US3105877A

US3105877A - Circuit for canceling oscillating in public address systems

Info

Publication number: US3105877A
Application number: US55396A
Authority: US
Inventors: Thomas P Miller; Solly L Fudaley; Raymond N Reinertson
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; International Telephone and Telegraph Corp
Priority date: 1960-09-12
Filing date: 1960-09-12
Publication date: 1963-10-01
Anticipated expiration: 1980-10-01

Description

Get. 1, 1963 T. P. MILLER E' Al CIRCUIT FR Emmi; OSCILLATING 1N PUBLC ADDRESS SYSIEIS Filed Sept. 12, 1960 INVENTORS.

BY laNfER 3,105,877 CmCUT FR CANCELWG OSCHILATING IN PUBiLiC ADDRESS SYSTEMS Thomas P. Miller, Mount Prospect, and Solly L. Futlaley and Raymond N. Reinertson, Chicago, 1li., assignors to International Telephone and Telegraph Corporation, New York, N.Y., a corporation of Maryland Filed Sept. 12, 1960, Ser. No. 55,396 8 Claims. (Cl. 179-1) This invention relates to public address systems and more particularly to circuits for canceling the oscillations sometimes caused by acoustical coupling between the input and output of such public systems.

A public address system is very often used to amplify and reinforce the voice of a person who is speaking into a microphone placed within acoustical range of an associated loud speaker that is broadcasting the sound of the voice. When the equipment is used under these conditions, the sound emanating from the loud speaker is fed back into the microphone vas noise which is reampliiied to cause oscillation. This fedback sound is commonly called acoustical coupling.

ln the past, systems for preventing oscillations caused by acoustical coupling have been unduly complex and have failed when used improperly. More particularly, in some systems it is necessary to cut-off or reduce the volume of nearby loud speakers each time that an adjacent microphone is used, thus requiring constant attention on the part of the user. Other systems use either directional or differential microphones to avoid acoustical coupling, thus preventing persons using the system for moving about freely. Finally, in these and other systems, the problems are only minimized because the system may oscillate if improperly used, as when a directional microphone is placed directly in front of a loud speaker.

Accordingly, an object of this invention fis to provide new and improved public address systems, and more parlticularly to provide public address systems having circuits for canceling the effects of acoustical coupling between the input and output thereof.

A further object of this invention is to develop a control signal from the voice currents appearing in a public address system for automatically canceling the effects of acoustical coupling between the microphones and the loud speakers used in the system.

A specific object of this invention is to provide a new and improved circuit for mixing two audio frequency signais. A more particular object is to cancel a band of frequencies comprising the mixed signals while transmitting a band of yfrequencies adjacent thereto, with the transmitted band having substantially full 'power up to the canceled band and a sharp cut-olf thereafter.

ln accordance with one .aspect of this invention, the public address system microphones and loud speakers are interconnected by means of a principal voice channel having an amplifier therein. Coupled to the principal channel and interposed between the microphones and ampliiier is a supplementary channel adapted :to bleed-off a high frequency portion of the voice signals appearing in the principal channel. The frequencies of this bledolf signal are the same as the resonant frequencies of the public address system. The bled-olf signals are then phase shifted by approximately 180 and fed into a mixer circuit which combines the original voice signals and the phase shifted signals with each other to eliminate a band of frequencies which is the same as the resonant frequencies of the system. Thus, acoustical coupling between the microphones #and loud speakers is at non-resonant frequencies and the system does not oscillate.

The above mentioned and other features and objects of this invention and the manner of obtaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. l shows by block diagram a public address system constructed'in accordance with this invention; and

FIG. 2 shows by schematic circuit diagram, the circuitry used to complete the hollow blocks of FIG. 1.

Where possible, simple terms are used md specific items are described hereinafter to facilitate as understanding of the invention; however, it should be understood that the use of such terms and references to such items are not to ybe construed as a disclaimer of the full range of equivalents normally given in paten-t lafw. For example, various semiconductor devices fare shown as PNP junction type devices; however, other electronic devices may be used also. Reference is made to a bandpass filter which passes frequencies in the voice range falling above 260() cycles per second. Other frequency ranges may also be accommodated. The bandpass filter is here shown as a coupling transformer tuned by a parallel capacitor while other filter circuits may be used also. Quite obviously, other examples could be selected to illustrate hofw the devices shown and described are entitled to a wide range of equivalents.

