rapid changes in 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 3! 5 are coupled to the receiving channel of the station circuitv at a point just following the signal amplifier tube 308. Briefly described, this network comprises a signal amplifier tube 322 having'its input electrodes 3221) and 3220 coupled to the output circuit of the amplifier '30! over a channel which includes thefrequency'discriminating circuit 3H5. This circuit, which comprises a coupling condenser 3! 1, a loading condenser 320, a voltage dividing resistor 3! 8 and two resistors 3!9 and 32!, is designed to favor frequencies lying within the lower portidn of the voice frequency range, and to discriminate against frequencies in the upper portion of this range. The output electrodes of the signal amplifier tube 322 are coupled through a coupling transformer 324 to the electrodes of a triode 326 which is connected to operate as a diode rectifier. A load resistor 328 shunted bya 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 33012 and 3300 of the control tube 330. In order to prevent the control circuit 3l5 from effecting a change in the gain of either of the two channel amplifier tubes 306 and 308 when the signal input to the station I! is below a predetermined value, a biasing battery 325 is provided in circuit with the tube '326. This battery functions to prevent the tube 326 from passing current until the signal input to the'station ex-.

ceeds the selected predetermined value.

The control tube 330 is provided with a space current path between its output electrodes 330a and 330s 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 apparentthat with the space current path of the tube 330 shunting the output circuit of th amplifier tube 308,, the signal current gain through the receiving amplifier 30! 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 spac 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.

Briefly considered, the power pack 335 comprises a voltage transformer 336 having a primary winding 33! which is arranged-to be connected to a source of commercial frequency current through a manually operable switch 338 and is provided with a high voltage secondary winding 339, a low voltage cathodeheating winding 340, and a low voltage cathode heating winding 34!.

fier tube342 having'its two anodesconnectedto the outer terminals of the'high voltage windin 339, and its cathode connected to a load- 'resistor 349" through a filter network which comprises a series chock 346, a pair of shunt connected filter.

' condensers 34'! and 348, and a bleeder resistor vide bias voltages for 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, is'nega-I tively applied'to the control electrode 30Gb of the tube 306 over a path which includes the filter resistor 364 and the lower section. of the voltage dividing or volume control'resistor 305. Thissection 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 Voltage of approximately 18.5 volts which is developed across the section 34312 of the resistor 343 during operation of the power pack 335, is negatively applied to the control electrode 33% of the tube. 330 over a path'which includes-the resistor 328. This section of the resistor 3'43 and the resistor 328 are bypassed for alternating currents by the condensers 33! and. 345. The entire voltage of approximately 270 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 309. Thisvoltage 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-Offswitch 36!, which is provided for rendering the control network 3l5 active or inactive as desired, this",

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 tothe anode of the transmitting amplifier tube 306 over a path which includes the re-, sistor 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 bypass condenser 32'! is connected in shunt with the portion of the resistor 349 from which the 110- volt anode potentials are derived, in order to preventthe alternating components of this voltage from being impressed upon the screen electrode 306d and the anodes 306a and 322a.

Cathode heating current is supplied to the cathode 3080 of the receiving amplifier tube 308 from the low voltage winding 34! of the transformer 336, it being noted in this regard that the mid-point of this winding is connected to ground over a path which includes the cathodebiasing resistor 35! and the biasing resistor 352 in series, whereby the cathode 308a is normally maintained at a potential which is substantially positive with respect to its associated control grid 30%. This biasing path is by-passed for alternating currents by a shunt connected condenser 353. Cathode The power pack also includes a full wave rectiheating cur n are up to the'cathodes of 331 of the transformer 336. More specifically, the

low voltage winding 35B is connected to supply heating current to the cathode 3260 of the tube 326 and is provided with a center tap which is connected to one terminal. of the load resistor 328. The low voltage winding 35'! is connected to'supply heatingcurrent to the cathode 3220 of the tube 322 and is provided with a tapped center point which is connected to ground through a'cathodebiasing network which comprises a resistor 359 shunted by a signal current by-pass condenser .360. The third low voltage winding 358 is connected to supplyv cathode heating current to the cathode 330c of the tube 330 and is provided with a tapped center point which is connected to ground through the cathode biasing resistor 352.

