GB2469269A

GB2469269A - A light emitting diode used as a limiter and high volume indicator for an audio transducer

Info

Publication number: GB2469269A
Application number: GB0905848A
Authority: GB
Inventors: Stephen Ronald Blincoe
Original assignee: LIMITEAR Ltd
Current assignee: LIMITEAR Ltd
Priority date: 2009-04-03
Filing date: 2009-04-03
Publication date: 2010-10-13
Also published as: WO2010112859A1; GB0905848D0

Abstract

A limiter circuit 10, between an audio input 11 and a sound transducer 12, employs one or more light emitting diodes 14, 16 as limiting means. Two LEDs 14,16 may be connected anti parallel to each other or an LED (26) may be connected across a diode bridge (Fig 2). A limiting LED (36, 46, 56) can be employed in a voltage multiplier circuit (Figs. 3, 4) or in an active circuit (Fig. 5). The circuit 10 may form part of a headphones cable device in which the LED is externally visible to indicate high sound levels. The limiting circuit 10 may be matched to different transducers. The device is designed to prevent hearing damage due to excessive sound volume levels in personal portable audio players.

Description

LIMITER CIRCUITS

The present invention relates to limiter circuits, for example circuits used in limiting sound levels in electronic audio devices.

As new legislation to control noise in the workplace comes into effect, the need for employers and employees to reappraise exposure to damaging sound pressure levels becomes more pressing. Even outside the workplace, with the dramatic increase in the use of personal listening devices, concerns about potential hearing damage with prolonged use are widespread. Thus there is a need to improve the limitation and visual indication of potentially damaging levels of sound exposure through personal listening devices.

It has been well established for many years that hearing can be permanently impaired through exposure to prolonged periods of high sound intensity, or by shorter bursts of extremely high sound intensity. Guidelines and regulations have been improved as and when new understanding of the effects of sound exposure emerges. Generally, acceptable levels have been reduced, and areas of application have broadened.

Irreversible hearing impairment is generally not measureable until years after exposure. The effect can be considered as accumulative, with contributions from both high-level prolonged exposures and intense short-term peak exposures. This places greater emphasis on the need for the assessment and control of sound pressure levels.

According to recent legislation a new Limit Value of 87dB(A) at the ear must not be exceeded, and hearing protection must be made available where the sound levels are at or above 8OdB(A), and strictly enforced where the sound levels reach or exceed 85dB(A).

This applies to all workplaces, and includes self-employed, and the music and entertainment business. Although no legislation for domestic and personal environments is in place, there is widespread concern about the prolonged exposure to high levels of sound, especially with live entertainment and personal listening devices.

There are several ways to deal with the situation of high ambient noise which would otherwise make it difficult to hear material on personal headsets. One is to use an ordinary headset at high volume levels. Disadvantages include: * Listening is especially likely to be at excessively high levels, leading to hearing damage.

* Others in vicinity can hear sound leakage from headset * Oblivious to verbal warnings of emergency Attenuating earplugs can be used in conjunction with a headset at high volume levels. This attenuates ambient noise effectively, especially if the plugs are custom-fitted.

Headset volume is boosted to overcome the attenuator and thus can be clearly heard above the ambient noise. Disadvantages include: * Others in vicinity can hear sound leakage from headset * Oblivious to verbal warnings of emergency * Listening at excessively high levels is also possible on these devices.

Noise cancellation headphones typically employ a sensing transducer (eg microphone) and active circuitry to detect ambient sounds and add a compensating signal to the audio feed so as to cancel the effect of ambient noise leaking through to the ear.

Their effectiveness can be limited, especially at low frequencies.

* Oblivious to verbal warnings of emergency * Listening to excessively high levels is also possible on these devices * They generally need batteries.

Ear defenders in the form of close-fitting, high isolation can-style headsets can be used with moderate volume levels; ambient noise is sufficiently attenuated to avoid interference. Disadvantages include: * Oblivious to verbal warnings of emergency * Listening at excessively high levels is also possible on these devices.

