Patent Publication Number: US-8995673-B2

Title: Audio power management system

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. application Ser. No. 12/725,941, filed Mar. 17, 2010, now U.S. Pat. No. 8,194,869, the disclosure of which is incorporated in its entirety by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to audio systems, and more particularly to an audio power management system for use in an audio system. 
     2. Related Art 
     Audio systems typically include an audio source providing audio content in the form of an audio signal, an amplifier to amplify the audio signal, and one or more loudspeakers to convert the amplified audio signal to sound waves. Loudspeakers are typically indicated by a loudspeaker manufacturer as having a nominal impedance value, such as 4 ohms or 8 ohms. In reality, the impedance of a loudspeaker varies with frequency. Variations in loudspeaker impedance with respect to frequency may be shown with a loudspeaker impedance curve, which is typically provided by the manufacturer with a manufactured model of a loudspeaker. 
     A loudspeaker, however, is an electromechanical device that is sensitive to variations in voltage and current, as well as environmental conditions, such as temperature and humidity. In addition, during operation a loudspeaker voice coil may be subject to heating and cooling dependent on the level of amplification of the audio content. Moreover, variations in manufacturing and materials among a particular loudspeaker design may also cause significant deviation in a loudspeaker&#39;s pre-specified parameters. 
     Thus, loudspeaker parameters such as the DC resistance, moving mass, resonance frequency and inductance may vary significantly among the same manufactured model of a loudspeaker, and also may change significantly as operating and environmental conditions change. As such, an impedance curve is created with a large number of relatively uncontrollable variables represented as if all these uncontrollable variables were fixed and non-varying. Accordingly, a manufacturer&#39;s impedance curve for a particular model of a loudspeaker may be significantly different from the actual operational impedance of the loudspeaker. In addition, an acceptable range of variations in the audio signal driving the loudspeaker may also vary based on the loudspeaker parameters of a particular loudspeaker and the operational conditions. 
     SUMMARY 
     An audio power management system may be implemented in an audio system to manage operation of devices such as loudspeakers, amplifiers and audio sources. Management of the devices in the audio system may be based on real-time customization of operational parameters of one or more of the devices in accordance with real-time actual measured parameters, and real-time estimated parameters. 
     Management of the ongoing operation of one or more devices in the audio system may be performed to accomplish both protection of the hardware, and optimization of system performance. Based on real-time estimated and actual operational capabilities of the specific hardware in the system, protective and operational threshold parameters that are developed in real-time specifically for the system hardware may be subject to ongoing adjustment as the system operates. Due to continuing adjustment of the operational and protective parameters, devices may be operated at, above, or below manufacturer specified ratings while minimizing or eliminating possible compromise of the integrity of the hardware, or operational performance of the audio system due to the thresholds being developed in real-time. 
     Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views. 
         FIG. 1  is an example block diagram of a power management system included in an audio system. 
         FIG. 2  is an example of loudspeaker modeling. 
         FIG. 3  is an example block diagram of a parameter computer included in the power management system of  FIG. 1 . 
         FIG. 4  is another example block diagram of the parameter computer included in the power management system of  FIG. 1 . 
         FIG. 5  is another example block diagram of the parameter computer included in the power management system of  FIG. 1 . 
         FIG. 6  is an example block diagram of a voltage threshold comparator included in the power management system of  FIG. 1 . 
         FIG. 7  is an example block diagram of a current threshold comparator included in the power management system of  FIG. 1 . 
         FIG. 8  is an example block diagram of a load power comparator included in the power management system of  FIG. 1 . 
         FIG. 9  is another example block diagram of a load power comparator included in the power management system of  FIG. 1 . 
         FIG. 10  is yet another example block diagram of a load power comparator included in the power management system of  FIG. 1 . 
         FIG. 11  is an example block diagram of a speaker linear excursion comparator included in the power management system of  FIG. 1 . 
         FIG. 12  is an operational flow diagram of the power management system of  FIG. 1 . 
         FIG. 13  is a second part of the operational flow diagram of  FIG. 12 . 
         FIG. 14  is a third part of the operational flow diagram of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is an example block diagram of a audio power management system  100 . The audio power management system  100  may be included in audio system having an audio source  102 , an audio amplifier  104 , and at least one loudspeaker  106 . An audio system that includes the power management system  100  may be operated in any listening space such as a room, a vehicle, or in any other space where an audio system can be operated. The audio system may be any form of multimedia system capable of providing audio content. 
     The audio source  102  may be a source of live sound, such as a singer or a commentator, a media player, such as a compact disc, video disc player, a video system, a radio, a cassette tape player, an audio storage device, a wireless or wireline communication device, a navigation system, a personal computer, or any other functionality or device that may be present in any form of multimedia system. The amplifier  104  may be a voltage amplifier, a current amplifier or any other mechanism or device capable of receiving an audio input signal, increasing a magnitude of the audio input signal, and providing an amplified audio output signal to drive the loudspeaker  106 . The amplifier  104  may also perform any other processing of the audio signal, such as equalization, phase delay and/or filtering. The loudspeaker  106  may be any number of electro-mechanical devices operable to convert audio signals to sound waves. The loudspeakers may be any size contain any number of different sound emitting surfaces or devices, and operate in any range or ranges of frequency. In other examples, the configuration of the audio system may include additional components, such as pre or post equalization capability, a head unit, a navigation unit, an onboard computer, a wireless communication unit, and/or any other audio system related functionality. In addition, in other examples the power management system may be dispersed and/or located in different parts of the audio system, such as following or within the amplifier, at or within the loudspeaker, or at or within the audio source. 
     The example power management system  100  includes a calibration module  110 , a parameter computer  112 , one or more threshold comparators  114 , and a limiter  116 . The power management system  100  may also include a compensation block  118  and a digital to analog converter (DAC)  120 . The power management system  100  may be hardware in the form of electronic circuits and related components, software stored as instructions in a tangible computer readable medium that are executable by a processor, such as digital signal processor, or a combination of hardware and software. The tangible computer readable medium may be any form of data storage device or mechanism such as nonvolatile or volatile memory, ROM, RAM, a hard disk, an optical disk, a magnetic storage media and the like. The tangible computer readable media is not a communication signal capable of electronic transmission. 
     In one example, the power management system  100  may be implemented with a digital signal processor and associated memory, and a signal converter, such as a digital to analog signal converter. In other examples, greater or fewer numbers of blocks may be depicted to provide the functionality described. 
     During operation, a digital signal may be supplied to the power management system  100  on an audio signal line  124 . The digital signal may be representative of a mono signal, a stereo signal, or a multi-channel signal such as a 5, 6, or 7 channel surround audio signal. Alternatively, the audio signal may be supplied as an analog signal to the power management system  100 . The audio signal may vary in current and/or voltage as the audio content varies over a wide range of frequencies that includes 0 Hz to 20 kHz or some range within 0 Hz to 20 kHz. 
     The power management system  100  may operate in the time domain such that time based samples or snapshots of the audio signal are provided to the calibration module  110 . The calibration module  110  may include a voltage calibration module  128  and a current calibration module  130 . The voltage calibration module  128  may receive a voltage signal indicative of a real-time actual voltage V(t) of the audio signal representative of the real-time voltage received at the loudspeaker  106 . The voltage signal may be proportional to the voltage of the audio signal. Due to variations in operational conditions and hardware, such as length and gauge of the wires carrying the audio signal, the real-time actual voltage V(t) is an estimate of the voltage at the loudspeaker  106 . In that regard, although receipt of the real-time actual voltage V(t) of the audio signal by the power management system  100  is illustrated as occurring between the limiter  116  and the amplifier  104 , the estimated voltage of the loudspeaker  106  may be measured at the loudspeaker  106 , at the amplifier  104  or anywhere else where a repeatable representation of the real-time actual voltage V(t) of the audio signal that is capable of being calibrated to be representative of an estimate of the voltage at the loudspeaker  106  may be obtained. 
