# Patent

## Title
Control of a loudspeaker output

## Abstract

A loudspeaker drive circuit comprises an input for receiving an audio signal and a signal processor for processing the audio signal before application to the loudspeaker. The signal processor processes the audio signal to derive a loudspeaker drive signal which results in the loudspeaker membrane reaching its maximum displacement in both directions of diaphragm displacement.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the priority under 35 U.S.C. §119 of European patent application no. 11154581.0, filed on Feb. 15, 2011, the contents of which are incorporated by reference herein.
This invention relates to the control of the output of a loudspeaker, and relates to a loudspeaker circuit and signal processing method.
An important cause of loudspeaker failures is a mechanical defect that arises when the loudspeaker diaphragm is displaced beyond a certain limit, which is usually supplied by the manufacturer. Going beyond this displacement limit either damages the loudspeaker immediately, or can considerably reduce its expected life-time.
Furthermore, the combination of the use of small loudspeakers and the demand for high acoustic output results in the need for loudness maximisation methods, but increases the risk of exceeding the diaphragm displacement limit, also referred to as the ‘cone excursion’ limit. Small loudspeakers are also unable to efficiently reproduce low frequencies, so that a so-called loudspeaker linearisation method is often used to correct for this low efficiency. However, this also increases the risk of going beyond the displacement limit.
There exist several methods to limit the displacement of the diaphragm of a loudspeaker to provide loudspeaker protection.
One category consists of methods that process the input signal with variable cut-off filters (high-pass or other), the characteristics of which are controlled via a feedback loop. A displacement predictor is used to control the filters. Another category uses a feedforward strategy in which the signal is processed with a bank of band-pass filters with adjustable gains in such a way that excess excursion is prevented (by attenuating only the frequency bands that cause most excursion).
The use of a low frequency shelving and notch filter has also been proposed in a feedforward approach (in U.S. Pat. No. 7,372,966), which is controlled via the displacement predictor in a feedforward manner, using a model of the loudspeaker to pre-process the signal before sending it to the amplifier/loudspeaker.
U.S. Pat. No. 6,201,873 describes a setup for protecting against excess audio distortion (mainly caused by cone excursion), or driving the loudspeaker to its maximum (to maximum cone excursion). It uses a cone excursion transfer function (or a maximal voltage transfer function) module, the output of which is used to control a variable gain on the input. U.S. Pat. No. 6,201,873 suggests the use of a transfer function to give maximal voltage leading to just acceptable distortion, which is the maximal voltage that can be applied per frequency without causing excess distortion. The distortion being considered is that arising from excess cone displacement (no distortion in the converter and amplifier is considered).
This invention is based on the recognition that in many loudspeakers, the positive (outward-moving) and negative (inward-moving) diaphragm displacement peaks are not symmetrical when the loudspeaker is driven towards its displacement limit. In the following description, it is arbitrarily assumed that the negative diaphragm displacement is larger in absolute value than the positive displacement when the loudspeaker is driven towards its displacement limit, but the opposite may be true. This is often due to the asymmetry of the nonlinear behaviour of the loudspeaker with respect to the diaphragm displacement.
If the (measured or predicted) diaphragm displacement is limited by means of a protection scheme, it is likely that only the negative displacement peak is limited, while the positive peak never reaches the displacement limit. Thus, a protection scheme will prevent the maximum loudness being reached.
According to the invention, there is provided a loudspeaker drive circuit comprising:
an input for receiving an audio signal;
a signal processor for processing the audio signal before application to the loudspeaker, and adapted to process the audio signal to derive a loudspeaker drive signal which results in the loudspeaker membrane reaching its maximum displacement in both directions of diaphragm displacement.
In this way, the invention provides a system to process an audio input signal (which could be used to drive a loudspeaker) such that the positive and negative displacement peaks are symmetrical, and that both reach their respective displacement limits. As a result, the peak-to-bottom displacement can be increased, which results in an increased sound level of the loudspeaker output.
It is noted that the “input for receiving an audio signal” is the input to the signal processor, and there may be other components between that input and an external input to the loudspeaker drive circuit.
The signal which is processed to derive the symmetrical displacement limit drive of the loudspeaker can comprise a signal which has been subjected to loudness maximisation. However, it may instead be a signal which has been subjected to processing (e.g., clipping) for loudspeaker protection. In both cases, it is a peak audio signal which is processed to ensure that the loudspeaker displacement limit is reach in both directions. Before the processing of the invention, the audio signal (either maximised or processed for protection) does not result in the maximum displacement in both directions. Typically, maximum displacement arises in one direction of diaphragm displacement only.
The circuit can comprise a maximisation unit for generating a loudspeaker drive signal which would drive the loudspeaker such that the maximum displacement in one direction of diaphragm displacement is reached. This can comprise a conventional loudness maximisation unit, which does not take into account the asymmetry of the loudspeaker.
Alternatively, the invention can be applied to an audio signal from a loudspeaker protection unit.
An adder can be provided for adding a dc offset to the output of the maximisation unit/protection unit. This dc offset then shifts the loudspeaker drive signal to provide the required symmetry which takes account of the nonlinear and asymmetrical characteristics of the loudspeaker.
To derive the asymmetrical characteristics, a diaphragm displacement predictor module can be used, and the dc offset can then be adjustable by the diaphragm displacement predictor module.
A gain element can be used for applying a gain to the output of the adder. This then ensures the displacement limit is reached. The gain can also be adjustable by a diaphragm displacement predictor module.
Instead of an adder and gain element, an asymmetrical limiter unit can be provided for applying a gain and asymmetrical limiter to the output of the maximisation unit. Again, the asymmetrical limiter unit can be controlled by a diaphragm displacement predictor module.
The circuit preferably comprises a loudspeaker, and it can be part of a portable device such as mobile telephone. The loudspeaker comprises a loudspeaker membrane which is displaceable from a rest position in two (opposite) directions.
The invention also provides a method of processing an audio input signal to derive a loudspeaker drive signal, comprising:
processing the audio signal to derive a loudspeaker drive signal which results in the loudspeaker membrane reaching its maximum displacement in both directions of diaphragm displacement.



Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:
FIG. 1 shows the signal processing adopted by the system and method of the invention;
FIG. 2 shows a first example of loudspeaker drive circuit of the invention;
FIG. 3 shows a second example of loudspeaker drive circuit of the invention;
FIG. 4 shows a third example of loudspeaker drive circuit of the invention;
FIG. 5 shows a fourth example of loudspeaker drive circuit of the invention; and
FIG. 6 shows a mobile telephone in which the loudspeaker system can be implemented.



The invention provides a loudspeaker drive circuit in which an audio signal is adapted before being applied to a loudspeaker. The adaptation is such an audio signal, when used to drive a loudspeaker for which the circuit is designed, results in the loudspeaker membrane reaching its maximum displacement in both directions of diaphragm displacement. In this way, the loudspeaker provides the greatest possible sound pressure, for example when it is driven to maximum loudness.
An example of the invention is described based on a digital implementation (requiring a digital-to-analog converter). It can also be implemented in the analog domain.
The invention can be applied to loudness maximisation or loudspeaker protection systems. The following examples are based on the application of the invention to systems which incorporate loudness maximisation.
For the following description, it is assumed that a loudness maximisation module processes an input signal in such a way that the processed signal drives the loudspeaker to, but not beyond, its negative peak displacement, and that the positive peak displacement does not reach the limit value. It can alternatively be that the positive peak is limited and that the negative peak does not reach the limit value, depending on the loudspeaker.
The positive and negative displacement peaks can have different values, and the proposed method can be used to align the positive and negative peaks to the respective displacement limits.
The basic principle is shown in FIG. 1.
In FIG. 1A, the diaphragm displacement is shown as a function of time for an arbitrary sound track, which is processed in such a way that the maximal (negative) displacement is at 1 mm, which is the displacement limit of the loudspeaker. The positive peak is much smaller (approximately 0.49 mm) due to the asymmetry of the nonlinear behaviour of the loudspeaker.
By adding a DC-value to the audio signal, the diaphragm displacement can be shifted upwards or downwards, and it can be chosen such that the positive and negative peak displacements are symmetrical, as shown in FIG. 1B.
The DC-value that is required for this can be predicted from a loudspeaker model, or it can be determined iteratively while measuring the diaphragm displacement. As a result, the positive and negative peaks are at ±0.74 mm, which is below the displacement limit.
An additional gain can be applied such that the positive and negative displacement peaks coincide with the respective displacement limits (+1 mm and −1 mm in this example).
FIG. 1C shows the effect of adding a DC-value and an additional gain.
A first embodiment of circuit of the invention is shown in FIG. 2.
The loudspeaker drive circuit comprises an input 10 for receiving an audio signal. This audio signal can be a digital signal which would be suitable for driving the speaker (after digital to analog conversion) or it can be an analog signal. The processing implemented by the additional circuit elements of the invention provide final adjustments to the audio signal.
The circuit comprises a signal processor for processing the audio signal before application to the loudspeaker. In the example of FIG. 2, the signal processor comprises an adder 13 for adding a dc value provided by a dc offset unit 14, and a gain element 16 for applying a gain. These units process the audio signal after a (conventional) loudness maximisation in unit 12. The purpose of the dc offset and gain is to derive a loudspeaker drive signal which results in the loudspeaker membrane reaching its maximum displacement in both directions of diaphragm displacement when the maximum loudness drive signal is provided.
The maximisation module 12 performs (fixed or adaptive) processing of the input signal 10. The DC-value 14 and an additional scaling 16 are added to the processed signal, and it is sent to a digital-to-analog converter 18 and the loudspeaker 20.
The DC- and gain-values are set such that the positive and negative peak diaphragm displacements both reach their respective limits. These values are fixed and can be determined for example by applying a given test sequence.
The test sequence comprises a drive signal which is multiplied by a gain. The gain is increased until the negative peak value of the excursion reaches the maximum allowed excursion (assuming the negative excursion peak is reached first).
The positive and negative excursion peaks at the point when the negative peak value of the excursion has been reached can be denoted by, respectively, x+ and x−. Next, the DC value is determined for which the positive peak excursion value equals (x+−x−)/2. In this way, the response is shifted until the positive and negative excursion values are equal (i.e. the shift from FIG. 1A to FIG. 1B).
