EP2375408B1

EP2375408B1 - A method of adapting noice reduction and a system for providing audio with noise reduction

Info

Publication number: EP2375408B1
Application number: EP10166978.6A
Authority: EP
Inventors: Jan Abildgaard Pedersen
Original assignee: Harman Becker Gepkocsirendszer Gyarto Kft
Current assignee: Harman Becker Gepkocsirendszer Gyarto Kft
Priority date: 2010-04-12
Filing date: 2010-06-23
Publication date: 2021-03-10
Anticipated expiration: 2030-06-23
Also published as: EP2375408A3; EP2375408A2

Description

The present invention relates to a method of adapting noise reduction when providing an audio signal and in particular in an environment, such as a vehicle, where the noise contents and noise level changes over time.
Usual noise reduction relies on a determination/estimation of the noise and subsequently "subtracting" the noise signal from the desired audio signal or adding the inverted noise signal thereto. The noise may be directly determined using sensors, such as microphones, or may be estimated, such as by determining the rpm of an engine and there from estimating the actual noise generated. In the latter example, a mathematical equation or a look-up table may be used for estimating the noise from the rpm determined.
Other types of systems act to adapt the audio provided on the basis of the noise generated by e.g. increasing the audio volume with increasing rpm of an engine.
In general, when using the same loudspeaker(s) and/or amplifier(s) for outputting the audio signal and the noise cancelling signal, the overall headroom for each of the signals will be reduced especially at large intensities/volumes. This reduces the quality of the audio signal provided, which is not desired. System using active noise cancellation technology in connection with signal limiters is known from US5469510A , JP2008137636A , JPH06282283 and US2009034748A1 . However, signal peaks can occur that may deteriorate the audio signal.
It is an object of the invention to adapt the resulting audio signal output to the noise generated and counteracted, but in a manner so that the audio signal is not compromised.
In a first aspect, the invention relates to a system for outputting an altered audio signal, the system comprising:

means for providing a noise reducing signal,
means for providing or receiving an audio signal,
means for outputting an altered audio signal based on the audio signal and the noise reducing signal, the outputting means being adapted to:
- o limit the noise reducing signal and the audio signal,
- o provide the altered audio signal by combining:
  - the limited audio signal with the limited noise reducing signal, wherein the outputting means are adapted to:
limit the audio signal so as to have a level/strength no higher than a first predetermined level/strength and
limit the noise reducing signal so that the combined, limited audio signal and noise reducing signal have a level/strength no higher than a second, predetermined level/strength, wherein the first predetermined level/strength is lower than the second predetermined level/strength, and
wherein the system further comprises additional means for limiting the altered audio signal.

