GB2249888A - Sound level control equipment - Google Patents

Abstract

The sound level of a sound system is automatically adjusted to compensate for environmental noise. A sound level adjusting device 21, such as a variable resistance device 30, is interposed between an amplifier and a loudspeaker of the system, and a microphone 22 is used to monitor environmental noise. The variable resistance device and the microphone are connected to a control circuit and this prevents over-adjustment of the sound level as noise increases by ensuring that a second adjusted increase in sound level only occurs when the noise level increase (say 4db) is greater than the first adjusted increase in sound level (say 3db). The system may be used for controlling the level of coin-operated music systems. <IMAGE>

Description

SOUND LEVEL CONTROL EQUIPMENT This invention relates to automatic sound level control equipment.

Coin-operated music playing machines (juke boxes) are commonly provided at sites, such as public bars and like establishments, where the amplitude of background noise can vary widely throughout a period of time. Thus the noise amplitude may be low at the start of an evening when there are few people present; the amplitude may rise to a much higher level later in the evening when there are many people present engaged in conversation; and there may be occasional large increases in amplitude for example when a group of people are laughing or celebrating loudly.

In our copending application No. 8900754.6 (Serial No. 2214013) there is described automatic level control equipment for use with a sound system having sound reproducing apparatus driven by an amplifier, said equipment comprising; a device interposable between the amplifier and the sound reproducing apparatus, the device being adjustable to change a characteristic of the output fed from the amplifier to the apparatus and hence the level of the sound produced thereby; a sound sensor for sensing sound in the environment of the sound reproducing apparatus; and a control circuit connected to the sensor and to the adjustment device to effect said adjustment of the device in correspondence with a selected parameter of the said environment sound, said control circuit including a filter device arranged to be responsive to a frequency or band of frequencies of sound produced by said sound system so as to modify said adjustment of said device in correspondence therewith.

With this arrangement automatic adjustment of the sound level can be achieved in a simple, convenient and reliable manner. Due to the provision of the filter device it is possible to reduce or eliminate pick-up by the sound sensor of a monitored frequency derived from the sound system rather than from background noise.

Whenever the monitored frequency is present in the output of the sound system, adjustment of the sound level may be inhibited, and, with a view to avoiding or at least minimising this inhibition of adjustment, our copending application No. 8927217.3 proposes automatic level control equipment for use with a sound system having sound reproducing apparatus driven by an amplifier, said equipment comprising: a device interposable between the amplifier and the sound reproducing apparatus, the device being adjustable to change a characteristic of the output fed from the amplifier to the apparatus and hence the level of the sound produced thereby; a sound sensor for sensing sound in the environment of the sound reproducing apparatus; and a control circuit connected to the sensor, the adjustment device and the output from the amplifier, said control circuit incorporating multiple sensor filters corresponding to different respective frequency bands of environment sound and being arranged to effect said adjustment of the device in correspondence with the level of said environment sound within at least one selected said frequency band; said control circuit further incorporating at least one output filter corresponding to a frequency band of amplifier output sound whereby said selection of the (or each) said frequency band of environment sound is determined in dependence on the level of said output sound within the respective frequency band.

With this arrangement it is possible to monitor environment sound at different frequency bands in correspondence with monitoring of a frequency band in the amplifier output whereby, for example, whenever the amplifier output has a content of the same frequency band as that being monitored in the environment sound, the circuit can operate to switch to a different monitored frequency band of the environment sound. In this way interrupted monitoring can be avoided or minimised.

With the control equipment of the above mentioned patent applications, in so far as the sound level output of the amplifier increases to follow increase in environmental sound level, the problem can arise in practice that this has the affect of causing increase in environmental sound. That is, if there is a prompt increase in amplifier output to compensate for increase in sound derived from persons talking, this may cause the persons to increase the volume of their conversation to match the increase in amplifier output, whereupon the amplifier output again increases and the overall sound level can become unduly loud.

An object of the present invention is to avoid or at least minimise this problem.

According to the invention therefore there is provided automatic level control equipment for use with a sound system having sound reproducing apparatus driven by an amplifier, said equipment comprising: a device interposable between the amplifier and the sound reproducing apparatus, the device being adjustable to change a characteristic of the output fed from the amplifier to the apparatus and hence the level of the sound produced thereby: a sound sensor for sensing sound in the environment of the sound reproducing apparatus; and a control circuit connected to the sensor and the adjustment device to effect said adjustment of the device in correspondence with level of the said environmental sound; characterised in that following an increase in said apparatus-produced sound level such sound level is not again increased until the sensed environmental sound level has increased by a greater amount than the said increase in said apparatus-produced sound level.

With this arrangement, it will be appreciated that in the case where environmental sound level increases to match increase in apparatus-produced sound level, there is no further increase in the latter sound level until there has been a further increase in environmental sound level signifying that environmental sound level is in fact increasing under its own impetus rather than being driven up by the action of the equipment.

By way of example, instead of having an arrangement whereby say a 3db increase in apparatus output followed by a 3db increase in environmental sound triggers a further 3db increase in apparatus output, with the present invention, after a 3db increase in apparatus output, an increase of say 4db in environmental sound would be required to trigger a further increase in apparatus output.

With regard to the adjustment device, this may be a variable resistance device. The sensor may comprise a microphone. The control circuit may comprise filters whereby monitoring of the environmental sound may be effected within one or more predetermined frequency bands. Reference is made to the aforementioned prior applications for further description of features which may be incorporated in the present invention.

The invention will now be described further by way of example only and with reference to the accompanying drawing which is a block circuit diagram of one form of equipment according to the invention.

