Method and apparatus for limiting power in multi-channel audio amplifier to protect power supply from overload

Abstract

A method for use in an audio system comprising a plurality of amplifier channels corresponding to the audio channels. Each of the amplifier channels receives a respective input audio signal and is powered by the same power source. The method comprises: obtaining (810) a value of an indicator, where the indicator represents an energy reserve of the power source; determining (830), for each of the amplifier channels, a gain to be applied to the respective input audio signal by an attenuation function and a clipping level to be applied to an output of the attenuation function by a clipping function; wherein, when the value of the indicator is below a threshold value, the gain decreases over time, and when the value of the indicator is above the threshold value, the gain increases over time to a maximum value, where the threshold value represents a nominal energy level at which the power supply is in nominal operation; wherein the clipping level is determined as a monotonic function of the value of the indicator.

Description

Technical Field

[0001] Various example embodiments are generally directed to methods and apparatus for use in an audio system including multiple amplifier channels. Background Art

[0002] In current audio systems, the number of audio channels is constantly increasing. Starting with 2-channel stereo, 5+1 or even 7+1 surround amplifiers are now common. Modern line array loudspeaker systems can use 48 channels. For optimal performance and for scientific purposes, there are wave field synthesis arrays with up to 832 independent channels. For size and cost reasons, it makes sense to connect many amplifier channels to only one power supply. Because not all channels are driven at their maximum power capacity at the same time, the average power of the channels determines the long-term power capacity of the common power supply.

[0003] The audio signal may represent, for example, music content. Since music content is dynamic, a power supply rated for maximum instantaneous power would be expensive and unnecessary. During power peaks, energy is provided by one or more bulk capacitors. The charge state of these bulk capacitors depends on the history of power demand (e.g., whether the bulk capacitors were depleted by previous power draws, or whether they had time to recharge during quiet portions of the audio signal).

[0004] To avoid shutoff, the typical solution is to use a dynamic range compressor or limiter to limit the root mean square (RMS) level of the audio signal to be amplified. Because the impedance of the load is frequency dependent and partially reactive, the relationship between the output signal level and the output power is not trivial. Therefore, in order to prevent the amplifier from shutting down, the limiter will need to be set conservatively, potentially failing to utilize the full output power capability of the system. Furthermore, by using such a limiter that operates independently on each audio channel, the multi-channel advantages of a shared power supply described above cannot be obtained.

[0005] In multi-channel amplifiers, it is common for power supplies and bulk capacitors to be shared between different amplification channels. If the power supply is accidentally overloaded, a protection mechanism shuts down the circuit to prevent damage.

[0006] What is needed is a limiting device that takes into account the power requirements of all amplified channels while minimizing audible distortion and preventing power shutoff. Summary of the invention

[0007] The protection scope is set forth by the independent claims. Embodiments, examples, and features described in this specification that do not fall within the protection scope, if any, are to be construed as examples useful for understanding various embodiments or examples that fall within the protection scope.

[0008] According to a first aspect, a device for use in an audio system comprising a plurality of amplifier channels of corresponding audio channels is disclosed. Each of the amplifier channels receives a corresponding input audio signal and is powered by the same power supply, the device comprising a device (e.g., a signal processing device or a signal processor) for performing a method, the method comprising: obtaining a value of an indicator, wherein the indicator represents an energy reserve of the power supply; for each of the amplifier channels, determining a gain to be applied by an attenuation function to the corresponding input audio signal and a clipping level to be applied by a clipping function to the output of the attenuation function; wherein when the value of the indicator is below a threshold, the gain decreases over time, and when the value of the indicator is above a threshold, the gain increases over time to a maximum value, wherein the threshold represents a nominal energy level of the power supply in nominal operation; wherein the clipping level is determined as a monotonic function of the value of the indicator.

[0009] The gain may be decreased over time at an attack rate. The attack rate may be a fixed attack rate corresponding to a fixed gain decrease per time period on a logarithmic scale. The attack rate may be a dynamic attack rate calculated based on the value of the indicator. The gain may be increased over time at a release rate. The release rate is a fixed release rate corresponding to a fixed gain increase per time period on a logarithmic scale. When the value of the indicator is below a threshold, the clipping level may be determined by applying a monotonic function of the output voltage of the power supply. The monotonic function may be a linear function of the output voltage of the power supply or a continuous function of the output voltage of the power supply.

[0010] The indicator may be proportional to the output voltage of the power supply, or proportional to the square of the output voltage of the power supply. The indicator may be proportional to the energy stored in the power supply. The gain varies between a minimum value and a maximum value. The monotonic function may be adjusted based on a mains voltage level signal received from the power supply.

[0011] For each amplifier channel, the device may include: a clipping device, which is connected to the input of the amplifier channel and is configured to apply a clipping function; an attenuation device, the output of which is connected to the input of the clipping device, wherein the attenuation device is configured to apply the attenuation function.

[0012] According to a second aspect, a method is disclosed for use in a multiple amplifier channel audio system comprising corresponding audio channels, each of the amplifier channels receiving a corresponding input audio signal and being powered by the same power supply, the method comprising: obtaining a value of an indicator, wherein the indicator represents an energy reserve of the power supply; for each of the amplifier channels, determining a gain to be applied by an attenuation function to the corresponding input audio signal and a clipping level to be applied by a clipping function to the output of the attenuation function; wherein when the value of the indicator is below a threshold, the gain decreases over time, and when the value of the indicator is above a threshold, the gain increases over time to a maximum value, wherein the threshold represents a nominal energy level of the power supply in nominal operation; wherein the clipping level is determined as a monotonic function of the value of the indicator.

[0013] The apparatus according to the first aspect may include a device for performing one or more or all steps of the method according to the second aspect. The device may include a circuit system (e.g., a signal processor) configured to perform one or more or all steps of the method according to the first aspect. The device may include at least one processor and at least one memory storing instructions, which, when executed by at least one processor, causes the device to perform one or more or all steps of the method according to the first aspect.

[0014] According to another aspect, a computer program is disclosed comprising instructions which, when executed by at least one processor, cause an apparatus to perform the method according to the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Example embodiments will be more fully understood from the detailed description and accompanying drawings given below, which are given by way of illustration only and thus do not limit the present disclosure.

[0016] Figure 1 is a schematic block diagram of a multi-channel amplification system having a power supply that delivers power to several amplifier channels utilizing a full-bridge output stage driving a bridge-tied load.