Asindicated above, any one of the three loudspeaker-microphone arrangements illustrated in Figsmla, 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 48! which is arranged to be primarily responsive to sound waves transmitted thereto along a horizontal path, anda 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 4000. which is provided with curved outer surfaces, and in order to minimize conductive or mechanical coupling 'between the two translating elements 40! and 402, the casing 400a is filled with acoustical damping material, such, for ex- 4 l a, and are surrounded by felt or other acoustical insulating material to minimize the conductive coupling therebetween. In the arrangement illustrated in Figs. 4b, 4c and 4d, the sound chamber adjacent the diaphragm of the loudspeaker 422 communicatesiwith two sets of sound passages 424a and 424b 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 openings. The transmitter elernent or microphone 42l is' disposed between the two sets .of sound passages 424a and 424i) 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

slightly downward to prevent sound reflected from the walls and ceiling of the. room in which the unit is located from being transmitted to the diaphragm of the microphone 42!. The loudspeaker 422 and the microphone 421 are housed within a casing 420a which is filled with acoustical insulating material, such as felt, to minimize the conductive coupling between the two elements. If desired, the insulating material provided within the casing 420a may be compressed or molded, and the sound passages 423, 424a and 424b may either be preformed therein during the molding process or may be cutin the formed block after the molding oi the structure is completed. As an alternative to this arrangement the two elements 42! and 422, each including its own sound directing passages, may be constructed as separate units, and the two units may then be housed within the casing 4200.. In the latter case a suitable amount of acoustical insulating material should be provided around each of the two units in order to minimize the conductive coupling therebetween.

In the operation of the loudspeaker-microphone arrangement illustrated in Figs. 4b, 4c and 4d of the drawings, sound waves developed during operation of the loudspeaker 422 and trans-. mitted through the sound passages 424a and 424b, diverge 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 42!. 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 I found that station transmitter-receiver combinawords, signal currents having frequencies corresponding to the response characteristic peaks and reproduced by the receiver element, cause the maximum response of the associated transmitter element. 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 two 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 equal to 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 of the drawings, thesecpassages are directed J5 Lper second, which corresponds to a wave length,

I of the opening and the center pole piece. magnetic circuit assembly is completed by means measured atordinary .room temperature, of approximately 22.3;inches. spacing between". the sound transmitting and re- In this case a mean ceiving openings of the loudspeaker and, 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 AM and 402, for example, is the size of the diaphragm used in the loudspeaker 402. In general,

it may-be statecl'that the smaller the loudspeaker diaphragm the less the acoustical coupling between the loudspeaker and its associated microphone; .In other words, a loudspeaker um't having a small effective diaphragm diameter 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 effective 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 difficult to" control. In'the system under consideration a transmission band ranging from about 350-cycles per second to approximately 33 cycles per sound is'utilized for voice current transmission'. Accordingly, a loudspeaker is provided which is equipped with a diaphragm having an effective diameter of less than 6 centimeters.

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 50!, a center pole piece 502, and a top plate 503. The parts 50!, 502 and 503 are preferably formed of Allegheny electric metal or other highly permeable magnetic material. The center pole piece 502 is provided with a lower portion 502a of reduced diameten'which extends within a centrally disposed opening provided in the lower plate 50L A sweat connection between the side walls of .this opening andthe idly to secure the center pole piece '502 to the bottom plate 50L At'its upper end the center pole piece 502 is provided with a portion 502!) -of reduced -diameter, which extends within a centrally disposed opening 503a formed in the top plate 503. As best shown in Figf of the drawings, the diameter 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 gap is formed between the adjacentside walls The of three clamping screws 50 iv which extend through openings provided in the bottom plate 50! and are threaded into tapped holes drilled in the top plate 503. These screws serve rigidly to clamp the annular permanent magnet 500 be- .tween the top and bottom plates 503 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 clamp- Fig. 8 arrangement"is used. By virtue of this frequencies is enhanced.

ing ring 508 and, 509 two paper, washers 5l0 and 5H, and six assembly screws 5l2. The

character of the diaphragm depends "upon whether thefldevice is to be used as a transmitting or receiving" element. If the device is to be .used as a'tran'smitter, the diaphragm is of one-piece'construction and is provided with an outerhatannular ring portion 505a which extends between and is cemented to the paper "washers i510 and 5H and is uniformly clamped around its periphery between these washersby the two clamping'rings508 and 509. If the device is to be used as a receiving element, the diaphragm construction illustrated in Fig. 8 of the drawings is usedQ In this construction only the bulbous portion of the diaphragm 505 r is constructed of molded or pressed fiber, the periphery of this element being secured to an annular ring 505b' formed of treated'silk which is disposed between and cemented'to the paper Washers 510 and'5l I. The purpose of this ar- 'rangement is to permit substantiallyfree vibration of the diaphragm element 505, particularly at the low -frequencies'of the operating frequency range, whereby the response of the device at these With the one-piece diaphragm arrangement, on the other hand, the diaphragmis considerably stiffer and, accordingly, the 1 response thereof, particularly at low frequencies, is substantially. less than whenthe increased stifinesaslowffreqdenoy noise components of sound rare prevented from producin any. substantial response. of -the device. I l 5.

A moving coil 5| 3.:isi'cemented to the-.- l0we bulbous portion "'of thediaphragm-5505 and is concentrically disposed within they air gap .de-

fined by the upper portion 502b ofthe,poleipiece F 502 and the opening 503a in the t'opplate..503.

