Circuit for canceling oscillating in public address systems

Oct 1, 1963

T. P. MILLER ETAL

3,103,877

CIRCUIT FOR CANCELING OSCILLATING IN PUBLIC annRRSR SYSTEMS

Filed Sept. 12, I960

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BY

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R.N.RWHER

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zrran&amp;r

United States Patent Office ' 3,195,377

Patented Oct. 1, 1963 ¹ 2

3,105,877

CIRCUIT FOR CANCELING OSCILLATING IN PUBLIC ADDRESS SYSTEMS

Thomas P. Miller, Mount Prospect, and Solly L. Fudaley * and Raymond N. Reinertson, Chicago, ffl., assignors to International Telephone and Telegraph Corporation, New York, N.Y., a corporation of Maryland

Filed Sept. 12, I960, Ser. No. 55,396

Claims. (CI. 179—1) . 10

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 15 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 20 back into the microphone as noise which is reamplified to cause oscillation. This fedback sound is commonly called “acoustical coupling.”

In the past, systems for preventing oscillations caused by acoustical coupling have been unduly complex and -5 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 ³⁰or 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 micro- ³⁵phone is placed directly in front of a loud speaker.

Accordingly, an object of this invention is to provide new and improved public address systems, and more particularly to provide public address systems having circuits for canceling the effects of acoustical coupling between ⁴⁰the 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 ⁴³loud speakers used in the system.

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

In accordance with one aspect of this invention, the ₆₅public 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 amplifier is a supplementary channel adapted to bleed-off ₆₀a high frequency portion of the voice signals appearing in the principal channel. The frequencies of this bledoff signal are the same as the resonant frequencies of the public address system. The bled-off signals are then phase shifted by approximately 180° and fed into a mixer circuit which combines the original voice signals and the ^bbphase 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-res- γθ onant 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. 1 shows by block diagram a public address system constructed in accordance with this invention; and

F1G. 2 shows by schematic circuit diagram, the circuitry used to complete the hollow blocks of FIG. 1.

Where possible, simple terms are used and 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 be construed as a disclaimer of the full range of equivalents normally given in patent law. For example, various semiconductor devices are 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 2600 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 how the devices shown and described are entitled to a wide range of equivalents.

Briefly, the principal components of the public address system shown in FIG. 1 include a pair of microphones 1®, 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-off a portion of the voice signal having a relatively narrow band width of frequencies that pass through a bandpass filter 15. The bled-off signal is then fed through phase shifter 16, which shifts the bled-off signal by approximately 180°. This phase shifted signal is applied through the mixer 12 where it is combined with and, therefore, subtracted from the original voice signal. The effect is to eliminate the band of frequencies at which the system is resonant. With this arrangement, the sound amplified at 17 ’and thereafter broadcast from a 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, reference is 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 2©. The resistors 21-23 match the impedance of the coupling transformer 2® to the impedance of the voice channel.

To bleed-off a portion of the voice frequency signals 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 25α 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 26 of any conventional design and a pair of amplifiers 28, 29, all of which are interconnected by a

3,105,877 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 applied to the mixer circuit 12.

The amplifier circuit 28 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 30 and the secondary 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. To match the high impedance of transformer 35 to the relatively low impedance of the second amplifier 29, a second coupling transformer 40 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 40 to provide impedance matching. If 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 0° 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 50, and a voltage dropping resistor 51 to battery. Connected to the emitter is a bypass and decoupling capacitor 52. The resistor 53 is a voltage dropping and loading device for matching the impedance of coupling transformer 50 to the impedance of the input to mixer circuit 12.

Means are provided for mixing 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 60, 61 each of which is connected in a common emitter configuration.

The base bias for transistor 60 is provided by the voltage divider including the series connected resistors 62, 63 connected between battery and ground while the base bias of transistor 61 is provided by the series circuit traced from battery through resistors 64, 65, and the secondary winding of transformer 20 to ground. The emitter of transistor 60 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 60. .

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 tran5 sistor 60. The output or collector electrode c of transistor 61 is coupled to the base or input electrode b’ of transistor 6® via a feedback capacitor 66 and the output or collector electrode c' of transistor 60 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 20 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 tran15 sistor 60 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 20 falls. Since the signals applied to base b and emitter e' are 180° 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 25 values are selected so that current flowing through the emitter-collector of transistor 61 tends to remain uneffected by input signals applied to its base throughout the entire frequency range of the phase shifted signals applied from phase shifter 16 through transistor 60 to the emitter of 30 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 33 upper limits of the frequency band passed through transistor 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 ⁴⁰ passed through transistor 61. Therefore, the output or collector electrode of transistor 61 is coupled to the input or base electrode of transistor 60 via a coupling capacitor 66. The capacitor 66 is selected to pass frequencies in the attenuated band and below the eliminated band. Thus, 45 in the frequency band passed by capacitor 66, the base bias of transistor 60 is raised or lowered to counteract the effects produced on the emitter-collector current by the phase shifted signals applied through network 16. Preferably, the signal applied through capacitor 66 to the 50 base electrode b' of transistor 60 has less effect upon its emitter-collector current flow than the signal applied to its emitter from phase shifter 16. Thus, any tendency for the cut-off point of the eliminated band to shift will be in favor of a cancelation of the noise frequencies that 55 cause circuit oscillations. Hence, the upper limit of frequencies passed through amplifier 61 are given a sharp cut-off without appreciable attenuation of lower than cut-, off frequencies. &gt;

The output signal of mixer circuit 12 is fed through a 60 coupling capacitor 70 to the output amplifier 17. The amplifier 17 includes a PNP transistor 71 connected in a common emitter configuration, the base bias of which is provided by a voltage divider including the resistors 72, 73 and 74 connected between the battery and ground.

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 across resistor 75. A decoupling capacitor 77 provides a bypass to ground. In the output circuit of transistor 71 is a coupling transformer 76 for .applying the amplified voice signals to one or more loudspeakers via conductors 79. The tuning of capacitor 78 determines the frequency response of the amplifier output.

In one circuit constructed in accordance with this invention, voice signals originating at microphones 10, 11

3,105,877 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 to 3000 cycles per second, bled-off 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 5-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.