[0017] Figure 2 is a schematic block diagram of a multi-channel amplification system having a power supply with two symmetrical supply voltages that deliver power to several amplifier channels using a half-bridge output stage driving a single-ended load.

[0018] Figure 3 is a schematic block diagram of a multi-channel amplification system including a limiting device having a full-bridge output stage driving a bridge-tied load according to one example.

[0019] Figure 4is a schematic block diagram of a multi-channel amplification system including a limiting device having a half-bridge output stage driving a single-ended load according to one example.

[0020] Figure 5A The gain applied by the attenuation device is shown over time for a fixed attack rate (AR), a maximum gain (MG) at zero rate (R=0), and a fixed release rate (RR), according to one example.

[0021] Figure 5B The gain applied by the attenuation device is shown over time for dynamic attack rates (AR1 to AR5) and a fixed release rate (RR), according to one example.

[0022] Figure 6 A flow chart of a method for calculating a gain applied by an attenuation device according to one example is shown.

[0023] Figure 7 A flow chart of a method for calculating a clipping level according to an example is shown.

[0024] Figure 8 A flow chart of a method implemented by a limiting device is shown.

[0025] Fig. 9 is an oscilloscope screenshot of a sinusoidal burst signal illustrating the effect of a limiting device on an output signal of a multi-channel amplification device and a voltage of a bulk capacitor according to one example.

[0026] Fig.10 is a comparison of two audio signals, one is a small signal version and the other is an amplified version with a power limiting algorithm applied according to one example.

[0027] Fig.11 is an oscilloscope screenshot of a burst signal showing the effect of a limiting device (including clipping release rate) on the output of a multi-channel amplification device and the voltage of a bulk capacitor according to one example.

[0028] It should be noted that these drawings are intended to illustrate the general characteristics of methods, structures and / or materials used in certain example embodiments and to supplement the written description provided below. The use of similar or identical reference numerals in the various drawings is intended to indicate the presence of similar or identical elements or features. DETAILED DESCRIPTION

[0029] Various example embodiments will now be described more fully with reference to the accompanying drawings, in which some example embodiments are shown. However, for the purpose of describing the example embodiments, the specific structural and / or functional details disclosed herein are merely representative. Therefore, these embodiments are shown in the accompanying drawings by way of illustrative examples, and will be described in detail herein, so as to provide a thorough understanding of various aspects. However, it will be appreciated by those of ordinary skill in the art that the example embodiments are capable of various modifications and alternative forms, and can be implemented without all the specific details. In addition, systems and processes can be shown in block diagrams so as not to obscure the example embodiments with unnecessary details. In other cases, known processes, structures, and techniques can be shown without unnecessary details to avoid obscuring the example embodiments.

[0030] Figure 1 is a schematic block diagram of a multi-channel amplification system 100 having a power supply 110 that delivers power to several amplifier channels 120, 130, 140 with a full-bridge output stage driving a bridge-tied load. The expressions "amplifier channel" and "amplification channel" used herein both refer to audio channels and designate functional blocks configured to perform amplification of corresponding input audio signals.

[0031] exist Figure 1 In the illustrated example audio system 100, a multi-channel audio amplifier comprises a power supply 110 connected to the mains via a plug or a fixed connector. The power supply 110 delivers power to a set of amplifier channels 120, 130, 140.

[0032] The amplifier channels 120, 130, 140 are connected to a power supply having at least a positive power connection 111 and a negative power connection 112. A bulk capacitor 115 at the output of the power supply is connected between the positive power connection 111 and the negative power connection 112. Each amplifier channel 120, 130, 140 receives a respective input audio signal 121, 131, 141 and delivers a respective output audio signal to a respective load 129, 139, 149. Such a load is typically a loudspeaker.

[0033] Figure 2 is a schematic block diagram of a multi-channel amplification system 200 having a power supply 210 with two symmetrical supply voltages that deliver power to a number of amplifier channels 220, 230, 240 with a half-bridge output stage driving a single-ended load.

[0034] and Figure 1 similar, Figure 2 The example audio system 200 shown includes a set of amplifier channels 220, 230, 240, but in this example, the amplifier channels have Figure 2Push-pull output configuration shown: There is a positive power supply connection 211, a negative power supply connection 212 and a third center connection 213 to circuit ground potential. A first bulk capacitor 216 is connected between the positive power supply connection 211 and the center connection 213. A second bulk capacitor 217 is connected between the center power supply connection 213 and the negative power supply connection 212.

[0035] In the case where several input audio signals and amplifier channel gains result in the corresponding amplifier channel outputting higher than average power, the power supply may be overloaded: the maximum power delivered by the power supply 110 or 210 is less than the total power drawn by all amplifier channels together. In this case, the bulk capacitors 115 or 216, 217 of the power supply will be discharged to deliver power from their stored energy, thereby reducing the supply voltage. Another reason for reducing the supply voltage is that in the case of a tightly regulated power supply, the power supply goes out of regulation due to reaching the limit of the maximum output current. In this case, the power supply is no longer a regulated power supply, but a current source with a maximum output current that is less than the load current. When the lower threshold of the supply voltage is reached, the power supply will shut down to prevent irregular operation or even damage. The interruption of the power supply results in muting of the output of the audio system.

[0036] A limiting device operating under the control of a common control device is introduced in order to limit the power drawn by the amplifier channels. Such a device is configured to limit the sum of the power consumption of all channels to the maximum value that the power supply is capable of delivering. Typically, a short-term overload of the power supply is possible until the energy storage of the power supply is discharged to its minimum value.

[0037] The common control device is configured to implement a control function (also referred to herein as a control law) to determine the amount of power limitation to be applied. The control function can be based on one or more values ​​of an indicator that can be dynamically calculated. The indicator can represent an energy reserve stored in a bulk capacitor that supports the output of the power supply. The physical parameter used to calculate the value of the indicator can be voltage or energy. The lower the value of the indicator, the greater the power limitation applied by the limiting device. The indicator can be proportional to the output voltage of the power supply, or proportional to the square of the output voltage of the power supply. The indicator can be proportional to the energy stored in the power supply.

[0038] Each limiting device may comprise an attenuation device configured to apply an attenuation function and a clipping device configured to apply a clipping function.

