EP3841668A1

EP3841668A1 - Controlling a limiter adapted for selectively suppressing an audio signal

Info

Publication number: EP3841668A1
Application number: EP18930864.6A
Authority: EP
Inventors: Anders Ekman
Original assignee: Dirac Research AB
Current assignee: Dirac Research AB
Priority date: 2018-08-24
Filing date: 2018-08-24
Publication date: 2021-06-30
Also published as: WO2020040676A1; EP3841668A4

Abstract

There is provided a controller (230) configured to control a limiter (220) adapted for selectively suppressing an audio signal processed by an audio processing chain (210) comprising a number, N ≥ 1, of audio processing blocks located in the signal path to the limiter (220). Each of the N ≥ 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. The controller (230) is configured to determine a control parameter for limiting the maximum suppression of the audio signal based on frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of a test signal as processed by the audio processing chain (210).

Description

CONTROLLING A LIMITER ADAPTED FOR

SELECTIVELY SUPPRESSING AN AUDIO SIGNAL

TECHNICAL FIELD

The proposed technology generally relates to audio processing, and more particularly to a method for controlling a limiter adapted for attenuating an audio signal, and a controller configured to control a limiter adapted for selectively suppressing an audio signal, an audio limiter system comprising such a controller, an audio processing system comprising such a controller, as well as a corresponding overall audio system and a computer program and computer-program product.

BACKGROUND

It is important to ensure safe listening levels of audio systems such as personal music players (PMPs).

By way of example, EU regulation EN 50332 [2], [3], [4] states how loud personal music players (PMP) can be and still play on a safe listening level for users. The regulation specifies that a test file is to be used (HD 438.1 S2), and that the user settings should be set in such a way as to maximize the playout level. The power spectral density of the test signal is shown in FIG. 2. The regulation contains limits [5], [6] and test procedures for both when players are sold together with headphones (100 dB(A) SPL) [2] as well as a limit for when the device is used with any headphone (150 mV max RMS output in 3.5 mm socket over a 32 ohm resistive load per channel) [3].

Personal music players such as mobile phones need to be limited in the playout level to comply with the regulation. One way to do that is to simply lower the playout level with a constant“gain” so that all user settings pass the safe levels, particularly the worst-case settings. The disadvantage of such an approach is that it lowers the playout level also when it is not needed, e.g., when the user settings are not set to the worst- case. Another approach is to use an adaptive gain, for example as applied by an adaptive limiter such as a RMS limiter, that should sit close to the end of the audio processing chain. RMS stands for’’Root Mean Square”, and slightly simplified, what it means in terms of audio is that it is an average over time. More specifically, RMS is the square- root of the arithmetic mean of the square of the signal. In other words, you square every sample value in the input, add them together, divide by the number of samples, and take the square root.

The adaptive gain of such a limiter takes a parameter, typically called threshold, that can be tuned such that the playout level of the test file complies with the regulation while still being as loud as possible. The threshold needs to be set such that the worst- case user settings are still safe. The advantage of the adaptive gain over the approach with constant gain previously described is that it adapts to the material being played, so that when a user plays material that has a low recorded RMS level, it can pass through unaffected. The constant lowering of the playout volume would affect also such material.

The prior art may be represented by references [1], [7], [8], and [9]

However, there is still a general demand for improved ways of ensuring safe playout levels, while allowing users to play as loud as possible within these limits.

SUMMARY

It is a specific object to provide a method for controlling a limiter adapted for attenuating an audio signal.

It is another object to provide a controller configured to control a limiter adapted for selectively suppressing an audio signal.

It is also an object to provide an audio limiter system comprising such a controller.

Yet another object is to provide an audio processing system comprising such a controller. Still another object is to provide a corresponding overall audio system.

It is also an object to provide a computer program and computer-program product. These and other objects are met by embodiments of the proposed technology.

According to a first aspect there is provided a method for controlling a limiter adapted for attenuating a first signal in the form of an audio signal processed by an audio processing chain comprising a number, N ³ 1 , of audio processing blocks located in the signal path to the limiter. Each of the N ³ 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. The method comprises:

determining an estimate of the amount of power that is added by the N > 1 audio processing blocks to a second signal in the form of a test signal, should it be passed through the audio processing chain, based on frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of the test signal;

determining a control parameter for controlling the limiter based on the determined estimate of the amount of power that is added to the test signal; and

determining a maximum suppression that can be applied by the limiter at least partly based on the control parameter.