In this context, an altered audio signal is an audio signal having one component being the audio signal to be provided to e.g. a listener and another component which is adapted to or desired to counteract or cancel out at least part of an audible noise present at the listener.
Naturally, an audio signal may be any type of signal, usually representing music, speech, rhythms, discussions, narrations, shows, or the like.
The audio signal may be received from an external source, such as a wireless signal received by an antenna, a cell phone or the like, an optically transmitted signal detected by an optical detector, or may be delivered by a local or internal source, such as from a CD, DVD, tape recorder, MP3/4 player, hard drive memory or a memory of any other type, such as a memory unit of a cell phone, a USB memory unit, flash memory, a RAM/ROM/PROM/EPROM/EEPROM or the like. Thus, the means for providing or receiving an audio signal may be adapted to derive the signal from any type of memory or storage unit and/or from an external source using an antenna or other means adapted to receive a wireless or optically transmitted signal.
A noise reducing signal is a signal which, when output, acts to counteract, extinguish and/or reduce audible noise. Usually, the outputting of the signal is the feeding of the signal to a loudspeaker or woofer.
A noise reducing signal may be provided or generated in a number of manners. In the extreme situation, the noise may be predicted or otherwise known, whereby the signal may be generated without any determination/detection or measurement of the actual noise. Usually, however, the noise reducing signal is generated on the basis of a determination, sensing or estimation of the actual noise.
Determining, sensing or estimating noise may be performed in a number of manners depending on whether the noise source is known and/or available. If the noise source is available, direct measurement of the noise may be performed at, in or on the noise source.
If the noise source can be modeled, the noise output or generated may be determined/estimated/approximated using other types or features of the noise source. One example is the noise generated by an engine, which may be determined/estimated/approximated based on a determination of the RPM of the engine. Thus, a mathematical model, or a look-up-table, may be used for generating the actual noise-reducing signal.
Alternatively, the noise may be determined indirectly, such as using force/vibration detection. Wind or tyre noise of a vehicle may be estimated on the basis of a velocity of the vehicle. Any type of sensors may be used, based on any suitable measuring technique, such as optical measurements, accelerometers, position sensors, rotation sensors, strain gauges, or the like.
A more general approach is to determine or sense the noise directly, such as using a microphone, accelerometer or the like. In this manner, knowledge of the noise source is not required, and noise from several sources may more easily be determined/detected. This also has the advantage that noise may be sensed/detected at different positions. In this situation, the sensed sound may be a combination of the audio signal and the noise, whereby it may be desired to subtract the audio signal to derive the noise. This is standard technology for a skilled person. This is described in further detail further below.
Naturally, combinations of the different sensing/determining/approximation methods may be used. An example is a car, where noise from the engine may be modeled based on e.g. a rpm-determination and wind noise and/or tire noise may be determined directly, such as using a microphone and subtracting the audio signal detected as well as engine noise.
From the noise determination/estimation, the noise reducing signal is provided which describes the noise and which, when added to an audio signal and provided to e.g. a loudspeaker, will act to counter-act the noise and thus reduce or remove the apparent noise.
Depending on the method desired and the calculation/processing speed, the noise reducing signal may be an inverted noise signal. If a sufficiently small delay is seen between determination/sensing of the noise signal and the outputting of the inverted signal, the outputting of this inverted signal will act to reduce the noise.
If a larger delay is experienced, for example, the inverted noise signal may, at least at some frequencies, act to amplify the noise, whereby another strategy may be to analyze different frequency components and their phases in the noise signal and derive a signal with the same frequency components but an opposite phase, and output this signal.
According to the invention, one or both of the noise reducing signal and the audio signal is/are limited with a level/strength of the audio signal.
Naturally the noise reducing signal may be limited simply by being provided with the desired level/strength. Alternatively, the limitation may be a reduction/increase of the level/strength thereof.
In this context, the level/strength of a signal is a value describing how loud the signal is, such as an amplitude or a mean sound pressure thereof.
This limitation acts to adapt the level/strength of the noise reducing signal in relation to the level/strength of the audio signal. Whether one signal is limited to have a reduced level/strength or the other is limited to have an increased level/strength is not important, as any combined signal may subsequently be limited (amplified to have a larger level/strength or be reduced in level/strength).
Thus, in this context, a "limitation" may both be an increase and a decrease in level/strength. It is noted that an increase/decrease in level/strength may be a linear scaling of the signal, such that all parts of the signal are linearly decreased/increased in level/strength. Alternatively, the limitation may be a non-linear increase/decrease in level/strength, such as the "local" limitation only of parts of a signal exceeding a given threshold, which is also denoted "clipping" or falling under a given threshold.
Subsequently, the signals are combined, such as by a simple summing or adding of the signals.
The limited audio signal is combined with the limited noise reducing signal.
In a preferred embodiment, the noise reducing signal is limited so that the level/strength of the combined signal fulfills predetermined criteria. These criteria may be a maximum level/strength or a level/strength falling within a predetermined interval, and the analysis may be performed on the basis of a mean level of the signal or a peak level of the signal.
According to the invention, the outputting means are adapted to:

limit the audio signal so as to have a level/strength no higher than a first predetermined level/strength and
limit the noise reducing signal so that the combined, limited audio signal and noise reducing signal have a level/strength no higher than a second, predetermined level/strength.