* Generally very expensive, and has potential health issues where shared use is practised Another possibility is the use of close-fitting ear inserts. The best passive isolation can be achieved through the use of close-fitting earplugs with integral transducers (audio driver). These are typically custom made for the individual and are inserted into the ear canal. They provide excellent isolation and superb sound quality. Their use has the following disadvantages: * Excessive signal levels can easily generate damaging sound levels, exacerbated by the proximity of transducers to the eardrum, and the relative difficulty of removing the plugs quickly. Such excess signals could be due to equipment faults, or by inserting a connector into a high-level source.

Users are particularly vulnerable to this.

* Simply listening at excessively high levels is just as easy on these devices as it is with others.

* Their effectiveness at blocking ambient noise also prevents hearing necessary instruction, such as a verbal warning of fire or other emergency.

Even when ambient noise is not a problem, certain forms of audio material, especially some genres of music, tend to be listened to at high volumes; this can lead to ear damage through prolonged or excessive exposure.

Any headset can be inadvertently plugged into an excessively high signal level.

This is exacerbated where close-fitting ear inserts are employed for the highest quality reproduction; they can take time to remove. There is a wide range of headset sensitivities, impedances, and there is also a wide range of signal sources. Mismatch and mistakes are easy.

The widespread use of personal listening devices such as iPod has resulted in extended period of exposure to potential loud volume levels. Their use also increases the risk of not being able to hear verbal warnings of emergency.

A particular instance in which there is the danger of sudden and unexpected exposure to excess noise levels is where an individual, such as a cameraman in a TV or film studio, receives several audio feeds into a single headset. If the volume setting is suitable for a relatively quiet source and a separate loud source intervenes, there is the danger of hearing damage.

There have been various prior proposals to limit sound levels by the incorporation of diodes in the signal path between the circuit input and the transducer, eg a headphone unit. US 4,224,470 discloses a headset with a limiter circuit including Zener diodes. WO 2007/112424 discloses headphones with an audio limiting circuit including Schotky diodes or germanium diodes.

There have also been prior proposals to provide visual indications of sound levels.

US 2007/0076907 discloses headphones with a supply conductor arrangement including a visual indication or display relating to sound levels. Different displays may be given for progressively more dangerous sound levels. US 4,538,296 discloses a protection circuit for limiting the sound pressure level provides by headphones incorporating a silicon control rectifier (SCR) which switches on when too high a volume level is reached. A light emitting diode (LED) indicator is provided which lights up when the SCR is triggered on.

Aspects of the present invention seek to provide a limiter circuit which employs fewer components. Other aspects of the invention seek to provide a circuit which is capable of limiting current or voltage levels while simultaneously giving a visual indication thereof.

According to a first aspect of the present invention, there is provided a limiter device comprising a limiter circuit having an input and an output with one or more limiting diodes therebetween, wherein the diode, or at least one of the diodes, is a light-emitting diode visible from the exterior of the device.

An advantage of such a device is that the same component(s) is/are used to provide the limiting function and visual indication thereof. This enables the circuitry to be installed in a compact device, such as an ear-piece or the connector plug or lead for a headset.

In preferred devices an audio signal source is connected to the input and a sound transducer is connected to the output, and the light-emitting diode illuminates to indicate high sound levels. Such a device is especially useful to people wearing earpieces or headphones since it allows them to recognise immediately when unacceptable sound levels are occurring.

The limiter circuit may take a number of forms. In one embodiment, it comprises at least one first light-emitting diode connected in parallel with the input and at least one second light-emitting diode connected in anti-parallel with said first light-emitting diode(s).

In another embodiment, it comprises a rectifier bridge circuit comprising a plurality of diodes is connected across the input, with at least one light-emitting diode connected to constitute a diagonal of the bridge circuits.

