News Column

Researchers Submit Patent Application, "Pre-Filtering for Loudspeakers Protection", for Approval

June 17, 2014



By a News Reporter-Staff News Editor at Information Technology Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Marguery, Philippe (Poisat, FR); Nagari, Angelo (Grenoble, FR); Sirito-Olivier, Philippe (Saint Egreve, FR), filed on June 28, 2012, was made available online on June 5, 2014.

The patent's assignee is St-ericsson Sa.

News editors obtained the following quote from the background information supplied by the inventors: "The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.

"Inductive loudspeakers often include a coil arranged around a magnetic core which is mechanically coupled with a membrane. Sound is produced by membrane displacements caused by magnetic core motion through inductive coupling to the coil which is controlled by an electrical signal oscillating at given frequencies.

"Loudspeakers converting thus an electrical signal into an acoustic signal can be endangered to malfunction or permanent destruction when they are solicited beyond their acceptable limits. If the electrical signal level is too high at specific frequencies, membrane displacement can be such that damage can occur, either by self-heating, mechanical constraint, or by demagnetization of the magnetic core. For instance, the coil of a loudspeaker can hit the mechanical structures of the device or the mobile membrane can be torn if the constraints are too high.

"In particular, these issues are very complex to solve for small inductive loudspeakers such as those in mobile devices such as mobiles or smart phones. Dimensions of those loudspeakers impact the heat dissipation and mechanical constraints.

"Moreover, being a mechanical oscillator, the loudspeaker may have a resonant frequency which amplifies the amplitude of the control signal at said frequency.

"In order to protect inductive loudspeakers against damages due to self-heating and excessive mechanical displacement of the membrane, non adaptive systems have been developed based on an 'a priori' prediction of the frequency response of the inductive loudspeakers.

"U.S. Pat. Nos. 4,113,983, 4,327,250 and 5,481,617 propose to use variable cut-off frequency filters driven by a membrane displacement predictor. The filter parameters are set according to a prediction of the loudspeaker membrane displacement response over frequency. Parameters are predicted based on a static model of the loudspeaker which is defined once in the life of the product.

"U.S. Pat. No. 5,577,126 proposes to use attenuators. The output of the displacement predictor is fed-back into the input signal, according to a feedback parameter computed by a threshold calculator, this parameter being calculated once in the life of the product.

"International patent application No. WO 01003466 proposes to use multi-frequency band dynamic range controllers. The input signal is divided into N frequency bands by a bank of band-pass filters. The energy of each frequency band is controlled by a variable gain before being summed together and input to the loudspeaker. A processor monitors the signal level in each frequency band and acts on parameters of each of the variable gain subsystems in order to limit the membrane displacement based on pre-calculated frequency response.

"Nevertheless, in case of variations of the loudspeaker transfer function over time, these solutions could not be able to adapt their parameters, as these parameters are calculated once in the life of the product. These variations may result from several factors: temperature, atmospheric pressure, ageing, humidity variations, etc. In contrast, an 'a priori' based compensation can not track the real time loudspeaker response, and a compensation filter can not be able to avoid loudspeaker damages in certain conditions."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "A first aspect of the present invention thus relates to a method of protecting an inductive loudspeaker (108) arranged to consume a current of a given value during reproduction of an audio stream.

"The method comprises: a/ filtering (801) a first part of the audio stream by applying a compensation filter to said first part of the audio stream; b/ inputting the filtered first part (OUT) of the audio stream to the inductive loudspeaker; c/ computing (802) at least a first estimation of a frequency response of the inductive loudspeaker based at least on: the filtered first part (OUT) of the audio stream; and the value of the current consumed (RET) by the inductive loudspeaker during reproduction of the filtered first part of the audio stream; d/ updating (805) characteristics of the compensation filter so as to attenuate a resonant frequency in the first estimated frequency response of the inductive loudspeaker.

"A part of an audio stream is a temporal subset of the audio stream. For instance, this subset can be an extract of 100 milliseconds of the audio stream. In one other embodiment, the subset can be, for instance, an extract of 23 ms (corresponding to 1024 samples at 44.1 kHz): this can relax memory size keeping low constraints on real time processing

"To 'apply a compensation filter to the part of the audio stream' generally means that the frequencies of the part of the audio stream are filtered according to the compensation filter.

"When it is stated that the filtered part of the audio stream is input to the inductive loudspeaker, it is to be construed that the inputting can be direct or indirect to the inductive loudspeaker. For instance, and as described in FIG. 1, the filtered part can transit via a 'digital to analog converter' and/or an amplifier before the inductive loudspeaker.

"To 'attenuate a resonant frequency in the estimated frequency response' means that the frequencies near the resonant frequency (or equal to this resonant frequency) is attenuated. For instance, the logarithm module of the filter can be substantially below 'zero' for frequencies near the resonant frequency.

"To 'update characteristics of the compensation filter' consists, for instance, in replacing the first compensation filter (respectively its parameters) with a second compensation filter (respectively its parameters) or in merging the first compensation filter with information of the second compensation filter (for instance, result of this modification can be the average filter computed with the first and second compensation filter).

"Hence, the updating of the compensation filter enables a feedback loop which can dynamically remove the resonant frequency of a loudspeaker. It ensures that the compensation filter evolves during time and life time of the loudspeaker (for instance due to heat or humidity) and avoiding any loudspeakers damages or deteriorations.

