US3409738A

US3409738A - Volume controlled audio program broadcasting

Info

Publication number: US3409738A
Application number: US448784A
Authority: US
Inventors: Ross M Heald; Donald L Hadden
Original assignee: RIMAC Ltd
Current assignee: RIMAC Ltd
Priority date: 1965-04-16
Filing date: 1965-04-16
Publication date: 1968-11-05
Anticipated expiration: 1985-11-05

Description

Nov. 5, 1968 R. M. HEALD ET AL 3,409,738

VOLUME CONTROLLED AUDIO PROGRAM BROADCASTING Filed April 16, 1965 United States Patent ABSTRACT F THE DISCLOSURE The level of an audio transmission channel is controlled by the ambient noise level at the loudspeaker. A slow attack time and a rapid decay time are used to prevent overcompensation. Feedback is prevented by sampling noise only within a very narrow band of the audio spectrum andusing a band rejection filter to eliminate that band from the transmission channel.

This invention relates to an audio program broadcasting system which is particularly advantageous for the broadcasting of background music and/or voice announcements in localized listening areas, such as restaurants, banks, offices and hospitals.

Background music systems have proved to be advantageous in a variety of practical applications. In restaurants background music helps to provide a pleasant atmosphere for dining. In offices or banks it enhances the efficiency of workers performing repetitive clerical tasks. In hospitals it tends to relax the patients. Ideally such a system should provide a music background which is audible, but not distracting, to people in the area being served since normally those people are there for a purpose other than to just to listen to music.

One of the problems associated with such systems heretofore has been the differences in the noise level in the listening area at different times of the day. For example, at the hours of peak activity in a restaurant, the noise level due to conversations, the clatter of dishes, etc., will be appreciably higher than at the hours of less activity, and it is virtually impossible to set the background music at a volume level Which will be satisfactory under all such widely different conditions. A background music level high enough to be audible at peak activity hours would be distractingly loud at other times and, conversely, a `background music level suitable for the less active hours would be virtually inaudible during the peak activity hours.

The present invention is directed to a novel broadcast system which overcomes this difficulty by providing a novel and advantageous arrangement for regulating the volume level of the background music or other audio program in accordance with the noise level in the listening area.

Accordingly, it is a principal object of this invention to provide a novel and improved audio program broadcasting system which is particularly advantageous for, but not limited to, the broadcasting of background music in localized areas, such as restaurants, banks, offices and hospitals.

Another object of this invention is to provide such a system having novel provision for automatically maintaining the volume of the background music at a suitable level with respect to the noise in the listening area.

3,409,738 Patented Nov. 5, 1968 Another object of this invention is to provide such a system which avoids undesired acoustic feedback between the audio program being broadcast and the control which regulates the broadcast level of that program, so that the control responds only to noise originating in the listening area and is unaffected by the audio program itself which is broadcast there.

Another object of this invention is to provide such a system which essentially responds only to a continuing noise level in the listening area and is substantially'insensitive to untypically loud noises of short duration.

Another object of this invention is to provide such a system which will shut off the broadcast program in case the regulating control fails.

Another object of this invention is to provide such a system having provision for selectively broadcasting local voice messages, such as announcements or paging calls, to the listening area.

Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment thereof, which is illus trated in the accompanying drawing. l

In the drawing:

FIGURE 1 is a block diagram of one embodiment of the present system; and

FIGURE 2 is a circuit diagram of a portion of this system.

Referring first to FIG. 1, the system shown therein includes an audio program source 10 located at a central station remote from the listening areas where the program is to be broadcast, such as restaurants, banks, offices or hospitals. The program to be broadcast usually will be composed primarily of background music selections, sometimes interspersed with voice programs, such as news or announcements. In this embodiment of the present system, the audio program is transmitted over a telephone line 11 to a receiver at each of the listening areas.