In another embodiment, a passive voltage multiplier circuit comprising a plurality of diodes is connected across the input, the multiplier circuit comprising said light-emitting diode.

In a further embodiment, a power supply circuit is connected across the input, the power supply circuit powering active limiting circuitry which is connected to at least one light-emitting diode.

The device may be conveniently located in a housing having a window or other aperture through which the light-emitting diode is visible from the exterior of the device.

The housing may be incorporated in a supply lead to the limiter circuit. This enables it to be conveniently viewed by a user. In preferred arrangements, the housing has external attachment means.

In other arrangements, the housing is a housing for the sound transducer.

There may be provided for the limiter circuit a connector lead with a plug, in which case the housing may be the plug housing.

The sound transducer may be selected from a plurality of sound transducers with different characteristics and means are provided for matching the limiting circuit to the selected transducer. This leads to flexibility of use of the device, so that it can be used by multiple users for a plurality of different applications. The means for matching the circuit to the selected transducer may operate in an automatic manner.

According to a second aspect of the present invention, there is provided a limiter circuit having an input and an output with one or more limiting diodes therebetween, wherein the diode, or at least one of the diodes, is a light emitting diode.

According to a third aspect of the present invention, there is provided a method of handling an electrical voltage or current wherein at least one light-emitting diode is used to limit the level of the electrical voltage or current.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig. 1 is a circuit diagram of a limiter circuit according to a first embodiment of the present invention; Fig. 2 is a circuit diagram of a limited circuit according to a second embodiment of the present invention; Fig. 3 is a circuit diagram of a voltage multiplier circuit in accordance with a third embodiment of the present invention; Fig. 4 is a circuit diagram of an alternative voltage multiplier circuit in accordance with a fourth embodiment of the present invention; and Fig. 5 is a circuit diagram of an active circuit in accordance with a fifth embodiment of the present invention.

Referring to the drawings, Fig. 1 shows a voltage limiting circuit 10 having an audio signal source 11 connected to its input and an earpiece 12 connected to its output.

Connected in parallel with source 11 and earpiece 12 is an anti-parallel combination of two diodes 14, 16. The diodes serve to limit excess voltage swings from source 11 in respective opposite directions. A first resistor RI serves to limit peak loading of the signal source 11. A second resistor R2 serves to limit the power with the earpiece. The resistance valve of R2 is selected to match the particular earpiece used.

As described so far, the circuit corresponds generally to that disclosed in WO 2007/112424. In the circuit of Fig. 1, however, the limiting diodes 14, 16 are both light-emitting diodes. The circuit 10 is located inside a housing (not shown) which has a window or other aperture which permits a user to view the LEDs 14, 16. The LEDs illuminate when they conduct to perform their limiting function so that the user can immediately see that the sound level is too high and that it has been necessary for the circuit to effect automatic noise limitation.

The housing is conveniently located in the lead connected to the earpiece and at sufficient spacing from the earpiece that the illumination of the LEDs can be seen in normal use. For example, the housing may have a clip to enable it to be attached to an article of clothing of the wearer in the wearer's line of sight. Once limitation has occurred, the wearer has the options of adjusting the amplitude of the incoming signal, removing the earpiece or (for example if the excess signal excursion is a single event) taking no action.

In all cases the wearer's hearing is adequately protected.

An advantage of the above arrangement is that the LEDs 14, 16 are used to simultaneously limit the signal and to provide a visual indication of their limiting action.

Thus the number of components of the circuit can be kept to a minimum. In addition, the energy produced by the limiting action is at least partly dissipated in producing the visual indication.

In a modification, light from only one of the LEDs 14, 16, is visible through the housing window, this being sufficient to give a useful indication. In a further modification the other LED is replaced by a diode which is not an LED.

Fig. 2 shows a second embodiment of the present invention in which a circuit 20 has a different diode arrangement 25. Here a rectifier bridge circuit comprises an individually-parallel arrangement of two diodes 23, 24, an LED 26, and two diodes 28, 29.