Briefly, the principal components of the publ-ic address system shown in FIG. l include a pair of microphones 1li, 11 connected to a mixer circuit 12 by Way of a principal voice channel 13. Coupled to the principal channel 13 is a supplementary channel 14 which bleeds-oli" a portion of the voice signal having a relatively narrow band width of frequencies that pass through a bandpass filter 1S. The bled-olf signal is then fed through phase shifter 16, which shifts the bled-off signal by approximately This phase shifted signal is applied through the mixer 12 where it is combined with and, therefore, subtracted from the original voice s-ignal. The effect is to eliminate the `band of frequencies at which the system is resonant. With this arrangement, the sound amplified at 17 tand thereafter broadcast from la loud speaker 18 does not include the frequencies which may cause the system to oscillate when fed back from the loud speaker into the microphones.

For a more complete understanding of the invention, referenceis made to the schematic diagram of FIG. 2. More particularly, the connection between the microphones and the output amplifier circuit 17 is completed from the principal channel 13 through a coupling transformer Zit. rl'he resistors 21-23 match the impedance of the coupling transformer 20 to lthe impedance of the voice channel.

To bleed-oli a portion of the voice frequency lsignals originating at the microphones, the supplementary voice channel 14 is connected between channel 13 and the primary winding of a transformer 25 in the bandpass filter circuit 15. Preferably, this transformer has a relatively high impedance to limit the current flowing from the channel 13 to the phase shifter 16. The transformer 25 is tuned by an adjustable capacitor 25a to pass a band of frequencies which coincides with system resonance. In one system, there were satisfactory results when the capacitor was adjusted to tune transformer 25 to pass all frequencies appearing in channel 13, which were higher than 2600 cycles per second.

Means are provided for shifting the phase of the signals bled-off the principal voice channel and passed through the bandpass filter 15. More particularly, the phase shifter 16 includes, as principal components, a phase shifter network V26 of any conventional design and a pair of amplifiers 2S, Z9, all of which are interconnected by a number of suitable coupling transformers. The phase shifter provides a control signal which is combined with the original voice signal in mixer circuit 12 to cancel the noise band. The amplifiers bring the phase shifted signal to a predetermined signal strength which is then appiied to the mixer circuit 12.

The amplifier circuit 23 includes a PNP transistor arranged in a common emitter configuration. A base biasing potential is applied to transistor 28 by a voltage divider including a resistor 3ft and the second-ary winding of a coupling transformer 31, the voltage divider being connected between battery and ground. The collector load of transistor 28 is completed to battery by way of the primary winding of a third coupling transformer 35 and a load dropping resistor 36. The base to emitter bias is provided by the impedance of the secondary winding 31. A decoupling capacitor 37 is connected between the primary winding of transformer 35 and ground.

In this circuit, the coupling transformer 35 is a high impedance device provided so that other voice channels may be connected in parallel and driven from a common phase shifter circuit, such as 16. T o match the high imedance of transformer 35 to the relatively low impedance of the second amplifier '29, a second coupling transformer 4t) is connected to the secondary winding of transformer 35. A loading resistor 41 is connected in parallel across both the secondary winding of transformer 35 and the primary winding of transformer 46 to provide impedance matching. lf there is no need to provide other parallel voice channels, both of the transformers 35, 40 may be omitted.

To provide a fine adjustment in the phase angle of the bled-off signal, a network including a capacitor 42 is connected in parallel and a pair of resistors 43, 44 are connected in series with the secondary winding of coupling transformer 40. As well known to those skilled in the art, current passing through a capacitor leads the voltage by a 90 phase angle. `Current passing through a linear resistor is in phase with the voltage. Therefore, the resultant impedance of circuit 42-44 has a phase angle between and 90. The exact angle is determined by the selection of the circuit components.

