Patent Application
20090034748
The present invention relates to ambient noise-reduction control systems, primarily for use with ear-worn speaker-carrying devices, such as earphones and headphones (such devices being referred to hereinafter as “ESDs” for convenience). The invention has especial, though not exclusive, application to ESDs intended for use in conjunction with portable electronic devices, such as personal music players and cellular phones.
The ambient noise to be reduced under the control of the invention is that occurring around an individual who is wearing an ESD. The ambient noise is detected by a microphone on (or inside) a housing that forms a part of the ESD, electronically inverted and filtered, and fed to the ESD's speaker, so as to create an acoustic signal which, in principle, is substantially equal in magnitude, but substantially opposite in polarity, to the ambient acoustic noise. Consequently, destructive wave interference occurs between the ambient acoustic noise and its inverse, generated via the speaker, and so the level of the ambient acoustic noise, as perceived by the listener, is reduced.
At present, some ESDs are wired directly to sound sources, such as personal music players and cellular phones, via short leads and connectors, and some are coupled to such sound sources via wireless links, using protocols such as the “Bluetooth” format. The present invention can be used with both wired and wireless couplings.
There are several distinct types or families of ESD in use at the present time, both as single, one-ear devices, and also as stereophonic pairs, as described below.
Types 1 and 5 both incorporate a form of acoustic seal in order to provide a degree of acoustic isolation for the wearer, but many find that this leads to various types of discomfort.
For example, Type 1 devices can be physically uncomfortable when lodged in the ear canal entrance for extended periods. Further, their acoustic isolating properties can be dangerous in terms of reducing wearers' awareness of their physical surroundings. Also, because the ear canal is effectively sealed, use of such devices in aircraft can cause ear “popping” and discomfort in response to changes in cabin pressure. Furthermore, if the housing of the device is brushed against an object, such as a pillow or item of clothing, a very loud (and distracting) mechanical transmission of the friction sound directly into the ear canal frequently occurs. Additionally, if the wearer is eating, the chewing noises are transmitted into the ear-canal via the mastoid bone, again creating a large and unpleasant acoustic signal.
Type 5 devices also strive to isolate the wearer's ears from the ambient and, whilst their construction is such that a small cavity is formed around the outer ear, such that the ear canal is not directly sealed itself, the ear canal is nevertheless coupled acoustically to this cavity. This sealed cavity around the ear is not ventilated, and therefore can quickly become warm, humid and uncomfortable. Also, if there are small imbalances between the left and right channel signals at low frequencies, which might be caused, for example, by two earphone devices not being seated perfectly symmetrically, unpleasant acoustic effects can be introduced by the occurrence of non-natural left-right phase differences which have been variously described as “phasey”, “sucking effect” and “ear-blocking”.
The sound isolation provided by acoustically sealed systems is a fixed feature: it can neither be varied, nor switched off. If a wearer wishes to hear the outside world briefly, say for conversation or for crossing a road, it is necessary to physically remove the ESD from the ear, and then replace it again afterwards. This is a major disadvantage in everyday usage, and can lead to potentially dangerous situations should wearers leave the devices in place continuously.
The present invention relates primarily to usage with device types (2), (3) and (4) in which there is some acoustic leakage present around the device itself, linking the wearer's ear to the ambient. Although this naturally makes electronic noise-reduction more difficult to achieve, the acoustic leakage affords a much more comfortable listening experience for the wearer, and this is a very important factor. The comfort factor of the pad-on-ear type of device is superior to the circumaural type in that (a) it is intrinsically relatively lightweight; (b) it allows natural air-flow and ventilation around the ears, thus avoiding sweatiness and irritation; and (c) it is not susceptible to artefacts associated with the actions of eating and chewing.
Another prime advantage of ESDs with relatively large acoustic leakage is that most of the human directional hearing capabilities remain intact, so that wearers still possess spatial hearing ability with the devices in place. Consequently, in the absence of loud music or noise-reduction signals, users can hear the world in a reasonably natural way. This is a safe default situation, unlike an acoustically sealed system that isolates a listener from the physical environment.
It is further important to note that existing ambient noise-reduction systems for ESDs are based on either one of two entirely different principles, namely the “feedback” method, and the “feedforward” method.