In accordance with this particular embodiment of the present invention, a band rejection filter 12 at the centralI station is connected between the output of the program source 10 and the end 13 of the telephone line 11 there. This filter eliminates from the outgoing audio program all signals falling within a predetermined frequency bandwidth. The location of this rejection lband or slot in the audio frequency spectrum is critical to the practical operating success of the present system, as explained hereinafter. In accordance with the present invention this rejection band is located between 500 and. 4000 cycles per second and is substantially less than one octave wide, preferably less than one-quarter octave. In the preferred embodiment the rejection slot or band is from substantially 1540 to 1760 cycles per second, which is less than one-quarter octave in width.

At the remotely located customers premises, the other end 14 of the telephone line is connected through a variable loss device 15 to an amplifier 16 leading to one or more loudspeakers 17 serving a particular listening area. Additional amplification stages (not shown) may be provided between the amplifier 16 and each loudspeaker 17. The variable loss device 15 and amplifier 16 together constitute a variable volume control arrangement which determines the acoustic volume level of the audio program at the listening area.

4One or more noise-sensing microphones 18 are located at typical locations in the same listening area to sense the 3 sound #level Ythere: While -each -such microphone A18 may pick up all sounds in the entire audio frequency range, its output is connected to a band pass filter 19 which passes only those signals falling entirely within the rejection band of the broadcast program, and preferably only those within a middle portion of this rejection band. In the preferred embodiment, the band pass filter 19 passes only signals within a pass band from 1610 to 1690 cycles per second. It will be apparent that none of the signal frequencies .passed by filter 19 `would be due to the audio program :being-.broadcast at the loudspeaker or loudspeakers 17 because all of these ysignal frequencies were eliminated from 'the programby thev rejection filter 12 at the program source-Therefore, allsignals passed by filter 19 will be dueentirely to noise originating at the listening area.

These noise signals are passed from filter 19 through .'.an amplifier 20 to a rectifier 21. The rectifier 21 produces a=D.C. control signal whose magnitude normally is proportionalto the magnitude of the amplified noise input signalsto lrectifier 21. This D.C. control signal is passed throught-another amplifier 22 to the variable loss device 15 in the audio broadcast program circuit.

A peak clipper 23 is connected between the output of amplifier 22'and rectifier 21. Its function is to limit the amplitude of the D.C. control signal applied to the variableloss device 15, as explained hereinafter.

A freeze control 24 is connected to rectifier so that the operator at the customers premises may, by operating this control, lock or freeze the D.C. control signal applied to the variable loss device 15, as explained hereinafter.

As explained in detail later on, the coupling between this noise feedback control circuit and the audio broadcast program circuit at the customers premises includes means providing a slow attack time constant when the noise level is increasing and a fast decay time constant when the noise level is decreasing. Accordingly, the audio program level increases relatively slowly in response to yan increasing noise level in the listening area, but it decreases substantially faster in response to a decrease in the noise level.

The portion of the receiver circuit at the customers premises which includes the variable loss device 15, amplifier 16, rectifier 21, amplifier 22, peak clipper 23, and freeze control 24 are shown in detail in FIGURE 2.

Referring to this figure, the amplified pass band noise -signal output from -amplifier 20 in FIG. 1 is applied across

terminals

25 and 26 in FIG. 2, terminal 26 being grounded, and terminal 25 being connected to a condenser 25a. A first rectifier diode 27 passes the negative half cycle of this noise signal, and a second rectifier diode 28 passes the positive half cycle of this noise signal. Diode 28 is in seriesl with a resistor 29 across diode 27. This arrangement corresponds to the rectifier block 21 in FIG. l and itconstitutes a full wave rectifier and voltage doubler for the noise signals falling within the pass band of'filter l19 in FIG. 1. A resistor 30 is connected between ground and the junction point 31 of diode 28 and resistor 29.

The output of this rectifier is connected through a pair of series-connected

resistors

31, 32 to the base of a P-N-P transistor 33. A condenser 34 is connected between the input side of resistor 31 and ground for the purpose of filtering the rectified noise signal. t A large capacity condenser 35 is connected between the 4base of transistor 33 and ground through a first set of normally closed contacts k-l of a relay K. Due to its large capacity, condenser 35 causes the D.C. variations at the vbase of transistor 33 to be of long time constant, preferably about one minute, when this 4D.C. voltage is rising due to increasing noise in the listening area. This is the slow attack time constant arrangement referred to previously.