According to a second aspect there is provided a controller configured to control a limiter adapted for selectively suppressing a first signal in the form of an audio signal processed by an audio processing chain comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter. Each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. The controller is configured to determine a control parameter for limiting the maximum suppression of the audio signal based on frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of a second signal in the form of a test signal as processed by the audio processing chain. Further, the controller is configured to determine a maximum suppression that can be applied by the limiter at least partly based on the control parameter

According to a third aspect there is provided an audio limiter system comprising a limiter adapted for selectively attenuating an audio signal processed by an audio processing chain comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter. Each of the N ³ 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. The audio limiter system further comprises a controller according to the second aspect.

According to a fourth aspect there is provided an audio processing system comprising an audio processing chain followed by a limiter adapted for selectively attenuating an audio signal processed by the audio processing chain. The audio processing chain comprises a number, N > 1 , of audio processing blocks located in the signal path to the limiter, and each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. The audio processing system further comprises a controller according to the second aspect.

According to a fifth aspect there is provided an audio system comprising an audio processing system according to the fourth aspect.

According to a sixth aspect there is provided a computer program for controlling, when executed by a processor, a limiter adapted for selectively suppressing a first signal in the form of an audio signal processed by an audio processing chain comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter. Each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. The computer program comprises instructions, which when executed by the processor, cause the processor to:

obtain frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of a second signal in the form of a test signal as processed by the audio processing chain;

determine a control parameter for limiting the maximum suppression of the audio signal based on the frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and the representation of the frequency response of the test signal as processed by the audio processing chain; and

determine a maximum suppression that can be applied by the limiter at least partly based on the control parameter.

According to a seventh aspect there is provided a computer-program product comprising a computer-readable medium having stored thereon such a computer program.

In this way, it is possible to ensure safe listening levels, e.g. according to safety regulations, while at the same time being able to play as loud as possible within these limits. In particular, the proposed technology enables to control the audio limiter in such a way that it considers not the worst-case settings, but rather the actual settings that the user currently has.

Other advantages will be appreciated when reading the following detailed description of non-limiting embodiment of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The embodiments, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

Fig. 1 is a schematic block diagram illustrating a simplified example of an audio system.

Fig. 2 is a schematic diagram illustrating an example of the Power spectral density of the particular test signal HD 438.1 S2.

Fig. 3 is a schematic diagram illustrating an example of an audio processing system according to an embodiment. Fig. 4 is a schematic flow diagram illustrating an example of a method for controlling a limiter adapted for attenuating an audio signal according to an embodiment.

Fig. 5 is a schematic block diagram illustrating an example of a controller, based on a processor-memory implementation according to an embodiment.

Fig. 6 is a schematic diagram illustrating an example of a computer-implementation according to an embodiment.

DETAILED DESCRIPTION

Throughout the drawings, the same reference designations are used for similar or corresponding elements. it may be useful to start with an audio system overview with reference to Fig. 1 , which illustrates a simplified audio system. The audio system 100 basically comprises an audio processing system 200 and a sound generating system 300. In general, the audio processing system 200 is configured to process one or more audio input signals which may relate to one or more audio channels. The filtered audio signals are forwarded to the sound generating system 300 for producing sound.

As mentioned, it is an object to ensure that a personal music player can pass safety regulations, e.g. as defined by the regulation EN 50332, which defines safe listening levels, while at the same time being able to play as loud as possible.

A basic idea is to provide a controller configured to control a limiter adapted for selectively suppressing a first signal in the form of an audio signal processed by an audio processing chain comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter. Each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. The controller is preferably configured to determine a control parameter for limiting the maximum suppression of the audio signal based on frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of a second signal in the form of a test signal as processed by the audio processing chain. Further, the controller is configured to determine a maximum suppression that can be applied by the limiter at least partly based on the control parameter.

Fig. 3 is a schematic diagram illustrating an example of an audio processing system according to an embodiment. Basically, the audio processing system 200 comprises an audio processing chain 210 having a number, N > 1 , of audio processing blocks located in the signal path to a limiter 220, as well as a controller 230 for controlling the limiter 220.

