Patent Publication Number: US-2022221937-A1

Title: Audio-haptic signal generator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/726,887, filed on Dec. 25, 2019, which claims priority to European Patent Application No. 19150625.2, filed on Jan. 7, 2019 and entitled “AUDIO-HAPTIC SIGNAL GENERATOR”. All of the aforementioned patent applications are hereby incorporated by reference in their entireties 
    
    
     TECHNICAL FIELD 
     This disclosure relates to an audio-haptic signal generator and a method of audio-haptic signal generation. 
     BACKGROUND 
     Human-Machine Interaction (HMI) relies increasingly on cross-modal user interaction, where multiple sensory modalities are involved. In particular, cross-modal auditive and tactile feedback may be used to enhance touch interfaces. A typical example is a “virtual button”, where button presses are detected by means of a touch sensor, and user feedback is provided by means of a vibration or haptic feedback and, usually, sound. A proper design of the tactile and auditive feedback makes it possible to mimic a real physical button. The combination of tactile and auditive feedback may be referred to as audio-haptics. Such a virtual button is available on some commercial smart phones. Physical buttons suffer from reliability issues due to ingress for example of dust and water, deteriorate over time, and add a physical constraint to the design of a device. Virtual buttons, on the other hand, are more reliable and relatively easy to integrate, multi-functional, and reprogrammable. 
     SUMMARY 
     Various aspects of the disclosure are defined in the accompanying claims. In a first aspect there is provided an audio-haptic signal generator for a haptic system, the haptic system comprising an amplifier coupled to a haptic actuator, the audio-haptic signal generator comprising: an audio input configured to receive an audio signal; a haptic input configured to receive a haptic signal; a controller configured to receive at least one of the haptic input signal, an amplifier state and a haptic actuator state; a mixer coupled to the audio input and the haptic input and having a mixer output configured to be coupled to a haptic actuator; wherein the controller is configured to control the mixer to process the audio signal dependent on at least one of a characteristic of the haptic signal, an amplifier state, and a haptic actuator state; and wherein the mixer is configured to mix the haptic signal and the processed audio signal and to output the mixed audio-haptic signal. 
     In one or more embodiments, the controller may be further configured to control the mixer to process the audio signal dependent on a characteristic of the audio signal. 
     In one or more embodiments, the controller may further comprise an amplifier sensor input configured to be coupled to the amplifier, wherein the controller is further configured to determine an amplifier state from a sensor signal received on the amplifier sensor input and to control the mixer to process the audio signal dependent on the amplifier state. 
     The amplifier state may comprise at least one of the amplifier boost voltage, the amplifier voltage clipping level, the amplifier current clipping level, the amplifier load current, and the amplifier die temperature. 
     In one or more embodiments, the controller may further comprise a haptic actuator sensor input configured to be coupled to a haptic actuator, wherein the controller is further configured to determine a haptic actuator state from a haptic actuator sensor signal received on the haptic actuator sensor input and to control the mixer to process the audio signal dependent on the haptic actuator state. 
     The haptic actuator state may comprise at least one of a haptic actuator resonant frequency, a haptic actuator excursion, a haptic actuator velocity, a haptic actuator acceleration and a haptic actuator voice coil temperature estimate. 
     In one or more embodiments, the mixer may be configured to process the audio signal by at least one of time-shifting the audio signal with respect to the haptic signal, applying a gain to the audio signal, applying a frequency-dependent gain to the audio signal, applying a frequency-dependent phase-shift to the audio signal, and high pass filtering the audio signal. 
     In one or more embodiments, the mixer may comprise an audio processor coupled to an adder wherein the audio processor input is coupled to the audio input, the audio processor output is coupled to a first input of the adder and the haptic signal input is coupled to a second input of the adder. 
     In one or more embodiments, the audio processor may comprise at least one of a delay adjuster, a gain adjuster and a phase adjuster. 
     In one or more embodiments, the audio processor may comprise a series arrangement of at least two of a delay adjuster, a gain adjuster and a phase adjuster coupled between the audio processor input and the audio processor output. 