     In  FIG. 1 , the audio signal is received by the DAC  120 , converted in real-time from a digital signal to an analog signal, and supplied on a real-time actual voltage line  134 . The DAC  120  may be any algorithm and/or circuit capable of converting digital data to analog data. In other examples, the audio signal may be an analog signal, and the DAC  120  may be omitted. The audio signal may be sampled at a predetermined rate such as 44.1 KHz, 48 KHz or 96 KHz. As used herein, the term “real-time” refers to processing and other operations that occur substantially immediately upon receipt of one or more samples or snapshots of the audio signal by the power management system  100  such that the power management system  100  is reactive to processing the continuous flow of audio content being received in the audio signal and generating corresponding outputs responsive to the continuous flow. 
     The current calibration module  130  may similarly receive a current signal indicative of real time actual current I(t) of the audio signal received at the loudspeaker  106 . A current sensor, such as a resistor across the input terminals of the loudspeaker  106 , a Hall effect sensor installed in, on or in nearby vicinity to the loudspeaker  106 , or any other form of sensor capable of providing a signal representative of current of an audio signal being supplied to the loudspeaker  106  may be used to obtain a variable voltage proportional to the real-time current that is representative of an estimate of the current received by the loudspeaker  106 . The real-time actual current I(t) may be supplied to the calibration module  110  on a real-time current supply line  136 . 
     The calibration module  110  may perform conditioning of the measured actual parameter(s). Conditioning may include band limiting the received measured actual parameter, adding latency and/or phase shift to the measure actual parameter, performing noise compensation, adjusting the frequency response, compensating for distortion, and/or scaling the measured actual parameter(s). The conditioned signal representative of current and the conditioned signal representative of voltage may be provided to the parameter computer  112  and one or more of the threshold comparators  114  as real-time signals on a conditioned real-time actual voltage line  138 , and a real-time actual current line  140 , respectively. 
     The parameter computer  112  may develop estimated operational characteristics for hardware contained in the audio system. Estimated operational characteristics may be developed by the parameter computer  112  using measured actual parameters, models, simulations, databases, or any other information or method to recreate operational functionality and parameters of devices in the audio system. 
     For example, the parameter computer  112  may develop an estimated speaker model in real-time for the loudspeaker  106  based on operating conditions of the audio system, such as the one or more conditioned measured actual parameters or one or more measured actual parameters. In one example, the parameter computer  112  may develop an impedance curve in real-time for the loudspeaker  106  at predetermined intervals, such as each time a predetermined number of samples of the one or more measured actual parameters are received. The developed impedance curve may be an estimate of the operational characteristics of the loudspeaker  106 . In another example, the parameter computer  112  may generate estimated operational characteristics, such as DC resistance, moving mass, resonant frequency, inductance or any other speaker parameters associated with a loudspeaker. In still other examples, other forms of operational characteristics may be implemented with the parameter computer  112 , such as fitting to enclosed loudspeaker models, crossover adaption models, or any other form of model representative of loudspeaker behavior. 
       FIG. 2  is an example equivalent circuit model representative of speaker parameters of the loudspeaker  106 . An input voltage (Vin)  202  may be supplied as the driving voltage of the loudspeaker  106 , which is equivalent to the real-time actual voltage V(t). An electrical input impedance of the loudspeaker  106  may be represented with a voice coil resistance (Re)  204  and a voice coil inductance (Le)  206 . The voice coil resistance Re  204  also may be representative of variations in the voice-coil temperature.  FIG. 2  includes an example curve illustrating the correlation between voice coil temperature and the voice coil resistance Re  204 . A motor flux density (Bl)  208  may be representative of the motional electromotive force of the loudspeaker  106 . An input current Iin  210 , which may be equivalent to the real-time actual current I(t) may flow as indicated through the transformer representing the motor of the loudspeaker  106 . 
     A mechanical impedance of the loudspeaker  106  that includes the mass, resistance, and stiffness of a loudspeaker suspension system included in the loudspeaker  106  may be represented with a mechanical inductance Mm  214 , a mechanical resistance Rm  216  and a mechanical compliance Cm  218 . The mechanical compliance Cm  218  may be representative of the stiffness or compliance of the loudspeaker  106 . Thus, the mechanical compliance Cm  218  also may be representative of changes in ambient temperature surrounding the loudspeaker  106 , and/or the temperature of the loudspeaker suspension system.  FIG. 2  includes an example curve illustrating the correlation between ambient temperature and the mechanical compliance Cm  218 . In other examples, other models may be used to model the speaker parameters of a loudspeaker. In addition, other models may be used to model other devices within the audio system. 
     The parameter computer  112  may not only determine the estimated real-time parameters, such as speaker parameters, but also may vary the determined estimated real-time parameters over time as the device, such as the loudspeaker  106  operates and the one of more measured actual parameters vary. As previously discussed, the parameter computer  112  may receive the one or more measured actual parameters in the time domain, however, the solutions representative of the estimated speaker parameters may be generated in the frequency domain. For example, the parameter computer  112  may use a fast Fourier Transform (FFT) to obtain the estimated impedance of the loudspeaker  106  in the frequency domain and solve for various speaker parameters using blocks of the audio signal divided into a predetermined size. In another example, in the time domain the estimate impedance of the loudspeaker may be calculated every predetermined number of samples, such as up to a sample-by-sample basis. Accordingly, as the one or more measured actual parameters vary, the estimated speaker parameters correspondingly may vary. 
       FIG. 3  is an example block diagram of the parameter computer  112  that includes a real-time parameter estimator  302  and a summer  304 . An audio signal is provided from an audio source on the audio source line  124 , which is used to drive the loudspeaker  106 . In this example, the parameter computer  112  receives samples of the real-time actual voltage V(t) of the audio signal (conditioned or unconditioned) on a real-time actual voltage line  306 . If the voltage is received via a digital to analog converter (DAC), the voltage may not be an actual voltage. Rather, the “actual” voltage may be an estimated voltage based on DAC voltage. In addition, the parameter computer  112  receives samples of the real-time actual current I(t) representative of the current received at the loudspeaker  106  (conditioned or unconditioned) on a real-time current line  308 . 
     The real-time parameter estimator  302  may be used in building a digital model of a device, such as the loudspeaker  106  by comparison of the real-time actual current I(t) to an estimated real-time current using the summer  304 . The comparison may occur each time a number of samples are received, on a sample-by-sample basis, or any other period of time that will provide real-time values as outputs. The estimated real-time current may be calculated by the real-time parameter estimator  302  based on the real-time actual voltage V(t). In  FIG. 3 , the estimated real-time current calculated by the real-time parameter estimator  302  may be subtracted from the real-time actual current I(t) to produce an error signal on an error signal line  312 . Alternatively, an estimated real-time voltage may be calculated by the real-time parameter estimator  302  based on the real-time actual current I(t), and compared to the actual real-time voltage to generate the error signal on the error signal line  312 . The real-time parameter estimator  302  may perform the calculations using filters that model the device parameters, such as speaker parameters, to arrive at an estimated real-time voltage or current. 
     In one example, the modeling performed with the real-time parameter estimator  302  may be load impedance based modeling using an adaptive filter algorithm that analyzes the error signal and iteratively adjusts the estimated speaker parameters as needed to minimize the error in real-time. In this example, the real-time parameter estimator  302  may include a content detection module  314 , an adaptive filter module  316 , a first parametric filter  318 , a second parametric filter  320 , and an attenuation module  322 . The real-time actual voltage V(t) of the audio signal may be received by the first parametric filter  318  on a sample-by-sample basis. The real-time actual current I(t) may similarly be received by the summer  304  on a sample-by-sample basis. 
     Accordingly, the adaptive filter module  316  may use the adaptive filter algorithm to analyze the error signal and iteratively and selectively adjust filter parameters in each of first and second parametric filters  318  and  320  to minimize the error. The algorithm executed by the adaptive filter module  316  may be any form of adaptive filtering technique, such as a least mean squares (LMS) algorithm, or a variant of an LMS algorithm. 
     The content detection module  314  may enable operation of the adaptive filter module  316  so that the adaptive filter module  316  does not operate when content included in the audio signal is not within predetermined boundaries. For example, the adaptive filter module  316  may be disabled by the content detection module  314  when only noise is detected in the audio signal so that stability of the adaptive filter module  316  is not compromised. 