Theterminal-ends of, this coil may be electrically connected'to lead-in wires in any desired manner although, preferably, the connections-are made at anchor posts which extend through and -aresecured to thelower bottom portionof the diaphragm505; 'In order to preserve the correct lateral and .axial spacing of the: moving coil 513 in the air gap of the magnetic circuit,

and to provideiadditional support for the diawithin the opening 503a, whereby an-annular air sides of the extended portion 502a is used rig-,

phragm 505, arstring supporting arrangement is provided. This arrangement comprises-three connected strings 514a, 5| 4b and 5l4c which Lextend through equiangularly spaced openings cut ,through the lower portion of the diaphragm member 505, and are joinedtogethenas indicated at-5l4d ,.within the dished cavity of this member; The free ends of the three strings are respectively anchored by clamping the same to "the lugs 5l5a,-5|5b and 5l5c which extend radially inward-and areturned upward from the supporting ring 506. Small clamping plates 5l5a,

, ingcollars 501 are used, to clamp the free ends 5l6b and 5I6c disposed beneath shortened spacof the strings 5l4a,- 514D and -5I4c against the lugs 5|.5a, 5 I5b and 5l5c. Preferably the three strings 5l4a, 5M!) and 5M0 are formed of silk fishline or the ;like,. and it will be; understood {that byappropriately tensioning the three strings therequired support for the moving coil 5l3 may 1' be obtained: 1 1

l I In order to 'seal thejannular cavity, formed be-j 7o tween the spacedlapart annular permanent magnet 500 and the polevpiece 502 fromthe acoustical: cavity 5| 1 adjacent the lower side of the' diaphragm 505, thereby; to-enhance the damping Y of the. diaphragm and thus produce a more uniform response thereof over the entire operating frequency range, a sealing: ring SIB is provided which snugly encloses the-upper end portion 5112b of the center pole piece 502,'and isclamped to the underside of the top plate 503-by means of assembly screws 519; 7, formed of rubben-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 -18 to the underside of the top plate 503 and then setting the assembly screws 5M'to clamp the annular permanent magnet 500 between the top and bottom plates 503 and 501. After the magnetic circuitstructure is thus-assembled the portionof the sealing ring 5l8 facing outward through the air gap of the structure may be painted with shellac or other sealingcompound, to provide an acoustical seal which blocks oi the cavity of the magnetic circuitstructure-from the cavity'5l'l adjacent the underside of the diaphragm. 505. The'diaphragm assemblyis prepared by cementing thetwo paper washers 5H! and 5H to'the top and bottom'surfaces of the fiatannular portion 505a. ofthe diaphragm 505, assuming that a transmittingelement is being constructed,- and by cementing the moving coil 513' to the lowercentral portion of the diaphragm member; If the device is to be used as a loudspeakingleceiver element, the. diaphragm structure illustrated-inns. 8 of the drawings is constructed by first cementing the'paper washers 510 and 5| lto the opposite peripheral surfaces of'the silkringillib; after which the inner upper edgcisurfaces of the silk-ring are cemented to thlitidtsid of the'diaphragm505.

Following the construction of: the: diaphragm structure, the three stringsSllw, 51th and 5l4c are drawn through the angularly spaced openings provided in the based the diaphragm 505. After. this operation :is performed, thesupportmg rin'g 'slls; the clamping 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 opening provided in the stack clamping-rings and diaphragm structure, the spacing collars 501', and the supporting ring 506. Initially the screws which extend through the clamping elements 516 are not tightened, but the.

alternate assembly screws which extend only through alternate spacing collars 501' are tightened suflicient'ly to provide a semi-rigid assembly;

Following this operation the ends of the strings 5l4a', 5H!) and i l lc' may be threaded between the clampingelements 516a, 51Gb and-515i: and

their associated'lugg 5l5a, 5152i and'5l'5c, and properly tensione'd to provide the required lateral and axial support'for the diaphragm 505.

Thereafter the assembly screws 512 may be tightened securely'to clamp the ends of the strings 514a, 5| 4b and'SIlc between the clamping elements 5|6a, 5l6b' and H50 and their respective associated lugs 5l5a, '5l5b and 5I5c. ,Thus the structure of the translating device is completed.

As pointed out above, each transmitting and receiving-element, as constructed in the manner just described, is preferably embeddedin a-block of acoustical insulating material to prevent the This ring ispreferably tion, and more specifically may be of any configuration illustrated inFigs. 4a, 4b, 4c, 4d and 4e of the drawings, is preferably preformed to the through. It has been found that the number therein, may be provided.