Typically, the second predetermined level/strength will be a maximum output level/strength. This level/strength may be dictated by other elements of the system, such as an amplifier used for subsequently amplifying the combined signal, in order to e.g. maintain optimum audio quality. The first predetermined level/strength is lower than the second one and may be determined so as to leave sufficient "headroom" for the noise reducing signal but still ensure sufficient audio signal level/strength and thereby quality.
According to the invention, the limiting means are adapted to limit, clip or scale the audio signal, i.e. before being combined. In this manner, it may be ensured that the level/strength of the audio signal is sufficiently low to allow "space" for the noise reducing signal - limited or not - while maintaining a maximum allowed or desired level/strength of the altered audio signal. In this embodiment, the audio signal may be limited, clipped or scaled to have a strength/level below a predetermined value, percentage or the like of a maximum desired level/strength of the altered audio signal. Then, the noise reducing signal may also be limited, if it does not already fall within the allowed "space" and thus has a maximum level within defined limits.
The advantage of this adaptation is that when noise reduction is needed, e.g. when an engine generates noise and no significant music levels are being played back then the noise reducing signal can be allowed to use almost all the dynamic headroom of the playback system. Thus, a good noise reduction is obtained when the audio signal level is low and the noise therefore audible. And when large levels of music are being played back, whereby the level/strength of the audio system is high, the noise reducing signal can be attenuated to avoid significant distortion of the sound output. Then, the noise will still be less audible due to the high level of the audio signal. Thus, both the noise reducing and the entertainment system can take full advantage of the audio system with only minimal interference.
It is noted that the noise may be generated by any type of source and usually may be heard both close to and farther away from the source.
Noise may be experienced outside, such as close to a factory, road, race track, railway track, airport, harbour, or the like. Thus, the present system may be desired when wishing to experience an audio signal in the vicinity of such installations. Then, music systems on porches, balconies, open air concerts or the like may advantageously use the present system. Also when using portable MP3/4 players or cell phones, the present system is advantageous. In one embodiment, however:

the means for providing/receiving the noise reducing signal comprise means for determining/estimating noise in a structure and providing the noise reducing signal from determined/estimated noise and
the outputting means are adapted to output the altered audio signal into the structure.

In the present context, the structure may be any type of structure, such as a building, a room/space in a building, a vehicle such as a car, bus, lorry or the like, or a vessel, such as a ferry, ship, boat or the like. Again, it is noted that the noise may be generated within the structure, such as an engine of a car, or may be generated outside the structure, such as a railway track located close to a house.
Usually, the system comprises a loudspeaker and means for providing the altered audio signal to the loudspeaker. Naturally, multiple loudspeakers may be used both to obtain stereo/surround sound or to obtain a desired sound level. In addition, different loudspeakers may have different frequency characteristics, so that a combination of speakers is desired to obtain a desired sound.
Naturally, one or more amplifiers may be used for amplifying the altered signal before being fed to the loudspeaker(s).
Depending on the noise source, such as the distance thereto, the noise cancellation or noise reduction may not be sufficient at all positions. This is equally relevant outside as within a structure. This disadvantage may be taken into account if the system further comprises a plurality of loudspeakers positioned in different first positions, such as within the structure, and wherein:

the means for receiving or providing the noise reducing signal are adapted to receive/provide noise signals for each of a plurality of different second positions in the structure and provide one or more noise reducing signals on the basis of received/provided noise signals,
the outputting means is adapted to generate a plurality of altered signals, each altered signal being generated on the basis of the audio signal and a noise reducing signal,