     The content detection module  314  may detect an energy level of content included in the audio signal within a predetermined frequency range or bandwidth. The predetermined frequency range may be based on estimated and/or actual operational characteristics the loudspeaker  106 . In one example, the predetermined frequency range may be from about zero hertz to a determined maximum frequency, such as a maximum possible estimated real-time resonance frequency of the loudspeaker  106 . In other examples, the frequency range may be from zero hertz to the manufacturer&#39;s advertised resonance frequency of the loudspeaker  106 . In still other examples, any other range of frequency may be applied as the predetermined frequency range. Detection of the energy level may be based on a predetermined energy level limit, such as a minimum energy level capable of being processed by the adaptive filter module  316 . In one example, the minimum energy level may be a minimum level of RMS voltage present in the audio signal. 
     Once enabled by the content detection module  314  based on the audio signal being within the predetermined boundaries, operation of the adaptive filter module  316  may continually solving to prevent local minimums in order be relatively quick and robust at converging any error between the estimated real-time parameter and the measured actual parameter to a predetermined level of error. The adaptive filter may continually solve during operation of the audio system to minimize error or it may be part of a multiplexed system where the algorithm adapts with some duty cycle. Operation of the adaptive filter module  316  may be seeded with initial values such as the design parameters of the speaker, the last known values from the algorithm, or a computed estimate of the parameters based on information supplied from one or more external sources, such as a reading from an ambient temperature sensor for example. 
     The initial filter values included in the first parametric filter  318 , the second parametric filter  320 , and the attenuation module  304  may be predetermined values previously selected in order to create a model of the loudspeaker  106  that approximates actual real-time operational characteristics of the loudspeaker  106 . The predetermined values may be stored in the respective filters and module, in the adaptive filter module  316 , in the parameter computer  112  or any other data storage location associated with the parameter computer  112 . The predetermined values can be based on testing of a representative loudspeaker  106 , testing of the actual loudspeaker  106  under lab conditions, last known operational values of the first parametric filter  318 , the second parametric filter  320 , and the attenuation module  322  from previous operation of the real-time parameter estimator  302 , a calculation based on an ambient temperature reading, or any other mechanism or procedure to obtain values that will allow the error (or differences) between the actual operational characteristics of the loudspeaker  106  and the estimated operational characteristics of the loudspeaker  106  to quickly converge to about zero or a predetermined acceptable level. However, the real-time parameter estimator  302  may include parameters to control how quickly the estimated operational characteristics are adjusted or evolved as the real-time actual values change. In one example, the estimated speaker parameters may evolve significant slower than the audio signal changes, for example one hundred microseconds to two seconds slower than changes in the audio signal based on sampling the audio signal at a predetermined rate. 
     The first and second parametric filters  318  and  320  may be any form of filter that can be used to represent or model all or some portion of operating parameters of a loudspeaker. In other examples, a single filter may be used to represent or model all or some portion of operating parameters of a loudspeaker. In one example, the first parametric filter  318  may be a parametric notch filter, and the second parametric filter  320  may be a parametric low-pass filter. The parametric notch filter may be populated with changeable filter parameter values, such as a Q, a frequency and a gain, to model loudspeaker admittance near a resonance frequency of the loudspeaker in real-time. The parametric low-pass filter may be populated with changeable filter parameter values, such as a Q, a frequency and a gain, to model loudspeaker admittance in a high frequency range of the loudspeaker. In an alternative example, the second parametric filter  320  may be omitted. Omission of the second parametric filter  320  may be due to the frequency range of the loudspeaker being modeled not needing such characteristics modeled, due to use of constant predetermined filter values to model loudspeaker admittance in a high frequency range of the loudspeaker, use of a constant to model loudspeaker admittance in a high frequency range of the loudspeaker, or any other reason that eliminates the need for the second parametric filter  318 . 
     The attenuation module  322  may be populated with a gain value to model DC admittance of the loudspeaker  106 . The gain value may be varied to account for DC offset in a value of the inductance of the loudspeaker. For example, in a nominally four ohm loudspeaker, the gain value may be about 0.25. Thus, as the real-time actual impedance of the loudspeaker  106  varies during operation, the gain value of the attenuation module  322  may be correspondingly varied in real-time to maintain an accurate estimate of the operational characteristics of the loudspeaker  106 . In one example, the attenuation model  322  may provide modeling of a DC offset in the admittance modeled by the second parametric filter. For example, as the error signal begins to flatten (converge) due to iterative real-time adjustments to the changeable values of the first parametric filter  318  and the second parametric filter  320 , the gain value of the attenuation module  322  may be adjusted by the adaptive filter module  316  to converge the error toward zero. 
     The estimated real-time parameters, such as estimated real-time speaker parameters may be provided on the estimated operational characteristics line  144 . Since the real-time parameter estimator  302  is directly developing the speaker parameters in real-time using parametric filters, curve fitting of filter parameters to obtain the speaker parameters is unnecessary. In addition, due to the continual solving to converge the error signal to substantially zero, if, for example, the actual characteristics of the loudspeaker vary during operation to the point where the resonance frequency has changed iterative adjustment of the changeable values in the first parametric notch filter  318  may occur to move the estimated center frequency included in the estimated operational characteristics to substantially match the actual resonance frequency of the loudspeaker  106 . 
       FIG. 4  is another example block diagram of the parameter computer  112  containing the real-time parameter estimator  302  and the summer  304 . An audio signal may be provided from an audio source on the audio source line  124 , which is used to drive the loudspeaker  106 . Similar to  FIG. 3 , the parameter computer  112  may receive samples of the real-time actual voltage V(t) of the audio signal (conditioned or unconditioned) on a real-time actual voltage line  406 . In addition, the parameter computer  112  may receive samples of the real-time actual current I(t) representative of the current received at the loudspeaker  106  (conditioned or unconditioned) on a real-time current line  408 . Also, the summer  304  may output a real-time error signal on an error signal line  412  representative of differences between the real-time actual current I(t) and a real-time estimated current. In other examples, the real-time error signal may represent the difference between the real-time actual voltage V(t) and a real-time estimate voltage. Due to the many similarities with the example parameter computer  112  of  FIG. 3 , for purposes of brevity, and to avoid repetition, the following discussion will focus mainly on differences between these two examples. 
     In  FIG. 4 , the real-time parameter estimator  302  may include a frequency controller  410 , a filter bank  414 , and a curve fit module  416 . The frequency controller  410  may receive estimated speaker parameters from the parameter computer  112 , such as a real-time estimated resonance frequency of the loudspeaker  106 . Based on the estimated speaker parameters, the frequency controller  410  may provide updated filter parameters to the filter bank  414 . The filter bank  414  may include a plurality of filters such that two filters cooperatively operate at one frequency. The two filters include a first filter for the voltage at that frequency, and a second filter for the current at that frequency. To get an impedance value at the frequency where a respective pair of filters is positioned, the results from the two filters are divided. Accordingly, each of the pairs of filters may provide one impedance value for one frequency, and it is a plurality of impedance values from the plurality of filters that may be populated with updated filter parameters in real-time to reflect an estimated impedance model for the loudspeaker  106 . In one example, each of the filters may be a discrete Fourier transform. In another example, each of the filters may be a Goertzel filter operating at a predetermined frequency. 
     Since each of the filters in the filter bank  414  converges to a different frequency ranging from about 20 Hz to 20 kHz, a speaker operational characteristic in the form of an impedance value for a single frequency may be derived by minimizing the error on the error line  412  at that single frequency. By minimizing the error in each of a plurality of the filters in the filter bank  414 , an estimated speaker impedance curve may be generated in real-time. Specifically, the error signal may be converged by iteratively adapting the filter parameters of the filters to obtain a frequency response curve with a shape substantially similar to a loudspeaker admittance. Following convergence, the curve fit module  416  may be executed to convert the filter parameters, which represent a set of admittance or impedance data points each being at different frequencies, to estimated operational characteristics of the loudspeaker  106  in the form of estimated speaker parameters. The estimated speaker parameters may be provided to the one or more threshold comparators  114  on the estimated operational characteristics line  144 . In addition, any other estimated operational characteristics may be supplied by the speaker parameters computer  112  to the threshold comparators  114  on the estimated operational characteristics line  144 . 
