A device and method are disclosed for modulating a power converter based on an audio signal to directly drive a speaker with a differential audio output signal. A first modulation signal and a second modulation signal are generated based on an input audio signal so that the first and second modulation signals are complementary signals to each other. In one embodiment, a feedback signal, such as an acoustic feedback signal from the speaker, is also used to generate the first and second modulation signals. A power supply voltage is modulated with the first modulation signal to generate a first voltage signal. The power supply voltage is also modulated with the second modulation signal to generate a second voltage signal. The first and second voltage signals form a differential audio signal that is used to drive the speaker. Alternatively, the power converter can drive a speaker with a single-ended output signal.

DETAILED DESCRIPTION

As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not to be construed as necessarily preferred or advantageous over other embodiments. Additionally, it will be appreciated that for simplicity and/or clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for illustrative clarity. Further, in some figures only one or two of a plurality of similar elements indicated by reference characters for illustrative c larity of the figure, whereas all of the similar element may not be indicated by reference characters. Further still, it should be understood that although some portions of components and/or elements of the subject matter disclosed herein have been omitted from the figures for illustrative clarity, good engineering, construction and assembly practices are intended.

Embodiments of the subject matter disclosed herein relates to an audio amplifier configured as a power converter having a reference that is modulated by an incoming audio signal and to directly drive a speaker with a differential audio output signal. Much of the cost of an audio amplifier product is associated with generating the power supply voltages for the amplifier output and embodiments of the subject matter herein provide a lower cost approach for generating an audio signal that can drive a speaker. Embodiments of the subject matter disclosed herein convert input power, which can be either AC power or DC power, into an amplified-audio signal that is delivered to speakers. High fidelity is provided by utilizing negative feedback to compare the signals delivered to the speakers (or alternatively an acoustic feedback from those speakers) to an incoming audio source signal.

In one exemplary embodiment, the subject matter disclosed herein is configured as a DC/DC power converter having a reference that is modulated by an audio signal and to directly drive a speaker with a differential audio output signal. In an alternative exemplary embodiment, the subject matter disclosed herein is configured as an AC/DC converter having a reference that is modulated by an audio signal and to directly drive a speaker with a differential audio output signal. Accordingly, because the subject matter disclosed herein directly modulates a power converter with an audio signal, the costs associated with an audio amplifier comprising a separate power supply and a separate Class-D amplifier are reduced. As a benefit, the subject matter disclosed herein instantaneously generates only the supply voltage that is needed as the output audio signal. Thus, the subject matter disclosed wherein provides an audio amplifier in which a listener is more or less listening to the power supply of the audio amplifier. An additional benefit is that this architecture is considerably more efficient than a conventional Class-D amplifier because it avoids losses from both power-supply generation and the switching amplifier. In yet another exemplary embodiment, a Switched-Mode Power Supply (SMPS) converts input power, which can be either AC power or DC power, to an audio signal instead of a DC output voltage. An audio input signal, which modulates the SMPS to provide an amplified-audio signal for driving speakers, is used in place of a DC reference signal.

FIG. 1depicts a block diagram of an exemplary embodiment of a switched-mode audio amplifier100that provides an audio output that is generated by directly modulating the input power supply according to the subject matter disclosed herein. A signal, which could be digital or analog, from an audio source that is to be amplified, is applied to a Digital Signal Processing (DSP) and Analog Processing101. One or more outputs from DSP and Analog Processing101are output to a High Voltage (HV) Driver102. One or more signals output from HV Driver102are input to a Switched-Mode Power Supply (SMPS)103. SMPS103receives power from an AC or a DC input. If the input power is AC, the AC power could be any voltage and/or frequency, such as 110 or 220V, 60 or 50 Hz commercial power. In an alternative exemplary embodiment, DSP and Analog Processing101are configured to drive SMPS103directly. In one exemplary embodiment, SMPS103outputs differential amplified-audio signals to speakers104aand104b. In an alternative exemplary embodiment, SMPS103outputs single-ended audio signals to speakers104aand104b. An acoustic pickup105is used to provide one or more feedback signals to DSP and Analog Processing101. Alternatively, one or more feedback signals could be generated directly from the amplified-audio signal driving speakers104aand104b. It should be understood that although two audio channels are shown, SMPS103could be configured to provide only a single differential amplified-audio signal.