The feedback method, described for example in U.S. Pat. No. 4,455,675, is based upon a closed-back, circumaural-type ESD. Inside the cavity that is formed between the ear and the inside of the ESD's shell, a miniature microphone is placed directly in front of the ESD's loudspeaker, and it is coupled back to the loudspeaker via a negative feedback loop (an inverting amplifier), such that it forms a simple servo system in which the loudspeaker is constantly attempting to create a null sound pressure level at the microphone. Although this principle is simple, it is quite difficult to implement efficiently in practice, especially in pad-on-ear format.
The feedforward method is disclosed, for example, in U.S. Pat. No. 5,138,664 and is depicted in basic form in
The basic feedforward method of ambient noise reduction is simple to implement, and a working system for use with ordinary earphones can be assembled at low cost using a simple electret microphone capsule and a pair of operational amplifiers to amplify and invert its analogue signal, prior to mixing, as at 13, with an audio drive signal, such as a music signal, fed to the ESD's speaker. This is done via an adjustable gain means (not shown), such as a potentiometer, in order to adjust the magnitude of the noise reduction component of the signal S to substantially equal that of the ambient noise. Some measure of noise reduction can be achieved with this method and, although current systems are far from perfect, this feedforward principle forms the basis of various commercially available noise-reducing earphones. However, in such systems, even when the noise reduction signal is optimally adjusted and balanced, there remains a considerable residual noise signal, and so it is common to observe that most commercial systems are only claimed to operate below about 1 kHz; i.e. with a bandwidth similar to that of the feedback method, and to provide relatively modest amounts of noise reduction.
Some currently available noise-reducing earphones allow the user to switch off the noise reduction function, whereupon the loudspeaker connections of the device are switched away from the output of the internal noise-reducing drive amplifier 15 directly to the audio input connections (shown schematically at 19), thus acting as a conventional earphone if the battery has expired, or in order to conserve battery energy.
The long-standing situation of poor noise reduction efficacy has been changed recently by the invention of a much more efficient feedforward noise reduction system based on the usage of accurately time-aligned signals and an associated microphone technology, described and claimed in UK Patent Application No. GB 0601536.6 and its counterpart International Patent Application No. PCT/GB2007/000120, both of which are assigned to the present applicant and incorporated herein in their entirety by reference. This system provides a greater degree of noise reduction, and is effective up to higher frequencies (e.g. in the order of 3.5 kHz) than prior-art systems.
It has now been determined by the inventor that there is considerable merit in providing for automatic variation in the amount or degree of ambient noise reduction that is effected, according to the circumstances under which the ESD is being used, and hence the present invention aims to provide a noise reduction control system in which the degree of ambient noise reduction imposed can be controlled by one or more external events; i.e. events which are outside the control of a wearer of the ESD, thus permitting various modes of operation to be implemented.
According to the invention there is provided a noise reduction control system for ESDs, comprising means for sensing ambient noise on its way towards an ear of a wearer of an ESD, for developing electrical signals indicative of said noise and for utilising said signals to reduce the amount of said ambient noise audible to a wearer of the ESD, and control means for setting a plurality of predetermined and discrete levels for said reduction; said control means further comprising response means for automatically responding to at least one controlling event, outside the control of said wearer, to set the degree of said reduction to a preselected one of said discrete levels.
Preferably the system further comprises means for inverting and filtering said electrical signals and for feeding said inverted and filtered signals to a loudspeaker means in said ESD in time for the loudspeaker means to generate sounds capable of interfering destructively with said sensed ambient noise when it arrives in the vicinity of said loudspeaker means.
It is further preferred that the system also comprises a source of further electrical signals relating to sounds intended for the listener's attention, and means for merging the further electrical signals with the inverted signals to create a composite signal for application to said loudspeaker means.
In one preferred example of the invention, the sounds intended for the listener's attention comprise music. Alternatively, or in addition, the sounds may be speech or other sounds received over a telecommunications link.
The response to the controlling event may be instantaneous, delayed or subjected to a time profile such as a ramp function.
In some preferred embodiments, said control means is adapted to separately control the said inverted signals and the said further electrical signals relating to sounds intended for the listener's attention. In such circumstances it is particularly preferred that the control means comprise a digital signal processor.
It is further preferred in some embodiments to provide plural channels for respective electrical signals relating to sounds intended for the listener's attention, for said control means to separately control the said signals in at least a first and a second of said channels.
The invention may receive external signals for the attention of the listener by way of direct electrical connections and/or through wireless communication.
Where a wireless communication is employed, it is preferred that such communication conforms to Bluetooth protocols.
The invention may beneficially be employed in association with an audio system providing 3D-audio virtualisation.