     Since each of the filters are operated at single frequency, there is no need for adaptive filtering as discussed with regard to  FIG. 3 . In addition, the level of computing power needed to converge the error signal is significantly less than the computing power needed with a Fast Fourier Transform (FFT) solution. For example, audio content in the form of a song may be provided on the audio signal line  406 , and one of the filters may ascertain the magnitude of energy in the audio signal at a selected frequency, such as 80 Hz. 
     In one example, the bank of filters included in the filter bank  414  may be distributed in a range of frequencies from about 20 Hz to about 20 kHz at one third octaves to accurately provide a sample of the frequency data. In another example, the filters within the filter bank may be distributed in predetermined locations, such as where the majority of the filters may be strategically positioned in a desired location, such as in the vicinity of the estimated resonance frequency of the loudspeaker  106 , while fewer filters may be distributed across the frequency range to capture the range of frequencies. Since the frequencies upon which the filters in the filter bank operate may be changed by changing the frequency parameter of individual filters in the filterbank  414 , the filters may be arrange within the frequency range so as to be placed at strategic locations useful in building an accurate estimate of the operational characteristics of the loudspeaker  106 . 
     The frequency parameters of individual filters may be changed manually by a user, automatically by the system, or some combination of manual and automatic to obtain desired locations of the filters along a frequency spectrum. For example, a user could group filters and make manual changes to the frequency of all of the filters in the group. Alternatively, the parameters computer  112  may detect an estimated resonance of the loudspeaker, as discussed later, and adjust the filter frequencies accordingly in order to optimize frequency resolution around the estimated resonance. In one example, the frequencies of the filters may be stored predetermined values. In another example, the frequencies may be dynamically updated in real-time by the parameter computer  112  as the estimated and actual operational characteristics, such as the resonance frequency, of the loudspeaker  106  vary during operation. In still another alternative, the parameter computer  112  may provide the frequencies on a predetermined time schedule, and/or in response to a predetermined percentage change in the estimated real-time operational characteristics of the loudspeaker  106 . 
       FIG. 5  is another example block diagram of the parameter computer  112  that includes the real-time parameter estimator  302  and the summer  304 . Similar to the previous examples, an audio signal is provided from an audio source on the audio source line  124 , which is used to drive the loudspeaker  106 . In addition, a real-time actual voltage V(t) (conditioned or unconditioned) is provided to the real-time parameter estimator  302  from the audio signal supplied on a real-time actual voltage line  506 . In addition, the summer  304  may similarly receive a real-time actual current I(t) (conditioned or unconditioned) supplied on a real-time current line  508 . The summer  304  may output an error signal representative of a difference in a measured actual parameter and an estimated real-time parameter in order to adjust an estimated speaker model indicative of estimated real-time operational characteristics of the loudspeaker  106 . The error signal may be output by the summer  304  on an error signal line  512  to the real-time parameter estimator  302 . Since this example is similar in many respects to the previously discussed examples of the power management system  100  and audio system of  FIGS. 3 and 4 , for purposes of brevity such information will not be repeated, rather the discussion will focus on differences from the previously discussed examples. 
     In  FIG. 5 , the real-time parameter estimator  302  includes an adaptive filter module  514 , a non-parametric filter  516 , and a curve fit module  518 . In this example, the adaptive filter module  514  may analyze the error signal and adjust filter parameters in the non-parametric filter  516  in real-time. The non-parametric filter  516  may be a finite impulse response (FIR) filter, or any other form of filter having a finite number of coefficients that is capable of modeling estimated operational characteristics of the loudspeaker  106  of another device in the audio system. By adaptive iteration of the coefficients in the non-parametric filter  516 , the error signal may be minimized in real-time. The rate of adaptation of the non-parametric filter  516  may be controlled by the adaptive filter module  514  so that evolution of the filter coefficients occurs relatively slowly with respect to the number of samples received. For example, iterative adaptation of the filter coefficients may occur in a range of 100 milliseconds to 2 seconds when compared to the rate of change of the audio signal. 
     The filter coefficients may be representative of a real-time estimate of an admittance of the loudspeaker  106  over a range of frequencies, such as from 20 Hz to 20 kHz. From the estimated admittance, estimated speaker parameters such as DC resistance, moving mass, resonance frequency, and inductance of the loudspeaker may be derived in real-time. Since the coefficients developed for the non-parametric filter  516  to estimate the operational characteristics of the loudspeaker  106  are not in a human readable form, the curve fit module  518  may be applied to fit the coefficients to a curve in order to obtain the estimated speaker parameters. Conversion of the filter coefficients to estimated speaker parameters allows use of the speaker parameters within the audio power management system  100 . The speaker parameters may be provided to the one or more threshold comparators  114  on the estimated operational characteristics line  144 . In addition, any other estimated operational characteristics may be supplied by the speaker parameters computer  112  to the threshold comparators  114  on the estimated operational characteristics line  144 . 
     In  FIG. 1 , the threshold comparators  114  may be selectively included in the power management system  100  to provide some form of management of operation of the loudspeaker  106 , the amplifier  104 , the audio source  102 , or any other component in the audio system. Management of operation may entail some form of protection of the loudspeaker  106 , the amplifier  104  and/or the audio source  102  from damage or other operation detrimental to the physical stability of the respective device, or other devices within the audio system. Alternatively, or in addition, management of operation may entail some form of operational control to minimize undesirable operation of the loudspeaker  106 , the amplifier  104  and/or the audio source  102  such as to minimize distortion or unneeded clipping. In addition, overall power consumption by the audio system, or individual components/devices within the audio system, may be minimized by adhering to power consumption targets or limits. 
     The threshold comparators  114  may use estimated parameters, such as speaker parameters developed by the parameter computer  112  along with real-time actual voltages V(t) (conditioned or unconditioned) and/or real-time actual currents I(t) (conditioned or unconditioned) to provide management of operation of the loudspeaker  106  and/or other devices in the audio system. Management of the devices may be based on development and application of one or more thresholds. The thresholds developed and applied by the threshold comparators  114  may be based on any combination of the real-time actual measured values, estimated parameters, limit values, and/or boundaries. In other words, the thresholds may be developed as a result of changing real-time operational characteristics and changing real-time calculation of limits or boundaries of one or more of the devices included in the audio system. 
     The parameter computer  112  may provide the estimated speaker parameters in real-time on the estimated operational characteristics line  144 . In addition, the real-time actual voltage V(t), and/or the real-time actual current I(t) may be provided to the threshold comparators  114  on the real-time actual voltage line  140  and the real-time actual current line  138 . The estimated speaker parameters, and the measured actual parameters may be provided to the threshold comparators  114  on a predetermined schedule, such as on a sample-by-sample basis, iteratively after a predetermined number of samples, or any other period of time that enables real-time calculation and/or application of limit values in order to develop and implement one or more thresholds. Development of the thresholds may include consideration of audio system operational parameter limits and/or audio system protection parameter limits. Accordingly, the audio power management system  100  may provide an equipment protection function, a power conservation function, and an audio sound output control function. 
     In that regard, following determination of threshold audio system operational parameters in real-time, the threshold comparators  114  may monitor on a real-time basis for the measured parameters to cross or reach the respective determined thresholds. Upon detecting in real-time that a respective threshold has been crossed, the respective threshold comparator  114  may independently provide a respective limiting signal to the limiter  116  on a respective limiter signal line  154 . 
     The limiter  116  may be any form of control device capable of adjusting the audio signal being provided on the audio signal line  124 . The limiter  116  may be triggered to adjust the audio signal in response to receipt of one or more limiting signals. As described later, the adjustments to the audio signal may be based on the particular threshold detector providing the limiting signal and/or the nature of the limiting signal being provided. The limiter  116  may operate as a digital device, such as within a digital signal processor. Alternatively or in addition, the limiter  116  may be an analog device and/or composed of electronic circuits and circuitry. Also, alternatively, or in addition, the limiter  116  may control a gain or some other adjustable parameter of the power amplifier  104 , the audio source  102 , or any other component in the audio system in response to receipt of one or more limiting signals. 