Thus, noise is determined/estimated at a plurality of second positions, even though each noise signal could be used for generating a particular, single altered signal which is then output to a loudspeaker, such as a loudspeaker positioned close to the pertaining second position, it is preferred that all noise signals are used for the generation of each altered signal.
In a preferred embodiment, the structure is a car, and speakers are positioned at each of the four main seats in the car as well as centrally (such as a subwoofer). Then, microphones may be positioned near or above the four main seats and generate noise signals from those positions.
As mentioned above, the limiting may act to ensure that the altered audio signal has a maximum or predetermined level/strength. However, due to there usually being a delay in the processing system, sudden bursts in the audio signal may occur and cause the altered audio signal to exceed this maximum limit. Thus, a too high signal is output to e.g. an amplifier, which may cause distortion of the sound.
Thus, preferably, the system further comprises additional means for limiting the altered signal, such as to stay below a predetermined maximum limit. In this situation, such processing delays are not detrimental to the system. Again, this limitation may be a scaling, clipping or any other desired type of limitation.
Naturally, this further limitation may be a standard limitation or attenuation of the signal.
A second aspect of the invention relates to a method of generating an altered audio signal, the method comprising:

providing a noise reducing signal,
providing or receiving an audio signal,
generating the altered audio signal based on the audio signal and the noise reducing signal by:
- o limiting the noise reducing signal and the audio signal,
- o providing the altered audio signal by combining:
  - the limited audio signal with the limited noise reducing signal, and
    - the further step of limiting the altered audio signal, wherein the limiting step comprises
      - limiting the audio signal so as to have a level/strength no higher than a first predetermined level/strength and
      - limiting the noise reducing signal so that the combined, limited audio signal and noise reducing signal have a level/strength no higher than a second, predetermined level/strength,
wherein the first predetermined level/strength is lower than the second predetermined level/strength.

The noise reducing signal may be estimated or predetermined or may be derived from a determination or estimation of the noise, which again may be performed as described above: directly, indirectly or modeled. A large number of manners are used. Each manner has its advantages and disadvantages and may be selected based on the actual situation.
The audio signal may be received from e.g. a remote source, such as a radio transmitter, a remote network, a cell phone transceiver or the like. Also, the audio signal may be provided from a source or storage locally or readily available, such as from a CD or DVD in a player, a MP3/4-player, a cell phone, or a local storage of any other type.
Again, the altered audio signal is based on the audio signal and the noise reducing signal, where at least one of these signals is limited on the basis of a level/strength of the audio signal. Again, the limiting may be an increasing of the level/strength of one of the representations or a reduction of the level/strength of the other - or both. Usually, the limiting is used to obtain a predetermined or desired relation between the level/strengths of the audio signal and the noise reducing signal.
In a preferred embodiment, even though the method may be carried out also in the open:

the step of providing/receiving the noise reducing signal comprises determining/estimating noise in a structure, and
the altered audio signal is output into the structure.

Usually, when noise cancellation or reduction is desired in a structure, the noise is determined or estimated in the structure. It is noted that the noise may be determined or sensed outside the structure, but then, preferably, an estimation step is used in order to estimate the noise inside the structure, as the structure will normally alter or affect the noise, such as the strength/level/amplitude thereof as well as a frequency content thereof.
Usually, the method further comprises providing the altered audio signal to a loudspeaker. Thus, the altered audio signal will be output as a combination of the audio signal and the noise reducing signal to the effect that the overall noise is reduced or cancelled out.
Naturally, multiple speakers may be used, and usually, one or more amplifiers are used for amplifying the altered audio signal before feeding this to the speaker(s).
In a particularly interesting embodiment, the method further comprises the steps of:

providing a plurality of loudspeakers positioned in different first positions,
receiving/providing noise signals for each of a plurality of different second positions in the structure, the step of receiving/providing the noise reducing signals comprising providing a plurality of noise reducing signals on the basis of the noise signals,
generating a plurality of altered signals, each altered signal being generated on the basis of the audio signal and a noise reducing signal, and
guiding each altered signal to one of the loudspeakers.