[0039] The attenuation device may be implemented by any type of attenuator / amplifier suitable for applying a gain (less than or equal to one in the linear domain, or equivalently, less than or equal to 0 dB in the logarithmic domain) to an input signal so as to reduce the amplitude of the input signal (e.g., a voltage reduction in the analog domain, or a numerical reduction in the digital domain). The attenuation device may be, for example, a voltage-controlled attenuator (VCA), a gain multiplier in a digital signal processing implementation, etc.

[0040] The clipping device may be implemented by any type of clipper suitable for reducing the amplitude (amplitude in absolute value) of the input signal to a given clipping level.

[0041] The clipping device may comprise, for example, a series resistor with two clamping diodes connected to a positive threshold voltage and a second clamping diode connected to an inverting (negative) threshold voltage. An improved version may use an operational amplifier on each clamping diode to compensate for the diode forward voltage.

[0042] It is also possible to use an attenuation device to simulate the clipping function of a clipping device. If the amplitude of the input signal exceeds the threshold, the threshold is divided by the amplitude. The result is the gain to be applied (limited to an upper value of 0dB) to obtain a clipped output signal.

[0043] For each audio channel, the output of the clipping device is connected to the input of the corresponding amplifier channel, and the output of the attenuation device is connected to the input of the corresponding clipping device. Each attenuation device is configured to apply a gain to the corresponding input audio signal. The gain is applied by the attenuation device, and when expressed in dB, the gain is, for example, negative or zero, or when expressed in the linear domain, the gain is higher than zero and lower than 1 or equal to 1. Each clipping device is configured to apply a clipping level to the signal generated by the attenuation device. The output signal of the clipping device is used to replace the input audio signal and is fed to the corresponding amplifier channel.

[0044] The indicator may be compared to a threshold value to calculate a gain to be applied to each audio channel in order to limit the total power drawn by the amplifier channel. The threshold value may represent a nominal energy level of the power supply in nominal operation. The threshold value may represent a nominal voltage reflecting the nominal energy level.

[0045] The common control device is configured to generate control signals for the attenuation device and the clipping device corresponding to the gain and the clipping level, respectively, determined by the control function based on the current value of the indicator.

[0046] When the value of the indicator is below a threshold, the gain is reduced (attenuated more), and when the value of the indicator is above a threshold, the gain is increased (attenuated less) up to a maximum value. The gain may be reduced over time at an attack rate. The attack rate may be a fixed attack rate corresponding to a fixed gain reduction per time period on a logarithmic scale. The attack rate may be a dynamic attack rate calculated based on the value of the indicator. The gain may be increased over time at a release rate. The release rate may be a fixed release rate corresponding to a fixed gain increase per time period on a logarithmic scale. The gain may vary between a minimum value (e.g., a fixed minimum value) and a maximum value (e.g., a fixed maximum value).

[0047] The clipping level is determined as a monotonic function of the value of the indicator. For example, when the value of the indicator decreases (lower energy reserve), the clipping level decreases (more clipping), and vice versa. For example, when the value of the indicator is below a threshold, the clipping level is determined by applying a monotonic function of the output voltage of the power supply. The monotonic function may be a linear function of the output voltage of the power supply, or have another shape or a continuous (e.g., piecewise linear) function of the output voltage of the power supply. The monotonic function may be determined based on a mains voltage level signal received from the power supply.

[0048] When the indicator is proportional to the voltage at the output of the power supply, the clipping level can be determined by applying a linear function to the voltage at the output of the power supply. The clipping level can be determined so that as long as the value of the indicator is above a threshold representing a nominal energy level, the clipping function has no effect on the audio signal fed to the clipping function (the clipping level is above a maximum value). The clipping function can act gradually around the clipping level (sometimes referred to as "soft clipping").

[0049] The value of the clipping level and / or the gain may be dynamically adjusted by the common control device.The clipping level and / or the gain may be dynamically adjusted according to the value of the indicator.

[0050] In case the total power consumption of all amplifier channels is temporarily too high, the energy reserve may decrease so quickly that a gain reduction alone may not be sufficient. Before a sufficient gain reduction is achieved, an attenuation device applying a gain with an attack rate may not prevent the power supply from shutting down. It is therefore necessary to have an instantaneous action mechanism. In order to act instantaneously, a clipping device is used in combination with an attenuation device and is configured so that a clipping level is applied at the beginning of a power peak, giving the attenuation device enough time to take over, i.e. to reach a sufficiently reduced gain, so that the value of the indicator no longer decreases.

[0051] However, the disadvantage of severe clipping is audible artifacts of longer duration. Here, the attenuation performed by the attenuation device with its attack ramp allows limiting the signal so that, for example, after a first time period (attack phase), for example 250 ms, limiting is to a level where clipping no longer occurs. This means that during a second time period (release phase) following the first time period, no clipping occurs (the clipping function has no effect on the signal received by the clipping device), and the limitation of the audio signal is produced by the gain applied by the attenuation function while still in the attack phase, when the energy reserve has not yet reached the nominal energy level. After an event with high peak power in several channels at the same time, the clipping device, which is operating momentarily, can return to a clipping level above the maximum signal amplitude. During a third time period, once the energy reserve has reached the nominal energy level, the gain is increased (for example, at a low rate to avoid pumping effects in the audio signal) up to a maximum gain level (for example, 0 dB).

[0052] The attack phase may start after a power consumption peak occurs that results in a discharge of energy from the bulk capacitor, the discharge being detected based on one or more values ​​of the indicator. If a new power consumption peak occurs, the release phase may be interrupted by a second attack phase. The release phase and the attack phase may alternate in time based on changes in the value of the indicator.

[0053] The combination of clipping and dynamic attenuation can maintain a high average power level, and therefore loudness, with the output signal having sufficiently low audible artifacts. The combination can react quickly enough to protect the power supply from a shutdown condition during high power output.

[0054] Figure 3 3 is a schematic block diagram of a multi-channel amplification system 300. The multi-channel amplification system 300 comprises a limiting device 322, 332, 342 before each amplifier channel 320, 330, 340, each limiting device 322, 332, 342 being controlled by a common control device 390 which is common to all amplifier channels and uses status information of the supply voltage. Figure 3 In the example, as in Figure 1 In the example, the full-bridge output stage drives the bridge-connected load.

