GB2289583A - Controlling audio level in dependence on ambient noise - Google Patents

Abstract

An adaptive audio system, such as may be used for public address, is provided with an electrical control of its output by means of a circuit whose signal source is a microphone which senses the level of ambient acoustic noise in its locality and which provides an electrical controlling voltage whose amplitude is a function of this level. By this means, the magnitude, or volume, of the system audio output is adapted to the ambient acoustic noise level as sensed by the microphone. The controlling circuit also includes provision for a pre-set threshold 6 and 7 which prevents the adaptive facility from coming into operation until a predetermined level of ambient acoustic noise is sensed by the microphone. This provision avoids continual changes in audio output level in response to operationally insignificant changes in sensed acoustic ambient noise. Time constants and biases may also be set and a display of signal level may be provided. Detection of passengers or rolling stock may also be used for control purposes. <IMAGE>

Description

Description of Invention.

This invention relates to an adaptive audio system for use with public address and other audio systems.

Referring to Fig. 1 hereto, an audio system, such as that commonly used for Public Address Systems at railway stations, airports and other public or private places, often consist of a signal source, such as a microphone, a source of recorded or synthesised speech and/or sound, an off air radio system, etc. 1, a pre-amplifier 2, a power amplifier 3, one or more loud speakers 7 and a power supply 5.

In many cases, such systems must operate in the presence of high and intermittent levels of acoustic noise such as are encountered at railway stations when trains enter or leave, and at airports when aircraft take off Under these circumstances, it may be difficult to set the level of loudness of the public address system such that announcements are intelligible against such high levels of acoustic noise, yet are unobtrusive when noise levels are less, especially at night.

The purpose of the invention to be described, and for which we pray that a patent may be granted, is to vary the level of volume of a public address or other audio system as a function of ambient acoustic noise so as to avoid causing a nuisance to passengers and others in the vicinity of the public address system, while providing a sufficiently high level during the presence of loud noises as to preserve audibility and intelligibility.

A prototype equipment, which is illustrated in block diagrammatic form in Fig.2., has been designed and constructed which achieves these objectives.

It consists ofthe following: 1. An electronic audio amplifier whose gain may be varied by means of an externally applied control signal 1.

2. An audio amplifier 2, which takes the signal from 1 above and amplifies it to a level suitable for driving one or more loudspeakers.

3. A microphone 4 and amplifier 5, the microphone being used to sense the level of ambient acoustic noise.

4. A rectifier and smoothing circuit 6 which produces a d.c. vole signal proportional to the ambient acoustic noise level sensed by the microphone.

5. Controls 7 to provide an off-set such that the variation in acoustic output level only occurs when the ambient acoustic noise exceeds a certain pre-set level and to set the normal level of the variable output control at a suitable intermediate point on its gain vs control voltage characteristic.

6. A source of electrical d.c. power 8.

5. Mode of Operation.

In practice, the apparatus operates as follows.

Referring to Fig.2., the audio input channel signal is applied to the input of the electrically controllable stage, the controlled output from which is applied to the input of the loudspeaker drive amplifier whose output drives one or more loudspeakers.

Ambient acoustic noise is sensed by a microphone, which after amplification, is applied to the primary of a d.c level isolating transformer.

The secondary voltage of the transformer is rectified to produce a d.c. voltage proportional to the level of acoustic noise sensed by the microphone.

Adjustments are provided for the attack and decay time constants ofthe rectifier, the output from which is applied to a blocking diode to which is also connected the gate of a field effect transistor connected as a source follower.

The positive from the rectified output from the transformer is connected to the wiping contact of a potentiometer which forms part of a potential divider connected across the main d.c. bias supply.

This enables the voltage at the cathode of the stand off diode d.c. blocking diode to be set so as to achieve the required stand - off( delay).

The d.c. voltage on the gate of the source follower is set by means of a potentiometer so as to set the operating point of the controllable gain stage appropriately.

The levels of signals in the audio and control channels may be monitored by means of multi-element bar graph displays - these may also be used for the purpose of setting the levels correctly during installation and commissioning and subsequently.

A variation of the basic system described above for which patent cover is also requested, is that in which compensation is made for the effects of winds which are commonly experienced in out-door situations, one effect of which can be to increase or decrease the level of sound perceived in the areas covered by the loudspeaker system due to the flow of air between the loudspeaker sound source and listener.

The method which we have invented to overcome, or at least mitigate, this effect is to transmit over one or more of the public address loudspeakers, an inaudible sub or ultra-sonic continuous signal whose amplitude is detected beyond the area to be served.

The detected amplitude, which varies in a manner similar to that of the main public address messages, etc., is fed back as a controlling signal to the adaptive audio system as described above so as to maintain the level cf the control tone al.d ,therefore, the level of the public address announctments. etc., substantally constant.

In order to prevent the interruption to the audio path between loudspeaker and remote detector due to the presence of trains or other temporary obstructions causing the adaptive system to interpret such an interruption as a very large rise in audio signal path attenuation and increasing the output from the loudspeakers accordingly, the presence of rolling stock in the station or area to be controlled is detected and the level of the audio system "frozen" at the current level until the obstruction is removed.

Alternative Implementations.

As an alternative to the implementation described above, it is possible to include a system of controlled switching on and off of selected loudspeakers in, say, a public address loudspeaker array.