Thus LED 26 forms a diagonal of the rectifier bridge circuit. Diodes 23, 24, 28, 29 are not LEDs and can be any suitable rectifying means, In terms of limiting excess signals and providing a visual indication thereof, the arrangement 25 functions in a similar way to the circuits of Fig. 1. An excess voltage signal excursion in one sense will cause a current to flow through diodes 23, 26 and 29 so that LED 26 will illuminate. An excess voltage signal excursion in the opposite sense will cause a current to flow through diodes 28, 26 and 24 so that LED 26 will illuminate.

An advantage of the embodiment of Fig. 2 is that only a single LED is required.

Moreover, it is possible for the window in the housing to be smaller, since light from only a single source needs to be visible.

Various modifications can be made to the above-described embodiments. For example, if desired, the individual LEDs may each be replaced by a plurality of LEDs or by an LED in series with a normal diode. The light path from each LED to the housing window may be via one or more of a lens, a mirror, an optical fibre and or through a panel, which may be transparent or translucent. Electronic means, such as a photodetector, may be provided for detecting illumination of the LED and for producing a further indication of the limiting function. Thus further indication may be a remote visual indication or an audible click or a beep. Alternatively the circuitry may automatically produce an audible click when limiting occurs.

For stereo circuits, or for earpieces with a plurality of independent feeds, a circuit according to Fig. 1 or Fig. 2 can be provided for each channel. The earpiece may be a custom-made or other device arranged to fit within the wearer's ear or can be part of a headphone set. The housing with the illuminated window can be provided at any desired location, eg or an article carried by the wearer, in particular on a support strap, or on a piece of equipment operated by the wearer. A pin, such as a safety pin, or other suitable convector may be used instead of a clip for attaching the housing.

Turning now to Fig. 3, there is shown a passive voltage multiplier circuit 30 in accordance with a third embodiment of the present invention. As with the embodiment of Fig. 2, the circuit comprises a plurality of normal diodes Dl to D4 and a single LED 36. A resistor Rl limits peak loading of the signal source 11. A resistor R2 limits power into the earpiece 12. A plurality of capacitors Cl-C4 is also provided which, together with diodes D1-D4 boost the low-level voltage into the limiting region of the LED 36.

Instead of the circuit shown in Fig. 3, a transformer or any other suitable passive device can be used to boost the voltage levels, in particular for low-signal applications.

Fig. 4 shows an alternative voltage multiplier circuit 50 in accordance with a fourth embodiment of the present invention. The circuit comprises normal diodes D3, D4 capacitors C3, C4 and a single LED 56.

Fig. 5 shows an active circuit 40 in accordance with a fourth embodiment of the present invention. Circuit 40 comprises a single LED 46. In this circuit both a switching step-up power supply 42 and active circuitry 44 are connected in parallel with source 11 and earpiece 12. An optional resistor Ri optionally limits peak loading of the signal source 11. Register R2 limits power into the earpiece. The power supply circuit 42 uses some of the energy from the incoming signal to power the active circuitry 44, which provides a limiting function through dumping energy into LED 46.

Instead of power supply circuits 42, another power source can be employed, such as a battery, an electro-mechanical generator, an energy harvester or a solar power source.

The single LEDs 36, 46, 56 in the embodiments of Figs. 3, 4 and 5 can be replaced by a plurality of LEDs.

Various modifications may be made to all the above devices. For example, the LED or one of the LEDs may be arranged to give out a relatively low level of light during acceptable sound levels and to give out a brighter light when the limiting function is being used. Suitable additional circuitry may be provided to convert some of the energy produced during operation of the limiting function to generate an audible warning.

To allow different earpieces 12 to be used with the limiter circuit a suitable connection socket may be provided and resistor R2 may be variable, eg by means of a switched arrangement, to set an appropriate level for the earpiece being used. The adjustment to match the resistor R2 to the selected earpiece may be effected by the user or automatically.