As mentioned above, an altered signal may be derived for one first position and on the basis of a noise reducing signal derived from a noise signal from one second position, which positions may correspond, so individual or local noise cancellation or reduction may be obtained at different positions, such as within a structure. Usually, however, all or more than one noise signal is used for generating each altered signal.
In one embodiment, the method further comprises the further step of limiting the altered signal, such as before amplifying it. This has the advantage that sudden burst in the altered signal do not cause the amplified signal to be distorted due to the amplifier being operated close to its upper limit.
According to the invention, the method comprises the step of limiting the audio signal before the combination. This is one manner of allowing the limiting step to not having to limit the audio signal but only the noise reducing signal in order to obtain a desired relation between the levels/strengths thereof while allowing a minimum "space" or headroom for the noise cancellation.
As mentioned above, the limiting of the determined/estimated noise reducing signal may be a generation of this signal with the desired level/strength.
According to the invention, the limiting step comprises:

limiting the audio signal so as to have a level/strength no higher than a first predetermined level/strength and
limiting the noise reducing signal so that the combined, limited audio signal and noise reducing signal have a level/strength no higher than a second, predetermined level/strength.

In the following, preferred embodiments of the invention will be described with reference to the drawing, wherein:

figure 1 illustrates major components of an embodiment of the invention and
figure 2 illustrates steps performed in the analyzer of the first embodiment.