"For instance, the updated characteristics of the compensation filter can define a band-stop filter adapted to attenuate the resonant frequency in the first estimated frequency response of the inductive loudspeaker.

"Thus, the implementation (circuit implementation or programming implementation) can be simple as this type of filter is common in electronics and filter domain.

"According to another embodiment, steps a/ to d/ can be repeated for a second part of the audio stream.

"For instance, this second part of the audio stream is a temporal subset of the audio stream following the above mentioned part (in step a/). Thus, the method can be reapplied, in a loop, for all subsets of the audio stream.

"Moreover, the compensation filter evolves while the reproducing of the audio stream and ensures a dynamic protection all over the reproduction of the audio.

"According to another embodiment, compensation filter is updated at step d/ only if a second estimated response of the loudspeaker is lower than a threshold. The second estimated response can be, for instance, computed by applying the estimation of a frequency response of the inductive loudspeaker to a third part of the audio stream.

"The threshold can be adjusted for a given loudspeaker. This threshold value can be fixed for a given type of loudspeaker and is not to be changed from one loudspeaker sample to another. It can be fixed before production on some phone during the tuning procedure.

"The third part of the audio stream can be advantageously the second part mentioned above.

"Consecutively, the compensation filter can be updated only if needed, i.e. only if the compensation performed by the previous compensation filter is not sufficient. In particular, if the second estimated response is lower than the threshold, it can mean that the frequency response of the loudspeaker has not changed significantly and that there is no need to change the second compensation filter to a new one. The threshold can also avoid equalization if spectral density of the signal is low and thus if there is no risk to damage the loudspeaker. This can offer optimum audio rendering avoiding cutting some frequencies of the audio signal if it is not needed.

"According to another embodiment, the value of the current consumed by the inductive loudspeaker during reproduction of the filtered part of the audio stream can be sensed by electronic circuit coupled to the inductive loudspeaker through a current mirror circuit.

"Current mirror circuit is a circuit designed to copy a current through one active device. For instance, such circuit can be a 'Wilson mirror' made with simple transistors.

"Thus, there is no need to use an element in series with the loudspeaker (sense resistor) which can decrease the maximum electrical power expected in the load and thus the maximum sound pressure level.

"A second aspect relates to a processing device, connected with a mixing signal unit comprising an inductive loudspeaker. The processing device includes: an input interface to receive a part of an audio stream; an input interface to receive a value of a current consumed by the inductive loudspeaker; an output interface to send a filtered part of an audio stream.

"In this embodiment, the processing device is configured to: a/ filter (801) a first part of the audio stream by applying a compensation filter to said first part of the audio stream; b/ input the filtered first part (OUT) of the audio stream to the inductive loudspeaker; c/ compute (802) at least a first estimation of a frequency response of the inductive loudspeaker based at least on: the filtered first part (OUT) of the audio stream; and the value of the current consumed (RET) by the inductive loudspeaker during reproduction of the filtered first part of the audio stream; d/ update (805) characteristics of the compensation filter so as to attenuate a resonant frequency in the first estimated frequency response of the inductive loudspeaker.

"A third aspect relates to an electronic device comprising a processing device as mentioned above. An electronic apparatus can be for instance a mobile phone, a smart phone, a PDA (for 'Personal Digital Assistant'), a touch pad, or a personal stereo.

"A fourth aspect relates to a computer program product comprising a computer readable medium, having thereon a computer program comprising program instructions. The computer program is loadable into a data-processing unit and adapted to cause the data-processing unit to carry out the method described above when the computer program is run by the data-processing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

"The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements and in which:

"FIG. 1 is a possible data flow for filtering an audio stream in a processing unit and in a mixing signal unit;

"FIG. 2 shows chart examples of different frequency responses of an inductive loudspeaker upon temperature variations;

"FIGS. 3a and 3b present the module and the phase of a possible modelled frequency response for an inductive loudspeaker;

"FIGS. 4a and 4b present the module and the phase of a possible 'adaptive loudspeaker protection' ('ALP') filter;

"FIGS. 5a and 5b present the module and the phase of a possible modelled frequency response for an inductive loudspeaker when the ALP filter is applied to the input audio stream;

"FIGS. 6a, 6b and 6c present respectively the module of a possible frequency response of a loudspeaker when solicited with a white noise (ideal pattern for transfer function estimation), the module of the corresponding compensation filter and the module of the loudspeaker when solicited with a white noise filtered with the compensation filter;

"FIGS. 7a, 7b and 7c present respectively the module of a possible frequency response of a loudspeaker when solicited with a jazz audio stream, the module of the corresponding compensation filter and the module of the loudspeaker when solicited with the jazz audio stream filtered with the compensation filter;

"FIG. 8 is an example of a flow chart illustrating steps of a process to filter dynamically an audio stream;

"FIG. 9 presents a module of a possible second order under-damped filter."

For additional information on this patent application, see: Marguery, Philippe; Nagari, Angelo; Sirito-Olivier, Philippe. Pre-Filtering for Loudspeakers Protection. Filed June 28, 2012 and posted June 5, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=3182&p=64&f=G&l=50&d=PG01&S1=20140529.PD.&OS=PD/20140529&RS=PD/20140529

Keywords for this news article include: St-ericsson Sa, Information Technology, Information and Data Processing.

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Source: Information Technology Newsweekly


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