of coats or the thickness of the layer of lacquer on the outside surfaces of the molded block determined to some extent, the acoustical permeability of the structure. Hence, by appropriately controlling the depth of the lacquer layer,

the acoustical characteristics of the sound transacoustical material 520 may be either round or 25- square in horizontal cross section, and is provided with an opening which snugly receives the side walls of the completed. transmitting or re-- ceiving element. If the unit is to operate as a nondirectional device, the assembly is completed by closing the opening in which the transmitting or receiving element is inserted with a silk screen 523 having approximately'the characteristic impedance of air, over which is placed a. thin sheet of aluminum 524 having a number of holesoi 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 inaccordance with the method briefly outlined above, and having sound transmitting passages of appropriate lengths and dimensions preformed A simple embodiment of-an acoustical insulating block provided with such passages is illustrated in Fig. 90f the drawings as being adapted for use in conjunction with theunitillustrated in Fig. 5 of thedrawings. -More specifically, the Fig. 9 arrangement comprises a block of acoustical insulating material 521 which is provided with a number of sound transmitting passages 522 molded or out there- "through along the axis of sound transmission to and from the diaphragm of the associated transmitting or receiving element. These passages are of the correct dimensions, 1. e., diameters and lengths, to provide for maximum sound energy transfer between the atmosphere and the cavity immediately adjacent the outer surface of the V diaphragm. The block of acoustical insulating transmission of sound waves and mechanical sulating material'52ll. This block oiinsulating material, whichmay be oithe desired-configuramaterial 52l may be secured to the block of insulating material 520 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.

1 From the foregoing explanation it willbe understood that a transmitting and a receiving eletively. It will also be understood that in the unit as thus formed the conductivecoupling 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 of the drawings, contributes materially to the acoustical attenuation between the transmitting and receiving elements.v This at tenuation may also be enhanced by utilizing the sound directing arrangements illustrated in Figs.

4b, 4c, 4d and 4c of the drawings in the construction of the block of insulating material, or by using either of the arrangements shown in Fig. 4a

or 4c of the drawings. In short, the construction of the transmitting and receiving elements aswell as the physical arrangement of these elements and the manner in which the elements are mounted, all contribute to the reduction in acous-.

tical coupling between theseelements as provided at each station of the system. This means that the transmitting and receiving amplifiers at each of the two connected stations l0 and II, as well as the channel amplifiers in the central repeater i2, may be operated with normal gain settings which are substantially greater than the permissible gain settings when-conventional microphone and loudspeaking arrangements 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 l0 and l I, 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 at the west station II for operation, the switch 338 is manually operated to its closed-circuit position wherein alternating current is delivered to the parallelconnected windings 355 and331 of the power transformers 354 and 336. With these transformers energized, low voltage alternating current is delivered to the cathodes of each of the tubes" 343', 322, 326, 330 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 the voltage developed across the high voltage secondary winding 339, and is passed through each of the two.

resistors 343 and 349 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 34! and 348. The direct voltage developed across the section 343a of the resistor '343 is negatively applied to the control electrode of the transmitting amplifier tube 306' to deter- 1 mine the operating point on the characteristic of this tube. Similarly, the voltage drop across the resistor section 3431) is negatively applied to the control electrode 33017 of the tube 330 to determine the normal space current flow through this tube.

Initially, the voltage dividing resistor 305 is so adjusted that the maximum signal output from the amplifier 300 consistent with stability of the station circuit is obtained" when heating current 5 of rated value traverses thecathode 3060 ofthe tube 306. Similarly, the voltage dividing resistor 30'! is initially adjusted so that the maximum signal output from the amplifier 30! consistent with stability of the station circuit is obtained when the amplifier 30| is operating with normal signal current gain therethrough. 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 300 and 30! and, hence, the gain setting of each amplifieris dependent upon the setting of the other amplifier. Assuming that the On-Off switch 36! oocupies the position illustrated in the drawings, the signal current output from the receiving amplifier 30| is in part determined by the setting of the adjustable voltage dividing resistor 30! 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 330 depends, in turn, upon the negative bias voltage impressed across the input electrodes 3301) and 3300 thereof. Normally this voltage is equal to the sum of the voltage drop across the resistor section 3432; and the voltage drop across the cathode biasing resistor '352. The total magnitude of this bias voltage is normally oi 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 thereof divides between the resistor 352 and the cathode 3060 of the transmitting amplifier tube 306. The

value of the resistor 352 may be so'chosen that rated current normally traverses the cathode 3060.

With the gain settings of the two amplifiers- 300 and 30l determined in the above-described manner, the circuit constantsof the station circuit are so chosen that the signal current gains through the two amplifiers 300 and 30! are substantially equal. Stability of the station circuit may be then obtained during installation of the system by adjusting the voltage dividers 305 and 301 until any tendency for the station circuit to oscillate is eliminated,

With the central repeater l2 conditioned for operation, but inactive, no signal currents are transmitted through either of the two amplifier sections of the duplex diode tube 23!, andhence no bias voltage is developed across the resistor 234. Accordingly the space current flow through the tubes 232 and 233 and their respective associated resistors 236a and 23611 is determined by thesettingof the adjustable cathode biasing resistors 235a and 235?). Each of these resistors is initially adjusted so that the space current flow through the associated tube develops a voltage of approximately [2 volts across the associated resistor 236a or 236b.- 'The voltage across the resistor 236a is negatively applied to the theresistor 23Gb is negatively applied to the control electrode 2|2b of the east-west amplifier tube 2i'2. I v