     In one or more embodiments, the controller may comprise: a state parameter calculator coupled to an amplifier sensor input and a haptic sensor input, and a having state parameter calculator output wherein the state parameter calculator is configured to determine an amplifier state from an amplifier sensor signal received on the amplifier sensor input and a haptic state from a haptic sensor signal received on the haptic sensor input; 
     an amplitude-frequency detector coupled to the audio input and the haptic input and having an amplitude-frequency detector output; at least one of a delay controller, a gain controller, and a phase controller; wherein the delay controller comprises a first delay controller input coupled to the state parameter calculator output, a second delay controller input coupled to the amplitude-frequency detector output, and a delay controller output coupled to the delay adjuster; the gain controller comprises a first gain controller input coupled to the state parameter calculator output, a second gain controller input coupled to the amplitude-frequency detector output, and a gain controller output coupled to the gain adjuster; and the phase controller comprises a first phase controller input coupled to the state parameter calculator output, a second phase controller input coupled to the amplitude-frequency detector output, and a phase controller output coupled to the phase adjuster. 
     Embodiments of the audio-haptic signal generator may be included in a haptic system include an amplifier having an input coupled to the output of the mixer and an output configured to be coupled to a haptic actuator. 
     Embodiments of the audio-haptic signal generator may be included in a human-machine interface, for example a touch panel or touch screen. 
     In a second aspect there is provided a method of generating an audio-haptic signal generation for a haptic system, the haptic system including an amplifier driving a haptic actuator, the method comprising: receiving an audio signal; receiving a haptic signal; processing the audio signal dependent on at least one of a characteristic of the haptic signal, and amplifier state, and a haptic actuator state; and mixing the processed audio signal and the haptic signal. 
     In one or more embodiments, the method may comprise processing the audio signal dependent on a characteristic of the audio signal. 
     In a third aspect there is provided audio-haptic signal generator comprising: an audio input configured to receive an audio signal; a haptic input configured to receive a haptic signal; a controller having a first controller input coupled to the haptic input; 
     an audio processor coupled to the audio input and a controller output; an adder coupled to an output of the audio processor output and the haptic input and configured to combine the haptic signal and processed audio signal; wherein the controller is configured to control the audio processor to process the audio signal dependent on a characteristic of the haptic signal; and the audio-haptic signal generator is configured to output the combined haptic signal and processed audio signal. 
     In one or more embodiments, the controller may further comprise an amplifier sensor input configured to be coupled to the amplifier, and a state parameter calculator coupled to the amplifier sensor input wherein the controller is further configured to determine an amplifier state and the audio processor is configured to process the audio signal dependent on the amplifier state. 
     In one or more embodiments, the controller may further comprise a haptic actuator sensor input configured to be coupled to the haptic actuator, and a state parameter calculator coupled to the haptic actuator sensor input wherein the controller is further configured to determine an amplifier state and the audio processor is configured to process the audio signal dependent on the amplifier state. 
     In one or more embodiments, the controller may comprise a second controller input coupled to the audio input and wherein the controller may be further is configured to control the audio processor to process the audio signal dependent on a characteristic of the audio signal. 
     In one or more embodiments, the audio processor may comprise at least one of a delay adjuster, a gain adjuster and a phase adjuster. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the figures and description like reference numerals refer to like features. Embodiments of are now described in detail, by way of example only, illustrated by the accompanying drawings in which: 
         FIG. 1  shows an audio-haptic signal generator according to an embodiment. 
         FIG. 2  illustrates the frequency response of an audio signal and a haptic signal which provide an example virtual button effect. 
         FIG. 3A  shows a graph illustrating the example haptic signal and audio signal of  FIG. 2  which in combination provide a virtual button effect. 
         FIG. 3B  illustrates a combined haptic actuator signal and the audio signal. 
         FIG. 3C  illustrates an example combined haptic actuator signal and delayed audio signal. 
         FIG. 4  illustrates an audio-haptic signal generator according to an embodiment. 
         FIG. 5  shows another audio-haptic signal generator according to an embodiment. 
         FIG. 6  illustrates an audio-haptic signal generator to according to an embodiment. 
         FIG. 7  illustrates a method of controlling a haptic actuator according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows an audio-haptic signal generator  100  according to an embodiment. The audio-haptic signal generator  100  includes a controller  110  and a mixer  120 . An audio signal input  102  may be connected to a first input of the mixer  120 . A haptic signal input  104  may be connected to a second input of the mixer  120 . The haptic signal input  104  may be connected to an input of the controller  110 . The controller  110  may have an amplifier sensor input  112  and a haptic actuator sensor input  114 . The controller  110  may have a control output  106  connected to a control input of the mixer  120 . 