The amplifier circuit 29 includes a PNP transistor connected in a common emitter configuration. The base bias for transistor 29 is provided by a voltage dividing network which may be traced from ground through a gain control potentiometer 46, a resistor 47, the primary Winding of a fourth coupling transformer t), and a voltage dropping resistor 51 to battery. yConnected to the emitter is a bypass and decoupling capacitor 52. The resistor 53 is avoltage dropping and loading device for matching the impedance of coupling transformer 50 to the impedance of the input to mixer circuit 12.

vMeans are provided for mining the voice signals occurring in the principal channel 13 and the phase shifted signals received from the phase shifter 16 to cancel the band of frequencies which coincide with the resonant frequencies of the public address system. More particularly,

the circuit for accomplishing this function includes a pair of PNP transistors 6i), 61 each of which is connected in a common emitter configuration.

The base bias for transistor 6ft is provided by the voltage divider including the series connected resistors 62, 63 connected between battery and ground while the base bias of transistor A61 is provided by the series circuit traced from battery through resistors v64, 65, and the secondary winding of transformer Z@ to ground. The emitter of transistor is biased by the voltage drop across resistor 53 and by the phase shifted signal applied via the coupling transformer 50. The collector load on transistor 60 results from the internal resistance of transistor 61. The collector to base bias on transistor 61 results from the voltage drop across resistor 65, while the base to emitter bias for transistor 61 is provided by the internal resistance of transistor 6&1.

4 The voice signals appearing in the principal channel 13 are applied across the coupling transformer 20 to the base or control electrode b of transistor 61 and the phase shifted signal is applied to the emitter electrode e of tranV sistor 69. The output or collcetor electrode c of transistor 61 is coupled to the base or input electrode b of transistor 6ft via a feedback capacitor 66 and the output or collector electrode c of transistor 69 is connected to the emitter electrode e of transistor 61.

The operation of the mixer circuit 12 is as follows: The original voice signals are applied across the inductive coupling of transformer 2t? to the base electrode b of transistor 61 and the phase shifted signals are applied to the emitter electrode e of transistor 60. The lower transistor 6i) functions as a variable resistor connected in the biasing circuit of the emitter electrode e of transistor 61. More specifically, as the current of the phase shifted control signal rises and falls the current flowing through the emitter-collector circuit of transistor 61 also rises and falls. Since the signals applied to base b and emitter e are out of phase, transistor 60 tends to cut down the emitter current of transistor 61 at the same time that the signal applied from channel 13 to its base b tends to raise the emitter current, and vice versa. The circuit values are selected so that cur-rent flowing through the emitter-collector of transistor 61 tends to remain unefected by input signals applied to its base throughoutthe entire frequency range of the phase shifted signals applied from phase shifter 16 through transistor 60 to the emitter of transistor 61. in other words, the frequencies of the phase shifted signal are eliminated from the signals passing from channel 13 through the mixer circuit 12 to the loud speakers.

Means are provided for giving a sharp cut-off at the upper limits of the frequency band passed through tram;

sistor 61. More specifically, with the control described thus far, there is a tendency for the phase shifted control signal not only to eliminate a band of frequencies, but also to attenuate signals of lower frequency in the band passed through transistor 61. Therefore, the output or collector electrode of transistor 61 is coupled to the input or base electrode of transistor 6i) via a coupling capacitor 66. The capacitor 66 is selected to pass frequencies in the attenuated band and below the eliminated band. Thus, in the frequency band passed by capacitor 66,'the base bias ofV transistor 60 is raised or lowered to counteract the effects produced on the emitter-collector current by the phase shifted signalsv applied through networkY 16.

Preferably, the signal applied through capacitor 66 to the base electrode b of transistor 60 has less effect upon its emitter-collector current fiow than the signal applied to its emitter from phase shifter 16. Thus, any tendency for the cut-ofi` point of the eliminated band to shift will vbe in favor of a cancelation of the noise frequencies thatk cause circuit oscillations. Hence, the upper limit of fre'4 quencies passed through amplifier 61 are given a sharp cutoff without appreciable attenuation of lower than cut-.y

The resistor 72 and the primary Winding of transformer 76 provide the load for the collector circuit of transistor '71. The emitter bias is provided by the voltage drop acrossresistor 75. 'A decoupling capacitor 77 provides a bypass to ground. q is a coupling transformer 76 for Yapplying the amplified voice signals to one or moreloudspeakers via conductors 79. The tuning of capacitor'78 determines the frequency response of the amplifier output.