The invention also encompasses personal music players or cellular telephone devices incorporating one or more components of any of the aforementioned inventive systems.
The present invention thus provides a system for variable, controllable ambient noise-reduction for an ESD user. It is especially suited for use with the efficient time-alignment system described and claimed in the aforementioned UK Patent Application, that is effective to frequencies up to, and beyond, 3 kHz, in contrast to the sub-1 kHz limit of presently available commercial products. Included amongst advantages of the invention are that the associated ESD type is both comfortable in use (being lightweight, not rigidly clamped to the head, and affording some ventilation), and that the amount or degree of noise-reduction that is effected may be electronically controllable; both of these characteristics being particularly desirable for earphones intended for use with mobile electronic devices. The noise-reduction level can be controlled in a binary “on-off” mode, or it can be switched directly between different pre-determined levels in a range, or it can be subjected to a time profile, such as a ramp function, to provide adjustment between levels on a “continuously variable” basis, either smoothly or in discrete increments.
The continuously variable function enables smoothly faded transitions to be made between different levels of ambient noise reduction, as will be described. For example, when initially activated, the ambient noise reduction can be caused to fade from its “off” value of reduction factor (RF) of 0%, to its maximum effect (RF=100%) over a period of, say, one second; thereby affording the listener a smoothly achieved transition, free from audible clicks and other unpleasant switching artefacts.
The control of noise-reduction level is exercised in response to the occurrence of selected events which themselves are outside the listener's control, thus enabling various automatic functions to be implemented for enhanced user satisfaction and safety.
In addition to directly connected ESD applications, the invention is also applicable to telephony applications such as radio-linked (Bluetooth) ESDs, where an incoming call can be used to trigger a pre-determined sequence of events, and where the cessation of the call can restore the original listening conditions for the user.
In practice, the amount of active noise reduction that can be achieved is limited by physical variables related to ESD placement and the like, and it varies with frequency over the range of operation. For the purpose of illustration it will be assumed, in the examples that follow, that a relatively efficient ambient noise reduction system is used, affording 20 dB of noise reduction at the eardrum of the ESD's wearer, and hence the RF of 100% corresponds to 20 dB noise reduction.
The invention can be applied to both feedback and feedforward types of ambient noise reduction, although it is best-suited to feedforward systems, where there is intrinsic acoustic leakage from the ambient to the eardrum. For clarity of description, the examples herein relate to the feedforward method. Also, for simplicity in the accompanying Figures, a single microphone system has been depicted, but it should be noted that a time-aligned, multi-microphone arrangement (for example of the kind described and claimed in the aforementioned UK Patent Application No. GB 0601536.6 and International Patent Application No. PCT/GB2007/000120) is preferred, because it is more effective in use.
In order that the invention may be clearly understood and readily carried into effect, certain embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:
a and 2b show respectively electronic analogue switching and gain-adjustment devices suitable for use in systems according to examples of the invention;
Whilst the following examples and drawings relate to an analogue circuit implementation, ambient noise reduction signal processing can alternatively or additionally be carried out in the digital domain; the invention being equally applicable to analogue and/or digital processing routes.
Some preferred embodiments of the invention provide an ambient-noise reduction system having a variable degree of reduction that can be controlled by one or more external events. Each controlling event can initiate one or more actions that control the degree of ambient noise reduction, using one or more of several different operating modes. Some illustrative examples of these controlling events, actions and operating modes are listed below, followed by descriptions of various embodiments and two automatic operating modes afforded by the invention.
In order to vary and control the amount or degree of noise reduction which is carried out, the magnitude of the noise reduction signal must be switched or reduced from its maximum, optimal value, to some other value, such as zero. It will be appreciated that this could be done at several points in the circuit of
One example of a suitable analogue switch is MAX325CPA (manufactured by Maxim), which is a double-pole double-throw switch based on coupled MOSFET devices having low Ron values (˜33Ω).
An arrangement for switching between different levels of ambient noise reduction is depicted in block form in
A further degree of sophistication can be achieved by applying a similar, controllable, variable-gain stage 28 to the music stage, prior to its summation with the noise signal, as shown in
Two automatic mode embodiments, already briefly referred to above as E4 (Auto-Noise Mode) and E5 (Auto-Music Mode) will now be described in more detail.