In this particular example, the audio processing system 200 is configured to receive an audio input signal x, which is fed into the audio processing chain 210, and the output signal y of the audio processing chain 210 is fed into the limiter 220, which in turn is configured to provide an audio output signal z. It should though be understood that additional function and/or processing blocks may be part of the audio processing system, e.g. in the signal path from audio input signal to audio output signal. For example, there may be one or more additional blocks and/or modules located downstream of the limiter.

As a matter of definition, the limiter 220 and the controller 230 may be regarded as a limiter system 250.

By way of example, the controller 230 may be configured to determine an estimate of the amount of power that is added by the N > 1 audio processing blocks to the test signal, should it be passed through the audio processing chain, based on the frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and the representation of the frequency response of the test signal. The controller may then be configured to determine the control parameter for limiting the maximum suppression of the audio signal based on the determined estimate of the amount of power that is added to the test signal.

As an example, the representation of the frequency response of the test signal may be a predetermined frequency response representation. The test signal is typically a standardized audio test signal. For example, the standardized audio test signal may be a test signal according to the EU regulation EN 50332.

In a particular example, the controller is configured to determine the control parameter for limiting the maximum suppression of the audio signal according to:

4P = ⁿbr ¹S₁ ¹iYMi² - ⁿbå‘£ ¹ ₁ix[i]i² where Y[k] = X[k}H₁[k\H₂[k] H_N[k], and X[k] denotes the frequency response of the test signal, and the frequency responses of the N ³ 1 audio processing blocks are represented

By way of example, the controller may be configured to limit the maximum suppression that can be applied by the limiter according to: where RMS_{y dB} is the Root Mean Square value of the audio signal processed by the audio processing chain, 0_dB is a threshold value and <P_dB = AP_dB + b_dB, where b_ds is a constant.

As an example, the limiter is a Root Mean Square, RMS, limiter.

According to a further aspect, there is thus provided an audio limiter system 250 comprising a limiter 220 adapted for selectively attenuating an audio signal processed by an audio processing chain comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter, wherein each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. According to the proposed technology, the audio limiter system 250 further comprises a controller 230 as described herein. According to yet another aspect, there is provided an audio processing system 200 comprising an audio processing chain 210 followed by a limiter 220 adapted for selectively attenuating an audio signal processed by the audio processing chain 210. The audio processing chain 210 comprises a number, N > 1 , of audio processing blocks located in the signal path to the limiter, wherein each of the N ³ 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. According to the proposed technology, the audio processing system 200 further comprises a controller 230 as described herein.

According to still another aspect, there is provided an overall audio system 100 comprising an audio processing system 200 as described herein.

By way of example, the audio system 100 may include a sound generating system using headphones or earphones. For example, the audio system may be a personal music player.

Fig. 4 is a schematic flow diagram illustrating an example of a method for controlling a limiter adapted for attenuating a first signal in the form of an audio signal according to an embodiment.

The audio signal is processed by an audio processing chain comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter, and each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block.

Basically, the method comprises:

S1 : determining an estimate of the amount of power that is added by the N > 1 audio processing blocks to a second signal in the form of a test signal, should it be passed through the audio processing chain, based on frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of the test signal; S2: determining a control parameter for controlling the limiter based on the determined estimate of the amount of power that is added to the test signal; and

S3: determining a maximum suppression that can be applied by the limiter at least partly based on the control parameter.

For example, the control parameter may be a control parameter for limiting the maximum suppression of the audio signal.

By way of example, the representation of the frequency response of the test signal may be a predetermined frequency response representation.

The test signal may be a standardized audio test signal. For example, the standardized audio test signal may be a test signal according to the EU regulation EN 50332.

In a particular example, the amount of power that is added by the N ³ 1 audio processing blocks is estimated by:

DR = - i¾ Sί,ipίΐi «b å¾ ¹i*rai². where Y[k] = X[k]H₁[k]H₂[k] ··· H_N[k], and X[k] denotes the frequency response of the test signal, and the frequency responses of the N ³ 1 audio processing blocks are represented by Hi[k], H₂[k], .. H_N[k].

For example, the maximum suppression that can be applied by the limiter may be limited according to: gdB where RMS_{y dB} is the Root Mean Square value of the audio signal processed by the audio processing chain, 0_dB is a threshold value and O_dB = DR_άB + b_dB where bdB is a constant. Optionally, the method further comprises the step of obtaining, for each of the N ³ 1 audio processing blocks, the frequency response estimate given the user settings currently in use when processing the audio signal.