     In operation the mixer output  108  may be connected to an amplifier  130  which may be for example a class-D audio amplifier, class A amplifier or class AB amplifier. The amplifier output  116  may be connected to a haptic actuator  140  which may be for example an electrodynamic actuator such as a linear resonant actuator (LRA), or a piezo electric actuator. The amplifier sensor input  112  may be connected to the amplifier  130 . The haptic actuator  140  may be connected to the haptic actuator sensor input  114 . 
     For an audio-haptic effect, an audio signal may be received on the audio signal input  102  and a haptic signal may be received on the haptic signal input  104 . An audio signal in this context may be considered as an auditive signal intended to primarily to generate a specific audible sound intended to be heard by a user when driving the haptic actuator. The audio signal may typically be designed to avoid generating vibrations that may interfere with the intended vibrations resulting from the haptic signal. 
     The haptic signal is a signal primarily intended to generate specific tactile or haptic feedback when used to drive a haptic actuator. There is no desired audible effect. 
     In operation the controller  110  may receive the audio signal, the haptic signal, an amplifier sensor signal and/or a haptic actuator sensor signal. 
     The amplifier sensor signal may for example directly indicate or be used to determine an amplifier state. The amplifier state may include one or more of an amplifier boost voltage, a voltage clipping level, a current clipping level, a current consumption value which for portable devices may indicate a battery current, and an amplifier integrated circuit die temperature. 
     The haptic actuator sensor signal may for example directly indicate or be used to determine a haptic actuator state. For a LRA, a haptic actuator state may include one or more of a resonant frequency, a LRA excursion, velocity or acceleration value or LRA voice coil temperature value. The haptic actuator sensor signal may for example be a signal representative of the current flowing into the haptic actuator. 
     The controller  110  may apply a control signal to the mixer  120  dependent on one or more of the haptic actuator state, the amplifier state, and a characteristic of the haptic signal in order that the resulting output of the mixer does not exceed the system limitations. The controller  110  controls the mixer  120  which may process the audio signal dependent on the control signal. 
     Example system limitations may include a limitation that the amplifier output voltage does not exceed an amplifier clip level, the current consumption does not exceed a battery current limit. Further limitation may include limiting LRA excursion so that it does not exceed predefined limits to prevent one or more of mechanical damage and audible mechanical artefacts and limiting LRA voice coil temperature to mitigate against thermal damage. 
     The characteristic of the audio signal or haptic signal may include for example one or more of the instantaneous signal value, the time-varying amplitude, the frequency spectrum or the peak value. The processing of the audio signal may include applying a delay to the audio signal with respect to the haptic signal. The processing of the audio signal may include applying a time-varying gain to the audio signal. The processing of the audio signal may include applying a frequency dependent amplitude change to the audio signal. The processing of the audio signal may include applying a frequency dependent phase change to the audio signal. The mixer  120  may process the signal to apply a high pass filter to the audio signal by applying a frequency dependent gain and phase shift. The mixer  120  may combine the adapted or processed audio signal with the haptic signal and output the mixed adapted audio and haptic signal on the mixer output  108 . The amplifier  130  which may also be referred to as a haptic driver may amplify the mixed adapted audio and haptic signal and drive the haptic actuator  140  with the combined signal. 
     It will be appreciated that in some examples, the controller  110  may control the mixer  120  to process the audio signal determined from only a characteristic of the haptic signal. In these examples, the amplifier sensor input  112  and the haptic actuator sensor input  114  may be omitted. In other examples the controller  110  may adapt the audio signal determined only from one of an amplifier state or a haptic actuator state. In these examples, the connection from the haptic signal input  104  to the controller  110  and one of the amplifier sensor input  112  and the haptic actuator sensor input  114  may be omitted. 
     The audio-haptic signal generator  100  may be implemented in hardware or a combination of hardware and software, for example by software executable on a microcontroller or digital signal processor. In some examples the amplifier  130  and the audio-haptic signal generator  100  may be included on the same device. 