In figure 1, major components of a preferred embodiment 10 of the invention for use in a car are described.
As is usual in cars 30, an audio signal may be provided as a flow or signal, such as generated by a radio wave receiver, a CD player, a DVD player, a network connection, an MP3/MP4 player, a cell phone, a computer, a laptop, a router, an e-reader, a memory holding audio information, or any other type of audio source.
This signal is fed to a unit 12 which introduces cross-over filters, equalizers etc, and divides the audio signal into individual signals for each of a number of, here six, speakers in the car 30.
Subsequent to the unit 12, circuits are provided for adapting the individual audio signals to meet specifications of the user/driver of the car 30, such as an adaptation of the volume 14 or other types of manipulations, which will be described further below. Naturally, no manipulations or adaptations are required, as the audio signal may be output as received.
The resulting, adapted signals usually are amplified and output into the interior of the car 30 using one or more speakers. Only a single amplifier 28 and a single speaker 34 are illustrated.
Naturally, any number of speakers 34 and amplifiers 28 may be used. Presently, desired types of manipulations are performed by a volume control, a so-called compressor which handles the variations of dynamics found in the music signal, and a limiter 18, the function of which is to ensure that the audio signal is kept below a predefined level before adding the noise reducing signal, as will be described below. The limiter 18 may ensure that the audio signal is kept e.g. 2 dB below Full Scale Digital (FSD) of a digital signal path or a predetermined limit below full analogue signal on an analogue signal path.
Naturally, the signal path may be a single audio signal path instead of the separate paths depicted in the figure. Then, if multiple speakers are used, the signal may be divided into signals for each speaker at any position in the figure.
In the figure, a total of six audio paths are illustrated between the audio source 12, volume control 14, compressor 16, limiter 18 and then to separate amplifiers and speakers, of which only one amplifier and one speaker is illustrated, for a total of six music channels to be presented in the car.
In addition to these standard components 12, 14, 16, 18, 26, 28 and 34 of many car HiFi systems, the present system 10 further comprises an analyzer 20 which receives the audio signal output of the limiter 18.
The analyzer 20 receives one of the audio signals output of the limiter 18 and determines a level thereof. This level is a value describing how loud the audio signal is in the car. This level is transmitted to a noise reducing circuit 22 which generates a noise reducing signal adapted to cancel noise in the car 30. The level may relate to the signal level of the audio signal or a maximum level or a noise reducing signal to be generated.
This cancelation/reduction may be performed in any desired manner, such as based on an actual determination or detection of the noise in the car using one or more microphones 32 and/or based on the Revolutions Per Minute (RPM) of the engine of the car 30, such as could be determined from the engine control system 36 of the car 30. Detected noise may itself be used for generating the noise reducing signal, such as by simple inversion, or may be used for the generation thereof. Usually, a determined RPM is used by a Digital Signal Processor (DSP) for generating a noise reducing signal based on any of a multitude of methods.
In the extreme situation, the noise may be so well known that it need not be determined/sensed but may be predicted/modelled sufficiently without such determination/sensing.
The determined noise reducing signal is limited on the basis of the value provided by the analyzer 20 and is subsequently added, in the adder 24, to the audio signal output of the limiter 18 and from which the value was derived.
The limiting/scaling of the noise reducing signal is an adaptation of the level or volume thereof and may be performed by simple multiplication or division of the noise reducing signal by the value received from the analyzer 20, so as to ensure that the sum of the audio signal and the noise reducing signal does not exceed a predetermined limit. In this manner, it is ensured that the amplifier 28 is not overloaded by the summed signal output of the adder 24, which could otherwise result in a deterioration of the audio signal quality. It is noted that the limiting of a signal may be a linear limiting of all parts of the signal or e.g. a clipping, whereby only parts of the signal exceeding, for example, a maximum level, are reduced.
Thus, the limiter 18 may be set to ensure that the audio signal does not take up the full signal level so that there is "room" for the noise reducing signal. In one situation, a further limiter 26 is provided in order to ensure that the summed signal does not overload the amplifier 28. Then, the setting of the limiter 18 may be lower compared to the setting of the limiter 26.
Naturally, the analyzer 20 and noise reducing circuit 22 will provide a signal delay, so that a sudden increase in the audio signal, which would then cause a reduction in a level of the noise reducing signal, will cause a short burst of high level summed signal, which may then be reduced by the limiter 26.
Thus, in general, the louder the audio (the higher the level of the audio signal), the lower will the resulting (limited) level of the noise reducing signal be. However, the louder the audio, the less discernible will the noise be, so the present technology will both ensure noise cancellation/reduction and first of all a high quality audio representation in the vehicle.
The settings of the limiters 18 and 26 may be chosen in many manners. In one situation, 2dB may be "reserved" by the limiter 18 for the noise reducing signal, to ensure a minimum "head room" for the noise reducing signal independently of the level of the audio signal. This, however, may effectively reduce the power output available from the amplifier for the audio signal from e.g. 125W to 79W. Therefore, less "head room" may be desired for the noise reducing signal to be able to have a louder audio signal. This will also depend on the actual vehicle and the amount of noise which requires cancellation/reduction.