FIG. 2depicts a functional block diagram of one exemplary embodiment of a switched-mode audio amplifier200that provides an audio output that is generated by directly modulating the input power supply according to the subject matter disclosed herein. Switched-mode audio amplifier200comprises a modulator and drive controller201, a first modulated boost regulator202and a second modulated boost regulator203. Modulator and drive controller201receives an audio source input204, such as a pulse-width modulated (PWD) audio signal, and outputs complementary drive signals205and206to modulated boost regulators202and203. Modulated boost regulators202and203are each coupled to a power supply voltage VPS, which is filtered in a well-known manner by a capacitor207. Because of the inherent differential nature of the design, variations that may occur in the value of VPSare first-order rejected by virtue of the CMRR (Common-Mode Rejection Ratio) of the design. Modulated boost regulators202and203operate as single-ended DC/DC converters that are respectively directly modulated by complementary drive signals205and206to generate two Class-D outputs. There are other ways that are possible to produce the differential drive to a speaker208in addition to modulating each side single ended. For example, it would also be possible to generate the signal driving speaker208differentially at the output of a single modulator and control the common-mode aspect of those signals to ensure that each of the two speaker inputs (i.e., the modulator outputs) stays above some desired minimum operating voltage. For example, the minimum voltage might be 0V, or perhaps 2V. The Class-D output of modulated boost regulators202and203are respectively filtered in a well-known manner by capacitors209and210and differentially coupled to speaker208. A microphone211located in proximity to speaker208provides an input for an acoustic feedback signal, which is coupled to modulator and drive controller201to close the feedback loop. In one exemplary embodiment, electrical feedback from the speaker inputs is used instead of acoustic feedback. An advantage of the acoustic feedback is that it would reduce the notoriously-poor linearity of speakers that can easily reach 2% non-linearity. Additional feedback signals can also be coupled from the differential outputs to modulator and drive controller201. Moreover, it is possible to employ acoustic and electrical feedback in yet another exemplary embodiment of the subject matter disclosed herein.

FIG. 3depicts a more detailed functional block diagram of an exemplary embodiment of a switched-mode audio amplifier300according to the subject matter disclosed herein. Switched-mode audio amplifier300comprises a modulator and drive controller301, a first modulated boost regulator302and a second modulated boost regulator303.

Modulator and drive controller301comprises a digital signal processor (DSP)321that is coupled to a high-voltage (HV) logic and drive circuit322. DSP321receives an audio source input304, such as a pulse-width modulated (PWD) audio signal, and outputs drive signals323and324to HV logic and drive circuit322. HV logic and drive circuit322conditions and converts signals323and324in a well-known manner from low-voltage signals to high-voltage signals305and306that are capable of driving modulated boost regulators302and303. In some exemplary embodiments, the HV logic portion and the drive portion of HV logic and drive circuit322may comprise a separate functional blocks.

Additionally, in one exemplary embodiment, DSP functional block321may comprise general-purpose microprocessor control functions. In some exemplary embodiments, the general control functions for DSP321may comprise in a separate functional block.

Modulated boost regulator302comprises inductor325, a switching Field Effect Transistor (FET)326and an active pass device327. Power supply VPS, which is filtered in a well-known manner by capacitor307, is coupled to one terminal of inductor325. The other terminal of inductor325is coupled to the drain terminal of FET326and to the source terminal of active pass device327. The gate terminal of FET326is coupled to HV drive signal305a. The source terminal of FET326is coupled to system common or ground (i.e., a return path for the VPSpower supply). The gate of active pass device327is coupled to HV drive signal305b, and the drain terminal of active pass device327is coupled to one terminal of filter capacitor309.

In one exemplary embodiment, HV drive signal305acomprises a PWM signal corresponding to audio source input304as processed by DSP321. HV drive signal305ahas also been conditioned and scaled in voltage to be capable of driving FET326between on and off states. As FET326is driven between on and off states, inductor325generates a stepped-up voltage from input power supply VPSto a desired output voltage level for driving speaker308. Additionally, as FET306is driven between on and off states, FET326operates as a Class-D amplifier for audio input signal304. Active pass device327, which is configured to operate as a diode, in combination with capacitor309low pass filters the output of ringing inductor325and FET326. HV drive signal305bcontrols the operation of active pass device327. Signals305aand305bare essentially in phase electrically, except for the addition of non-overlapping timing to avoid both FET326and FET327being on at the same time.