     The inventors of the present disclosure have appreciated that a haptic actuator such as a LRA which is traditionally intended only to produce haptic effects may be controlled by the audio-haptic signal generator  100  to simultaneously output a combined audio and haptic signal for example for an audio haptic effect without overdriving the LRA while not degrading the haptic performance. This may avoid the requirement for an additional loudspeaker or using an expensive multi-function transducer. The inventors of the present disclosure have further appreciated that by adapting or processing the audio signal, the haptic performance of a haptic actuator such as a LRA may be unaffected by the additional audio signal. The inventors of the present disclosure have further appreciated that the user experiences may be more robust to signal modifications on the auditive signal component of the audio-haptic feedback signal than the haptic signal component. 
     An example operation of the audio-haptic signal generator  100  for a virtual click audio-haptic signal may be further understood with reference to  FIGS. 2, 3A, 3B and 3C .  FIG. 2  shows an example frequency spectrum  150  of haptic signal  152  and an audio signal  154  for a typical virtual button effect. The x-axis indicates frequency on a logarithmic scale from 50 Hz to 20 kHz. The y axis indicates the amplitude in dB. The haptic signal  152  is concentrated at low frequencies below 1 kHz and particularly below 300 Hz. The audio signal  154  is concentrated at higher frequencies typically above 2 kHz. 
       FIG. 3A  shows a plot of the audio and haptic virtual click signals  160 . The haptic signal is shown by line  162 . The audio signal is shown by line  164 . The y-axis shows the normalized amplitude between −1 and +1 In this example, amplifier voltage clipping occurs for sample values beyond this range. The x-axis shows time varying between 0 and 50 milliseconds. 
       FIG. 3B  shows illustrates the audio-haptic signal  170 . Line  172  shows the combined audio signal  162  and haptic signal  164  resulting from simple mixing. This results in amplifier clipping as indicated in regions  174  and  176 . 
       FIG. 3C  shows the audio-haptic signal  180 . Line  182  shows the combined audio signal  162  and haptic signal  164  resulting from mixing by the audio-haptic signal generator  100 . The audio-haptic signal generator  100  may determine from the detected amplitude of the haptic signal that amplifier clipping is likely to occur if the audio signal is mixed at the same time and so delays the audio signal, in this example by 6 milliseconds before mixing. The result is that amplifier clipping is avoided and as the delay is relatively small, there may be no perceptible difference to a user. 
       FIG. 4  shows an audio-haptic signal generator  200  according to an embodiment. The audio-haptic signal generator  200  includes a controller  210  and a mixer  230 . The mixer  230  includes an audio processor  220  and an adder  234 . The audio processor  220  may include a delay adjuster  222  having an input connected to an audio signal input  202 . The delay adjuster  222  may have an output  224  connected to a gain adjuster  226 . The gain adjuster  226  may have an output  228  connected to a phase adjuster  232 . The phase adjuster  232  may have an output  208  connected to a first input of the adder  234 . The second input of the adder  234  may be connected to a haptic signal input  204 . The controller  210  may have an output  206  connected to the delay adjuster  222 , the gain adjuster  226  and the phase adjuster  232 . The controller  210  may have a first input connected to the audio signal input  202 . The controller  210  may have a second input connected to the haptic signal input  204 . The output of the adder  234  may be connected to the mixer output  212 . The mixer output  212  may be connected to an amplifier (not shown) which may drive a haptic actuator (not shown) such as a linear resonant actuator. 
     The controller  210  controls the audio processor  220  to adapt the audio signal dependent on at least one of a characteristic of the haptic signal and a characteristic of the audio signal. The characteristic of the haptic and audio signals may include for example one or more of the instantaneous signal value, the time-varying amplitude, the frequency spectrum or the peak value. 
     The audio processor  220  may process the audio signal by applying a delay to the audio signal with respect to the haptic signal with delay adjuster  222 . The delay adjuster  222  may be implemented as a delay line element in hardware or software in combination with hardware. In some examples the delay adjuster may delay the audio signal until the haptic signal amplitude is below a predetermined threshold. 
     The audio processor  220  may process the audio signal by applying a time-varying gain to the audio signal or applying a frequency dependent amplitude change to the audio signal with gain adjuster  226 . In some examples, the gain adjuster  226  may apply a smoothed sample-based gain control typically implemented in dynamic range controllers or limiters. Alternatively, or in addition, in some examples the gain adjuster  226  may include a first stage of a variable gain or filter-bank with variable gains per sub-band implemented using an analysis filter-bank or fast fourier transform (FFT) followed by a second stage including variable gain stage and in third stage including a synthesis filter-bank or an inverse fast fourier transform (iFFT) or similar technique. 