A diode 36 is connected across

resistors

31 and 32 and its polarity is such that when the D C. voltage at the base .f4 I." 12.14 f* of transistor 33 is falling, lduefto decreasing no'iseirr the listening area, this diode conducts current and provides a relatively fast decay time constant, preferably about 10 seconds.

With this arrangement therefore, the D.C. voltage at the lbase of transistor 33 will increase slowly in response to increasing noise in the listening area, but will decrease at a substantially faster rate inresponse to decreasing noise in the listening area.

A resistor 37 is connected between the emitter of transistor 33 and ground. Also, a resistor 38 and a diode 39 are connected in series with leach other between-this'emitter and ground, Diode'3 9 is afvaria'ble loss device, having the characteristic that 'an increase 'ini thev direct current through it will proportionatelvredu 4itsAQCfj resistance.

The collector of transistorr33 is connected directly to the base of an N-P.-N transistor- 40. Both are connected through a feedback circuit composed of a resistor 41 and diode 42 to the junction lpoint'43 between -

resistors

31 and 32. The junction point 44 between resistorV 41 and diode 42 is connected directly to the collector of transistor 40 and also is connected to ground through parallel-'connected resistor 45 and 'condenser 46.

The emitter of transistor 40 is connected through! a resistor47 to a junction point 48. The collector of transistor 33 is connected to this same junction point 48 through a series-connected resistor 49 and diode 50 of one polarity, and also through series-connected resistor 51, opposite polarity diode 52 and resistor 53. A negative battery terminal 53a is connected directly tothe anode of diode 52 and to the upper end of resistor 53 in FIG. 2. A pair of parallel-connected resistors 54 and 55 are connected between junction point 48 and the aforementioned junction point 31.

Transistor 33 provides the amplification stage of the rectified noise signal amplifier block 22 in FIG. 1.

Diode 39 is part of the variable lossdevice block 15 in FIG. 1.

Resistor 30' provides an initial bias which makes up for the initial voltage required to insure that transistor 33 is just on the verge of conduction in the absence of a noise control signal.

In operation, the rectified noise signal at the base 0f transistor 33 produces a D.C. signal at its collector, which is applied to the base of transistor 40. Diodes 50 and 52 provide temperature compensation in this circuit. The amplified D.C. signal appearing at the collector of transistor 40 is in phase with the input signal to the' base of transistor 33. That is, an increase in the D.C. input signal to the base of transistor 33 will produce an amplified increase in the D.C. voltage at the collector of transistor 4 0. This amplified D.C. voltage is fed back via diode 42 to the junction point 43 between

resistors

31 and 32.

This circuit provides a peak-clipping action (as represented by the block 23 in FIG. 1) for the noise signals as follows:

If a sudden peak of the noise signal appears at the output of the rectifier it will not get through at a high level to charge up condenser 35 unduly because it will be clipped at a level determined by the D.C. voltage on the collector of transistor 40, which is fed back to junction point 43.`

Resistors

31, 32, 49, 51, 41 and 45 determine this level. In the preferred embodiment, the circuitry is such that noise signals greater than 10 decibels above the average noise level then prevailing are clipped.

The audio program input signals coming in over the telephone line 11 in FIG. 1 are applied between an input terminal 60 and ground in FIG. 2. These program input signals pass through an adjustable volumecontrol resistor 61 and a coupling condenser 62 to the base of a P-N-P transistor 63, which provides the first amplification stage of the amplifier block 16 in FIG. l. The emitter of this transistor is connected to ground through a Iresistor 64 and also through a capacitor 65 and the aforementioned diode 39.

A resistor 66 is connected between the adjustable contact on the volume control resistor 61 and ground. A resistor -67 is connected between the base of transistor 63 and ground. A pair of .

resistors

68 and 69 are series-connected between the collector and base of transistor 63.

