Patent Grant
9332343
An embodiment of the invention is related to speaker impedance estimation techniques. Other embodiments are also described.
Knowledge of the electrical input impedance of an individual speaker driver can be used to for example predict the operating temperature of the speaker so as to better manage long term reliability of an audio system of which the speaker is an important part. A typical technique for computing speaker driver input impedance senses the input voltage and senses the input current (using a current sense resistor), and then computes their ratio to obtain the impedance.
In portable electronic audio systems that have multiple speakers and multiple amplifiers, which are examples of multichannel audio systems, protecting the battery from temporary but excessive current demands, and meeting a finite power budget in view of the battery's limitations, generally requires controlling the total current that is drawn by the audio subsystem. As a result, there is often a need for a current sense element that can sense the shared or total current used by the audio subsystem.
An embodiment of the invention is a shared current sensing and speaker impedance estimation infrastructure in a multi-channel audio system that uses certain types of test signals to help estimate the individual speaker impedances. A shared current sensing element in the audio system is used to estimate (or compute, using digital signal processing techniques) the electrical input impedance of each speaker, without having to sense the individual speaker current or amplifier output current. This approach may help save significant manufacturing costs, as well as printed circuit board area and power consumption, by essentially removing the individual speaker driver current sensing infrastructure (from each audio channel). By eliminating the individual current sensing requirement (where the amplifier output current or the speaker driver input current would have been sensed), a wider range of audio amplifiers may be considered for the audio subsystem design.
In one embodiment of the invention, the speaker driver input voltage is a known variable, either via direct voltage sensing of the amplifier output node or the speaker driver input node voltage, or by estimating the amplifier output voltage or speaker driver input voltage, in view of the source audio channel test signal and an amplifier model (assuming linearity and the absence of amplifier clipping events). The shared current sense element indicates the total power supply current that feeds two or more amplifiers that are sharing the same power supply rail. Test signals are applied to the amplifier inputs, while the above measurements and calculations are made, in order to compute for example the dc (or, alternatively, very low frequency) electrical input impedance of each of the speaker drivers, without having to sense individual input currents of the speaker drivers.
The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary.
The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. Also, in the interest of conciseness, a given figure may be used to illustrate the features of more than one embodiment of the invention, or more than one species of the invention, and not all elements in the figure may be required for a given embodiment or species.
Several embodiments of the invention with reference to the appended drawings are now explained. While numerous details are set forth, it is understood that some embodiments of the invention may be practiced without these details. In other instances, well-known circuits, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description.
Referring to either
Each of the audio amplifiers is coupled to receive a respective audio channel signal. These may be from an audio source device such as a telephony device or a digital media player device. The N audio channel signals may have been up-mixed from a fewer number of original channels, or they may be a down mix of a greater number of original channels. The audio source device that produces the N audio channel signals may be integrated with the rest of the audio system, for example, as part of a laptop computer. In many instances, the speakers shown in the figures here may be built-in speakers, that is built into the housing of the consumer electronics device, although as an alternative one or more of the speakers may be external or detachable. In yet another embodiment, the audio source device may be in a different housing than the amplifiers and speakers, such that the N audio channel signals are delivered to the amplifier through a wired or wireless audio communication link.
Regardless of the particular implementation, the relevant audio system or audio subsystem may have a data processor (e.g., a programmed microprocessor, digital signal processor or microcontroller) that obtains a measure of input voltage, Vhat1, Vhat2 . . . VhatN for each of the drivers. The data processor computes an estimate of electrical input impedance of each of the speaker drivers, Zhat1, Zhat2 . . . ZhatN, using the sensed shared current (provided by the current sense element) and the measure of input voltage Vhat1, Vhat2 . . . VhatN that is associated with that particular driver, while the amplifiers are being driven by test signals (not shown in
As part of an audio signal processing system, the programmed data processor (see
Once the input voltage measurements Vhat1, Vhat2 . . . VhatN have been obtained, together with the sensed shared current, the programmed data processor can compute the estimates of electrical input impedance Zhat1, Zhat2 . . . ZhatN, where these estimates may represent linear time invariant impedance that varies as a function of frequency. These may computed in real-time, while the audio amplifiers are driving their respective speaker drivers in accordance with their respective audio channel test signals. A real-time measure of the individual speaker input impedances can be calculated without requiring a current sense infrastructure at the individual speaker level.
Referring now to
The equation to be solved for estimating the impedance of each speaker driver has the following general form
where Isharedi is the contribution to the total supply current by amplifier Ai, Vi is the speaker driver input voltage for that amplifier, and Zi, the sole unknown, is the speaker driver input impedance. Ti is a predetermined mathematical expression that relates the output current of the amplifier Ai to its power supply input current Ii, supp. A mathematical expression for Ti can be readily derived using circuit modeling and network analysis techniques that in effect characterize the audio amplifier Ai, so as to relate the audio amplifier output current (or speaker driver input current that is associated with each amplifier) to the amplifier's input supply current Ii, supp. This model may also include temperature dependence where the model changes depending upon the operating temperature of the amplifier.