     The audio processor  220  may process the audio signal by applying a frequency dependent phase change to the audio signal with phase adjuster  232 . In some examples, the phase adjuster  232  may include all-pass filters to adapt the audio signal so that the audio signal is out of phase with respect to the haptic signal when the effect of the haptic signal alone is close to one of the system limits. 
     The adder  234  may combine the processed audio signal output from the audio processor  220  with the haptic signal and output the mixed processed audio and haptic signal on the mixer output  212 . The amplifier (not shown) which may also be referred to as a haptic driver may amplify the mixed adapted audio and haptic signal and drive the haptic actuator (not shown) with the combined signal. 
     The audio-haptic signal generator  200  may be implemented in hardware or a combination of hardware and software for example software executable on a microcontroller or digital signal processor. 
     The audio-haptic signal generator  200  may allow simultaneous output of a combined audio and haptic signal for example to produce an audio-haptic effect without overdriving a haptic actuator such as a LRA typically designed for haptic output only, while maintaining the haptic performance. Furthermore, by adapting the audio signal, the haptic performance of a haptic actuator such as a LRA may be unaffected by the additional audio signal. The audio-haptic signal generator  200  may process the audio signal dependent on both the audio signal characteristic and the haptic signal characteristic. This may allow the controller  210  to more accurately adjust the delay, gain and phase of the audio signal than by using the haptic signal only. 
       FIG. 5  shows an audio-haptic signal generator  300  according to an embodiment. The audio-haptic signal generator  300  includes a controller  310  and a mixer  330 . The mixer  330  includes an audio processor  320  and an adder  334 . An audio signal input  302  may be connected to an input of the audio processor  320 . An output  308  of the audio processor  320  may be connected to a first input of the adder  334 . A haptic signal input  304  may be connected to a second input of the adder  334 . The output of the adder  334  may be connected to mixer output  312 . The haptic signal input  304  may be connected to a first input of the controller  310 . The audio signal input  302  may be connected to a second input of the controller  310 . The controller  310  may have an amplifier sensor input  314  and a haptic actuator sensor input  316 . The controller  310  may have a control signal output  306  connected to a control input of the audio processor  320 . 
     In operation the mixer output  312  may be connected to an amplifier  340  which may be for example a class-D audio amplifier. The amplifier output  322  may be connected to a haptic actuator  350  which may be for example a linear resonant actuator (LRA). The amplifier sensor input  314  may be connected to the amplifier  340 . The haptic actuator  350  may be connected to the haptic actuator sensor input  316 . 
     For an audio-haptic effect, an audio signal may be received on the audio signal input  302  and a haptic signal may be received on the haptic signal input  304 . The audio signal is an auditive signal intended primarily to generate an audible output from the haptic actuator. The haptic signal is primarily intended to generate tactile or haptic feedback from the haptic actuator. 
     In operation the controller  310  may receive the haptic signal, the audio signal an amplifier sensor signal and a haptic actuator sensor signal. 
     The amplifier sensor signal may for example directly indicate or be used to determine an amplifier state. The haptic actuator sensor signal may for example directly indicate or be used to determine a haptic actuator state. 
     The controller  310  may apply a control signal to the audio processor  320  dependent on the haptic actuator state, the amplifier state, and a characteristic of the haptic signal and of the audio signal in order that the resulting output of the mixer  330  does not exceed the system limitations. 
     The processing of the audio signal by audio processor  320  may include applying a delay to the audio signal with respect to the haptic signal. The processing of the audio signal by audio processor  320  may include applying a time-varying gain to the audio signal. The processing of the audio signal by audio processor  320  may include applying a frequency dependent amplitude change to the audio signal. The processing of the audio signal by audio processor  320  may include applying a frequency dependent phase change to the audio signal. The adder  334  may combine the processed audio signal with the haptic signal and output the mixed processed audio and haptic signal on the mixer output  312 . The amplifier  340  which may also be referred to as a haptic driver may amplify the mixed adapted audio and haptic signal and drive the haptic actuator  350  with the combined signal. 
     The audio-haptic signal generator  300  may allow mixing audio haptic signals without overdriving the system or degrading haptic performance. The audio-haptic signal generator  300  may allow a haptic actuator such as a LRA originally designed for haptic output only to be used with audio-haptic signals. Furthermore, since the controller  310  controls the audio processor  320  based on a characteristic of the audio signal and of the haptic signal, the amplifier state and the haptic actuator state, the control of the audio signal may be more accurate in staying within the system limitations of the amplifier and of the haptic actuator. 