As an example, the limiter may be a Root Mean Square, RMS, limiter.

Expressed slightly differently, a basic idea is to use spectral information from processing blocks in an audio processing chain to control an“adaptive gain” that sits after those processing blocks. The processing blocks are subject to user settings, and the user settings can affect the frequency response of each block. Each block reports a frequency response estimate for a discrete number of frequency bands, given the user settings that is currently in use. These frequency response estimates are used to form an estimate of the total frequency response from all processing blocks that lies before the adaptive gain. The total frequency response together with an estimate of the frequency content of the test signal, e.g. a test signal used for EN 50332, gives an estimate of how much additional power that is added to the test signal, should it be run through such a processing chain. This value can be used to limit the maximum suppression that is applied by the adaptive gain. This means that the adaptive gain is bounded not to suppress more than is needed by the known processing blocks, and more specifically, the user settings of those processing blocks. If the user settings are such that no additional power is added to the test signal, the adaptive gain backs off and does not suppress the signal.

For a better understanding, the invention will now be described with reference to additional, non-limiting examples.

In a sense, the invention enables control of the“adaptive gain” in a way such that it considers not the worst-case settings, but the actual settings that the user has. In this way, the playout level is maximized, while still being safe.

As described, the audio processing chain contains N processing blocks with known frequency responses that are parameterized using Hi[k], Ftejk], .... HN[k], These representations may typically be discrete Fourier transforms (DFTs) of a fixed length nb: H[k] = å;ih[p]e-^¾i^kP/¾, where h[p] denotes the time impulse response of the processing block, and k is the frequency bin index.

Denote the frequency response of the test signal with X[k]. In the control block, we then have an estimate of the frequency response Y[k] of the signal y just before the adaptive gain, should we feed the system with the test signal:

Y[k] = X [k] H i [k] H ₂ [k] ··· H_N[k], and we can calculate the power that is added by the processing blocks 1 , .... N; and this should be the amount of suppression that is needed

The adaptive gain block measures the RMS level of the y signal where n is the number of time samples within a frame (a short time segment of the audio signal).

If the RMS level in dB is larger than a threshold 0dB, then the output is suppressed by an adaptive gain g s < 0: where all values are in dB. The threshold 0dB is tuned such that the audio output reaches the maximum allowed listening level RMSiimit:

We further limit the maximum suppression that can be applied by the adaptive gain by introducing a parameter F_άB so that the gain is defined by: and use the information from the control block on how much suppression that is needed, given the user settings:

F(ΪB = AP(JB + b(jB_> where the constant bdB is included to account for imperfections in the frequency estimates, to give headroom so that the adaptive gain in fact makes the device fulfill the regulation. The constant should be small, typically between 0 and 1 dB.

In this way, the adaptive gain is bounded to only suppresses up to the amount that is needed from the actual user settings of the processing blocks, as opposed to taking into account the worst-case settings that the user can have.

Illustrative examples

Consider the case of a PMP that is subject to EN-50332 regulation. The device contains an equalizer that lets the user boost certain frequency bands to obtain a certain preferred audio experience. When the test signal is played, as specified, at -10 dBFS digital level, the Sound Pressure Level (SPL) is measured to be 100 dB(A) from the headphones of the device, when the user has the equalizer set to a flat frequency response, which we denote setting S. When the user sets the equalizer to setting T, which is the setting that creates the highest possible SPL, the SPL at the headphones increases to 1 10 dB(A). Since this SPL is in violation of the regulation, the manufacturer needs to impose a limiter in the audio processing chain. The limiters threshold is set to tune the output to be maximum 100 dB(A) in all conditions, and it needs to be able to attenuate 10 dB to be able to pass the test with worst case user settings.

The user listens to highly compressed music, with digital levels of -3 dBFS. With the equalizer in setting S, the SPL at the headphone is measured to be 107 dB(A). The limiter will reduce the SPL to 100 dB(A), which it can do since it has a maximum attenuation of 10 dB.

With the invention, the control unit would calculate the frequency response of the equalizer to give no additional power to the test signal given setting S, DR = 0. Thus it would not attenuate the signal at all, outputting a SPL of 107 dB(A), hence 7 dB louder than the reference method.

Should the user play the highly compressed music with setting T, which can give a SPL of 117 dB(A), both the invention and the reference method would use the maximum suppression of 10 dB, resulting in 107 dB(A) SPL from the headphones.