     The limiter  116  may also include stored parameters for use with one or more of the limiting signals to adjust the audio signals. Example parameters include an attack time, a release time, a threshold, a ratio, an output signal level, a gain, or any other parameters related to adjusting the audio signal. In one example, different stored parameters may be used by the limiter  116  in limiting the audio signal depending on the limiting signal, and/or the threshold comparator  114  providing the limiting signal. Accordingly, each of the threshold comparators  114  may provide limiting signals that include information identifying the type of limiting signal and/or the one of the threshold comparators  114  from which the limiting signal was produced. For example, the limiter  116  may include input mapping that corresponds to the threshold comparators  114  such that limiting signals received on a particular input are known by the limiter  116  to be from a particular one of the threshold comparators  114  based on the input mapping. In another example, the limiting signals may include an identifier of the respective threshold comparator  114  transmitting the respective limiting signal. In addition, or alternatively, each of the different limiting signals may include an action identifier indicating what action the limiter  116  should take upon receiving a particular type of limiting signal. The action identifier may also include parameters, such as gain values or other parameters to use in limiting or otherwise adjusting the audio signal or a device in the audio system. 
     Operation by the limiter  116  to adjust the audio signal may be performed in real-time based on limiting signals provided from the threshold comparators  114 . The limiter  116  may also operate to adjust the audio signal in real-time in response to limiting signals from two or more different threshold comparators  114 . In one example, such adjustments responsive to different limiting signals from different threshold comparators  114  may be performed at substantially the same time to adjust the audio signal. 
     The compensation block  118  may also optionally be included in the audio power management system  100 . The compensation block  118  may be any circuit or algorithm providing phase delay, time delay, and/or time shifting to allow real-time operation of the limiter  116  without distortion of the audio signal. As described later, the compensation block  118  may also cooperatively operate with the individual threshold comparators  114  to perform different types of compensation of the audio signal dependent on the nature of the limiting signal being provided by a particular threshold comparator  114 . In addition or alternatively, the compensation block  118  may be selectively activated and deactivated based on the limiting signal being provided by a respective threshold comparator  114 . The compensation block  118  may also be selectively adjusted based on estimated operational characteristics of the loudspeaker  106  provided by the parameter computer  112 . 
     In  FIG. 1 , the threshold comparators  114  may include any one or more of a voltage threshold comparator  146 , a current threshold comparator  148 , a load power comparator  150  and a speaker linear excursion comparator  152 . In other examples only one, or any sub-combination, of the above-identified threshold comparators  114  may be included in the audio power management system  100 . In still other examples, additional or alternative threshold comparators, such as a sound pressure level comparator, or any other form of comparator capable of developing a threshold to manage operation of one or more components of the audio system may be included in the audio power management system  100 . 
       FIG. 6  is a block diagram example of a voltage threshold comparator  146 , the limiter  116 , and the compensation block  118 . The voltage threshold comparator  146  may include an equalization module  602  and a voltage threshold detector  604 . The audio signal may be supplied to the compensation block  118  on the audio signal line  124 . In addition, the real-time actual voltage V(t) (conditioned or unconditioned) of the audio signal may be supplied to the equalization module  602  on a real-time actual voltage line  606 . In this example, the compensation block  118  may operate as a phase equalizer to maintain the phase consistently between the sensed voltage signal and the audio signal during operation of the voltage threshold comparator  146  to prevent overshoot in the audio signal due to phase lag in the signals passing through  146 . 
     In  FIG. 6 , the equalization module  602  may operate based on not only the real-time actual voltage V(t), but also based on estimated real-time operational characteristics provided from the parameter computer  112  on the speaker parameters line  144 . In one example, the estimated real-time operational characteristics may be a stored predetermined value. In another example, the estimated real-time operational characteristics may be dynamically updated in real-time by the parameter computer  112  as the estimated and actual operational characteristics of the loudspeaker  106  vary during operation. In still another alternative, the parameter computer  112  may provide the estimated real-time operational characteristics on a predetermined time schedule, and/or in response to a predetermined percentage change in the estimated real-time operational characteristics. 
     The equalization module  602  may include a filter, such as narrow band all pass filter, a peak notch filter, or any other filter capable of modeling the resonance of a loudspeaker. The filter may include adjustable filter parameters, such as a Q, a gain, and a frequency. The filter parameters of the filter may be varied by the equalization module  602  as the estimated real-time operational characteristics such as a real-time estimated resonance frequency, of the loudspeaker  106  varies. Variations in the filter may adjust a magnitude of signal energy in certain frequencies such that at some frequencies the real-time actual voltage V(t) of the audio signal is attenuated, while at other frequencies the real-time actual voltage V(t) is accentuated. The variations in the filter may occur on a sample-by-sample basis, every predetermined number of samples, or at any other time period. 
     The resulting output of the equalization module  602  is a filtered or equalized real-time voltage signal in the frequency domain that has been compensated based on the real-time estimated resonance frequency of the loudspeaker  106 . The filtered real-time actual voltage V(t) may be provided as a compensated real-time voltage signal on a compensated voltage line  606  to the voltage threshold detector  604 . 
     The voltage threshold detector  604  may determine if thresholds are exceeded at any of a predetermined number of frequencies based on the compensated real-time voltage signal. A loudspeaker is capable of handling relatively large magnitudes of voltage in an audio signal near the resonance frequency of the loudspeaker, and has relatively lower voltage magnitude handling capability further away from the resonance frequency. The compensation by the equalization module  602  reflects the varying voltage handling capability of the loudspeaker  106  within the frequencies as the estimated resonance frequency of the loudspeaker  106  changes during operation. 
     The speaker parameter computer  112  may provide a continuous frequency based boundary curve that is provided as a limit for the voltage threshold detector  604  to use in developing the threshold. The boundary curve may initially be a stored curve that may be adjusted in realtime by the parameter computer  112  based on the real-time actual measured values and/or the estimated real-time operational characteristics. The parameter computer  112  may provide the adjusted boundary curve to the voltage threshold detector  604  on a predetermined time schedule, and/or in response to a predetermined percentage change in the boundary curve. Alternatively, the stored boundary curve may be provided to the voltage threshold detector  604  for use by the voltage threshold detector. In addition, or alternatively, the voltage threshold detector  604  may adjust the received boundary curve in real-time based on the received real-time actual voltage V(t), and the estimated real-time operational characteristics. When the voltage threshold detector  604  identifies a signal level of the filtered real-time actual voltage V(t) that exceed the boundary curve the threshold determined by the voltage threshold detector  604  is exceeded. In response, a corresponding limiting signal may be generated by the voltage threshold detector  604  and provided to the limiter  116 . Based on the particular limiting signal provided, the limiter may take a pre-specified action. For example, dependent on the particular limiting signal, the limiter  116  may perform gain reduction or clipping of the audio signal. As such, using the real-time estimated resonance frequency of the loudspeaker  106 , distortion and/or physical damage of the loudspeaker may be minimized. Moreover, efficient operation may be optimized, which optimizes energy efficiency, due to frequency based consideration of the real-time actual voltage V(t) based on an estimated real-time resonance frequency of the loudspeaker  106 . Using this approach, the equalization module  602  can develop and provide a varying, frequency sensitive filtered voltage signal to the voltage threshold detector  604 . 
       FIG. 7  is an example block diagram of the current threshold comparator  148  and the limiter  116 . The real-time actual current I(t) (conditioned or unconditioned) may be supplied to the current threshold comparator  148  on a real-time actual current line  708 . The current threshold comparator  148  may develop a threshold by comparison of the real-time actual current I(t) to an audio system boundary parameter, such as an audio system protection parameter. The audio system boundary parameter may be a stored value of current, which is not dynamically changed during operation of the audio power management system  100 . Alternatively, the audio system boundary parameter may be a changeable boundary value. In one example, the audio system boundary parameter may be a derived estimated real-time parameter, such as an estimated real-time current derived by the parameter computer  112  based on a measured actual parameter, such as the real-time actual voltage V(t) and an estimated real-time impedance of the loudspeaker  106 . The estimated real-time current may be used by the current threshold comparator  148  in developing and applying the threshold. In other examples, the estimated boundary value may be derived by the current threshold comparator  148  from all estimated values, tables, and/or any other means to develop the threshold. 