Modulated boost regulator303is similar to modulated boost regulator302and comprises a ringing inductor328, a switching FET329and an active pass device330. Power supply VPSis coupled to one terminal of inductor328. The other terminal of inductor328is coupled to the drain terminal of FET329and to the source terminal of active pass device330. The gate terminal of FET329is coupled to HV drive signal306a. The source terminal of FET329is coupled to system common or ground (i.e., a return path for the VPSpower supply). The gate of active pass device330is coupled to HV drive signal306b, and the drain terminal of active pass device330is coupled to one terminal of filter capacitor310.

In one exemplary embodiment, HV drive signal306acomprises a PWM signal corresponding to audio source input304as processed by DSP321. HV drive signal306ahas also been scaled in voltage to be capable of driving FET329between on and off states. As FET329is driven between on and off states, inductor328generates a stepped-up voltage from input power supply VPSto a desired output voltage level for driving speaker308. Similar to FET326, as FET329is driven between on and off states; FET329operates as a Class-D amplifier for audio input signal304. Active pass device330, which is configured to operate as a diode, in combination with capacitor310low pass filters the output of ringing inductor328and FET329. HV drive signal306bcontrols the operation of active pass device327.

HV drive signals305and306are modulated so that they are complements of each other and so that the signals appearing on capacitors309and310are compliments of each other. That is, drive signals305and306produce a differential output signal at the outputs of first and second modulated boost regulators302and303that is coupled to speaker308. In one exemplary embodiment, HV drive signals are modulated using a pulse-width modulation (PWM) technique. In another exemplary embodiment, HV drive signals305and306are modulated using another modulation technique, such as, but not limited to, a pulse-density modulation (PDM) technique. In another exemplary embodiment, the HV signals305and306may be modulated using a hybrid technique, such as using PDM at low signal levels and PWM at higher signal levels. Generally, it is important to include a non-overlapping timing between signals205aand305b—and likewise between signals306aand306bto ensure that the two aspects of the boost regulator (charging up the input inductor) and outputting current into the output capacitor are not attempted at the same time.

Modulated boost regulators202and203(302and303) are characterized as “boost” regulators because for the exemplary embodiments depicted inFIGS. 2 and 3, the output signal driving speaker208(308) is greater than the fixed input voltage VPS. The subject matter disclosed herein is not so limited and the modulators may be of a different type. For example, in an embodiment in which the output signal driving speaker208,308is less than the fixed input voltage VPS, modulated regulators202and203(302and303) would be configured to be “buck” regulators.FIG. 4depicts an exemplary embodiment of a switched-mode audio amplifier400in which modulated regulators402and403comprise modulated buck regulators. In an embodiment in which the output signal driving speaker208,308can be greater than, equal to, and less than fixed input voltage VPS, modulated regulators would be single-ended primary-inductor converter (SEPIC) type regulators.FIG. 5depicts an exemplary embodiment of a switched-mode audio amplifier500in which modulated regulators502and503comprise modulated SEPIC-type regulators. Additionally, it should be understood that one exemplary embodiment of the subject matter disclosed herein comprises a single modulated regulator formed from any of the like-kind modulated regulators depicted inFIGS. 1-5that outputs a differential output that drives speaker208(308). It should also be understood that the subject matter disclosed herein can alternatively be configured as an AC/DC converter having a reference that is modulated by an audio signal and to directly drive a speaker with a differential audio output signal.

Referring again toFIG. 3, in one exemplary embodiment, a first feedback network is coupled to the output of modulated boost regulator302, and a second feedback network is coupled to the output of modulated boost regulator303. The first feedback network comprises a resistor331, a resistor332, a switch333, and a sampling capacitor334. Resistors331and332form a resistor divider network that appropriately scale the first feedback signal for subsequent processing. Switch333, under control from timing and control342, passes the scaled feedback signal335to capacitor334, which is input to a first input to a multiplexer (MUX)341.

The second feedback network comprises a resistor336, a resistor337, a switch338, and a sampling capacitor339. Resistors336and337form a resistor divider network that appropriately scale the second feedback signal. Switch338, under control from timing and control343, passes the scaled feedback signal340to capacitor339, which is input to a second input to MUX341.