The embodiment illustrated comprises a housed unit 21 with a microphone 22, and input and output terminals 23, 24 respectively for connection to an amplifier and to a loudspeaker.

The microphone is connected via a preamplifier stage 25 to a filter and detection stage 26, which is operational at one or more narrow frequency bands in the range 500-600 Hz. The output of the stage 26 is fed to a microprocessor-based control circuit 28 incorporating an analogue-to-digital converter. The pre-amplifier stage 25 is also connected to a re-set input of the control circuit 28 via a noise switch 29. The control circuit 28 controls an audio attenuation device 30 which is interposed between the input and output terminals 23, 24.

A power supply 31 is provided for operation of the unit 21 and this comprises a rechargeable Ni-Cd battery together with circuitry for charging the battery from audio signals at the input terminals 23.

The operation of the unit is as follows: After installation? the unit 21 is permanently powered. Back up power during a standby or idle state (i.e. when there is no audio signal at the input terminals 23) is derived from the battery in the power supply 31. Only the microprocessor-based control circuit 28 (PIC-processor) in a 'sleeping' mode, and the noise-switch 29 with associated circuitry (forming part of the pre-amplifier stage 25) are powered and they have a very low level of comsumption.

In this mode, the pre-amplifier stage 25 is configured to amplify over a wide frequency bandwidth and responds to any noise picked up by the microphone 22. The output of the pre-amplifier stage 25 is compared with a predetermined value. If the rate at which this value is exceeded, exceeds a predetermined rate, a re-set circuit is initiated which 'wakes up' the PIC-processor.

Immediately the processor is awakened it does the following: inhibit the re-set circuit; re-configure the pre-amplifier stage 25 into a broad low-pass filter; connect power supply to the entire unit; commence main operation.

Approximately every second, simultaneous reading of ambient noise (at a narrow frequency band centred on 510 Hz) and detection of any component of the same frequency band in the audio signal at the input terminals 23 is carried out.

Assuming that no 510 Hz component is present in the audio signal, a figure for the average ambient noise level is calculated (in decibels). At approximately 30 second intervals the attenuation level is calculated using the average ambient noise figure. If an alteration of attenuation is indicated, this is effected. If there is a 510 Hz component in the audio signal alteration of attenuation is inhibited or monitoring may be effected at a different frequency band as discussed in the aforementioned prior applications.

The ambient noise level is measured by filtering the signal after pre-amplification using a very narrow 3Hz bandwidth filter. The filtered signal is then rectified and smoothed before passing it to the analogue-to-digital converter and the control circuit 28. The control circuit 28 reads the digital value as the ambient level.

The control circuit 28 stores in memory the last implemented positive attenuation level. Thus, for example, if the last attenuation implemented corresponded to an increase in sound level of 3db, this value of 3db is stored in memory. At the same time the increase in ambient noise level over the level at which there was last an alteration of attenuation is monitored and stored.

These stored parameters are interrelated and adjustment of attenuation such as to increase the level of sound output is only effected in the case where the increase in ambient noise level is greater than the last implemented positive attenuation by a predetermined margin (which may be a percentage of the last implemented positive attenuation level or which may be a fixed numerical value). Thus, for example, where the last implemented positive attenuation corresponded to 3db, the required increase in ambient noise level to initiate further attenuation adjustment may correspond to say 4db.

The attenuation adjustment then implemented may correspond to that required to match the increase in ambient noise level (in the above example 4db) or it may be a different figure (e.g. a predetermined figure of say 3db) or a figure which is a predetermined numerical value or percentage less than or more than the increase in ambient noise level.

If the level of ambient noise at the monitored frequency has been at or near zero for a period of time, the microprocessor turns off the power supply to unnecessary parts of the circuit and the microprocessor returns to its 'sleep' mode. The attentuation device will be at its maximum level in response to the detected minimum level of ambient noise. The relative volume of audio output to ambient noise is set by adjustment of an attenuating potentiometer in the microphone pre-amplification stage 25.

During operation, the audio signal is sufficient to power the complete circuit and also recharge the Ni-Cd battery. If there is no audio signal within a predetermined time after being 'awakened' the microprocessor sets a maximum attenuation of music and resumes its 'sleep' in order to preserve battery power.

The above described embodiment may be used in the context of a music system where people-derived background noise is monitored. The invention is however not restricted to this and it may be used with other systems (e.g. a speech amplification system) and/or to monitor other background noise or for use with vehicle audio systems.

By requiring there to be a predetermined excess of ambient noise before adjustment is effected it can be ensured that there is little or no tendency for the equipment to give rise to increase in ambient noise level.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only.

Claims (5)

1. Automatic level control equipment for use with a sound system having sound reproducing apparatus driven by an amplifier, said equipment comprising: a device interposable between the amplifier and the sound reproducing apparatus, the device being adjustable to change a characteristic of the output fed from the amplifier to the apparatus and hence the level of the sound produced thereby: a sound sensor for sensing sound in the environment of the sound reproducing apparatus; and a control circuit connected to the sensor and the adjustment device to effect said adjustment of the device in correspondence with level of the said environmental sound; characterised in that following an increase in said apparatus-produced sound level such sound level is not again increased until the sensed environmental sound level has increased by a greater amount than the said increase in said apparatus-produced sound level.

2. Equipment according to claim 1 wherein the adjustment device is a variable resistance device.

3. Equipment according to claim 1 or 2 wherein the sensor comprises a microphone.

4. Equipment according to any one of claims 1 to 3 wherein the control circuit comprises filters whereby monitoring of the environmental sound is effected within one or more predetermined frequency bands.

5. Automatic level control equipment substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.