A single analyzer 20 and noise reducing circuit 22 may be provided for generating the noise cancellation in the vehicle 30. This may be on the basis of the audio signal in a single channel as illustrated in figure 1 or on the basis of the audio signals of all channels. However, an analyzer 20 and noise reducing circuit may be provided for each speaker 34 or more than one speaker 34, so as to better provide the noise cancelling sound in the vehicle 30. Also, in this situation, the signal strength required from each speaker 34 to cancel the noise (when the noise reducing signal is output by multiple speakers 34) will be lower, whereby a better audio quality (and louder audio) may be obtained.
Naturally, multiple microphones 32 may be present in the vehicle 30. In fact, one microphone 32 may be provided per speaker 34, per main seat in the vehicle 30, or per set of analyzer 20 and noise reducing circuit, so that the different noise in different positions in the car 30 may be determined and cancelled out. In one situation, the noise reducing signal for a given speaker 34 may be generated from the microphone 32 which is the closest to the speaker 34 or from signals from multiple microphones 32 in order to estimate, from the microphone signals, the noise signal at the position of the speaker 34 or of a listener. Naturally, the sound output of all speakers 34 will be sensed by all microphones 32, so the output of each speaker 34 should be taken into account when generating the noise reducing signal. Usually, a system having four microphones 32 and four speakers 34 will result in the solving of a number of equations with a number of unknown parameters. This, however, is a known technique.
Naturally, individual units of figure 1 may be, as is also described above, left out or be positioned at other positions in the flow direction of the audio signals. It is preferred, however, that the compressor 16 is positioned before the adder 24 in the signal path (downwards in the figure) so as to not affect the noise reducing signal provided by the circuit 22.
In figure 2, the operation principle of the Analyzer 20 is illustrated. This principle builds on five steps:
Firstly, the absolute value of the audio signal from the limiter 18 is derived in step 50:
Output(t) = ABS(input(t))
The output is simply calculated as the absolute value of the input yielding a strict positive number. The input is the music signal for any one of the 5 woofers (WF - left front, WF - right front, WF - left back, WF - right back, Subwoofer), which is generated by the complete audio flow.
Then, a peak detection is made in step 52:
output = MAX(input(t)), 0sec < t < 1.0sec
The output is the maximum value of the input during a period of 1.0sec. The output is only calculated every integer multiple of 1.0sec, i.e. at the termination of each block of 1.0sec. (end of peak detection period). The length of the peak detection period can be adjusted.
There after, in step 54, it is determined how much digital head room can be used for the noise reducing signal. This is performed by comparing to the max Digital Value, which is the max value which is to be output by the adder 24 in order to still maintain the desired audio quality: $output = Max Digital Value - Peak Value$
The calculated Peak Value is subtracted from the Max Digital Value to work out how much digital head room can be reserved for the noise reducing signal.
In step 56, it is ensured that there is always "room" for the audio signal:
output = input bound to the range [ (0.0057 x FSD) : (0.8 x FSD) ]
The output is calculated as the input restricted (bound to) the range from (0.0057 x FSD) to (0.8 x FSD). This ensures that some digital head room is always reserved for the music signal, i.e. -14 dB rel. FSD and that a small digital head room (-44.8 dB rel. FSD) is reserved for a noise reducing signal even when the music is at maximum level. The minimum allowed noise reducing signal level "Min. ANC" and the maximum allowed noise reducing signal level "Max. ANC" can be adjusted, depending on the actual needs and e.g. the noise level of the vehicle.
The final step, step 58, is a slew rate limiter:
output = tracing towards input at a specified slew rate
The current output is the former output incremented or decremented by a specific number or step size to move the current output closer to the input value (tracing). This process or step acts to limit the increase/decrease of the noise reducing signal (also called attack rate) to avoid the situation where the audio signal decreases abruptly but just as abruptly increases again. In this situation, the changes of the noise reducing signal strength are limited by the slew rate limiter.
The specific number, which is the step size, here corresponds to a slope of +/-FSD/(2x0.05sec). Only if the former output is within +/- (specific number/2), the output remains unchanged.
The output of this block is the "Max ANC signal", which imposes a limit on how large the noise reducing signal is allowed to be. The maximum slew rate for increasing "Max. ANC" and decreasing "Max. ANC" can be adjusted individually, i.e. attack rate and release rate.
Naturally, the analyzer 20 and noise reducing circuit 22 may be provided in a number of manners, such as as special purpose electronics, ASICs or the like, or on the same or different processors or DSPs. If multiple analyzers 20 and noise reducing circuits 22 are provided, any number of processors/DSPs or the like may be used, depending on the capabilities thereof and the programming efficiency etc. of the software defining these processes or circuits.
Even though the present invention and embodiments primarily have been directed toward noise cancellation in a car or vehicle, the same technology may with advantages be used in any type environment. A number of structures or crafts generate noise themselves, such as trains, busses, ferries, ships or the like having engines or other elements creasing noise. However, also external noise sources may generate noise which is desired cancelled. For example, noise from the surroundings may be desired cancelled out within a building or room, or outside, such as on a soccer field, on a porch or the like. Naturally, the noise detection or estimation means may vary from situation to situation. If the noise is generated by an element to which access is possible, the rpm-detector mentioned above may be a possibility. This may, naturally, be replaced by another type of sensor adapted to detect or sense noise from that particular type of element.
Alternatively, the noise may be detected or sensed using microphones, vibration sensors or the like. Alternatively, the noise may be estimated or modelled without requiring sensing of any kind.
Then, the same noise detection or noise cancellation signal(s) may be used in different environments for noise cancelling different audio signals, such as an audio signal generated by a TV in one room of a house or ship and a radio signal provided in another room of the house/ship.