As already mentioned, diode 39 is a variable loss device whose A.C. impedance varies inversely with the D.C. curkrent through it. The D.C. current through diode 39 is proportional to the lrectified noise signal input to transistor 33. Accordingly, if this D.C. current through diode 39 increases, its A.C. impedance will decrease and the gain of transistor 63 will increase because of the reduction in the emitter negative feedback. Conversely, if the D.C. current through diode 39 decreases, its A.C. impedance will increase and the gain of transistor 63 will decrease. Thus, an increase in the noise signal will increase the gain of transistor 63, and a decrease in the noise signal will decrease the gain of transistor 63. This relationship is substantially linear over a 30 decibel range in the preferred embodiment of the present system.

If the D.C. control signal disappears, due to a failure in the noise signal feedback portion of this circuit, then the music program will be shut off completely. This is advantageous in that such an equipment failure will shut off the program, instead of permitting it to rise to a maximum volume level as would be the case if the coupling between the noise signal feedback portion and the audio program portion of the receiver were through a Variable gain (as contrasted with Ia variable loss) device.

The collector of transistor 63 is connected through a coupling condenser 70 t-o the base of a P-N-P transistor 71, which provides the final amplification stage in the amplifier block 16 of FIG. 1. The emitter of transistor 71 is grounded. A resistor 72 is connected between the base and -collector of transistor 71. A resistor 73 and condenser 74 are series-connected between the collector of transistor 71 and ground. The junction point between

resistors

68 and 69 is connected directly to the junction point between resistor 73 and condenser 74.

The collector of transistor 71 is connected through a coupling condenser 75 to a program output terminal 76 leading to the one or more loudspeakers 17 (FIG. l) at the listening area. A resistor 77 is connected between this terminal and ground.

A second, normally-open set of contacts k-2 operated by Irelay K is connected across resistor 77.

Relay K is under the control of a normally open switch 78 adapted to be closed manually by an operator at the customers premises. Switch 78 and relay K provide the freeze control designated by the block 24 in FIG. 1. The energization circuit for the coil of relay K in FIG. 2 is from a positive power supply terminal 79, through a resistor 80, the coil of relay K, and switch 78 to ground. A condenser 81 and resistor 82 are connected in series with each other across this relay coil.

When switch 78 is closed manually, this completes the energization circuit for the c-oil of relay K. This Irelay now opens its first set of contacts k-1 and closes its second set of contacts k-2.

The closing of contacts k-Z short-circuits the program output terminals and thereby stops the audio program signal being -fed to the loudspeakers 17 at the listening area.

The opening of relay contacts k-1 open-circuits condenser 35, preventing it from discharging and thereby freezing the D.C. voltage on it. Consequently, when the operator releases switch 78 and the latter -opens to deenergize relay K, the D.C. noise signal voltage at condenser 35 will be at the same value as just before switc'h 78 was closed. Therefore, the volume level of the audio program, when it resumes in response to the opening of switch 78, will remain substantially the same as it was before being interrupted by the closing of switch 78.

Switch 78 preferably is a push-to-talk switch associated with a local microphone 25 (FIG. 1), which may be used for paging or other local announcements at thecustomers premises. As shown in FIG. 1, this local microphone may be connected through an amplier 26 to a local paging or announcement speaker 27 at the listening area.

As already indicated, when switch 78 is closed, the user may speak through this local announcement system and while he does so the normal audio program is off and its volume level is frozen so that it will resume at the same level when switch 78 is released.

Alternatively, the local announcements may be made through the loudspeaker or loudspeakers 17 which broadcast the normal audio program. In this alternative arrangement, the normal audio program is muted by contacts of the press to talk microphone. Referring to FIG .1, this arrangement includes, in series, a local microphone 25', an amplifier 26 and a band rejection filter 12 feeding into the program input terminal 60 in FIG. 2, so that the local announcement is broadcast while the music is suppressed. The filter 12 rejects all signals within the same frequency band (e.g., 1540-1760 c.p.s.) as the rejection filter 12 at the central program source.