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!, the signal voltagedeveloped in the moving coil of this microphone is transmitted portion of this voltage appearing across the lower nal current amplifier 105.

resistor 234.

portion of the voltage dividing resistor 305is impressed upon the input electrodes 306D and 306a of the transmitting amplifier tube 306. The signal currents as amplified by the tube 306 are transmitted through the coupled windings of the 201D and 201C of the west east amplifier tube 201 in parallel with the primary winding of the coupling transformer 221. 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 13. From this point the signal currents are transmitted over the line 13 and through the hybrid system 102 to the input circuit of the sig- These currents as amplified by the amplifier 105 are transmitted to the moving coil of the loudspeaker 101 for ;-reproduction.

The signal voltage as impressed between the input electrodes 223*?) and 2230 of the amplifier tube 223 through the coupling transformer 221 and the voltage divided 222, is amplified-through this tube and impressed through the coupling transformer 224 across the anode 231a and the cathode 231c of the duplex diode 231. More specifically, the voltage appearing across the secondary winding of the transformer 224 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 230, the anode 231a, the space current path between this anode and the cathode 2310 and the resistor 234 to the lower terminal of the indicated transformer winding. This pulsating current is smoothed through the action of the condenser 236 which shunts the Thevoltage thus developed across the resistor section 234a is negatively applied to -the control electrode of the tube 232 to decrease the space current-flow through this tube and thus lower the voltage drop across the biasing resistor 230a. When this voltage drop is' decreased the negative bias on the control electrode 2011; of the active west-east signal amplifier tube 201 is lowered to increase the amplification factor of this tube. At the same time that the gain through the active west-east channel 200 of the repeater 12 is thus increased, the gain of the inactive east-west channel 201 is correspond- -ingly decreased. Thus it will be noted that the voltage drop across the resistor section 23% is positively applied to the control electrode of the tube 233, whereby the space currentfiow through this tube is increased. As a result the voltage drop across the biasing resistor 23% is-increased. Since this voltage is negatively applied to the control electrode 212!) of the tube 212, it will be understood that a corresponding decrease inthe signal current gain through'the amplifier tube 212 occurs. 1 From the above explanation it will be understood that the increase in signal current gain through the active west-east transmission chan- 75 nel .200 is accompanied by a corresponding decrease in the gain through the inactive east-west transmission channel 201 of the central repeater 12. Thus the stability of the repeater network is preserved. In this regard it will be noted that the network includes a substantially closed signal trol amplifier tube 221.

current circuit which comprises the two channels 200 and 201 and the coupling paths through the two hybrid systems 202 and 203, and that if the two amplifier tubes 20! and 212 are both operated at the high gain settings required during signal current transmission, the total gainaround 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 may be 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 which 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 14 is terminated to arrest the signal input to the control network 220, the bias voltages across the two resistor sections 234a and 23411 are reduced to zero, whereby the normal bias voltages across the two biasing resistors 235a and 23Gb are restored. The signal current gain throug I .the amplifier tube 201 is thus decreased to normal and the signal current gain through the amplifier tube 212 is increased to its normal value.

The rate at which the bias voltages across the values of this network are proportioned to provide the necessary hangover period at each signal current break-off point, in order to prevent syllable clippin which would otherwise occur.

When sound waves are transmitted to the microphone provided at the east station 10, corresponding signal currents are developed in the output circuit of the signal current amplifier 104 and are transmitted through the hybrid system 102, over the line 13 and through the hybrid system 202 to the parallel-connected input circuits of the channel amplifier tube 212 and the con- These signal currents as amplified by the channel amplifier 201 are transmitted through the coupling transformer213 and the hybrid system 203 to the line 14. From this point they are transmitted over the line 14 and through the hybrid system 302 to the input circuit of the receiving amplifier 301 provided at the west station 1 1. After being amplified by the receiving amplifier tube 308, the signal currents are transmitted through the coupling transformer 309 and over the cord conductors 431 to the moving coil of the loudspeaker 402 for reproduction.

The signal voltage as impressed between the inputelectrodes of the amplifier tube 22'! through the coupling transformer 225 and the voltage divider 22B is amplified by this tube and impressed through the coupling transformer 22B across the anode 231d and the cathode 231e of the duplex diode 231. 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 whichmay be traced as extending from the upper terminal of this winding by way of the resistor 229, the

trode of the tube 232 to increase the space current flow through this tube and thus increase the voltage drop across the biasing resistor 233a. When this voltage drop is increased the negative bias on the control electrode 2371) of the'inactive west-east signal amplifier tube 201 is increased to decrease the amplification factor of this tube. At the same time that the gain through the mac- 'tive west-east channel 200 of the repeater i2 is thus decreased, the gain of the active east-west channel 20! is correspondingly increased. Thus it will be noted that the voltage drop across the resistor section 23413 is negatively applied to the control electrode of the tube 233 so that the space current fiow through this tube is decreased. As