     The derived estimated real-time parameter, may be provided on the estimate operational characteristics line  144  to the current threshold comparator  148 . In other examples, the threshold audio system parameter may be any other estimated real-time parameter provided from the parameter computer  112 , which may be used by the current threshold comparator  148  to derive a threshold. For example, an estimated real-time voltage and an estimated real-time impedance may be provided to the current threshold comparator  148  by the parameter computer  112  to allow the current threshold comparator  148  to derive an estimated real-time current. In one example, the estimated real-time parameter(s) may be a stored predetermined value. In another example, the estimated real-time parameter(s) may be dynamically updated in real-time by the parameter computer  112  as the estimated and actual operational characteristics of the loudspeaker  106  vary during operation. In still another alternative, the parameter computer  112  may provide the estimated real-time parameter(s) on a predetermined time schedule, and/or in response to a predetermined percentage or degree of change in the estimated real-time parameter(s). 
     During operation, when the threshold is exceeded based on the real-time actual current I(t) (conditioned or unconditioned) of the audio signal, the current threshold comparator  148  may output a limiting signal to the limiter  116 . The limiter  116 , based on the specific limiting signal provided may act to adjust the audio signal. For example, the limiter may act as a voltage limiter to maintain current in the audio signal below the threshold. Since the real-time actual current I(t) is representative of the current flowing in the loudspeaker  106 , operation of the feedback loop represented by the current threshold comparator  148  and the limiter  116  may be fast enough to “catch” a relatively fast rising current in the audio signal prior to causing undesirable operation of the loudspeaker  106 . In this regard, the current threshold comparator  148  may also use previously received real-time actual current I(t) samples to interpolate for future samples. In this way, the current threshold comparator  148  may perform a predictive function and provide limiting signals to the limiter  116  to “head off” undesirable levels of current in the audio signal when the threshold is exceeded. In this way, the current threshold comparator  148  may operate to protect loudspeaker operation, such as a woofer loudspeaker that could be low pass filtered at a predetermined frequency, such as about 200 Hz for example. In addition, protection of the amplifier  104  from over current conditions may be accomplished by holding down the current in the audio signal. 
       FIG. 8  is an example block diagram of the load power comparator  150  that includes an example of the calibration module  110  and an example of the limiter  116 . The load power comparator  150  may include a multiplier  802  and a time averaging module  804  that includes a short average module  806  and a long average module  808 . The calibration module  110  may include the voltage calibration module  128  and the current calibration module  130 . An audio signal provided on the audio signal line  124  may be provided to the limiter  116 . In  FIG. 8  the limiter  116  includes an instantaneous power limiter  810 , a long term power limiter  812  and a short term power limiter  814 . 
     The real-time actual voltage V(t) of the audio signal may be supplied to the voltage calibration module  128  on a real-time actual voltage line  818 . The voltage calibration module  128  may include a voltage gain module (Gv)  824 , a voltage time delay module (T)  826  and a voltage signal conditioner Hv(x)  828 . Each of the voltage gain module  824 , the voltage time delay module  826  and the voltage signal conditioner  828  may include pre-stored predetermined settings to calibrate the real-time actual voltage V(t) signal. The real-time actual voltage V(t) signal may be calibrated with the voltage calibration module  128  by applying a predetermined gain with the voltage gain module  824  to scale the voltage, a delay with the voltage time delay module  826  by applying a time delay or time shift, and correcting for response variations with the voltage signal conditioner  828 . In other examples, the parameters in the voltage gain module  824 , the voltage time delay module  826  and the voltage signal conditioner  828  may be developed and adjusted in real-time by the parameter computer  112 . 
     The real-time actual current I(t) may be supplied to the current calibration module  130  on a real-time actual current line  820 . In  FIG. 8  the current calibration module  130  includes a current gain module  832  and a current signal conditioner (Hi(z))  834 . The real-time actual current I(t) signal may be calibrated with the current calibration module  130  by applying a predetermined gain with the current gain module  832  to scale the current and correct for response variations with the current signal conditioner  834 . In other examples, the parameters in the current gain module  832  and the current signal conditioner  834  may be developed and adjusted in real-time by the parameter computer  112 . In still other examples, one or both of the voltage calibration module  128  and the current calibration module  130  may be omitted. In addition, the voltage calibration module  128  and the current calibration module  130  of  FIG. 8  may be applied to condition the real-time actual voltage V(t) and real-time actual current I(t) for the parameter computer  112  or any other of the threshold comparators  114 . 
     In  FIG. 8 , during operation, the conditioned real-time actual voltage V(t) and the conditioned real-time actual current I(t) may be supplied in real-time to the multiplier  802 . The output of the multiplier  802  may be an instantaneous power value (P(t)=V(t)*I(t)) representative of the power output (P(t)) to the loudspeaker  106  in real-time. In other examples, one or neither of the conditioned real-time actual voltage V(t) and the conditioned real-time actual current I(t) may be supplied to the multiplier  802  along with one or more estimated operational characteristics. 
       FIG. 9  is a block diagram of another example of the of the load power comparator  150  that includes the limiter  116 . The limiter  116  receives the audio signal on the audio signal line  124 . In addition, the load power comparator  150  may receive the real-time actual current I(t) (conditioned or unconditioned) on a real-time current line  908 , and estimated operational characteristics on the parameter computer line  144 . In this example, the estimated operational characteristics may include an estimated speaker parameter in the form of an estimated resistive portion R(t) or real(Z) of a loudspeaker impedance Z(t). In one example, the estimated resistive portion R(t) may be a stored predetermined value. In another example, the estimated resistive portion R(t) may be dynamically updated in real-time by the parameter computer  112  as the estimated and actual operational characteristics of the loudspeaker  106  vary during operation. In still another alternative, the parameter computer  112  may provide the estimated resistive portion R(t) on a predetermined time schedule, and/or in response to a predetermined percentage change in the estimated resistive portion R(t). 
     Changes in the resistive portion R(t) of the loudspeaker are indicative of heating and cooling of the voice coil in the loudspeaker  106 . Increases in the real-time estimated resistance R(t) indicate increasing temperature of the voice coil, and decreasing real-time estimated resistance R(t) indicates decreasing temperature of the voice coil. 
     In  FIG. 9 , the load power comparator  150  includes a square function  902 , the multiplier  802 , and the time averaging module  804 . The square function  902  may receive and square the real-time actual current I(t), and provide the result to the multiplier  802  for multiplication with the estimated real-time impedance R(t) of the loudspeaker  106 . The result of this operation (P(t)=I(t) 2 *R(t)) may be provided to the time averaging module  802  in order to derive an estimated instantaneous power value, an estimated short term power value, and a long term power value. It is to be noted that use of the estimated real-time impedance R(t) and the real-time actual current I(t) may provide increased accuracy when compared to use of actual or estimated real-time voltage V(t) and the real-time actual current I(t) to derive the estimated power since voltage drop considerations are unnecessary when estimated real-time impedance R(t) is used to determine power. The difference in accuracy can be significant if the distance between the location of sampling the real-time actual voltage V(t) and the location of the loudspeaker create voltage drop due to line losses. 
     In  FIGS. 8 and 9 , the load power comparator  150  may use the instantaneous output power (estimated or actual) from the multiplier  802  to develop a long term average power value and a short term average power value as part of the development and application of thresholds related to output power. Development of the long and short term average power values may be based on a predetermined number of samples of the instantaneous output power that are averaged over time. The number of samples, or the period of time over which the samples are averaged may be from 1 millisecond to about 2 seconds for the short term average power values, and may be from about 2 seconds to about 180 seconds for long term average power values. 
     The instantaneous power may be compared against a determined instantaneous power limit value by the load power comparator  150  to determine if the derived instantaneous threshold has been eclipsed. In addition, the short term average power values and the long term average power values may be compared against a determined short term limit value and a determined long term limit value to determine if the derived short term threshold and the derived long term threshold have been surpassed. When a respective developed threshold is exceed based on a respective power value, a respective limiting signal may be generated by the load power comparator  150  and provided to the limiter  116 . The limiting signals may include an identifier indicating the instantaneous power limiter  810 , the short term power limiter  814  or the long term power limiter  812 . Alternatively, the limiting signals may be provided as different inputs to the limiter  116  to identify the signals as being designated for the instantaneous power limiter  810 , the short term power limiter  814  or the long term power limiter  812 . In other examples, any other method may be used to identify the different limiting signals, as previously discussed. 