By way of example, where a public address system is provided at a railway station, airport, etc., for the purpose of communicating with passengers or others, it may be advantageous to employ a series of distributed loudspeakers mounted, say, on lighting standards or other supports which are only switched into operation when the nearby presence of a human is detected.

Such means of detection may be, for instance, a Passive Infra Red detector, a microwave or ultra-sonic detector, etc.

In this way, nuisance which may be caused to neighbours to the station, airport, etc., may be avoided by operating the loudspeakers only when required by the presence of passengers, etc., and then only when the current level of acoustic noise is taken into account. For safety or other reasons, it may be prudent to arrange for one or two, strategically placed loudspeakers to be operational at all times, independently of the detection of the presence of passengers.

The control system described in the above uses low cost analogue techniques.

It is possible to provide the control function using digital techniques. Such techniques, while somewhat more expensive than the analogue methods already described, have advantages in the flexibility and sophistication of the control algorithms which may be employed.

As an example, the output from the control channel rectifier can be applied to the input of an A/D Converter, the digital output of which can then be manipulated under program control, using either a dedicated data processor or a PC, say.

The processed digital data can then be converted back into the analogue control signal required by the analogue controllable gain stage already described or, alternatively, a digitally controlled attenuator can be used directly.

Claims (1)

1. - Claims.

   An audio amplifier means, a signal amplitude controlling means, a loudspeaker driving amplifier, a sound level sensing microphone and associated amplifier, a d.c. level isolating and signal rectification means, a threshold level, attack and decay and bias level setting means and signal level indicating means and one or more loudspeakers, together with an electrical power supply.

   An apparatus according to Claim 1 above, wherein the operation of individual loudspeakers is additionally controlled by means of switches operated by such means as passive infra-red sensors, microwave or ultra-sonic sensors, visible light sensors, etc.

   An apparatus according to the foregoing claims which, additionally or alternatively controls the level of the audio output from the loudspeakers so as to compensate for the effects of wind.

   An apparatus similar to that described in 7.1. above, but in which the output from the control channel rectifier is converted into a digital equivalent ofthe voltage appearing at the output of the rectifier by means of an analogue-to-digital converter (A/D Converter ) after which the digital signal is processed by means of a data processor which might be a microprocessor. personal computer, etc. The processed data may then be used either to generate an analogue control voltage as is required by the analogue control circuit described above, using a digital-to-analogue converter (D/A Converter) or, alternatively, used to control the magnitude of the signal input to the main power amplifier by means of a digitally controlled attenuator.The solution most appropriate to any given application will depend upon the specific operational requirement and financial constraints.

   Amendments to the claims have been filed as follows 1. A means for the minirnising of noise pollution at railway stations, sports stadia, airports, public places, etc., by the use of an adaptive public address system consisting of one or more public address audio amplifiers whose acoustic output levels are adapted to the prevailing levels of unwantcd acoustic noise, together with a number of loudspeakers which are nonnally disconnected from the amplifiers mentioned above and which are switched into operation when associated detection means sense the nearby presence of those for whom the public address announcement are intended.

   Z. An adaptive public address system as claimed in Claim 1 whereby the sound power level at which announcements are made is varied according to prevailling levels of ambient noise using an electronic circuit consisting of an audio amplifier whose gain may be varied by means of an externals applied control voltage obtained from the output stage of a rectifying and smoothing circuit designed to produce a d.c. voltage which is proportional to the ambient noise level sensed by one or more microphones.

   3. An adaptive public address system as claimed in Claim t and Claim 2 which uses an electronic circuit which allows for the setting of a threshold corresponding to a minImum ambient noise level, below which the system does not adapt to changes in ambient noise level.

   4. An adaptive public address system as in Claim 1, Claim 2 and Claim 3 employing an electronic circuit which allows for the setting of attack and decay times for the control circuits, thus allowing the ambient noise sensing circuits to discriminate against short duration ambient noise events which do not significantly reduce the intellegibility of g given public address announcement.

   5. An adaptive public address system as claimed in Claim 1 above wherein the switching of individual loudspeakers is operated by such means ai passive infka-red sensors, microwave or ultra-sonic sensors, visible tight sensors or any other means.

   6. An adaptive public address system as claimed in Claim 1, Claim 2, Claim 3.

   Claim 4 and Claim 5 which controls the level of Lhe audio output from the loudspeakers so as to compensate for the effects of wind.

   7. An adaptive public address system as claimed in Claim 1 and Claim 2 but in which the output from the control channel rectifier is converted into a digital cquivalent of the voltage appearing at the output of the rectifier by means of an analogue to digital converter ( A/D Converter ) after which the digital signal is processed by means of a data processor which might be a mloro- processor. personal computer, etc. The processed data may then be used either to generate an analogue control voltage as is required by the analogue control circuit described above, using a digital to analogue converter (DIVA Converter ) or, alternatively, used to control the magnitude of the signal input to the main power amplifier by means of a digitally controlled attenuator.

   8. A system of controlled switching ( on and off), as in Claim 5, of one or more selected loudspeakers In an adaptive or non-adaptive public address system, upon the detection of the nearby presence of a listener, in order to minimise noise pollution by making public address announcements only when those for whom the announcements are intended are present.