a result, the voltage drop across thebiasing resistor 23612 is decreased. Since this voltage is negatively applied to the control electrode 2l2b of the tube 2l2, it willbe understood that a decrease in the magnitude of this voltage produces a corresponding increase in the signal cur-3.,

rent gainthrough the amplifier tube 2 [2. Thus it will be seen that when signal currents are transmitted in the east-west direction from the station I to thestation Ii,thesignal current gain through the active channel is increased the required amount for satisfactory reproductionpf the signal currents by the loudspeaker 402 in use, and this signal current gain is accompanied by a corresponding decrease in the gain of the inactive west-east channel 200, whereby the stability of;

the central repeater I2 is preserved. It will also be understood from the above explanation that each timesignal current transmission over the line l3 from the east station I0 is terminated to arrest the signal input to the control network 220,,

the bias voltages across the two resistor sections 234a and 2341) are reducedto zero after a short time interval which is determined by the time constant of the shunt circuit including the resistor 234 and the condenser 236. Accordingly, the bias voltages across the two biasing resistors 236a and 23Gb are again equalized at their respective normal values, whereby the signal current gainthroughthe amplifier tube 201 is increased to normal and the signal current gain through the amplifier tube 2l2 is decreased to its normal value.

Referring now more particularly to the method of controlling the network 315 provided at the west station I I, it will be noted that a' portion of the signal voltage appearing across the output circuit of the receiving amplifier tube 308 is impressed across the input electrodes 32 2b'and 3220 of the amplifier tube 322 through the contacts of the switch 36! and the frequency discriminating network 3I6. Aspointed out above, this network is designed to favor signal currents having frequencies in the band ranging fromapproximately 300 cycles persecond to approximately 1200 .cycles per second, In thlsregardit will be understood that the' ratio of the signal voltages across the two'resistors 3!!) and SM is determined by the reactanceof the condenser 320. At relatively low signal voltage frequencies the reactance of this condenser isrelatively high so that a relatively large proportion of the available voltage drop across the two series-connected resistors 3 i9 and 32I appears across the resistor 32L 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 3E9. Thus by properly proportioning the constants of the network3l6, this network may be operated to favor signal currents of the frequencies within the band indicated. The reason for providing a discriminating network 3H3 which favorssignal current frequencies within the low end of the-operating range-is to make the control network 3l5 primarily responsive tothe band of frequencies at which the major portion of thevoice'current energy is produced. Thus the predominant portion of the frequencies used in ordinary speech lies within the band ranging from 300 to approximately 1200cycles per second. i

The signal voltage as developed across the resistor 32l 'is amplified by the tube 322, and the amplified signal currents cause corresponding inducedvoltages in the secondary winding of the coupling transformer 324, which voltages are applied across the anode and cathode of the rectifying tube 326. This tube is biased by means of the battery 325 to a point .on its operating charthe amplifier tube 308 exceeds a predetermined value, determined by the voltage of the biasing value. Thus, so long as the signal voltage across the output circuitof the receiving amplifier tube 308 does not exceed a predetermined value, the biasing battery 325. prevents the control network .3l5from changing the gain of either of the .two

amplifiers 30| or .302. When, however, the signal voltage appearing across the output circuit of battery 325,the signal voltageacross the secondary winding of the transformer 324 becomes sufiicientto cause current to traverse the space current path of the tube 326. This currenttraverses the biasing resistor 328, is smoothed by the condenser 329, and flows in a direction such that the resulting direct voltage across the resistor 328 is additive with respect to the bias voltage across the section 343b, of the resistor 343. Ac-

" cordingly the net bias voltage across the input electrodes 3302) and 3300 of the, control tube 330 is increased to increase the resistance'of the space current path of this tube. Incidentto this increase in space current resistance of the tube 330, the shunting effect of the tube on the output circuit of the receiving'amplifier tube 308 is correspondingly decreased to produce anincrease in the signal current gain through the amplifier 30 I. This decrease in the shunting effect of the tube 330 is supplemented byan increase in the amplification, factor of the tube 308 occasioned by an increase in the voltage on the anode 308a. Thus when the space current flow through the tube 330 is decreasedthe voltage drop across the resistor .which the control circuit 315 starts inversely to change the gains of the two amplifiers 300 and 301, may be adjusted, through suitable adjustment of the voltage divider 318, 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 3011 and 301. Hence, after the required signal current gain through the two tubes is once established, the voltagedivider 318may be adjusted to provide the minimum signal voltage input to the amplifier tube 322 consistent 1 with the required control of the station circuit.