     The limit values for comparison to the instantaneous, short term and long term power may be stored predetermined values. Alternatively, the limit values may be dynamically updated in real-time based on estimated operational characteristics provided to the load power comparator  150  from the parameter computer  112  on the estimated operational characteristics line  144 . For example, the real-time loudspeaker parameters of the loudspeaker  106  may be used by the load power comparator  150  to derive the limit values as real-time varying values. Alternatively, the limit values may be stored values, or derived in real-time by the parameter computer  112  and provided to the load power computer  150 . In still another alternative, the parameter computer  112  may provide the limit values on a predetermined time schedule, and/or in response to a predetermined percentage change in the limit values. 
     Loudspeakers inherently have thermal time constants regarding the level of heating and cooling, as a function of power input via an audio signal. Since real-time power input to the loudspeaker may be estimated, threshold protection of the loudspeaker from undesirable heating may be avoided. Moreover, threshold protection from such undesirable heating may be achieved, while still allowing maximum operational flexibility due to the real-time or static limit values reflecting the actual acceptable instantaneous, short term, and long term power input ranges for a specific loudspeaker. Use of the real-time actual and estimated parameters to calculate the power and the limit values and determine if the thresholds have been exceeded may account for fluctuations in ambient temperature, variations in manufacturing, and any other factors that affect desirable maximum power thresholds for a specific loudspeaker. 
       FIG. 10  is another example block diagram of the of the load power comparator  150  that includes the limiter  116 . The limiter  116  receives the audio signal on the audio signal line  124 . In addition, the load power comparator  150  may receive estimated operational characteristics on the parameter computer line  144 . In this example, the estimated operational characteristic include an estimated speaker parameter in the form of an estimated resistive portion R(t) or real (Z) of a loudspeaker impedance Z(t). In one example, the estimated resistive portion R(t) may be a stored predetermined value. In another example, the estimated resistive portion R(t) may be dynamically updated in real-time by the parameter computer  112  as the estimated and actual operational characteristics of the loudspeaker  106  vary during operation. In still another alternative, the parameter computer  112  may provide the estimated resistive portion R(t) on a predetermined time schedule, and/or in response to a predetermined percentage change in the estimated resistive portion R(t). Since the load power comparator  150  may operate to develop and apply the thresholds at a relatively slow rate due to calculation of a moving average, the estimated resistive portion R(t) may be sampled at a relatively slow rate. 
     The load power comparator  150  includes a moving average module  1002 . In the case where the estimated resistive portion R(t) is provided on the parameter computer line  144  as a dynamically updated parameter, the moving average module  1002  may receive and average the estimated resistive portion R(t) over a determined time period. Since estimated resistive portion R(t) is indicative of changes in voice coil temperature, deriving a moving averaging of the estimated resistive portion R(t) with the moving average module  1002  may be used to monitor long term heating of the voice coil of the loudspeaker  106 . 
     The moving averaging of the estimated resistive portion R(t) may be compared against one or more boundary values indicative of a desired resistive portion R(t) of the loudspeaker  106  by the load power comparator  150  to determine if a threshold has been eclipsed. When the moving averaging of the estimated resistive portion R(t) exceeds one of the boundaries indicating that the threshold has been crossed, a limiting signal may be generated by the load power comparator  150  and provided to the limiter  116  that is indicative of the threshold being exceeded. Upon receipt of the limiting signal, the limiter  116  may take action to minimize undesirably high temperatures and/or undesirable low temperatures of the voice coil. The boundary value for comparison to the estimated resistive portion R(t) may be a stored predetermined value. Alternatively, the boundary value may be dynamically updated in real-time based on estimated operational characteristics provided to the load power comparator  150  from the parameter computer  112  on the estimated operational characteristics line  144 . For example, the real-time loudspeaker parameters of the loudspeaker  106  may be used by the load power comparator  150  to derive the boundary as a real-time varying value. Alternatively, the boundaries may be a stored value, or derived in real-time by the parameter computer  112  and provided to the load power computer  150  for use in monitoring the thresholds. In still another alternative, the parameter computer  112  may provide the boundaries on a predetermined time schedule, and/or in response to a predetermined percentage change in the boundary values. 
     The limiter  116  may apply attenuation to the audio signal to reduce the magnitude of the audio signal and avoid overheating of the voice coil of the loudspeaker  106 . Alternatively, or in addition, the limiter  116  may apply gain to the audio signal in order to compensate for compression of the audio content in the audio signal. In another alternative a combination of compensation for compression by selectively applying gain to the audio signal, and selectively applying attenuation may be used. For example, when a first threshold is exceeded based on receipt of a corresponding first limiting signal, the limiter  116  may apply gain to the audio signal to compensate for compression. When a second threshold is exceeded and a corresponding second limiting signal is provided indicating that the voice coil temperature is continuing to increase, the limiter  116  may apply attenuation to the audio signal to avoid undesirable levels of temperature in the voice coil of the loudspeaker  106 . 
       FIG. 11  is an example block diagram of the speaker linear excursion comparator  152  that includes the limiter  116  and the compensation block  118  to develop thresholds used in management of loudspeaker voice coil excursions. The compensation block  118  includes a time delay  1102  and a phase equalizer  1104 . The time delay  1102  may provide delay or time shifting of the audio signal to provide additional time for the audio power management system  100  to manage undesirable excursions by the voice coil of the loudspeaker. The phase equalizer  1104  may provide phase compensation as needed to maintain the phase relationship between the audio signal and the real-time actual voltage V(t) within the audio power management system  10 . The real-time actual voltage V(t) (conditioned or unconditioned) of the audio signal may be supplied to the speaker linear excursion comparator  152  on a real-time actual voltage line  1106 . The speaker linear excursion comparator  152  includes a speaker excursion model  1110  and an excursion threshold detector  1112 . 
     The speaker excursion model  1110  receives the real-time actual voltage V(t) and estimated operational characteristics from the parameter computer  112  on the operational characteristics line  144 . In  FIG. 11 , the operational characteristics received by the speaker excursion model  1110  include an estimated mechanical compliance Cm(t) and an estimated voice coil resistance Re(t). The estimated mechanical compliance Cm(t) and the estimated voice coil resistance Re(t) may be used by the speaker excursion model  1110  to derive a real-time electro-mechanical speaker model representative of the loudspeaker  106 . In other examples, additional operational characteristics, such as one or more of the estimated speaker parameters included in  FIG. 2  may also be provided by the parameter computer  112  to the speaker excursion model  1110 . Based on application of the real-time actual voltage V(t) to the real-time electro-mechanical speaker model, the speaker excursion model  1110  may derive a predicted excursion of the voice coil of the loudspeaker  106  in response to the audio signal. 
     The excursion of the voice coil may be predicted based on integration over time of the estimated mechanical velocity of the voice coil in response to the real-time actual voltage V(t). In addition, or alternatively, the speaker excursion model  1110  may use a frequency dependent transfer function, such as a filter, to perform real-time computation of predicted voice coil excursion per volt of the real-time actual voltage V(t). Using the estimated mechanical compliance Cm(t) and the estimated voice coil resistance Re(t), the predicted excursion may account for loudspeaker specific operational characteristics due to variations in production, age, temperature, and other parameters affecting voice coil excursion during real-time operation of the loudspeaker  106 . The predicted excursion may be provided to the excursion threshold detector  1112 . 
     The excursion threshold detector  1112  may compare the predicted excursion to a boundary representative of the maximum desirable excursion of the voice coil to determine if the developed threshold has been exceeded. The boundary may be a predetermined value stored in the excursion threshold detector  1112 . Alternatively, the boundary may be stored in the parameter computer  112  and provided to the excursion threshold detector  1112  on the operational characteristics line  144 , or stored anywhere else in the audio system. In addition or alternatively, the boundary may be dynamically updated in real-time by the parameter computer  112  as the estimated and actual operational characteristics of the loudspeaker  106  vary during operation. In still another alternative, the parameter computer  112  may provide the boundary on a predetermined time schedule, and/or in response to a predetermined percentage change in the boundary. 