If desired, the system may have a local audio program source 10a at the customers premises in addition to, or in place of, the central program source 10. This local program source feeds through a band rejection filter 12a to the program input terminal 60 in FIG. 2. Filter 12a rejects all signals within the rejection band, preferably 1540-1760 c.p.s. Where both the local program source 10a and the central program source 10 are in the same system, a suitable switching arrangement (not shown) will be provided for selectively connecting and disconnecting them alternatively in the local sound system, so that when one is on the other is off, and vice Versa.

In the operation of the present system, the background music or other normal audio program signals coming from either the central program source 10 or the local program source 10a are fed through the local sound system at the customers premises to lthe loudspeaker or loudspeakers 17 serving a given listening area. A predetermined rejection slot or band of signal frequencies is rejected from this audio program before it is applied to the local system.

At the listening area, one or more microphones 18 sense the noise there, and the noise signals sov sensed are applied to the band pass filter 19, which` passes only the noise signals falling within a predetermined frequency band, which is entirely within the rejection slot or band kept out of the audio program. Consequently, all such noise signals passed by filter 19 will have originated at the listening area and none will be due to the audio program broadcast there. These noise signals are then rectified to provide a D.C. control signal which controls the volume level of the audio program being broadcast there in such a manner that the audio program will be maintained at a predetermined level with respect to the noise level in the listening area. For example, the audio program level may be maintained above, or below, or substantially the same as, the ambient noise level over a wide range of the noise level. Also, if desired, the response of the system may be such that, beginning with a condition of Zero or minimum noise, the audio program level may increase faster or slower or at the same rate as a gradual increase in the noise level (as distinguished from an abrupt and transient noise increase).

The frequency range of the rejection slot or band in the audio program is of particular importance. It has been found that the lower frequency limit of this rejection slot must be not lower than substantially 500 cycles per second. At frequencies below 500 c.p.s. the acoustic power of such signals is relatively great. The noise-sensing microphones 18 normally would respond more or less linearly to such low frequency sounds, whereas the human ear would not. Therefore, the effect which such low frequency sounds would have on the volume control system for the audio program signals would be greatly disproportionate to their actual audibility to the human ear. Consequently, such low frequency noises, even though not readily noticeable or annoying to people in the listening area, would produce an excessive increase in the audio program vol urne level to compensate for them, which in many cases would be more annoying than the noise itself. That is, the control would overcompensate for such low frequency noise signals.

If, on the other hand, the rejection slot or band in the audio program were above 4000 c.p.s., the system would not work as satisfactorily, either. The noise harmonics in this region are low in level, intermittent in nature, and highly directional. For such high frequencies the noise pickup device would have to be too expensive to obtain an omnidirectional pattern. Otherwise, only very local sounds would be picked up. As well, distortion products in the music power amplifier could produce noise frequencies in the noise-sensing band and thus bypass the rejection filter. This could cause the music level to run away. Finally, it has been found that these frequencies are not as representative of all types of noises as those below 4 kc.

In accordance with the present invention the rejection slot in all cases is between 500 and 4000 c.p.s. and is substantially narrower than one octave, and preferably narrower than about one-quarter octave. With such an arrangement, the quality of the background music does not suffer appreciably because the rejection slot is sufiiciently narrow and is above the fundamental frequencies of the majority of musical notes being played. Consequently, at worst the loss of the overtones or harmonics falling within this rejection band will cause the identity of the instrument which produced the fundamental tone to be lost. However, normally the fundamental note itself will not be lost, nor will lbe overtones which fall outside the rejection band, so that the quality of the background music is not noticeably diminished. The preferred rejection slot or band is from substantially 1540 to 1760 cycles per second, which is less than one-quarter of an octave.