The value for the gain element is then determined as the gain factor that is required to increase the peak excursion value to the maximally allowed value (i.e. the shift from FIG. 1B to FIG. 1C).
The test sequence can be applied to one speaker of a given speaker design, which assumes that the speaker nonlinearities are uniform across samples of the same design. A laser displacement meter can be used to measure the excursion points, and thereby determine when the excursion limit has been reached in one direction (by comparison with a manufacturer-supplied value), as well as measuring the excursion reached in the other direction.
To implement the invention in a loudspeaker protection system, the loudness maximisation unit 12 is replaced by a protection unit. This suffers the same problem that it gives rise to a drive signal which only makes the loudspeaker reach its displacement limit in one direction.
A second embodiment is shown in FIG. 3. The DC-shift and gain are replaced by an asymmetrical limiter implemented by an asymmetrical limiter unit 22. The limiter applies a gain and limits the positive and negative peaks to different values. The parameters of the asymmetrical limiter are set such that the positive and negative peak diaphragm displacements both reach their respective limits. These values can be determined in the same way as outlined above.
A third embodiment is shown in FIG. 4. It is an extension of the first embodiment. The maximisation module 12 consists of a processing block 24 that is controlled by an excursion module 26, which consists of a diaphragm displacement predictor module, which obtains a measurement or a prediction of the diaphragm displacement. A number of control signals can be derived from this measurement or prediction, including control parameters for the dc offset unit 14. This block can be used to control the DC- and gain-value that are required to render the peak displacements symmetrical and corresponding to their respective limit values.
For instance, the gain can be set to a fixed value, and the DC-value can be set to zero if the expected peak diaphragm displacement is below threshold, and to a fixed value otherwise. If the excursion module is able to predict the asymmetrical excursion peaks (for instance, if the excursion module takes into account the asymmetry of the nonlinear behaviour of the loudspeaker), the required DC-value can be estimated in the excursion module.
The adaptive processing (maximisation and/or protection) can for example use filters, an adaptive gain element and a dynamic range compression module. Typically, the parameters of the processing blocks are controlled in such a way that the expected peaks of the diaphragm displacement coincide with the maximally allowed values (maximisation), and/or that the diaphragm displacement does not exceed the maximally allowed value (protection). The adaptive gain can thus be dependent on the predicted diaphragm displacement in response to the actual audio signal.
A fourth embodiment is shown in FIG. 5. It is an extension of the second embodiment. The maximisation module 12 consists of a processing block that is controlled by an excursion module 26, which again consists of a measurement or a prediction of the diaphragm displacement. This block can then be used to control the parameters of the asymmetrical limiter function. For instance, the gain can again be set to a fixed value and the positive and negative limiting levels can be set to predefined levels if the expected peak diaphragm exceeds a threshold value. If the excursion module is able to predict the asymmetrical excursion peaks (for instance, if the excursion module takes into account the asymmetry of the nonlinear behaviour of the loudspeaker), the required limiting levels can be estimated in the excursion module.
The invention can be used in combination with loudness maximisation algorithms that incorporate a loudspeaker protection by limiting the diaphragm displacement. The effect will be an increase of the gain that can be applied without exceeding the displacement limit.
The maximisation unit can be conventional, for example a known approach is outlined in U.S. Pat. No. 6,201,873. The loudness maximisation unit can always maximise loudness, or does it can only come into play if there is a maximum volume setting applied to the speaker. For example, the highest volume can correspond to maximisation, while the lower volumes used fixed gains. Another possibility is that all volume levels are derived from a maximised signal, so that volume control provides attenuation of the maximised signal.
As mentioned above, the invention can be applied in the analog domain instead of the in the digital domain.
The invention can be used for maximising the loudness in sound reproduction systems, while protecting the loudspeakers. An important application is in mobile phones, where lower-quality loudspeakers are often employed, but high acoustic output is desired.
FIG. 6 shows a mobile phone 30 including the loudspeaker system 32 of the invention.
Various modifications will be apparent to those skilled in the art.