Some of the energy produced by the limiting can be used to provide a visual indication while the active circuitry 44 alters the signal level by gain change rather than by clipping (this covers limiters which operate by manipulating gain rather than clipping or non-linear compression). Dynamic coupling (eg capacitive) may be employed to improve handling of signal transients. Capacitive coupling can be used to improve operation with "peaky" signals such as speech. This is of particular relevance with reference to the embodiment of Fig. 5.

Other forms of temporal filtering than capacitiveldynamic coupling may be used in controlling the signal limiting and/or the signal gain.

Other forms of signal processing may be employed to control the effective signal energy levels. This can be a relatively sophisticated monitoring system, limiting sound pressure levels over an extended period such as several hours. A period monitoring function may be provided; this can give an indication or manipulate signal levels when a predetermined accumulated threshold is reached.

Other non-linear or adjustable-gain devices may be used in limiting the signal.

A switch device can be employed to facilitate local sound from a microphone and/or other audio sources to be added to the signal.

The earpiece can be integral to a headset or another form of personal listening device.

Means may be provided to permit a selectable or variable threshold for the signal-level limiting effect to be set.

The present invention is not limited to the audio signal limiting arrangements described above. In accordance with the present invention, a light-emitting diode can replace any of the diodes in the above-mentioned prior art documents which undertake a limiting function. Devices according to the present invention are not limited to audio applications. In accordance with the invention a light-emitting diode can be used to limit any electrical signal.

The features of the various described embodiments and modifications thereto may be combined or interchanged as desired.

Claims

I. A limiter device comprising a limiter circuit having an input and an output with one or more limiting diodes therebetween, wherein the diode, or at least one of the diodes, is a light-emitting diode visible from the exterior of the device,
2. A device according to claim 1, wherein an audio signal source is connected to the input and a sound transducer is connected to the output, and the light-emitting diode illuminates to indicate high sound levels.
3. A device according to claim 1 or 2, comprising at least one first light-emitting diode connected in parallel with the input and at least one second light-emitting diode connected in anti-parallel with said first light-emitting diode(s).
4. A device according to claim 1 or 2, wherein a rectifier bridge circuit comprising a plurality of diodes is connected across the input, with at least one light-emitting diode connected to constitute a diagonal of the bridge circuit.
5. A device according to claim 1 or 2, wherein a passive voltage multiplier circuit comprising a plurality of diodes is connected across the input, the multiplier circuit comprising said light-emitting diode.
6. A device according to claim 1 or 2, wherein a power supply circuit is connected across the input, the power supply circuit powering active limiting circuitry which is connected to at least one light-emitting diode.
7. A device according to any preceding claim, wherein the device is located in a housing with a window through which the light-emitting diode is visible from the exterior of the device.
8. A device according to claim 7, wherein the housing is incorporated in a supply lead to the limiter circuit.
9. A device according to claim 8, wherein the housing has external attachment means.
10. A device according to claims 2 and 7, wherein the housing is a housing for the sound transducer.
11. A device according to claim 7 wherein there is provided for the limiter circuit a connector lead with a plug, and the housing is a housing for the plug.
12. A device according to claim 2, wherein the sound transducer is selected from a plurality of sound transducers with different characteristics and means are provided for matching the limiting circuit to the selected transducer.
13. A device according to claim 12 wherein said matching means are automatic.
14. A limiter device substantially as herein described with reference to any of Figs. I to of the accompanying drawings.
15. A limiter circuit having an input and an output with one or more limiting diodes therebetween, wherein the diode, or at least one of the diodes, is a light emitting diode.
16. A limiter circuit substantially as herein described with reference to any of Figs. ito of the accompanying drawings.
17. A method of handling an electrical voltage or current wherein at least one light-emitting diode is used to limit the level of the electrical voltage or current.
18. A method of handling an electrical voltage or current substantially as herein described with reference to any of Figs. 1 to 5 of the accompanying drawings.