     Based on the developed threshold, when the predicted excursion exceeds the boundary, a limiting signal is provided to the limiter  116 . The limiter  116  may apply clipping to the audio signal in the time domain in response to receipt of the limiting signal. In addition, or alternatively, the limiter may apply soft clipping to the audio signal in the time domain in response to receipt of the limiting signal. Soft clipping may be used to smooth the sharp corners of a clipped signal, and reduce high order harmonic content in an effort to minimize undesirable auditory effects associated with clipping an audio signal. In addition, or alternatively, the limiter may reduce the gain of the audio signal, such as in the audio amplifier in response to receipt of the limiting signal. 
     In order for the speaker linear excursion comparator  152  and the limiter  116  to “stay ahead” of undesirable actual excursions of the voice coil in the loudspeaker  106 , the latency of modeling of the speaker excursion model may be minimized. In addition, the time delay block  1102  may be used to provide a look ahead capability that may involve predictive interpolation of future real-time actual voltage V(t) of the audio signal. 
       FIG. 12  is an example operational flow diagram for the audio power management system  100  with reference to  FIGS. 1-11 . At block  1202 , the audio power management system  100  is powered up, and the one or more of the threshold comparators  114  are populated with stored settings. The stored settings may be the last known values from previous operation or predetermined stored values. An audio signal is provided to the power management system  100  on the audio signal line  144  at block  1204 . At block  1206 , the audio signal is sampled to obtain the real-time voltage signal V(t) and the real-time current signal I(t). At block  1208 , the real-time voltage signal V(t) and the real-time current signal I(t) may be calibrated with the calibration module  110  and the operation proceeds to block  1210 . 
     Alternatively, the calibration of the real-time voltage signal V(t) and the real-time current signal I(t) may be omitted and the operation proceeds directly to block  1210 . At block  1210  the parameter computer  112  receives and uses the real-time voltage signal V(t) to derive a real-time estimated current. The real-time estimated current is derived based on estimated operational characteristics, such as the estimated operational characteristics of the loudspeaker  106 . The real-time estimated current is compared to the real-time current signal I(t) at block  1212 . At block  1214 , it is determined if greater than a pre-determined difference (error) exists between the estimated real-time current and the real-time actual current I(t). If yes, the operation adjusts the estimated operational characteristics and returns to block  1210  to recalculate the estimated real-time current based on the adjusted operational characteristics. 
     Referring to  FIG. 13 , if at block  1214 , the difference in real-time estimated current and the real-time actual current I(t) are within an acceptable predetermined range (converge), at block  1216  the estimated operational characteristics, such as the estimate speaker parameters are made available for use as estimated real-time parameters by the threshold comparators  114  in performing threshold development and monitoring. In other examples, such as when a current amplifier is used, the real-time actual current I(t) may be used to derive a real-time estimated voltage, which is compared to the real-time actual voltage V(t). 
     At block  1218  it is determined which of the threshold comparators  114  are operable in the audio power management system  100 . If the voltage threshold comparator  146  is operable in the audio power management system  100 , at block  1222 , the estimated real-time parameters are selectively provided to the voltage threshold comparator  146 . The filter parameters of the voltage threshold comparator  146  are adjusted based on the estimated real-time parameters at block  1224 . At block  1226  the real-time actual voltage V(t) is filtered by the voltage threshold comparator to align the real-time actual voltage V(t) over the range of frequency with the estimated resonance frequency of the loudspeaker  106 . Accordingly, the filtered real-time actual voltage V(t) may be adjusted according to the estimated real-time resonant frequency of the loudspeaker in order to represent the available operational capability of the loudspeaker based on the estimated resonance frequency. 
     At block  1228 , a changeable or static limit value representative of a frequency dependent desired voltage level may be received from the parameter computer  112 , derived by the voltage threshold comparator  146 , and/or retrieved from some other location. The filtered real-time actual voltage V(t) may be compared to the limit value, such as by curve fitting, at block  1230 . It is determined if the filtered real-time actual voltage V(t) exceeds the threshold at block  1232 . If no, the operation returns to block  1222 . If at block  1232  the filtered real-time actual voltage V(t) exceeds the threshold, a limiting signal is provided to the limiter  116  at block  1234 . At block  1236  the limiter adjusts the audio signal, and the operation returns to block  1222 . 
     Returning to block  1220 , if the current threshold comparator  148  is operable in the audio power management system  100 , at block  1240 , the current threshold comparator  148  receives the real-time actual current I(t). In addition, the current threshold comparator  148  may selectively receive the changeable or static boundary value representative of a maximum desired current at a predetermined interval from the parameter computer  112 , selectively derive the maximum desired current, and/or retrieve the maximum desired current from some other storage location. At block  1242 , the current threshold comparator  148  may compare the real-time actual current I(t) to the boundary value. It is determined at block  1244  if the real-time actual current I(t) exceeds the boundary value at block  1244 . If not, the operation returns to block  1240 . If at block  1244 , the real-time actual current I(t) exceeds the threshold, a limiting signal is generated and provided to the limiter  116  at block  1246 . At block  1248  the limiter adjusts the audio signal, and the operation returns to block  1240 . 
     Returning again block  1220 , if the load power comparator  150  is operable in the audio power management system  100 , at block  1252 , the load power comparator  150  receives at least one of the real-time actual current I(t) and real-time actual voltage V(t) (conditioned or unconditioned). In addition or alternatively, the load power comparator  150  may selectively receive estimated real-time parameters such as estimated real-time speaker parameters from the parameter computer  112 . Further, the load power comparator  150  may receive the changeable or static limits representative of desired levels of power at a predetermined interval from the parameter computer  112  or some other storage location or derive the changeable or static limits. At block  1254 , the load power comparator  150  may calculate instantaneous power based on the real-time estimated and/or actual current or voltage. 
     The calculated instantaneous power may be used to update short average power and the long average power values at block  1256 . At block  1258 , the instantaneous, short term and long term calculated power may be compared to respective limits. It is determined if the instantaneous power, the short term power, or the long term power exceeds the respective thresholds at block  1262 . If not, the operation returns to block  1252 . If at block  1262  any or all of the instantaneous power, the short term power, or the long term power exceeds the respective thresholds, the load power comparator  150  generates corresponding limiting signal(s) and provides the corresponding limiting signal(s) to the limiter  116  at block  1264 . At block  1266 , the limiter  116  adjusts the audio signal accordingly based on the received limiting signal(s). 
     Returning again block  1220 , if the speaker linear excursion comparator  152  is operable in the audio power management system  100 , at block  1270 , the speaker linear excursion comparator  152  receives the real-time actual voltage V(t) (conditioned or unconditioned) and estimated real-time parameters such as estimated real-time speaker parameters from the parameter computer  112 . Further, the load power comparator  150  may receive one or more of the changeable or static boundaries representative of desired excursion levels of the voice coil of the loudspeaker  106  from the parameter computer  112  or some other storage location, or derive the changeable or static boundaries. At block  1272 , the estimated excursion is derived by application of the real-time actual voltage V(t) and estimated real-time parameters to the real-time electro-mechanical speaker model. The estimated excursion is compared to the boundaries at block  1274 . At block  1276  it is determined if any of the thresholds have been exceeded. If not, the operation returns to block  1270 . If any of the thresholds have been exceeded at block  1276 , then at block  1278  corresponding limiting signals are generated and provided to the limiter  116 . At block  1280 , the limiter  116  adjusts the audio signal according to the respective limiting signals received. 
     As previously described, the audio power management system  100  provides management of loudspeakers, amplifiers, audio sources and any other components in an audio system. By using real-time measured actual parameters, the audio power management system  100  may customize management of the various components in the audio system. In the case of protective management, the audio power management system  100  may develop and adjust various protective thresholds for individual devices in real-time to allow maximum operational capability of the respective devices while still maintaining operational parameters, such as the audio signal within limits that would otherwise have undesirable detrimental effects on the hardware of the audio system. In the case of operational management, the audio power management system may optimize power consumption, performance, and functionality by adjusting operational thresholds for individual devices in real-time to minimize distortion, clipping and other undesirable anomalies that may otherwise occur. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.