[0055] Figure 4 4 is a schematic block diagram of a multi-channel amplification device 400. The multi-channel amplification system 400 comprises a limiting device 422, 432, 442 before each amplifier channel 420, 430, 440, each limiting device 422, 432, 442 being controlled by a common control device 490 which is common to all amplifier channels and uses information of the power supply voltage. Figure 4 In the example, as in Figure 2 In the MOSFET, the half-bridge output stage drives a single-ended load.

[0056] In order to Figure 3 For simplicity in the diagram, the diagram is drawn with a power supply 310 having only one supply voltage (between 311 and 312), supplying power to a set of amplifier channels 320, 330, 340 using bridge-tied loads 329, 339, 349, requiring only one supply voltage.

[0057] It is also possible to have a single-ended push-pull output stage at an amplifier that requires both positive and negative supply voltages, such as Figure 4 Here, power supply 410 has a positive supply voltage (between 411 and 413) and a negative supply voltage (between 412 and 413) that share a common ground at a center connection 413 therebetween. In this case, the state of the power supply is given by the minimum of the absolute values ​​of the positive supply voltage and the negative supply voltage.

[0058] exist Figure 3 or Figure 4 In each audio channel of the amplifier, a limiting device 322, 332, 342, 422, 432, 442 receives a corresponding input audio signal 321, 331, 341, 421, 431, 441 of the relevant audio channel and delivers a corresponding limited audio signal 324, 334, 344, 424, 434, 444 to replace the corresponding input audio signal 321, 331, 341, 421, 431, 441 to the corresponding amplifier channel 320, 330, 340, 420, 430, 440.

[0059] Each limiting device 322, 332, 342, 422, 432, 442 comprises an attenuation device followed by a clipping device.

[0060] Each attenuation device receives an input control signal from the common control device 390, 490, which is configured to control the value of the gain applied by the associated attenuation device. The control signal may be the same for all audio channels. If the gain is set to a maximum value of 0 dB (i.e. corresponding to a multiplication factor of 1), the input audio signal remains unchanged.

[0061] Each clipping device receives an input control signal from the common control device 390, 490, which is configured to control the value of the positive clipping level and the negative clipping level applied by the associated clipping device. For each clipping device, the maximum positive saturation threshold and the negative saturation threshold (i.e., the maximum positive and negative values ​​of the clipping level) are set to the maximum signal level that can be processed by the corresponding amplifier stage at its input. Typically, this maximum signal level is limited by the supply voltage, which is divided by the amplifier voltage gain of the amplifier channel.

[0062] The common control device 390 , 490 may use different information which may be used to generate the input control signal.

[0063] exist Figure 3 In the embodiment of the invention, the supply voltage at the bulk capacitor of the supply (between 311 and 312) is used and transmitted to the common control device 390. Figure 4 In the case of two symmetrical supply voltages and a push-pull configuration, two supply voltages are used (between 411 and 413, and between 412 and 413), and the minimum of the absolute values ​​of these two voltages is transmitted to the common control device 490.

[0064] Several types of control signals may be generated by a common control device to control gain and clipping levels.

[0065] The gain can be adjusted by applying an attack rate during the attack phase (e.g., a linear decrease as a function of time when the gain is expressed in dB) and / or by applying a release rate during the release phase (e.g., a linear increase as a function of time when the gain is expressed in dB). The gain control signal is the same for all audio channels. The gain can vary between a maximum value (e.g., equal to 0 dB, i.e., no attenuation) and a minimum value. The attack rate and / or the release rate can be fixed or determined based on the difference between the indicator and a threshold representing a nominal energy level. The absolute value of the release rate can be less than the attack rate, so that the release phase lasts longer than the attack phase. A compromise can be found between reacting too quickly (resulting in low average power) or too slowly (resulting in too long a gain reduction period, in which additional clipping of the input audio signal occurs). As long as the energy reserve represented by the indicator is above the threshold, the gain is in the release phase and it is limited to the maximum gain (e.g., equal to 0 dB, i.e., no attenuation).

[0066] The gain may decrease (i.e., more decay) as a function of time during a first time period (attack phase) and may increase (i.e., less decay) as a function of time during a second time period (release phase). For example, during the attack phase, the gain decreases at a constant attack rate (e.g., a predefined attack rate) or at a dynamic attack rate. The attack rate and the release rate represent the change in gain over time (increase or decrease, respectively). The dynamic attack rate may be adjusted based on the difference between the threshold and the current value of the indicator.

[0067] For example, Figure 5AAs shown, the attack rate (AR) and the release rate (RR) are fixed, and the attack phase alternates with the release phase. During the release phase, the gain is limited to a maximum gain value (MG, e.g., 0 dB). This limitation can result in a portion of the release phase having a zero gain change rate (R=0), while the gain remains at its maximum gain value (MG).

[0068] For example, Figure 5B As shown, the dynamic attack rate is adjusted according to the difference between the value of the indicator and the threshold value. When the energy reserve represented by the indicator decreases, the difference increases, and the dynamic attack rate also increases, and vice versa.

[0069] Figure 5B A curve illustrating the variation of the gain is shown, wherein a first slope corresponds to a first attack rate AR1 (a linear decrease in gain when expressed in dB) during a first time period, a second attack rate AR2 during a second time period. Likewise, other attack rates AR3, AR4, AR5 may be used during subsequent corresponding time periods. In this example, the dynamic attack rate is updated at the transition from AR1 to AR2, from AR2 to AR3, and so on. The updating of the dynamic attack rate may occur together with a control update rate at which the value of the indicator is evaluated by a common control function.

[0070] Figure 6 650-1, a value of an indicator is obtained. The indicator may be a power supply voltage measured at a bulk capacitor, in this example, the power supply voltage V supply In step 650-2, the indicator value is compared with the threshold value V th For comparison, the threshold V th is the voltage level for nominal operation.

[0071] If the value of the indicator is lower than the threshold, step 650-3 is performed after step 650-2. Otherwise, if the value of the indicator is higher than or equal to the threshold, step 650-6 is performed after step 650-2.

