GB2237463A

GB2237463A - Automatic volume control system responding to ambient noise

Info

Publication number: GB2237463A
Application number: GB8923298A
Authority: GB
Inventors: John Moffatt
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-10-16
Filing date: 1989-10-16
Publication date: 1991-05-01
Anticipated expiration: 2009-10-16
Also published as: GB8923298D0; GB2237463B

Abstract

An automatic volume control system which adjusts the level of an audio electrical signal in dependence on ambient noise level comprises a filter 1 for deriving low frequency components from the audio signals, which are then applied to a gate 3 where they are intermittently gated. An electrical signal produced from ambient noise detected by a sensor 10 is sampled by a gate 13 during the gating period of gate 3 and a sampled level of the noise signal is maintained from one gating period until the next gating period and is applied to a gain control circuit 7 to control the level of the audio electrical signal in dependence on each sampled level of the noise signal. The gating and sampling actions preferably occur at irregular time intervals under the control of a logic and timing circuit 16, assessing circuit 15 and clock 17. <IMAGE>

Description

AUTOMATIC VOLUME CONTROL SYSTEM The present invention relates to an automatic volume control system for audio electrical signals.

It is an object of the present invention to provide an automatic volume control system which will adjust the level of an audio signal in dependence on the level of ambient noise.

According to the present invention, an automatic volume control system for controlling the level of an audio electrical signal in dependence on ambient noise comprises means for deriving low frequency components from said audio signal, means for intermittently gating said low frequency components, means for producing an electrical signal from detected ambient noise, means for sampling the electrical noise signal during said gating periods, means for maintaining a sampled level of said noise signal from one gating period until the next gating period and means for controlling the level of said audio electrical signal in dependence on each sampled level of said noise signal.

Preferably the audio electrical signal is initially divided into low frequency components and high frequency components in order to facilitate the gating of said low frequency components. The high and low frequency components are then recombined after the low frequency components have been gated and before the level of the audio signal is controlled.

Advantageously the gating and sampling actions are arranged to occur at irregular time intervals which has the effect of making the changes in audio signal level appear less obtrusive than if they occurred at regular intervals.

The means for sampling the electrical noise signals may include gating means controlled by a logic and timing circuit which in turn is controlled from an assessing circuit which is fed by the low frequency components of the audio signals and determines the point at which said low frequency components are gated. A clock circuit is provided to control the assessing circuit and logic and timing circuit.

In the case of stereo audio electrical signals, the low frequency components are derived from each of the channels and are added before being applied to the assessing circuit. The low frequency components of each channel are gated and each channel is controlled by a single noise derived signal and logic and timing circuit.

The same concept would apply to complex audio systems employing more than two audio signal channels.

Switch means may be provided which permit the automatic volume control system to be selectively switched in and out of operation.

The system according to the present invention finds particular application in radios installed in vehicles which frequently have to operate in conditions of persistent and variable ambient noise, for example noise from other traffic or from roadworking equipment such as road drills. However,the invention may also be employed in other equipment such as the sound channel of television receivers, domestic hi-fi systems and radios as well as two-way radio communication and telephone systems.

The invention will now be further described, by way of example, with reference to the accompanying drawing which is a block circuit diagram of one embodiment of the system according to the invention as applied to a stereo audio amplifier.

The system to be described is a two channel processor which is capable of adjusting the audio output levels in response to variable local background noise. The system operates at signal levels, as distinct from power levels, and is preferably inserted immediately prior to the normal manual volume control(s).

Referring to the drawing the left and right stereo audio channels each apply the input audio signal to an LF filter 1 and HF filter 2. Each LF filter 1 feeds a gate 3 and a further filter 4; whilst each HF filter 2 feed a phase shift circuit 5. The outputs from filter 4 and phase shift circuit 5 are both applied to an adder 6, which in turn feeds a gain control stage 7 which controls the gain of a power amplifier 8 feeding a loud speaker 9.

Ambient noise is detected by a dynamic transducer 10 and applied through a preamplifier 11 and band pass filter 12 to one input of a gate 13. An output from each of the LF filters 1 is fed to an adder circuit 14 which in turn feeds an assessing circuit 15 whose output is applied to a logic and timing circuit 16. This latter circuit is controlled by a clock 17 and has an output fed to control the gate 13. The output from gate 13 is fed via a detect and scaling circuit 18 to a hold latch 19 whose output controls the gain control stages 7 in the right and left stereo amplifier channels. The hold latch 19 is also controlled by an output from the logic and timing circuit 16.

The low pass and high pass filters 1 and 2 need slopes of no more than 12dB per octave which is readily provided around one active stage per filter. The crossover point is preferably positioned in the 180 to 250Hz region.