In one circuit constructed lin accordance with this invention, voice signals originating atY microphones 10,11

In the output circuit of transistor 71' l and applied over principal channel 13 had a frequency range of approximately 200 to 3000 cycles per second. In this exemplary public address system, the resonant frequency range extended from about 2600 to 3000 cycles per second. Therefore, a frequency band of 2600 t 3000 cycles per second, bled-orf in supplementary channel 14, was applied through the phase shifter circuit 16 to the mixer circuit 12. In the mixer circuit 12, the original Voice signals appearing in channel 13 and the phase shifted signals fed through circuit 16 were combined with each other to cancel output signals in the resonant band. With acoustical coupling between the loudspeakers and the microphone, the system did not break into oscillation when a loudspeaker having a -watt audio output was positioned immediately above and behind the head of a person speaking into the microphone.

While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

We claim:

l1. A public address system having a resonant frequency band width, means for transmitting voice frequency signals through said system, said signals including frequency falling in said resonant band width, means in said system for canceling that portion of the signals which includes frequencies falling in said band width, said canceling means including a pair of semiconductor devices, one of said devices having an input electrode coupled to be energized by said voice signals and an output electrode coupled to an input electrode of the other of said devices, an output electrode of said other device being connected to control the bias on a third electrode of -said one device, means for applying at least a portion of said voice signals phase shifted to a third electrode of said other device, and means also coupled to the output electrode of said one device for broadcasting sounds corresponding to said voice frequencies less said canceled band Width.

2. A public address system having a resonant frequency band width, means for transmitting voice frequency signals through said system, said signals including frequency falling in said resonant band width, means in said system for canceling that portion of the signals which includes frequencies falling in said band width, said last named means including a pair of scmicoductor devices, one of said devices having an input electrode coupled to be energized by said voice signals and an output electrode coupled to an input electrode of the other of said devices via a capacitor, said capacitor being tuned to pass frequencies below the upper limit of said band width, an output electrode of said other device being connected to control the bias on a third electrode of said one device, means for applying a portion of said voice signals falling in said band width, phase shifted to a third electrode of said other device, and means also coupled to the output electrode of said one device for broadcasting sounds corresponding to said voice frequencies less said canceled band width.

t3. A circuit for preventing oscillation responsive to acoustical coupling in a public address system comprising at least a microphone and a loudspeaker interconnected by an amplifier, there being acoustical coupling between said loudspeaker and said microphone, means interposed between said microphone and said amplifier for bleedingoff a portion of the voice signals transmitted from said microphone to said amplifier, means for shifting the phase of said bled-off signal, means including a mixer circuit for combining said phase shifted signals and said voice signals, said mixer circuit comprising a pair of transistors, one of said transistors having its input electrode coupled to said microphone and an output electrode coupled to an input electrode of the other of said transistors, an output electrode of said other transistor being connected to control the bias on a third electrode of said one transistor,

means for applying said phase shifted signal to a third electrode of said other transistor, and means for driving said loudspeaker by said combined signal emanating from said mixer circuit.

4. A circuit for preventing oscillation responsive to Iacoustical couplings in a public address system comprising =a principal voice channel for carrying voice frequency signals, means including an amplifier in said principal voice channel for amplifying signals appearing therein, a loud speaker coupled to be driven by said amplified signals, la supplementary voice channel coupled to said principal voice channel for bleeding-olf a portion of the signals appearing therein, bandpass filter means in said supplementary channel for passing a band of frequencies which coincides with the resonant frequencies of the public address system, Kmeans for shifting the phase of said voice signals passed thro-ugh said bandpass filter means, Kand a mixer circuit comprising a pair of transistors, one of said transistors having an input electrode coupled to said principal voice channel and an output electrode coupled to an input electrode of the other of said transistors, -an output electrode of said other transistor being connected to control the bias on a third electrode of said one transistor, iand means for applying said phase shifted signal to -a thir-d electrode of said other transistor.

5. A circuit for preventing oscillation responsive to acoustical couplings in la public address system comprising -a principal voice channel for carrying voice frequency signals, means including an' iamplifier in said principal v-oice channel for amplifying signals appearing therein, a loud speaker coupled to be drive by said amplified signals, la supplementary voice channel coupled to said principal v-oice channel for bleeding-off la portion of the signals appearing therein, bandpass filter means in said supplementary channel for passing a band of frequencies which coincides with the resonant frequencies of the public address system, means for shifting the phase of said voice signals passed through said bandpass filter means, and a mixer circuit comprising a pair of transistors, one of -said transistors having an input electrode coupled to said principal voice channel Kand an output electrode coupled via a capacitor to an input electrode of the other off said transistors, said capacitor being tuned to pass frequencies below the upper limit of said band Y of frequencies, an output electrode 'of said other transistor being connected to control the bias on' la third electrode of said one transistor, `and means for applying said phase shifted signal to a third electrode of said other transistor.