Claims

A system for outputting an altered audio signal, the system comprising:
- means (32) for providing a noise reducing signal,

- means (12) for providing or receiving an audio signal,

- means (18, 20, 22, 24) for outputting an altered audio signal based on the audio signal and the noise reducing signal, and means for limiting the altered audio signal;
whereby the outputting means (18, 20, 22, 24) is adapted to
o limit the noise reducing signal and the audio signal,

o provide the altered audio signal by combining:
• the limited audio signal with the limited noise reducing signal, wherein the outputting means (18, 20, 22, 24) are adapted to:
- limit the audio signal so as to have a level/strength no higher than a first predetermined level/strength and the system characterized in that the outputting means is adapted to

- limit the noise reducing signal so that the combined, limited audio signal and noise reducing signal have a level/strength no higher than a second, predetermined level/strength,

wherein the first predetermined level/strength is lower than the second predetermined level/strength.
A system according to claim 1, wherein:
- the means (32) for providing the noise reducing signal comprise means (22) for determining/estimating noise in a structure and providing the noise reducing signal from determined/estimated noise and

- the outputting means (18, 20, 22, 24) are adapted to output the altered audio signal into the structure.
A system according to claim 1, further comprising a loudspeaker (34) and means (28) for providing the altered audio signal to the loudspeaker (34).
A system according to any of claims 1-3, further comprising a plurality of loudspeakers positioned in different first positions, and wherein:
- the means (32) for providing the noise reducing signal are adapted to receive/provide noise signals for each of a plurality of different second positions in the structure and provide one or more noise reducing signals on the basis of received/provided noise signals,

- the outputting means (18, 20, 22, 24) is adapted to generate a plurality of altered signals, each altered signal being generated on the basis of the audio signal and a noise reducing signal, the system comprising means for guiding each altered signal to one of the loudspeakers (34).
A method of generating an altered audio signal, the method comprising:
- providing a noise reducing signal,

- providing or receiving an audio signal,

- generating the altered audio signal based on the audio signal and the noise reducing signal by:
o limiting the noise reducing signal and the audio signal,

o providing the altered audio signal by combining:
• the limited audio signal with the limited noise reducing signal, an then further limiting the altered audio signal, wherein the limiting step comprises
- limiting the audio signal so as to have a level/strength no higher than a first predetermined level/strength, the method characterized in that the limiting step further comprises

- limiting the noise reducing signal so that the combined, limited audio signal and noise reducing signal have a level/strength no higher than a second, predetermined level/strength,

wherein the first predetermined level/strength is lower than the second predetermined level/strength.
A method according to claim 5, wherein:
- the step of providing the noise reducing signal comprises determining/estimating noise in a structure, and

- the altered audio signal is output into the structure.
A method according to claim 5 or 6, further comprising providing the altered audio signal to a loudspeaker (34).
A method according to any of claims 5-7, further comprising the steps of:
- providing a plurality of loudspeakers positioned in different first positions,

- receiving noise signals for each of a plurality of different second positions in the structure, the step of providing the noise reducing signals comprising providing a plurality of noise reducing signals on the basis of the noise signals,

- generating a plurality of altered signals, each altered signal being generated on the basis of the audio signal and a noise reducing signal, and

- guiding each altered signal to one of the loudspeakers.