       FIG. 6  shows an audio-haptic signal generator  400  according to an embodiment. The audio-haptic signal generator  400  includes a controller  410  and a mixer  430 . The mixer  430  includes an audio processor  420  and an adder  434 . 
     The audio processor  420  may include a delay adjuster  422  having an input connected to an audio signal input  402 . The delay adjuster  422  may have an output  424  connected to a gain adjuster  426 . The gain adjuster  426  may have an output  428  connected to a phase adjuster  432 . The phase adjuster  432  may have an output connected to the audio processor output  408 . The audio processor output  408  may be connected to a first input of the mixing module  434 . The second input of the adder  434  may be connected to a haptic signal input  404 . 
     The controller  410  may have an amplitude and frequency detector  440  connected to the audio signal input  402  and the haptic signal input  404 . The amplitude and frequency detector output  448  may be connected to a delay controller  442 . The output of the delay controller  442  may be connected to a delay control output  406 . The amplitude and frequency detector output  448  may be connected to a gain controller  444 . The output of the gain controller  444  may be connected to a gain control output  406 ′. 
     The amplitude and frequency detector output  448  may be connected to a phase controller  446 . The output of the phase controller  446  may be connected to a phase control output  406 ″. 
     The delay control output  406  may be connected to the delay adjuster  422 . The gain control output  406 ′ may be connected to the gain adjuster  426 . The phase control output  406 ″ may be connected to the phase adjuster  432 . 
     The controller  410  may have an amplifier sensor input  414  and a haptic actuator sensor input  416  connected to a state parameter calculator  450 . The state parameter calculator  450  may have an output  456  connected to the delay controller  442 , the gain controller  444  and the phase controller  446 . 
     In operation the mixer output  412  may be connected to an amplifier (not shown) which is connected to a haptic actuator such as a linear resonant actuator. For an audio-haptic effect, an audio signal may be received on the audio signal input  402  and a haptic signal may be received on the haptic signal input  404 . The amplitude and frequency detector  440  may determine characteristics of the audio signal and the haptic signal. These characteristics may include the instantaneous signal value, the time varying amplitude, peak amplitude and frequency spectrum. The amplitude and frequency detector  440  may output the characteristics of the audio signal and the haptic signal to the delay controller  442 , the gain controller  444  and the phase controller  446 . 
     The state parameter calculator  450  may receive an amplifier sensor signal from the amplifier (not shown) on the amplifier sensor input  414 . The state parameter calculator  450  may receive a haptic sensor signal from the haptic actuator (not shown) on the haptic actuator sensor input  416 . 
     The state parameter calculator  450  may determine an amplifier state value and a haptic actuator state value from the respective amplifier sensor value and the haptic actuator sensor value. In some examples, the state parameter calculator  450  may determine an estimate of excursion for a LRA and voice coil temperature from a haptic actuator current value. In other examples, the state parameter calculator  450  may determine an estimate of excursion for a LRA and voice coil temperature from an amplifier current value together with a model of the amplifier behaviour. The state parameter calculator  450  may provide haptic state and amplifier state values to the delay controller  442 , the gain controller  444  and the phase controller  446 . 
     The delay controller  442  may generate a delay control output value on the delay control output  406  determined from the audio and haptic signal characteristics provided by the amplitude and frequency detector  440 , and the haptic actuator and amplifier states provided by the state parameter calculator  450 . For example, when the sum of the instantaneous signal values of the audio and of the haptic signals exceeds the amplifier voltage clipping level, the delay controller  442  may control the delay adjuster  422  to apply a delay to the audio signal such that the peak value of the audio signal may be shifted in time. 
     The gain controller  444  may generate a gain control output value on the gain control output  406 ′ determined from the audio signal characteristics, the haptic signal characteristics, the haptic actuator state, and the amplifier state. The gain adjuster  426  may adjust the gain of the audio signal dependent on the gain control output value. For example, when the sum of the instantaneous signal values of the audio and of the haptic signals exceeds the amplifier voltage clipping level, the gain controller  444  may control the gain adjuster  426  to apply an attenuation corresponding to a gain value lower than 0 dB. 