Another advantage of the present system is that it responds slowly to an increase in the noise level in the listening area and also clips the noise peaks which are greatly above the average noise level. The slow attack time constant and the noise peak-clipping action in the control portion of the present system together minimize the effect on the audio program volume level of sudden loud noises of short duration in the listening area, such as the noise caused by a tray of dishes dropped in a restaurant. Also the relatively fast decay time constant arrangement in the present system insures the audio program volume level will drop promptly in response to a decrease in the noise level in the listening area.

While a presently preferred embodiment of the present system has been described in detail and illustrated in the accompanying drawing, it is to be understood that the invention is susceptible of other embodiments differing from the particular arrangement shown and that various modifications, omissions and refinements which depart from the disclosed embodiment may be adopted Without departing from the spirit and scope of the present invention. For example, the band rejection filter 12 or 12' shown associated with the audio program source may be connected instead in the audio program receiver at the customers premises, if desired. Also, the program may be transmitted from the central program source by radio communication, such as FM multiplex, instead of by a telephone line, as shown.

We claim:

1. An audio program broadcasting system for use with an audio program source, said system comprising:

loudspeaker means, for broadcasting the audio program from said source to a listening area;

means connected ahead of said loudspeaker means for rejecting all program signals of frequencies Within a rejection band located between 500 and 4000 cycles per second and substantially fless than one octave wide;

means for sensing the noise in said listening area;

and means coupled to said sensing means and responsive only to noise signals falling substantially entirely within said rejection band for regulating the volume level of the audio program broadcast by said loudspeaker means to the listening area.

2. An audio program broadcasting system for use with an audio program source, said system comprising:

loudspeaker means for broadcasting the audio program from said source to listening area;

band rejection filter means connected ahead of said loudspeaker means and operative to reject program signals of frequencies within a rejection band located between 500 and 4000 cycles per second and substantially less than one octave wide;

volume control means connected ahead of said loudspeaker means iand operative to control the volume level of the audio program broadcast at the listening area;

noise-sensing means at said listening area;

band pass filter means connected to said noise-sensing means and operative to pass therefrom only signals falling substantially entirely within said rejection band;

and means coupling the output of said band pass tlter means to said volume control means for regulating the latter in accordance with the noise level in said listening area.

3. A background music broadcasting system for use with a background music program source, said system comprising:

loudspeaker means for broadcasting the music program from said source to a listening area;

band rejection filter means connected ahead of said loudspeaker means and operative to reject all program signals of frequencies within a rejection band located between 500 and 4000 cycles per second and less than substantially one-quarter octave wide;

variable volume control means connected ahead of said loudspeaker means and operative to control the volurne level of the music program broadcast at said listening area;

noise-sensing microphone means at said listening area;

band pass filter means connected to the output of said microphone means and operative to pass therefrom only signals falling substantially entirely within said rejection band;

and means coupling the output of said band pass filter means to said volume control means for regulating the latter in accordance with the noise level in said listening area so as to provide in said listening area volume level of the music program having a predetermined relationship to the noise level thereat.

.4: A background music broadcasting system comprrsing:

:a background music program source;

a band rejection filter connected between said music program source and said loudspeaker means and operative to reject all program signals of frequencies within a rejection band from substantially 1540 to 1760 cycles per second;

variable volume control means connected between said music program source and said loudspeaker means and operative to control the volume level of the music program broadcast at said listening area;

noise-sensing microphone means at said listening area;

a band pass filter connected to the output of said microphone means and operative to pass therefrom only signals falling substantially entirely within said rejection band;

and means coupling the output of said band pass filter to said volume control means for regulating the latter in accordance with the noise level in said listening area so las to provide in said listening area a volume level of the music program at a predetermined relationship with respect to the noise level thereat.

5. An audio program broadcasting system for use with an audio program source, said system comprising:

' noise-sensing microphone means at said listening area;

and means coupling the output of said band pass filter means 'to said volume control means for regulating the latter in accordance with the noise level in said listening area so as to maintain the volume level of the audio program broadcast in said listening area;

i a predetermined relationship with respect to the noise level thereat, said coupling means including a slow attack ltime constant means and noise peak-clipping means for reducing and slowing the response of said volume control means to an increase in the noise level at said listening area and fast decay time constant means for providing a quick response of said volume control means to a decrease in the noise level at said listening area.