Processing blocks without frequency representation

If the audio processing, e.g. as illustrated in Fig. 3, includes other, unknown blocks, 3^rd party blocks, and so forth, it is possible to take any of the following example approaches:

1 ) Estimate the impulse response of the processing block by feeding (a copy of) it with a Dirac delta function, and calculate the frequency response estimate needed to use the described invention.

2) Increase margin bds to account for the unknown components in the audio processing chain. The worst-case setting of the unknown processing block depends on the user settings of the known processing blocks. a. To keep the margin bdB as small as possible, we would have to consider all these combinations of user settings of the known blocks, as well as the unknown blocks, to create the margin bdB as a function of user settings of the known processing blocks. For each unique user setting of the known processing blocks, there is a worst-case setting of the unknown blocks, that give rise to a certain power increase. This would result in a mapping from known block user settings to bdB, which can be of considerable size and complexity. b. If the unknown blocks can be considered to give only small contributions to the output power when the audio chain is fed with the test signal, we can simply give a small increase to the margin bdB. This increase can be tuned empirically by exploring combinations of user settings to find the smallest possible bdB that makes the device pass the regulation.

It will be appreciated that the methods and arrangements described herein can be implemented, combined and re-arranged in a variety of ways.

For example, embodiments may be implemented in hardware, or in software for execution by suitable processing circuitry, or a combination thereof.

The steps, functions, procedures, modules and/or blocks described herein may be implemented in hardware using any conventional technology, such as discrete circuit or integrated circuit technology, including both general-purpose electronic circuitry and application-specific circuitry.

Alternatively, or as a complement, at least some of the steps, functions, procedures, modules and/or blocks described herein may be implemented in software such as a computer program for execution by suitable processing circuitry such as one or more processors or processing units.

Examples of processing circuitry includes, but is not limited to, one or more microprocessors, one or more Digital Signal Processors (DSPs), one or more Central Processing Units (CPUs), video acceleration hardware, and/or any suitable programmable logic circuitry such as one or more Field Programmable Gate Arrays (FPGAs), or one or more Programmable Logic Controllers (PLCs).

It should also be understood that it may be possible to re-use the general processing capabilities of any conventional device or unit in which the proposed technology is implemented. It may also be possible to re-use existing software, e.g. by reprogramming of the existing software or by adding new software components.

Fig. 5 is a schematic block diagram illustrating an example of a controller, based on a processor-memory implementation according to an embodiment. In this particular example, the controller 230 comprises a processor 231 and a memory 232, the memory 232 comprising instructions executable by the processor 231 , whereby the processor is operative to implement the aspects of the proposed technology described herein.

Optionally, the controller 230 may also include an input/output (I/O) device 233 to enable input and/or output of relevant data such as input parameter(s) and/or resulting output parameter(s).

It is also possible to provide a solution based on a combination of hardware and software. The actual hardware-software partitioning can be decided by a system designer based on a number of factors including processing speed, cost of implementation and other requirements.

Fig. 6 is a schematic diagram illustrating an example of a computer-implementation according to an embodiment. In this particular example, at least some of the steps, functions, procedures, modules and/or blocks described herein are implemented in a computer program 425; 435, which is loaded into the memory 420 for execution by processing circuitry including one or more processors 410. The processor(s) 410 and memory 420 are interconnected to each other to enable normal software execution. An optional input/output device 440 may also be interconnected to the processor(s) 410 and/or the memory 420 to enable input and/or output of relevant data such as input parameter(s) and/or resulting output parameter(s). The term‘processor’ should be interpreted in a general sense as any system or device capable of executing program code or computer program instructions to perform a particular processing, determining or computing task.

The processing circuitry including one or more processors 410 is thus configured to perform, when executing the computer program 425, well-defined processing tasks such as those described herein.

The processing circuitry does not have to be dedicated to only execute the above- described steps, functions, procedure and/or blocks, but may also execute other tasks.