     The phase controller  446  may generate a phase control output value on the phase control output  406 ″ determined from the audio signal characteristics, the haptic signal characteristics, the haptic actuator state, and the amplifier state. The phase adjuster  432  may adjust the phase of the audio signal dependent on the gain control output value. 
     For example, when the sum of the instantaneous signal values of the audio and of the haptic signals exceeds the amplifier voltage clipping level, the phase controller  446  may control the phase adjuster  432  to apply an all-pass filter to the audio signal such that the peak value of the audio signal may be shifted in time. 
     The adder  434  may combine the processed audio signal output from the audio processor  420  with the haptic signal and output the mixed processed audio and haptic signal on the mixer output  412 . The amplifier (not shown) which may also be referred to as a haptic driver may amplify the mixed adapted audio and haptic signal and drive the haptic actuator (not shown) with the combined signal. 
     The audio-haptic signal generator  400  may be implemented in hardware or a combination of hardware and software for example software executable on a microcontroller or digital signal processor. 
     The audio-haptic signal generator  400  may allow simultaneous output of a combined audio and haptic signal for example to produce an audio-haptic effect without overdriving a haptic actuator such as a LRA typically designed for haptic output only, while maintaining the haptic performance. Furthermore, by adapting the audio signal, the haptic performance of a haptic actuator such as a LRA may be unaffected by the additional audio signal. The audio-haptic signal generator  400  may process the audio signal dependent on both the audio signal characteristic, the haptic signal characteristic, the amplifier state and the haptic actuator state. This may allow the controller  410  to more accurately adjust the delay, gain and phase of the audio signal than by using the haptic signal, the amplifier state, or the haptic actuator state only. 
     In some examples any one or two of the delay adjuster  422 , the gain adjuster  426  and the phase adjuster  432  may be omitted together with the corresponding the delay controller  442 , gain controller  444  and phase controller  446 . In some examples the delay adjuster  422 , the gain adjuster  426  and the phase adjuster  432  may be arranged in a different order between the audio input  402  and the audio processor output  408 . 
       FIG. 7  shows a method of generating an audio haptic signal  500  for a haptic system including an amplifier and a haptic actuator according to an embodiment. In step  502  an audio signal is received. In step  504 , a haptic signal is received. In step  506  the audio signal is processed dependent on at least one of a characteristic of an audio signal, a characteristic of a haptic signal, an amplifier state and a haptic actuator state. In step  508  the processed audio signal is mixed with the haptic signal. The method  500  may allow audio and haptic signals to be combined and used by an amplifier to drive a haptic actuator intended to be used only for haptic signals without overdriving the system or degrading the haptic performance. This may avoid the requirement of a separate loudspeaker for auditive haptic effects. In some examples, the method  500  may be applied in real-time. In some examples, the method  500  may be applied to generate the audio-haptic signal for later use. 
     An audio-haptic signal generator for a haptic system including an amplifier coupled to a haptic actuator is described. The audio-haptic signal generator includes an audio input configured to receive an audio signal; a haptic input configured to receive a haptic signal and a controller configured to receive at least one of the haptic input signal, an amplifier state and a haptic actuator state. A mixer is coupled to the audio input and the haptic input. The mixer has an output configured to be coupled to a haptic actuator. The controller controls the mixer to process the audio signal dependent on at least one of a characteristic of the haptic signal, an amplifier state, and a haptic actuator state. The mixer is configured to mix the haptic signal and processed audio signal and to output the mixed haptic signal and processed audio signal. 
     Embodiments of the linear resonant actuator controller and method of controlling a linear resonant actuator may be included in mobile devices such as smart phones, smart watches portable medical devices, wearable devices, laptop computers, tablets. Embodiments may be included in any device using HMI with haptic and audio feedback. For example, the device may be a control panel for an industrial control, automotive control or domestic control system. 
     The audio-haptic signal generators described herein may for example be implemented in a touch panel using a single haptic actuator for many application domains, such as automotive and Internet of things (IoT) where physical buttons may be replaced by virtual buttons. 
     Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
     Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination. 
     In some example embodiments the set of instructions/method steps described above are implemented as functional and software instructions embodied as a set of executable instructions which are effected on a computer or machine which is programmed with and controlled by said executable instructions. Such instructions are loaded for execution on a processor (such as one or more CPUs). The term processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. A processor can refer to a single component or to plural components 
     The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 
     For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.