The gating functions can be achieved by any one of a number of techniques from a single transistor to a symmetrical CMOS switch. The filter circuit 4 is helpful in removing residual disturbances from the gating and any phase shift between the separated LF and HF signal components must be matched by circuit 5 before recombining the components.

The final gain control section 7 may employ either totally analogue techniques or part digital techniques where the control information may be parallel or serial logic. A scaled derivative of the noise measurement is used to adjust the output levels but the control range is inherently limited by power amplifier capability. For example a 15dB level increase requires 32 times the output power. A range of 15 or 16dB represents a practical limit and the latter may be divided into eight 2dB steps using some kind of ladder arrangement. This size of step is not obvious on speech or music and the gain stability can be as good as the ladder itself.

The circuit elements 10 to 19 have the task of measuring noise and generating control information. The dynamic transducer 10 is followed by the steep-sided narrow band pass filter 12 centred in the 90 to 130Hz region where significant speed related engine noise predominates. These preselected noise components are then measured, scaled, and finally held as control information until updated.

The low frequency components from both L and R input signals are mixed in the adder 14 and fed to the assessing circuit 15 which must choose a suitable point to gate the low frequencies and initiate the sample, hold and update sequence. The process is important to the success of the system and requires fast assessment of several waveform parameters if the gain variation is to be relative unobtrusive. The clock 17 counts off a suitable wait time before allowing the assessor 14 to find the next acceptable interrupt point. The same clock then times the interval and enables sampling hold, and update in due order.

Any required change is automatically passed on to the programmable gain control circuits 7. The combination of a clocked wait time with a speech or music dependent find time results in non-regular sampling which is advantageous in rendering the gain control process relatively unobtrusive. The continuous signal paths for the higher frequencies provide further enhancement for this effect.

As mentioned previously, switch means may be provided for bringing the automatic gain control system into and out of operation. Such switch means could, for example, take the form of a simple push button or could be part of a manual volume control.

It will be seen that the system according to the invention basically operates to sample environmental noise level during a gating period of the low frequency component of an audio signal and then maintains a randomly updated control of the audio signal level by sampling and hold techniques.

The system may be readily built into new audio equipment or alternatively it may be added to existing equipment.

It will also be appreciated that the system could operate to control the audio gain in the reverse manner.

Claims

1. An automatic volume control system for controlling the level of an audio electrical signal in depedence on ambient noise comprising means for deriving low frequency components from said audio signal, means for intermittently gating said low frequency components, means for producing an electrical signal from detected ambient noise, means for sampling the electrical noise signal during said gating periods, means for maintaining a sampled level of said noise signal from one gating period until the next gating period and means for controlling the level of said audio electrical signal in dependence on each sampled level of said noise signal.

2. A system as claimed in claim 1, in which the audio electrical signal is initially divided into low frequency components and high frequency components in order to facilitate the gating of said low frequency components.

3. A system as claimed in claim 2, in which the high and low frequency components are recombined after the low frequency components have been gated and before the level of the audio signal is controlled.

4. A system as claimed in claim 1, 2 or 3 which the gating and sampling actions are arranged to occur at irregular time intervals so as to have the effect of making the changes in audio signal level appear less obtrusive than if they occurred at regular intervals.

5. A system as claimed in any preceding claim, in which the means for sampling the electrical noise signals includes gating means controlled by a logic and timing circuit which in turn is controlled from an assessing circuit which fed by the low frequency components of the audio signals and determines the point of which said low frequency components are gated.

6. A system as claimed in any preceding claim, in which a clock circuit is provided to control the assessing circuit and logic and timing circuit.

7. A system as claimed in any claim, in which the audio electrical signals are stereo audio electrical signals and the low frequency components are derived from each of the channels and are added before being applied to the assessing circuit.

8. A system as claimed in claim 7, in which the low frequency components of each channel are gated and each channel is controlled by a single noise derived signal and logic and timing circuit.

9. A modification of the system as claimed in claimss 7 and 8, in which the audio system is a complex audio system employing more than two audio signal channels.

10. A system as claimed in any preceding claim, in which switch means are provided which permit the automatic volume control system to be selectively switched in and out of operation.

11. A system as claimed in claim 2 or 3, including a phase shift circuit for matching the phases of the separated high frequency and low frequency components before they are recombined.

12. A system as claimed in any preceding claim in which the means of controlling the level of the audio electrical signals in dependence on each sampled level of the noise signal comprises a programmible gain control circuit or circuits.

13. A system as claimed in claim 10, in which the switch means forms part of a manual volume control.

14. An automatic volume control system substantially as hereinbefore described with reference to the accompanying drawings.

15. A vehicle radio including a system as claimed in any preceding claim.

16. A radio telephone including a system as claimed in any of the preceding claims 1 to 14.

17. A high fidelity sound reproducing system as including a system as claimed in any of the preceding claims 1 to 14.

18. A television receiver including a system as claimed in any of the preceding claims 1 to 14.