6. A background music broadcasting system comprising:

a background music program source;

band rejection filter means connected between said music program source and said loudspeaker means j and operative to reject all program signals of fre- 'quencies within a rejection band from substantially 1540 to 1760 cycles per second;

variable volume control means connected between said music program source and said loudspeaker means and operative to control the volume level of thev music program broadcast at said listening area; noise-sensing microphone means at said listening area;

and means coupling the output of said band pass filter means to said volume control means for regulating the latter to increase the volume level of the music program in response to increasing noise and to decrease the volume level of the music program in response to decreasing noise in said listening area so as to provide in said listening area a volume level of the music program at a predetermined relationship with respect to said noise level thereat, said coupling means including slow attack time constant means and noise peak-clipping means for reducing and slowing the response of said volume control means to an increase in the noise level at said listen- Ying area and fast decay time constant means for providing a quick response of said volume control means to a decrease in the noise level at said listening area.

7. An audio program broadcasting system for use with an audio program source, said system comprising:

loudspeaker means for broadcasting the audio program from said source to a listening area;

band rejection filter means connected ahead of said loudspeaker means and operative to reject all audio program signals of frequencies within a rejection band located between 500 and 4000 cycles per second and less than substantially one-quarter octave wide;

variable volume control means connected ahead of said loudspeaker means and operative to control the volume level of the audio program broadcast at said listening area;

noise-sensing microphone means at said listening area;

means coupling the output of said band pass filter means to said volume control means for regulating the latter in accordance with the amplitude level of the noise signals passed by said band pass filter `means so as to provide at said listening area an audio program volume level having a predetermined relationship with respect to the noise level thereat;

and said volume control means including a variable loss device controlled by said coupling means and operative to shut off the audio program broadcast at said listening area in the absence of an output signal from said band pass filter means.

8. A system according to claim 7, wherein:

said volume control means includes an amplifier for amplifying the audio program signals, and a diode connected in said amplifier to control the latters gain;

and said coupling means includes a rectifier for rectifying the noise output 'signals from said band pass filter means to provide a D.C. control signal whose magnitude is proportional to the amplitude of said noise output signals, and means for applying said D.C. control signal to said diode;

said diode having an A.C. impedance which varies inversely with said D.C. control signal and being operative to increase the gain of said amplifier in response to an increase in said D.C. control signal and to decrease the gain of said amplifier in response to a decrease in said D.C. control signal.

9. A background music broadcasting system comprising:

a background music program source;

band rejection filter means connected between said music program source and said loudspeaker means and operative to reject all music program signals of frequencies within a rejection band from substantially 1540 to 1760 cycles per second;

noise-sensing microphone means at said listening area;

means coupling the output of said band pass filter means to said volume control means for regulating the latter in accordance with the amplitude level of the noise signals passed by said band pass filter means so as to provide at said listening area a volume level of the music program broadcast there having a predetermined relationship with respect to the noise level thereat;

and said volume control means including a variable loss device controlled by said coupling means and operative to shut ofi the music program at said listening area in the absence of an output signal from said band pass filter means.

10. A background music broadcasting system comprising:

a background music program source;

a band rejection filter connected between said music program source and said loudspeaker means and operative to reject all music program signals of frequencies within a rejection band from substantially 1540 to 1760 cycles per second;

noise-sensing microphone means at said listening area;

a band pass filter connected to the output of said microphone means and operative to pass therefrom only signals falling entirely within said rejection band;

means coupling the output of said band pass filter to said volume control means for regulating the latter in accordance with the amplitude level of the noise signals passed by said band pass filter so as to provide at said listening area a volume level of the music program broadcast there having a predetermined relationship With respect to the noise level thereat, said coupling means including slow attack time constant means and noise peak-clipping means for reducing and slowing the response of said volume control means to an increase in the noise signal passed by said band pass filter and fast decay time constant means for providing a quick response of said volume control means to a decrease in the noise signal passed by said band pass filter;

and said Volume control means including a variable loss device controlled by said coupling means and operative to shut oli the music program at said listening area in the absence of an output signal from said band pass filter.