It will also be understood from the above explanation that after the control circuit 315 starts to operate, the extent to which the signal current gains of the two amplifiers 300 and'301 are inversely changed depends, within limits, upon the magnitude of the signal currents incoming to the west station 11' overthe line 14. Preferably, the circuit constants of the network 315 are so adjusted that the amount of the decrease gain of the-signal amplifier 3011 which occurs in response to any given Change in the input signal intensity is just sufficient to maintain the stability of the station circuit. -As indicated by the above explanation, the gain control action realized through operation of the network 315 isnot effective until the-signal input to the loudspeaker 1132 exceeds a predetermined value. By virtue of this arrangement, the'control network '3 1 is rendered substantially non-responsive to noise cur- 5 rents resulting from background noises which may be'-acoustic'ally imposed'onthe system 'or to noise currentsresulting from electrical transients imposed on the system, which currents are usually of a low order of magnitude, -Since such noise currents'areusually' of low frequency, the couplin'g condenser 31'! maybe chosen of such value as to prevent-any substantial'portion of there- "slllting voltage *across the output circuit'of the amplifier tube308 fromappearing'across the voltage dividing resistor 318. Thus, the response of the network 315 islimitedto speech or other actual signal "current transmission over the -two connected lines'of the system in'an east-west direction. Further, the constants of the station cir cuit are so chosen'that-the variation in gain'of *the amplifier300,-efiected through operation of the control network 31 5, are limited toa definite range which will include all cases of normal sigparent that amplified-signal voltages appearing across the output circuit of the tube 308 are not impressed upo-nthe frequency discriminating circuit 316. Thus the control network 315 is rendered completely inactive, with the result that current of a substantially constant magnitude is caused to traverse the cathode 31360 of the transmitting signal amplifier tube 306 regardless of the level of signal current transmission to the station 11. While it will be understood that the specifications of the circuit provided at the station H may vary according to the design of 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 ;Commercia1 type 47 Tube 308 Commercial type lA5G Tube 322 Commercia1 type 26 Tube 326 Commercial type 26 Tube 330 "Commercial type 10 Tube 342 Commercial type 80 Condenser 363, 0.25 microfarad Condenser 365 50.0 microfarads Condenser 317 0.001 microfarad 5 Condenser. 320 0.002 microfarad Condenser 323 0.05 microfarad Condenser 327 8.0 microfarads Condenser 331 0.25 microfarad Condenser 329 0.01 microfarad Condenser 345 8.0 microfarads Condenser 344--.; 8.0 microfarads Condenser 348 16.0 microfarads Condenser 347 8.0 microiarads Condenser 353-; 5.0 microfarads Condenser 360--... ..5.0 micro-farads Resistor 305 r 250,000 ohms Resistor 364 500,000 ohms Resistor 366' n 500,000 ohms 4 Resistor 307' "250,000 ohms Resistor 362 1250 ohms Resistor 31 8 "250,000 ohms Resistor 319 100,000 ohms Resistor 321 500,000 Ohms- -Resistor 328 5 00,000 ohms Resistor 343- 250 m Resistor 349 25,000 ohms Resistor 350 1 5,0'00 ohms Resistor 351 r 420 ohms .50 Resistor 352 ohms Resistor 359 1000 ohms Battery 325 "4.5 volts Although only one-control network 315, which responds to signalcurrents incoming tothe station 11 to control the gains of the-amplifiers 3110 "and 381, has been illustrated in the drawings, it will be understood that a second control network of like arrangem'entand responsive to signal currents transmitted from'the microphone 401 "to the amplifier 3l10,'maybe provided if necessary. In suchcase, the input and output circuits of the second control network will be bridged across'the output circuit'of the afiiplifier tube306 and-the space current traversing-the control't'ube'oi the second networkwill determine the cathode heating currentof the receivingam plifier tube. With such an arrangement theseco'nd control circuit will function to-i'ncrease the gain of the-transmitting amplifier tube 30'6andto-decrease the '-.gain of the receiving :a mplifier tube when sound.

waves-exceeding a predeterminedvalue aretransinitted to the microphone 401.

The loudspeaking system-shown in the drawings of the present application is' claiined in the copendin'g application of Gabriel- Giannini,-' Serial No. 419,165, filed November 14,1941, now Patent No. 2,341,539, granted February 15,1944.

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

Whatv is claimed is: f

1. In a transmission system, a pair of variable gain signal current channels'one of said channels including an amplifier tube having a cathode, a circuit for energizing said cathode, means controlled by signal currents traversing the other of said channels for changing the gain through said other channel in one sense and. for controlling the current traversing said circuit to change the gain through said one channel in the opposite sense, and means for preventing the operation of said last-named means until the signal input to said other channel exceeds a,

predetermined value.

2. In a transmission system, a pair of signal current channels, one of said channels including an amplifier tube having a cathode, a control tube having output electrodes defining a space current path which is bridged across the other of said channels, whereby the signal current gain through said other channel is changed as the resistance of said space current path is varied, means for energizing said cathode with a current which varies in accordance with the resistance of said space current path, whereby the signal current gain through said one channel is changed as the resistance of said space current path is varied, and means controlled by signal currents traversing said other channel for controlling the resistance of said space current path.