In a particular embodiment, the computer program 425; 435 comprises instructions, which when executed by the processor 410, cause the processor 410 to perform the tasks described herein. More specifically, there is provided a computer program 425; 435 for controlling, when executed by a processor 410, a limiter adapted for selectively suppressing a first signal in the form of an audio signal processed by an audio processing chain comprising a number, N ³ 1 , of audio processing blocks located in the signal path to the limiter, wherein each of the N ³ 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block. Such a computer program 425; 435 comprises instructions, which when executed by the processor, cause the processor 410 to:

obtain frequency response estimates of the audio processing b!ock(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of a second signal in the form of a test signal as processed by the audio processing chain;

determine a maximum suppression that can be applied by the limiter at least partly based on the control parameter. The proposed technology also provides a carrier comprising the computer program, wherein the carrier is one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or a computer-readable storage medium.

By way of example, the software or computer program 425; 435 may be realized as a computer program product, which is normally carried or stored on a non-transitory computer-readable medium 420; 430, in particular a non-volatile medium. The computer- readable medium may include one or more removable or non-removable memory devices including, but not limited to a Read-Only Memory (ROM), a Random Access Memory (RAM), a Compact Disc (CD), a Digital Versatile Disc (DVD), a Blu-ray disc, a Universal Serial Bus (USB) memory, a Hard Disk Drive (HDD) storage device, a flash memory, a magnetic tape, or any other conventional memory device. The computer program may thus be loaded into the operating memory of a computer or equivalent processing device for execution by the processing circuitry thereof.

The procedural flows presented herein may be regarded as a computer flows, when performed by one or more processors. A corresponding apparatus may be defined as a group of function modules, where each step performed by the processor corresponds to a function module. In this case, the function modules are implemented as a computer program running on the processor.

The computer program residing in memory may thus be organized as appropriate function modules configured to perform, when executed by the processor, at least part of the steps and/or tasks described herein.

Alternatively it is possible to realize the function modules predominantly by hardware modules, or alternatively by hardware, with suitable interconnections between relevant modules. Particular examples include one or more suitably configured digital signal processors and other known electronic circuits, e.g. discrete logic gates interconnected to perform a specialized function, and/or Application Specific Integrated Circuits (ASICs) as previously mentioned. Other examples of usable hardware include input/output (I/O) circuitry and/or circuitry for receiving and/or sending signals. The extent of software versus hardware is purely implementation selection. The embodiments described above are merely given as examples, and it should be understood that the proposed technology is not limited thereto. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the present scope as defined by the appended claims. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.

REFERENCES

[1] WO 2015/128696

[2] EN 50332-1 - Sound system equipment: Headphones and earphones associated with personal music players - Maximum sound pressure level measurement methodology - Part 1 : General method for "one package equipment".

[3] EN 50332-2 - Part 2: Matching of sets with headphones if either or both are offered separately, or are offered as one package equipment but with standardised connectors between the two allowing to combine components of different manufacturers or different design.

[4] EN 50332-3 - Part 3: measurement method for sound dose management.

[5] EN 60950-1 : Information technology equipment - Safety - Part 1 : General requirements

[6] EN 60065 - Audio, video and similar electronic apparatus - Safety requirements.

[7] US 2016/0268989

[8] US 2008/0013751

[9] EP 3 229 497

Claims

1. A method for controlling a limiter (220) adapted for attenuating a first signal in the form of an audio signal processed by an audio processing chain (210) comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter (220), wherein each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block,

wherein the method comprises:

determining (S1 ) an estimate of the amount of power that is added by the N > 1 audio processing blocks to a second signal in the form of a test signal, should it be passed through the audio processing chain, based on frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of the test signal;

determining (S2) a control parameter for controlling the limiter (220) based on the determined estimate of the amount of power that is added to the test signal; and

determining (S3) a maximum suppression that can be applied by the limiter (220) at least partly based on the control parameter.

2. The method of claim 1 , wherein the control parameter is a control parameter for limiting the maximum suppression of the audio signal.

3. The method of claim 1 or 2, wherein the representation of the frequency response of the test signal is a predetermined frequency response representation.

4. The method of any of the claims 1 to 3, wherein the test signal is a standardized audio test signal.

5. The method of claim 4, wherein the standardized audio test signal is a test signal according to the EU regulation EN 50332.

6. The method of any of the claims 1 to 5, wherein the amount of power that is added by the N > 1 audio processing blocks is estimated by: where Y[k] = X[k]H₁[k]H₂[k] · H_N[k], and X[k] denotes the frequency response of the test signal, and the frequency responses of the N ³ 1 audio processing blocks are represented by Hi[k], Hilk], .. HNP (].