11. A background music program broadcasting system comprising:

a program broadcast station;

a receiving station remote from said broadcast station;

a background music program source at said broadcast station, a band rejection filter at said broadcast station connected to the output of said program source and operative to reject all music program signals of frequencies within a rejection band between 500 to 4000 cycles per second and substantially less than one octave wide;

a telephone line connected between said broadcast station and said receiving station for transmitting the music program signals between them;

loudspeaker means at said receiving station for broadcasting the music program signals to a listening area thereat;

variable volume control means at said receiving station connected between the telephone line and said loudspeaker means and operative to control the volume level of the music program broadcast to said listening area;

noise-sensin g means at said listening area;

a band pass filter connected to the output of said noisesensing means and operative to pass therefrom only signals falling substantially entirely -within said rejection band;

and means coupling the output of said band pass filter to said volume control means for regulating the latter in accordance with the amplitude level of the noise signal output from the band pass filter to provide a music program volume level at said listening area at a predetermined level With respect to the noise level thereat. y 12. A system according to claim 11 and further comprising:

a voice microphone at said receiving station;

a loudspeaker coupled to the output of said microphone for broadcasting voice messages to said listening area;

and means operable selectively to shut off the music program at said listening area while said microphone is in use and to maintain the music program level in said volume control means at substantially the value which it had just before use of the microphone began.

13. A system according to claim 11 and further cornprising:

a Voice microphone at said receiving station;

and means coupling the output of said microphone to said volume control means for passing voice signals to said loudspeaker means to be bradcast at said listening area, said coupling means including a band rejection filter for rejecting voice signals within said rejection band.

14. A background music program broadcasting system comprising:

a program broadcast station;

a receiving station remote from said broadcast station;

a background music program source at said broadcast station, a band rejection filter at said broadcast station connected to the output of said program source and operative to reject all music program signals of frequencies within a rejection band from substantially 1540 to 1760 cycles per second;

noise-sensing microphone means at said listening area;

and means coupling the output of said band pass filter to said volume control means for regulating the latter in accordance with the amplitude level of the noise signals passed by said band pass filter to provide a music program volume level at said listening area at a predetermined level with respect to the noise level thereat.

15. A system according to claim 14, wherein said volume control means includes a variable loss device controlled by said coupling means and operative to shut off 60 the music program broadcast at said listening area in the absence of an output signal from said band pass filter.

16. A system according to claim 14, wherein:

said volume control means includes an amplifier for amplifying the audio program signals, and a rectifier diode connected in said amplifier to control the latters gain;

and said coupling means includes a rectifier for rectifying the noise output signals from said band pass filter to provide a D.C. control signal whose magnitude is proportional to the amplitude of said noise output signals, and means for applying said D.C. control signal to said diode;

said ydiode having an A.C. impedance which varies inversely with said D.C. control signal and being operative to increase the gain of said amplifier in response to an increase in said D.C. control signal References Cited and to decrease the gain of said amplier in re- UNITED STATES PATENTS sponse to a decrease in said D.C. control signal. T

17. A system according to claim 14, wherein said tron "171979 coupling means includes slow attack time constant means 5 2392218 1/1946 Anderson 179:18 and noise peak-clipping means for reducing and slowing 2616971 11/1952 Kannenberg' 79. 1`8 the response ofrsaid volume control means to an increase 3:02:504 2/196-2 Stroud et aT 179 1 X'R in the noise output signal from said band pass filter and 3,160,707 12/1964 Meyers l 179 1 8 fast decay time constant means for providinga quick response of said volume control means to a decrease in 10 KATHLEEN H' CIiAFFY Pnmmy Exammer the noise output signal from said band pass filter. R. P. TAYLOR, Assistant Examiner.