6. A mixer circuit for combining two electrical signals comprising `a pair of transistors, one of said transistors having lan input electrode coupled to be energized by one of said signals and an output electrode coupled to an input electrode of the other of said transistors, an output electro-de of said other transistor being connected to control the bias on a third electrode of said one transistor, means for applying the other of said signals to `a third electrode of said other transistor, and an output circuit coupled to the output electrode of said one transistor.

7. In -a mixer circuit for combining two electrical signals utilizing means for phase shifting one of said signals relative to the other, the combination therewith comnals, one of said signals including a narrow band of fres quencies, Vthe other of said signals having a wide band Width which includes said narrow band width, the combination comprising a pair of transistors, one of said transistors having an input electrode coupled to he energized 'by saidk other signal :and its output electrode coupled via a capacitor to an input electrode of the other of said transistors, said capacitor bein-g tuned to pass frequencies in said wide band width immediately outside of said narrow band, an output electrode of said other transistor being connected rto control the bias on a third electrode of said one transistor, means for applying said narrow band width in phase shifted relation to said wide band to a third electrode of said other transistor, tand an output circuit means coupled to the output electrode of said one transistor.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Van N ostrand, The International Dictionary of Physics Y and Electronics (pages 133-134 relied on).

Claims

1. A PUBLIC ADDRESS SYSTEM HAVING A RESONANT FREQUENCY BAND WIDTH, MEANS FOR TRANSMITTING VOICE FREQUENCY SIGNALS THROUGH SAID SYSTEM, SAID SIGNALS INCLUDING FREQUENCY FALLING IN SAID RESONANT BAND WIDTH, MEANS IN SAID SYSTEM FOR CANCELING THAT PORTION OF THE SIGNALS WHICH INCLUDES FREQUENCIES FALLING IN SAID BAND WIDTH, SAID CANCELING MEANS INCLUDING A PAIR OF SEMICONDUCTOR DEVICES, ONE OF SAID DEVICES HAVING AN INPUT ELECTRODE COUPLED TO BE ENERGIZED BY SAID VOICE SIGNALS AND AN OUTPUT ELECTRODE COUPLED TO AN INPUT ELECTRODE OF THE OTHER OF SAID DEVICES, AN OUTPUT ELECTRODE OF SAID OTHER DEVICE BEING CONNECTED TO CONTROL THE BIAS ON A THIRD ELECTRODE OF SAID ONE DEVICE, MEANS FOR APPLYING AT LEAST A PORTION OF SAID VOICE SIGNALS PHASE SHIFTED TO A THIRD ELECTRODE OF SAID OTHER DEVICE, AND MEANS ALSO COUPLED TO THE OUTPUT ELECTRODE OF SAID ONE DEVICE FOR BROADCASTING SOUNDS CORRESPONDING TO SAID VOICE FREQUENCIES LESS SAID CANCELED BAND WIDTH.

6. A MIXER CIRCUIT FOR COMBINING TWO ELECTRICAL SIGNALS COMPRISING A PAIR OF TRANSISTORS, ONE OF SAID TRANSISTORS HAVING AN INPUT ELECTRODE COUPLED TO BE ENERGIZED BY ONE OF SAID SIGNALS AND AN OUTPUT ELECTRODE COUPLED TO AN INPUT ELECTRODE OF THE OTHER OF SAID TRANSISTORS, AN OUTPUT ELECTRODE OF SAID OTHER TRANSISTOR BEING CONNECTED TO CONTROL THE BIAS ON A THIRD ELECTRODE OF SAID ONE TRANSISTOR, MEANS FOR APPLYING THE OTHER OF SAID SIGNALS TO A THIRD ELECTRODE OF SAID OTHER TRANSISTOR, AND AN OUTPUT CIRCUIT COUPLED TO THE OUTPUT ELECTRODE OF SAID ONE TRANSISTOR.