A microphone311, which is placed in proximity to speaker308, provides an acoustic feedback signal342by sampling the acoustic backwave of speaker308. In an alternative exemplary embodiment, microphone311could be configured to sample the acoustic frontwave of speaker308. Acoustic feedback signal342is input to a third input to MUX341. In one exemplary embodiment, microphone311is placed in proximity to speaker308, such as within the enclosure for speaker308, to sense the actual speaker acoustic output. The acoustic feedback signal is then fed back through MUX341and DSP321to improve the sound quality of speaker308. That is, DSP321applies corrections to drive signals305and306that account for the speaker enclosure cabinet impulse response, the impulse response of microphone311, the spectral and dynamic speaker errors, or any desired spatialization or equalization, or a combination thereof. Acoustic feedback signal342can be calibrated to ensure that the signal is a faithful representation of the frontwave acoustic signal radiated from the front of speaker308. In one exemplary embodiment, such calibration can comprise, but is not limited to, a characterization of the relationship between the frontwave and the backwave signals, a characterization of microphone311, and a characterization of the frequency response of the backwave enclosure so that sources of error between the frontwave of speaker308and the signal picked up by microphone321are characterized and calibrated.

The specific feedback circuit details may vary depending on the specific system requirements for getting the signals at the drains of FET327and FET330fed back to the system DSP (or microprocessor). In some exemplary embodiments, the signals output from FETs327and330may need to be scaled (such as by, for example, resistors331,332,336and337.) In some exemplary embodiments, MUX341may or may not be used. In some exemplary embodiments, a Nyquist analog-to-digital (ADC) may be used and samplers333and338may not be used. The important concept here is that the signals at the outputs of the modulated regulators can be fed back to the amplifier control and used as negative feedback.

Timing and control343outputs a sampling signal344and a MUX selection signal345. Sampling signal344controls the timing of the sampling of feedback signals335and340. MUX selection signal345controls which input to MUX341is passed through to Analog-to-Digital Converter (ADC)346. ADC346generates a digital signal representation of the selected feedback signal in a well-known manner, which is input to DSP321. DSP processes the various feedback signals in combination with audio source input304in a well-known manner to generate drive signals323and324to HV logic and drive circuit322. In one exemplary embodiment, DSP321determines a difference signal between the incoming digital audio and the digitized feedback signal (or signals). The difference signal is then applied to a loop filter function that provides gain and noise shaping in a well-known manner. In one exemplary embodiment, the output of DSP321is a gained-up and filtered error signal that is converted in a well-known manner into a PWM or PDM signal. In another exemplary embodiment, DSP321provides auto-calibration functionality, such as, but not limited to, Common Mode Rejection Ratio (CMRR) calibration.

FIG. 6depicts a flow diagram for one exemplary process600for generating a differential audio output by directly modulating a power converter by an audio signal according to the subject matter disclosed herein. At601, a first modulation signal and a second modulation signal are generated based on an input audio signal. According to the subject matter disclosed herein, the first and second modulation signals are complementary to each other in order to produce a differential audio output signal. In one exemplary embodiment, the first and second modulation signals are generated based on the input audio signal and at least one feedback signal. In one exemplary embodiment, the at least one feedback signal comprises an acoustic feedback signal of a speaker coupled to the differential audio signal. In another exemplary embodiment, the at least one feedback signal includes a first feedback signal corresponding to the first stepped-up voltage signal and a second feedback signal corresponding to the second stepped-up voltage signal. In yet another exemplary embodiment, the first and second modulation signals comprise pulse-width modulated (PWM) signals or pulse-density modulated (PDM) signals, or a combination thereof. At602, a power supply voltage is modulated with the first modulation signal to generate a first stepped-up voltage signal. At the same time at603, the power supply voltage is modulated with the second modulation signal to generate a second stepped-up voltage signal. At604, the first and second stepped-up output signals are output as a differential audio output signal based on the complementary first and second modulation signals. Although the process disclosed inFIG. 6relates to a stepped-up power converter, the subject matter disclosed herein is not so limited and can relate alternatively to a step-down power converter or a SEPIC power converter.

Although the foregoing disclosed subject matter has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced that are within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the subject matter disclosed herein is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.