[0072] If the indicator is below the threshold, the method continues at step 650-3, determining the attack rate. This may be Figure 5A This can also be a fixed attack rate such as Figure 5B For the dynamic attack rate, the weighting factor w can be calculated as the ratio of the two differences, where w = (V th -V supply ) / (V th -V min ). In this example, the nominal voltage is V th=155V, and the minimum voltage for safe operation is V min = 130 V. The weighting factor w is therefore a number between 0 and 1. For a (logarithmic) maximum attack rate of -0,2085 dB / ms and a gain update rate of 24 kHz, the maximum attack rate corresponds to a factor FA on a linear scale max = 0,999. Using the current value of w, the dynamic attack rate is calculated, corresponding to the factor FA d =1-w·(1-FA max ).

[0073] In step 650-4, the gain is reduced by the factor so that the updated value of the gain is gain·FA d As an optional step 650-5, the gain may be limited to a minimum gain value, for example to a value of 0,01. The minimum gain value may help to shorten the duration of the subsequent release phase. As shown in step 650-8, this gain is then used for all attenuation devices until the next control update, for example the next execution of step 650-1 of obtaining a new value for the indicator. The control update rate of evaluating the value of the indicator may be slower than the gain update rate of updating the value of the gain.

[0074] If the indicator is above or equal to the threshold, the method continues with step 650-6. The gain is increased at a release rate, for example a fixed release rate of 20,85 dB / s and a gain update rate of 24 kHz, corresponding to a factor FR=1,0001 on a linear scale. The gain is increased by this factor: gain=gain·FR. In step 650-7, the gain is limited to a maximum gain value, for example to a value of 1 on a linear scale. This gain is then used for all attenuation devices until the next control update, as shown in step 650-8.

[0075] Figure 7 1 shows a flow chart of a method for calculating a clipping level of a clipping function using a control function according to an example. In step 750-1, the calculation is based on the supply voltage measured at the bulk capacitor. In step 750-2, the linear control law V new =c·V supply +V offset , as V supply The function calculates the new voltage level. The coefficient c and the voltage V offset It can be pre-calculated from the rail voltage RMS level at the input of the power supply and the load impedance connected at the output of a given amplifier channel.

[0076] For example, to determine the coefficients of a linear control law, two operating points are used. The first operating point is when the supply voltage is at the nominal level (V supply =Vth ), the linear control law returns to V new =V th +V ripple , where the power supply has V th = 155V nominal voltage, for example, at V ripple =5V and changes from 150V to 160V. The second operating point is when the power supply voltage is at the minimum level (V supply =V min ), the linear control law returns to the voltage level V new , so that the connected load impedance can operate with any signal at that voltage level without exceeding the long-term maximum average power level per channel. new These two conditions allow for the precalculation of c and V for each amplifier channel and its corresponding load impedance. offset .

[0077] As two optional steps 750-3 and 750-4, in step 750-3, the current clipping level is increased at a fixed clipping release rate (in dB / s) CRR, for example CRR = 16 dB / s. For discrete time implementation, the value of the clipping level release factor can be selected according to the control update rate. In step 750-4, the new voltage level V new is limited to be less than or equal to the clipping level. In this way, the power supply voltage V supply Fluctuations in the CRR have less effect on the clipping level. However, if the CRR is chosen to be too slow, clipping may continue for an unnecessarily long period of time until the attenuation caused by the gain reduction can take over. Steps 750-3 and 750-4 may both be used, or both may be omitted.

[0078] As shown in step 750-5, the new voltage level V new Used as the clipping level for all clipping devices until the next control update, e.g. when a new value for the supply voltage at the bulk capacitor is evaluated by the common control function.

[0079] The clipping level may be determined by the common control device based on the indicator.The clipping level of each audio channel may be determined as a function of the connected load impedance of the amplifier channel in question and the supply voltage, which is the voltage at the output of the power supply.

[0080] As long as the indicator is above the threshold, the gain is limited to a maximum value (eg, 0 dB, no attenuation) during the release phase, and likewise the clipping level is set to a level at which clipping is inactive (and may be limited to a maximum level).

[0081] Regarding the determination of the indicator, several algorithms can be used alone or in combination.

[0082] According to the algorithm, the value of the indicator is obtained by sampling the voltage at the output of the power supply. A good marker for the available energy reserve is the power supply output voltage (at Figure 3 between 311 and 312; or Figure 4 between 411 and 413, and between 412 and 413; Figure 4 The available energy reserve can therefore be calculated as E = 0.5·C. B ·(V supply ) 2 , where C B is the effective capacitance of the bulk capacitor, and V supply is the voltage at the output of the power supply. The control function is configured to calculate gain and clipping levels to be applied to some or all audio channels based on the available energy reserve.

[0083] The common control device may receive as input the value of the indicator representing the bulk capacitor voltage and calculate gain and clipping levels to be applied to some or all audio channels.

[0084] Example implementations of control functions and adjustment of thresholds and parameters are provided below.

[0085] The control function may compare the measured supply voltage to a threshold value set at 155 V. If the measured supply voltage is above 155 V, the gain release stage is applied.

[0086] If the measured supply voltage is below 155V, a certain amount of more attenuation is applied to the gain over time (at the gain update rate). Typical attack rates for dynamic compressors for music signals range from 0.04dB / ms to 3dB / ms. In an embodiment suitable for music signals, the attack rate of the gain is calculated dynamically and is not set to a fixed value. A linear control equation can be used. Therefore, the maximum attack rate is reached at 130V, which is 25V below the threshold of 155V. For example, the attack rate can be set to 0.25dB / ms. If the measured voltage is only 5V below the threshold (the measured supply voltage is 150V), the rise rate is set to 0.05dB / ms. Using analog hardware circuitry, the control function can be implemented with some comparators, differential amplifiers and integrators to match the requirements of the attenuation device (e.g., voltage-controlled attenuator, VCA). The control function can be implemented in a programmable device, such as a microcontroller with memory, an FPGA or CPLD or DSP, etc. In this case, the supply voltage can be provided by an analog-to-digital converter.

[0087] If the measured voltage is greater than the 155V nominal operating voltage, a smaller attenuation should be applied, returning from its current value to 0dB (no attenuation) at a fixed release rate in the range of 5dB / s to 250dB / s, with a preferred release rate of 25dB / s.

[0088] In one or more embodiments, the calculated attenuation is applied to all channels of the multi-channel amplifier. However, in other embodiments, it may be interesting to use a priori knowledge to exclude some audio channels from attenuation because they are known not to consume much power. Thus, if the audio system drives an active crossover speaker system, it may make sense to attenuate only the subwoofer and bass channels, and keep the midrange and high frequency channels of the system loud (by design, they only draw a small amount of power). In other embodiments, the channels with the highest instantaneous output power may be selected to attenuate them first.