3. In a transmission system, a pair of signal current channels, one of said channels includtrol tube having output electrodes defining a space current path which is bridged across the other of said channels, whereby the signal current gain through said other channel is varied directly in accordance with the resistance of said space current path, means for energizing said cathode with a current which varies inversely in accordance with the resistance of said space current path,,whereby the signal current gain through said one channel is varied inversely in accordance with the resistance of said space current path, and means for controlling the resistance of said space current path directly in accordance with the amplitude of the signal input to said other channel.

4. In a transmission system, a pair of signal current channels, one of said channels including an amplifier tube having a cathode, a control tube having output electrodes defining a space current path which is bridged across the other of said channels, whereby the signal current gain through said other channel is varied ing an amplifier tube having a cathode, a con the operation of said last-named means until the amplitude of the signal input to said other channel exceeds a predetermined value.

5. In a transmission system, a pair of signal current channels, .one of said channelsi including an amplifier tube having a cathode, a control tube having output electrodes defining a space current path, a resistor shunting said cathode and lbridged across the other of said channels .in series with said space current path, whereby the signal current gain through said other channel, is varied directly in accordance with the resistance of said path and the current traversing said cathode is varied inversely in accordance with the resistance of said space current path, and means for varying the resistance of said space current path directly in, accordance with the amtube having output electrodes defining a space current path, a resistor shunting said cathode and bridged across the other of said channels in series with said space current path, whereby with the resistance of said space current path,

means for varying the resistance of said space current path directly in accordance with the amplitude of the signal input to said other channel, and means for preventing the operation of said last-named means untilthe amplitude of the signal input to said other channel exceeds a predetermined value.

'7. In a transmission system, a pair of signal current channels each including an amplifier tube, the tube in one of said channels including a cathode, a controLtube having output electrodes defining a space current path connected in parallel with the space current'path 0f the amplifier tube in the other of said channels, a resistor shunting said cathode and connected in seriesfwith said parallel connected space current paths, whereby the signal current gain through said other channel is varied directly in accordance with the resistance of the space current path of said control tube and the cunrent traversing said cathode is varied inversely in accordance with the resistance of the space current path of said control tube, and means for varying the resistance of the space current path of said control tube directly in accordance with the amplitude of the signal input to said other channel.

8. In a transmission system, a pair of signal current channels each including an amplifier tube, the tube in one of said channels including a cathode, a control tube having output electrodes defining a space current path connected in parallel with the space current path of the amplifier tube in the other of said channels, a resistor shunting said cathode and connected in series with said parallel connected space current paths, whereby the signal current gain through said other channel is varied directly in accordance with the resistance of the space current path of said control tube and the current traversing said cathode is varied inversely in accordance with the resistance of the space current path of said control tube, mean for varying the resistance of the space current path of said control tube directly in accordance with the amplitude of the signal input to said other channel, and means for preventing the operation of last-named means until signal input to said other channel exceeds a'p'redetermined value.

10. In a transmission system, a pair of signal currentchannels each including an amplifier tube having a cathode, a cathode resistor included in the space current path of one of said tubes and shunted by the cathode of the other of said tubes. a variable resistance path shunting the space current path of said one tube, and means controlled in accordance with the signal input to the other of said channels for controlling the resistance of said variable resistance path.

11. In a transmission system, a pair of signal current channels each including an amplifier tube having a cathode, a cathode resistor included in the space current path of one of said tubes and shunted by the cathode of the other of said tubes, a variable resistance path shunting the space current path of said one tube, means controlled in accordance with, the signal input to the other of said channels for varying the resistance of said variable resistance path, and means for preventing the operation of said last-named means until the signal input to said other channel exceeds a predetermined value.

12. In a transmission system, a pair of variable gain signal current channels, a control net-' work in which the input and output circuits are coupled to one ofsaid channels at the same point and including means controlled from said point in accordance with the signal input to said one channel vfor inversely varying the gains of said two channels, the gain of said one channel being controlled at said point, and means included in said network for preventing the operation of said last-named means until the signal input to said one channel exceeds a predetermined value.

13. In a transmission system, a pair of signal current channels each including an amplifier tube, the tube in one of said channels including :a cathode, a circuit for energizing said cathode, and a control network in which the input and output circuits are coupled to the other of said channels at the same point following the tube in said other channel and including means controlled in accordance with the signal input to said other channel for varying the signal current gain through said other channel and for varying the current traversing said circuit.

14. In a transmission system, a pair of signal current channels each including an amplifier tube, the tube in one of said channels including a cathode, a circuit for energizing said cathode, a control network in which th input and output circuits are coupled to the other of said channels at the same point following the tube in said other channel and including means controlled in accordance with the signal input to said other channel for varying the signal current gain through said other channel and for varying the current traversing said circuit, and means included in said network for preventing the operation of said last-named means until the signal input to said other channel exceeds a predator. mined value.

GABREL M. GIANNINI. 'BEN EISENBERG.

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