In one embodiment, each of the audio channel test signals is a test tone that is centered at a different frequency. If desired to be inaudible, the frequency (spectral) content of each test signal may be designed to be below the human audible range. The resulting sensed shared current will contain a number of peaks each of which roughly aligns (in frequency) with a respective one of the test tones, due to the power supply current draw of the respective amplifier. This embodiment is illustrated in
Ishared_1 (produced by the filter bank)=T1*V1/Z1
where T1 is an expression that relates the output current of amplifier A1 to its input supply current (as explained earlier). Note that as a result of the effectively “orthogonal” nature of the test signals, each amplifier is fed its own or “unique” test signal and so there is no need to solve any simultaneous equations. Also, in many cases the speaker driver impedance estimate is of interest in just one or perhaps no more than a few adjacent frequency bins. As a result, the mathematics task of the data processor can be simplified greatly by using for example the Goertzel algorithm to obtain the frequency domain versions of Ishared_i and V1(t), V2(t), . . . , rather than a Discrete Fourier Tranform (DFT). More generally, the impedance estimation process performed by the programmed data processor here may have the following operations: filtering the sensed shared current to produce a number of filtered output signals each being aligned with a respective one of the different frequencies; and computing the estimate of the electrical input impedance of each of the speaker drivers using one of the filtered output signals and the measure of input voltage of the speaker driver that is associated with said one of the filtered output signals.
In another embodiment, each of the audio channel test signals is a unique phase-modulated or phase-encoded test signal. As a result, the sensed shared current will contain a modulation signature, for each modulated test signal, that is due to the power supply current draw of the respective amplifier. This embodiment is illustrated using the example constellation diagram in
Ishared_2 (produced by the demodulator)=T2*V2/Z2
where T2 is an expression that relates the output current of amplifier A2 to its input supply current (as explained earlier). Note that as a result of the effectively “orthogonal” nature of the test signals, each amplifier is fed its own or “unique” phase-encoded test signal and so there is no need to solve any simultaneous equations. The test signals may be generated by the programmed data processor using any suitable phase modulation technique. More generally, the impedance estimation process performed by the programmed data processor here may have the following operations: where each of the audio channel test signals is a unique phase modulated test signal, the sensed shared current is phase demodulated into a number of demodulated output signals; and the estimate of the impedance of each of the speaker drivers is computed using one of the demodulated output signals and the measure of input voltage of the speaker driver that is associated with said one of the demodulated output signals.
In yet another embodiment, the N audio channel test signals contain test content that are in effect time division multiplexed. In other words, when the N test signals are supplied to their respective amplifiers, the amplifiers are driven with test content one at a time. For convenience, the test content may be the same in each signal only shifted in time so that none of them overlaps with another—these are depicted by two examples in
Ishared_3 (produced by the demultiplexer)=T3*V3/Z3
where T3 is an expression that relates the output current of amplifier A3 to its input supply current (as explained earlier), and Ishared_3 and V3 are given by their frequency domain versions. Note that as a result of the effectively “orthogonal” nature of the test signals, each amplifier is fed its own or “unique” test signal and so there is no need to solve any simultaneous equations. More generally, the impedance estimation process performed by the programmed data processor here may have the following operations: where each of the audio channel test signals has test content that is shifted in time (or time-multiplexed) so that none of the test content in the test signals overlaps in time with another test content, the sensed shared current is first demultiplexed (in accordance with the known timing with which the test signals were produced) into a number of for example burst-like output signals; the estimate of the impedance of each of the speaker driver is computed using one of the output signals and the measure of input voltage of the speaker driver that is associated with said one of the pulse output signals. It should be noted here that while the time-division multiplexing technique may be used in place of the frequency-shifting and phase-encoding techniques described earlier, an alternative is to combine it with either the frequency-shifting or phase-encoding techniques so that the test content in either of those cases is applied one at a time (sequentially or randomly) to the amplifiers, which may make it easier to extract the test content from the sensed shared current.
As explained above, an embodiment of the invention may be a machine-readable medium (such as microelectronic memory) having stored thereon instructions, which program one or more data processing components (generically referred to here as a “processor”) to perform the digital audio processing operations described above including arithmetic operations, filtering, mixing, inversion, comparisons, and decision making. In other embodiments, some of these operations might be performed by specific hardware components that contain hardwired logic (e.g., dedicated digital filter blocks). Those operations might alternatively be performed by any combination of programmed data processing components and fixed hardwired circuit components.
While certain embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that the invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. For example, although the description above refers to techniques for estimating individual speaker impedances, this should be understood as also encompassing the alternative but equivalent mathematical construct of computing individual speaker admittances, where admittance is the inverse of impedance and is typically defined as Y=1/Z. The description is thus to be regarded as illustrative instead of limiting.
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