[0089] Although the described control scheme allows a good and musical-sounding power supply, the supply voltage-dependent attenuation of the music signals in the different audio channels may not prevent the switch-off state in any case. The reason is that a good sound quality is only achieved with moderate rise rates of typically 0,25 dB / ms. To guarantee system safety, additional transient action mechanisms are necessary.

[0090] Here, the clipping level (corresponding to the clipping voltage) is used for calculation. There may be a linear control function between the supply voltage and the clipping level corresponding to the maximum output signal voltage that can be reached until the clipping device will clip its input signal accordingly. For this control function, other control functions than the linear control function are possible, but they should be taken as V supply is a monotonically increasing function of the value of .

[0091] As a numerical example, use a ch = 8 channels and deliver P supply = 2500W common power supply audio system. The power supply has V th =155V nominal voltage, changes from 150V to 160V, where V ripple =5V, and the idle voltage V idle =177V. If no power is drawn from the power supply, the power supply voltage V supply Rising to V idle . Assume the power supply is lower than V supply =V th +V ripple All supply voltages deliver a constant power value P supply Assuming that the losses in the amplifier stage can be ignored, each channel has P ch =P supply / Nch = 312,5W steady-state output power budget. For safe operation of the amplifier, the supply voltage must not drop below V min = 130V or less. Assume that the load impedance of all audio channels is R load =8Ω.

[0092] There must always be at least V between the supply voltage and the maximum signal voltage at the amplifier output. diff =5V margin (or voltage difference). In order to establish a linear control law for the clipping level voltage, two operating points can be defined. First, at the nominal supply voltage V th The signal voltage corresponding to the high clipping level is V high =V th -V diff =150V. Second, at the lowest power supply voltage V min Even for a square wave (rectangular) output signal shape, the power dissipation per channel must not exceed P ch Therefore, the voltage corresponding to the low clipping level is V low , calculated as the product P ch ·R load The square root of R load is the total load impedance of the channel, giving V low =sqrt(P ch ·R load )=50V. Using these two operating points, a linear control function can be defined to calculate the clipping level depending on the supply voltage, V clip =c·V supply -V offset , where V supply is the current value of the power supply voltage used as an indicator. The coefficient of the linear control function is c = (V high -V low ) / (V th -V min )=4. The offset of this linear control function is V offset =c·V th -V high =470V.

[0093] Thus, at a 130V supply voltage (shutdown threshold), the clipping device allows only a 50V maximum voltage at the output of the amplifier channel. For an 8Ω load and a signal with a clipped rectangular shape, the power per channel is (50V)2 / (8Ω)=312,5W. Multiplying by the number of channels gives the total power that the power supply has to deliver at a 130V supply voltage. Thus, an audio system with 8 channels is protected if the power supply can deliver 2500W at a 130V supply voltage. Due to the efficiency of the amplifier channels, the power must be slightly higher. A technician designing such an amplification system can easily adapt these example values ​​to the needs of a specific audio system.

[0094] At a supply voltage of 155V, the clipping level is calculated by a linear equation to be a 150V output clipping voltage. This is a reasonable margin to allow for between the supply voltage and output voltage that a typical amplifier can achieve. When the clipping level is above the maximum output level, the clipping device has no effect, i.e. it does not change the output signal.

[0095] Figure 8 A flow chart of a method implemented by a limiting device according to an example is shown. The method is used in an audio system including a plurality of amplifier channels corresponding to audio channels. Each of the amplifier channels receives a corresponding input audio signal and is powered by the same power supply.

[0096] In step 810, a value of an indicator is obtained. The indicator represents the energy reserve of the power source.

[0097] In step 830, for each of the amplifier channels, a gain to be applied by the attenuation function to the corresponding input audio signal and a clipping level to be applied by the clipping function to the output of the attenuation function are determined. When the value of the indicator is below a threshold, the gain is reduced, and when the value of the indicator is above a threshold, the gain is increased to a maximum value. The threshold may represent a nominal energy level of the power supply in nominal operation. The clipping level may be determined as a monotonic function of the value of the indicator.

[0098] To demonstrate this operation, Fig. 9 A falling supply voltage 901 is shown as a system reaction to a burst signal at all audio inputs. Fig. 9 is an oscilloscope screenshot of a sinusoidal burst signal 902 illustrating the effect of the limiting device on the output signal of the multi-channel amplification device and the voltage of the bulk capacitor.

[0099] At the beginning, the output signal 902 is clipped by the clipping device due to the low supply voltage value after the sudden voltage drop as a reaction to the high power demand. After 100ms, the gain reduction takes over, which means that no signal clipping occurs after this moment. The output signal is now sinusoidal again. The reduced amplitude (and therefore reduced output power) leads to the recovery of the supply voltage. When the supply voltage reaches V supply =V th When , the control function releases the gain and the sinusoidal output voltage rises slightly. In the steady-state region, the time intervals of attack and release can alternate so that the attenuation level is almost constant. (If the indicator remains above the threshold, it means that V supply >V th , then the gain can be maintained during the release phase while it is limited to its maximum value. ) The supply voltage often drops below the threshold of 155V. Therefore, the gain decreases again at a rise rate of 0,01dB / ms at 154V, then 0,05dB / ms at 150V and reaches 0,25dB / ms at a 130V supply voltage.

[0100] If the load impedance is, for example, 4Ω instead of 8Ω, a different control law must be evaluated. At a clipping voltage of 35,36V, 4Ω achieves a power of 312,5W. This must be achieved at a supply voltage of 130V.

[0101] In an audio system with different load impedances, 2 or more clipping levels may be calculated, one for each load impedance. Also for each different load impedance, a corresponding control signal is generated from a common control device to the clipping device in the amplifier channel. While the gain may be the same for all channels, the clipping level may depend on the attached load, which may be different for different channels (e.g., 4 ohms or 8 ohms).