The purpose of this mode of operation is to allow a wearer of an ESD to hear all the low-level ambient sounds, but to limit excessively loud sounds. For example, if the wearer is walking through a town, it would be advantageous (and safer) to hear the sounds of cars approaching, people tanking and the like, but, when walking past a very noisy construction site, or when a high-speed train passes at a railway station, it would be very desirable to reduce these brief occurrences of very loud noise to a more comfortable level.
This can be achieved using the circuit shown in
The uppermost graph of
Initially, the ambient noise level is quite low, at 55 dB, but at point A, a noisy truck parks besides the wearer, raising the ambient noise level to 70 dB. This exceeds the activation threshold, and so the DSP 30 adjusts the gain of the noise reduction stage to implement 10 dB of ambient noise reduction, thus reducing the perceived noise level at the wearer's eardrums to 60 dB. At point B, the truck moves on, the ambient noise level reverts to its original 55 dB, thus falling below threshold, and the DSP 30 switches the noise reduction off again. At point C, the wearer passes a noisy construction suite which generates an ambient noise level of 80 dB, and so the DSP effects −20 dB noise reduction, bringing the perceived level by the listener to be only 60 dB once again. However, this −20 dB is the maximum degree of available noise reduction, and so a further rise in ambient noise level, just before marker D, causes the perceived noise level to increase linearly beyond the 60 dB target. The purpose of mentioning this latter is to illustrate the benign nature of the system, in that overloading beyond the maximum noise reduction capability does not cause detrimental artefacts.
The purpose of this mode of operation is to switch on maximum ambient noise reduction when the music is playing, and to switch it off when the music channel is silent, in order that the wearer can hear all ambient sounds when the music is not playing.
For example, if the wearer of an ESD is travelling on a noisy underground train, it would be very desirable (and safer) to hear all of the local environmental sounds in between music tracks, or in the silent pauses during radio programmes. Additionally, if the wearer wished to alight from one train, and then catch another, it would be advantageous to pause the music track, at which point the ambient noise reduction would be caused to be switched off, thus allowing the wearer to hear normally whilst negotiating the transition between trains, and then when the music is switched on again, the noise reduction is also activated automatically to maximum effect.
This is easier than having to switch both the music and the noise reduction off and on separately, and it is safer for the wearer than leaving the ambient noise reduction switched on continuously.
This example is illustrated in
The uppermost graph of
Initially, and up to point P, the music is switched off, and the music signal lies below the threshold level, causing the ambient noise reduction to be switched OFF. Consequently, the wearer hears the environmental ambient noise, at a level of about 75 dB. At point P, the wearer switches the music ON, which exceeds the threshold level, causing the noise reduction to be switched ON, thus reducing the perceived noise at the wearer's eardrum from 75 dB to 55 dB. At point Q the first music track stops and there is a short interval before the following track; the music signal falls below threshold, causing the noise reduction to switch OFF, and hence the wearer hears the ambient environment as normal, between tracks. At point R, the second music track starts, thus causing the ambient noise reduction to be switched ON again.
It will be understood that, without the noise reduction, the music and noise would have similar sound pressure levels at the eardrum of the wearer, making the music almost impossible to appreciate. At point S, the wearer wishes to leave the carriage and cross the platform to another train, and so the wearer pauses the music, causing the level to fall below threshold, thereby causing the DSP to switch off the noise reduction and allow the wearer to hear the ambient environment as normal, providing the safe default condition.
A further embodiment is depicted in
This arrangement permits implementation of a sophisticated audio management system on, for example, an ESD in the form of a wireless stereo earphone set networked to a cellular phone that is also equipped with a personal stereo (MP3) music player capability, as follows.
This sequence of events allows smooth, automatic transitions between the multiple audio channels, and makes for a pleasant and comfortable user experience.
As regards all of the embodiments of the invention herein described, it will be appreciated that electronic components of the control system can be housed in the ESD, or in a separate housing or “pod” connected (directly or wirelessly) thereto, or may be distributed between the ESD and the pod, depending upon factors such as design choice and operational convenience.
Alternatively, the electronic processing, or one or more components associated therewith, can be integrated into a mobile electronic device such as a cellular phone handset or an MP3 personal music player.
Furthermore, any embodiment of the invention may be utilised in association with an audio system providing 3D-audio virtualisation.
Although the invention has been described with reference to certain specific embodiments, these embodiments have been provided, by way of example only, to illustrate the scope and advantages of the present invention, the scope of which is not intended to be restricted to the details of any such embodiment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0606630.2 | Apr 2006 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB07/01099 | 3/28/2007 | WO | 00 | 8/13/2008 |