7. The method of claim 6, wherein the maximum suppression that can be applied by the limiter (220) is limited according to: gdB where RMS_{y dB} is the Root Mean Square value of the audio signal processed by the audio processing chain, 0_dB is a threshold value and F_άB = AP_dB + b_dB, where bde is a constant.

8. The method of any of the claims 1 to 7, wherein the method further comprises obtaining, for each of the N > 1 audio processing blocks, the frequency response estimate given the user settings currently in use when processing the audio signal.

9. The method of any of the claims 1 to 8, wherein the limiter is a Root Mean Square, RMS, limiter.

10. A controller (230) configured to control a limiter (220) adapted for selectively suppressing a first signal in the form of an audio signal processed by an audio processing chain (210) comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter (220), wherein each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block,

wherein the controller (230) is configured to determine a control parameter for limiting the maximum suppression of the audio signal based on frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of a second signal in the form of a test signal as processed by the audio processing chain, wherein the controller (230) is configured to determine a maximum suppression that can be applied by the limiter (220) at least partly based on the control parameter.

1 1. The controller of claim 10, wherein the controller (230) is configured to determine an estimate of the amount of power that is added by the N > 1 audio processing blocks to the test signal, should it be passed through the audio processing chain, based on the frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and the representation of the frequency response of the test signal, and

wherein the controller (230) is configured to determine the control parameter for limiting the maximum suppression of the audio signal based on the determined estimate of the amount of power that is added to the test signal.

12. The controller of claim 10 or 1 1 , wherein the representation of the frequency response of the test signal is a predetermined frequency response representation.

13. The controller of any of the claims 10 to 12, wherein the test signal is a standardized audio test signal.

14. The controller of claim 13, wherein the standardized audio test signal is a test signal according to the EU regulation EN 50332.

15. The controller of any of the claims 10 to 14, wherein the controller (230) is configured to determine the control parameter for limiting the maximum suppression of the audio signal according to:

AP = ⁱ³båS₁iy[i]i² - ⁿbå ,^B ₁ix[i]i², where Y[k] = X[k]H₁[k]H₂ [k] ··· H_N[k], and X[k] denotes the frequency response of the test signal, and the frequency responses of the N > 1 audio processing blocks are represented by Hi[k], H2[k], .. H_N[k].

16. The controller of claim 15, wherein the controller (230) is configured to limit the maximum suppression that can be applied by the limiter according to: where RMS_{y dB} is the Root Mean Square value of the audio signal processed by the audio processing chain, 0_dB is a threshold value and <t>_dB = AP_dB + b_dB, where bdB is a constant.

17. The controller of any of the claims 10 to 16, wherein the limiter (220) is a Root Mean Square, RMS, limiter.

18. An audio limiter system (250) comprising a limiter (220) adapted for selectively attenuating an audio signal processed by an audio processing chain (210) comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter (220), wherein each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block,

wherein the audio limiter system (250) further comprises a controller (230) according to any of the claims 10 to 17.

19. An audio processing system (200) comprising an audio processing chain (210) followed by a limiter (220) adapted for selectively attenuating an audio signal processed by the audio processing chain, wherein the audio processing chain (210) comprises a number, N > 1 , of audio processing blocks located in the signal path to the limiter (220), wherein each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block,

wherein the audio processing system (200) further comprises a controller (230) according to any of the claims 10 to 17.

20. An audio system (100) comprising an audio processing system (200) of claim 19.

21. The audio system of claim 20, wherein the audio system (100) comprises a sound generating system using headphones or earphones.

22. The audio system of claim 20 or 21 , wherein the audio system (100) is a personal music player.

23. A computer program (425; 435) for controlling, when executed by a processor (410), a limiter (220) adapted for selectively suppressing a first signal in the form of an audio signal processed by an audio processing chain (210) comprising a number, N > 1 , of audio processing blocks located in the signal path to the limiter, wherein each of the N > 1 audio processing blocks is subject to user settings that may affect the frequency response of the processing block, wherein the computer program (425; 435) comprises instructions, which when executed by the processor (410), cause the processor (410) to:

obtain frequency response estimates of the audio processing block(s) given the user settings currently in use when processing the audio signal, and a representation of the frequency response of a second signal in the form of a test signal as processed by the audio processing chain (210);

24. A computer-program product comprising a non-transitory computer-readable medium (420; 430) having stored thereon a computer program (425; 435) of claim 23.