[0102] Depending on the characteristics of the power supply, the maximum rated power of the power supply can depend on the available mains voltage. The operating ranges of 100V, 120V and 230V are used all over the world. The clipping level calculation can be applicable to these ranges. To achieve this, the information of the mains voltage is transmitted to the clipping level calculation device together with the signals 350, 450. Therefore, in a specific embodiment, for different loads of 4Ω and 8Ω, there can be 3 groups of control equations for 100V, 120V and 230V. The clipping level calculation equation is selected according to the actual operating conditions. According to the mains voltage, even the continuous adaptation of the clipping level calculation equation is also possible. Those skilled in the art can adapt the clipping level calculation to the needs of the power supply used in their multi-channel amplification system.

[0103] exist Fig. 9In the example shown, the burst signal starts at time 0ms. The output voltage of this signal exceeds 150V. As a result of this load, the supply voltage drops from its idle voltage V idle =177V suddenly drops to slightly above V min =130V supply voltage. The threshold is set to V th =155V. The common control device maps the supply voltage to a time-varying clipping level. In the time interval from 0ms to 100ms, the output signal is clipped and does not have a sinusoidal shape. In addition, the gain applied by the attenuation device is reduced at an attack rate close to 0,25dB / ms. Therefore, a gain reduction of almost 25dB is reached after 100ms. At this point, the amplitude of the burst signal has been attenuated to a signal peak below the clipping level. Because the signals of all channels are reduced, the supply voltage rises back to a stable state, fluctuating slightly around the nominal voltage. The computing device reacts to the voltage above the threshold by increasing the gain at a release rate of 25dB / s. Therefore, the output signal amplitude rises slightly. This results in a slightly lower supply voltage. From 300ms onwards, the gain applied by the attenuation device is almost constant, because the release phase is interrupted by the attack phase, so that the supply voltage fluctuates around the nominal voltage (which is related to the threshold level) (within the range of the ripple voltage).

[0104] Fig.10 is a comparison of two music signals. The bottom graph is the original signal 910 played back at a low level. At the top is the same signal 920 played back at a much higher level to overload the power supply, so that the common control device calculates the clipping level to cut the high output amplitude, and calculates the gain to attenuate the music signal in all channels.

[0105] Here, the original signal 910 is at a sufficiently low level that the supply voltage does not drop into the range where the clipping and attenuation devices work. When the input signal is provided to the multi-channel amplification system with a much greater gain, the supply voltage drops suddenly when the amplitude of the music signal becomes large. The clipping level is then reduced instantaneously. This results in a first time period (attack phase) with a duration of 60ms, which shows severe clipping of the signal peak. During this time, the gain applied by the attenuation device is further reduced. At the end of this first time period, the signal is sufficiently attenuated to show the same amplitude shape as the original signal shown below.

[0106] exist Fig. 9 and Fig.10It can be observed that the supply voltage has some signal and supply frequency related ripples which "modulate" the calculated clipping level of the audio signal. In another embodiment, it is possible to store the minimum value of the previously calculated clipping level and apply a clipping release rate of, for example, 16 dB / s to it. If there is a new minimum value of the clipping level, the stored clipping level is overwritten. This can be implemented as an approximation with a capacitor charged by a current source and a diode that can discharge or clamp the capacitor voltage directly to the calculated clipping voltage calculated from the supply voltage. The capacitor voltage is then used to set the clipping voltage in each amplification channel. This can of course also be implemented in software when the signal chain is implemented digitally and the amplifier has digital inputs and a digital-to-analog converter or a digital PWM modulator that receives a digital input signal is used.

[0107] Fig.11 is an oscilloscope screenshot of a burst signal showing the effect on the output signal 940 of a multi-channel amplification device using a fixed release rate of the calculated clipping level combined with minimum operation of the clipping level derived from the instantaneous supply voltage 930.

[0108] During the time period when the output signal 940 is clipped, there is a "flat top" from the clipping action. No modulation from the supply voltage ripple is seen. The disadvantage of this embodiment is that the time interval during which the signal is clipped may be longer. The reason is that by calculating the clipping level from the supply voltage instant, the clipping level rises faster.

[0109] Those skilled in the art will appreciate that any functions, engines, block diagrams, flow charts, state transition diagrams, flow charts, and / or data structures described herein represent conceptual diagrams of illustrative circuit systems that embody the principles of the present invention. Similarly, it will be appreciated that any flow charts, flow charts, state transition diagrams, pseudocodes, etc. represent various processes that can be substantially represented in a computer-readable medium and thus executed by a computer or processing device, whether or not such a computer or processor is explicitly shown.

[0110] Although a flowchart may describe operations as a sequential process, many operations may be performed in parallel, concurrently, or simultaneously. Moreover, some operations may be omitted, combined, or performed in a different order. A process may be terminated when its operations are completed, but may also have additional steps not disclosed in the drawings or description. A process may correspond to a method, function, procedure, subroutine, subprogram, etc. When a process corresponds to a function, its termination may correspond to the function returning to the calling function or main function.

[0111] Each described function, engine, block, step described herein may be implemented in hardware, software, firmware, middleware, microcode, or any suitable combination thereof.

[0112] When implemented in software, firmware, middleware, or microcode, instructions for performing the necessary tasks may be stored in a computer-readable medium, which may or may not be included in a host device or a host system. Instructions may be transmitted on a computer-readable medium and loaded onto a host device or a host system. The instructions are configured to cause the host device or host system to perform one or more functions disclosed herein. For example, as described above, according to one or more examples, at least one memory may include or store instructions, and the at least one memory and instructions may be configured to cause the host device or host system to perform one or more functions together with at least one processor. In addition, the processor, memory, and instructions act as a device for providing or causing the host device or host system to perform one or more functions disclosed herein.

[0113] The host device or host system may be a general purpose computer and / or computing system, a special purpose computer and / or computing system, a programmable processing device, an audio system, a machine, or the like.

[0114] An instruction may correspond to a computer program instruction, a computer program code, and may include one or more code segments. A code segment may represent any combination of a process, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or an instruction, a data structure, or a program statement. A code segment may be coupled to another code segment or a hardware circuit by transmitting and / or receiving information, data, an independent variable, a parameter, or a memory content. Information, independent variables, parameters, data, etc. may be transmitted, forwarded, or transmitted via any suitable technology including memory sharing, message passing, token passing, network transmission, etc.