## Claims

1. A loudspeaker drive circuit comprising:
an input for receiving an audio signal; and
a signal processor for processing the audio signal before application to a loudspeaker having a membrane, and adapted to process the audio signal to derive a loudspeaker drive signal which results in the loudspeaker membrane moving asymmetrically and reaching its maximum displacement in both directions of displacement, wherein based on a prediction of asymmetrical excursion peaks the signal processor incorporates a DC offset in the loudspeaker drive signal in each direction of excursion of the loudspeaker membrane to limit a movement of the loudspeaker membrane asymmetrically in each direction, the DC offset is determined based on a measurement of a maximum excursion of the loudspeaker membrane in the each direction, and drive a loudspeaker with the loudspeaker drive signal, wherein the DC offset is set to zero and a gain of the audio signal is set to a fixed value if expected loudspeaker membrane displacement is below the maximum excursion.

2. A circuit as claimed in claim 1, further comprising one of:
a maximisation unit for generating the audio signal as a loudspeaker drive signal which would drive the loudspeaker such that the maximum displacement in one direction of displacement is reached; and
a loudspeaker protection unit for generating the audio signal as a protected loudspeaker drive signal.

3. A circuit as claimed in claim 2, further comprising an adder for adding a dc offset to an output of the maximisation unit or the loudspeaker protection unit.

4. A circuit as claimed in claim 3, further comprising a diaphragm displacement predictor module, and wherein the dc offset is adjustable by the diaphragm displacement predictor module.