[0115] When provided by a processor, the functionality may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors (some of which may be shared). The term "processor" should not be interpreted as referring exclusively to hardware capable of executing software, and may implicitly include one or more processing circuits, whether programmable or not. A processor or similarly a processing circuit may correspond to a digital signal processor (DSP), a network processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a system on a chip (SoC), a central processing unit (CPU), a quantum processor, an arithmetic logic unit (ALU), a programmable logic unit (PLU), a processing core, programmable logic, a microprocessor, a controller, a microcontroller, a microcomputer, any device capable of responding and / or executing instructions in a defined manner and / or according to defined logic. Other conventional or customized hardware may also be included. A processor or a processing circuit may be configured to execute instructions suitable for causing a host device or a host system to perform one or more functions disclosed herein for a host device or a host system.

[0116] Computer readable medium or computer readable storage medium can be any storage medium suitable for storing computer or processor readable instructions.Computer readable medium can more generally be any storage medium capable of storing and / or containing and / or carrying instructions and / or data.Computer readable medium can be portable or fixed storage medium.Computer readable medium can include one or more storage devices, such as permanent mass storage device, magnetic storage medium, optical storage medium, digital storage disk (CD-ROM, DVD, Blu-ray, etc.), USB key or dongle or peripheral device, permanent mass storage device such as disk drive, hard disk drive (HDD), solid state drive (SSD), memory card, read-only memory (ROM), core memory, flash memory, or any other non-volatile memory, or any combination thereof.

[0117] In the present description, the phrase "device configured to perform one or more functions" or "device for performing one or more functions" may correspond to one or more functional blocks including circuit systems suitable for performing or configured to perform the relevant functions. The block may perform the function by itself, or may cooperate and / or communicate with other one or more blocks to perform the function. "Device" may correspond to or be implemented as "one or more modules", "one or more devices", "one or more units", etc. The device may include a circuit system configured to perform the relevant functions. The device may include at least one processor and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured to use the at least one processor to cause the device or system to perform the relevant functions.

[0118] As used in this application, the term "circuit" or "circuitry" may refer to a hardware circuit implementation only, a combination of hardware circuits and software, a hardware circuit that requires software (e.g., firmware) to operate, a processor. The term "circuit" or "circuitry" may, for example, cover digital signal processor (DSP) hardware, network processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), etc. A circuit system may be or include, for example, hardware, programmable logic, a programmable processor that executes software or firmware, and / or any combination thereof (e.g., a processor, a control unit / entity, a controller) to execute instructions or software and control the sending and receiving of signals, and a memory that stores data and / or instructions.

[0119] Although the terms first, second, etc. may be used to describe various elements in this article, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. For example, without departing from the scope of the present disclosure, the first element may be referred to as the second element, and similarly, the second element may be referred to as the first element. As used herein, the term "and / or" includes any and all combinations of one or more related listed items.

[0120] The terms used herein are used only for the purpose of describing specific embodiments and are not intended to be limiting. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "includes", and / or "including" when used herein specify the presence of stated features, integers, steps, operations, elements, and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts, and / or groups thereof.

[0121] Although various aspects of the present disclosure have been specifically shown and described with reference to the above embodiments, it will be understood by those skilled in the art that various additional embodiments may be conceived by modifying the disclosed machines, systems, and methods without departing from the scope of the disclosure. Such embodiments should be understood to fall within the scope of the present disclosure as determined based on the claims and any equivalents thereof.

Claims

1. An apparatus for use in an audio system comprising a plurality of amplifier channels for corresponding audio channels, each of the amplifier channels receiving a respective input audio signal and powered by a same power supply, the apparatus comprising means for performing a method comprising: - obtaining (810) a value of an indicator, wherein said indicator represents an energy reserve of said power source; - for each of the amplifier channels, determining (830) a gain to be applied by an attenuation function to the respective input audio signal and a clipping level to be applied by a clipping function to an output of the attenuation function, wherein the gain decreases over time when the value of the indicator is below a threshold, and the gain increases over time to a maximum value when the value of the indicator is above the threshold, wherein the threshold represents a nominal energy level at which the power supply is in nominal operation; Therein, the clipping level is determined as a monotonic function of the value of the indicator.

2. The device according to claim 1, wherein: The gain decreases over time at an attack rate.

3. The device according to claim 2, wherein: The attack rate is a fixed attack rate corresponding to a fixed gain reduction per time period on a logarithmic scale.

4. The device according to claim 2, wherein: The attack rate is a dynamic attack rate calculated according to the value of the indicator.

5. The device according to any one of claims 1 to 4, wherein: The gain increases over time at a release rate.

6. The device according to claim 5, wherein: The release rate is a fixed release rate corresponding to a fixed gain increase per time period on a logarithmic scale.

7. The device according to any one of claims 1 to 6, wherein: When the value of the indicator is below the threshold, the clipping level is determined by applying a monotonic function of the output voltage of the power supply.

8. Apparatus according to any one of the preceding claims, wherein: The monotonic function is a continuous function of the output voltage of the power supply.

9. Apparatus according to any one of the preceding claims, wherein: The indicator is proportional to the output voltage of the power supply, or proportional to the square of the output voltage of the power supply.

10. The apparatus according to any one of claims 1 to 8, wherein: The indicator is proportional to the energy stored in the power source.

11. The device according to claim 1, wherein: The gain varies between a minimum value and the maximum value.

12. The device according to claim 1, wherein: The monotonic function is adjusted based on a mains voltage level signal (350, 450) received from the power supply.

13. The apparatus according to claim 1, wherein: For each amplifier channel, the device includes: - a clipping device connected to an input of the amplifier channel and configured to apply the clipping function; - an attenuation device, an output of which is connected to an input of the clipping device, wherein the attenuation device is configured to apply the attenuation function.

14. A method for use in an audio system comprising a plurality of amplifier channels for corresponding audio channels, each of the amplifier channels receiving a respective input audio signal and powered by a same power supply, the method comprising: - obtaining (810) a value of an indicator, wherein said indicator represents an energy reserve of said power source; - for each of the amplifier channels, determining (830) a gain to be applied by an attenuation function to the respective input audio signal and a clipping level to be applied by a clipping function to an output of the attenuation function, wherein the gain decreases over time when the value of the indicator is below a threshold, and the gain increases over time to a maximum value when the value of the indicator is above the threshold, wherein the threshold represents a nominal energy level at which the power supply is in nominal operation; Therein, the clipping level is determined as a monotonic function of the value of the indicator.

15. A computer program comprising instructions which, when executed by at least one processor, cause an apparatus to perform the method according to claim 14.

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