5. A circuit as claimed in claim 3, further comprising a gain element for applying a gain to an output of the adder.

6. A circuit as claimed in claim 5, further comprising a diaphragm displacement predictor module, and wherein the gain is adjustable by the diaphragm displacement predictor module.

7. A circuit as claimed in claim 2, further comprising an asymmetrical limiter unit for applying a gain and asymmetrical limiter to an output of the maximisation unit or loudspeaker protection unit.

8. A circuit as claimed in claim 7, further comprising a diaphragm displacement predictor module, and wherein the asymmetrical limiter unit is controlled by the diaphragm displacement predictor module.

9. A loudspeaker system comprising a circuit as claimed in claim 1 and the loudspeaker.

10. A portable device comprising a loudspeaker system as claimed in claim 9.

11. The portable device of claim 10 wherein the device is a mobile telephone.

12. A method of processing an audio input signal to derive a loudspeaker drive signal, comprising:
generating a loudspeaker drive signal to move a loudspeaker membrane asymmetrically and to cause the loudspeaker membrane reach its maximum displacement in both directions of displacement, and driving a loudspeaker with the loudspeaker drive; and
incorporating a DC offset in the loudspeaker drive signal in each direction of excursion of the loudspeaker membrane to limit a movement of the loudspeaker membrane asymmetrically in each direction, wherein the DC offset is determined based on a prediction of asymmetrical excursion peaks and a measurement of a maximum excursion of the loudspeaker membrane in the each direction, wherein the DC offset is set to zero and a gain of the audio signal is set to a fixed value if expected the loudspeaker membrane displacement is below the maximum excursion.

13. A method as claimed in claim 12, further comprising:
generating a loudspeaker drive signal which would drive the loudspeaker such that the maximum displacement in one direction of diaphragm displacement is reached;
adding a dc offset to the loudspeaker drive signal; and
applying a gain.

14. A method as claimed in claim 12, further comprising:
generating a loudspeaker drive signal which would drive the loudspeaker such that the maximum displacement in one direction of diaphragm displacement is reached, and
applying an asymmetrical limiter to the loudspeaker drive signal.

15. A method as claimed in claim 14, wherein at least one of the dc offset, the gain, and the limiter is adjustable by a diaphragm displacement predictor module.

16. The method of claim 12, wherein the loudspeaker membrane is characterized by having a positive diaphragm displacement and a negative diaphragm displacement as the respective directions of displacement, and the positive diaphragm displacement and the negative diaphragm displacement are not symmetrical, and wherein the step of processing the audio input signal includes generating an asymmetric signal that limits excursion of the loudspeaker membrane to the maximum positive displacement and to the maximum negative displacement.

17. The method of claim 12, wherein processing the audio input signal to derive the asymmetrical loudspeaker drive signal includes:
processing the audio input signal to provide a symmetrical signal; and
adding gain to the audio input signal to provide the asymmetrical loudspeaker drive signal, and thereby causing the loudspeaker membrane to reach its maximum displacement in both directions of displacement.

18. The loudspeaker drive circuit of claim 1, wherein the loudspeaker membrane is characterized by having a positive diaphragm displacement and a negative diaphragm displacement as the respective directions of displacement, and wherein the signal processor is configured and arranged for processing the audio signal before application to the loudspeaker limit movement of the loudspeaker membrane to its maximum displacement in both directions of displacement, the maximum displacement in the positive direction being different than the maximum displacement in the negative direction.

19. The loudspeaker drive circuit of claim 1, wherein the loudspeaker membrane is characterized as having respective maximum displacements that are asymmetrical, and the signal processor limits the displacement in at least one of the directions to a value that is less than the maximum displacement in the at least one of the directions in a condition wherein the audio signal is applied to the loudspeaker without the audio signal being previously processed by the signal processor.

20. The loudspeaker drive circuit of claim 1, wherein the loudspeaker membrane is characterized as having respective directions of displacement that are asymmetrical, and wherein the signal processor is configured and arranged for processing the audio signal by adjusting a gain thereto before application to the loudspeaker to cause the loudspeaker membrane to reach its maximum displacement in both directions of displacement.