Patent Application
20140369529
The subject matter disclosed herein relates to audio amplifier devices. More specifically, the subject matter disclosed herein relates to an audio amplifier that is configured as a power converter having a reference that is modulated based on an audio signal and that directly drives a speaker with a differential audio output signal.
The subject matter disclosed herein is illustrated by way of example and not by limitation in the accompanying figures in which like reference numerals indicate similar elements and in which:
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.
Modulator and drive controller 301 comprises a digital signal processor (DSP) 321 that is coupled to a high-voltage (HV) logic and drive circuit 322. DSP 321 receives an audio source input 304, such as a pulse-width modulated (PWD) audio signal, and outputs drive signals 323 and 324 to HV logic and drive circuit 322. HV logic and drive circuit 322 conditions and converts signals 323 and 324 in a well-known manner from low-voltage signals to high-voltage signals 305 and 306 that are capable of driving modulated boost regulators 302 and 303. In some exemplary embodiments, the HV logic portion and the drive portion of HV logic and drive circuit 322 may comprise a separate functional blocks.
Additionally, in one exemplary embodiment, DSP functional block 321 may comprise general-purpose microprocessor control functions. In some exemplary embodiments, the general control functions for DSP 321 may comprise in a separate functional block.
Modulated boost regulator 302 comprises inductor 325, a switching Field Effect Transistor (FET) 326 and an active pass device 327. Power supply VPS, which is filtered in a well-known manner by capacitor 307, is coupled to one terminal of inductor 325. The other terminal of inductor 325 is coupled to the drain terminal of FET 326 and to the source terminal of active pass device 327. The gate terminal of FET 326 is coupled to HV drive signal 305a. The source terminal of FET 326 is coupled to system common or ground (i.e., a return path for the VPS power supply). The gate of active pass device 327 is coupled to HV drive signal 305b, and the drain terminal of active pass device 327 is coupled to one terminal of filter capacitor 309.
In one exemplary embodiment, HV drive signal 305a comprises a PWM signal corresponding to audio source input 304 as processed by DSP 321. HV drive signal 305a has also been conditioned and scaled in voltage to be capable of driving FET 326 between on and off states. As FET 326 is driven between on and off states, inductor 325 generates a stepped-up voltage from input power supply VPS to a desired output voltage level for driving speaker 308. Additionally, as FET 306 is driven between on and off states, FET 326 operates as a Class-D amplifier for audio input signal 304. Active pass device 327, which is configured to operate as a diode, in combination with capacitor 309 low pass filters the output of ringing inductor 325 and FET 326. HV drive signal 305b controls the operation of active pass device 327. Signals 305a and 305b are essentially in phase electrically, except for the addition of non-overlapping timing to avoid both FET 326 and FET 327 being on at the same time.
Modulated boost regulator 303 is similar to modulated boost regulator 302 and comprises a ringing inductor 328, a switching FET 329 and an active pass device 330. Power supply VPS is coupled to one terminal of inductor 328. The other terminal of inductor 328 is coupled to the drain terminal of FET 329 and to the source terminal of active pass device 330. The gate terminal of FET 329 is coupled to HV drive signal 306a. The source terminal of FET 329 is coupled to system common or ground (i.e., a return path for the VPS power supply). The gate of active pass device 330 is coupled to HV drive signal 306b, and the drain terminal of active pass device 330 is coupled to one terminal of filter capacitor 310.
In one exemplary embodiment, HV drive signal 306a comprises a PWM signal corresponding to audio source input 304 as processed by DSP 321. HV drive signal 306a has also been scaled in voltage to be capable of driving FET 329 between on and off states. As FET 329 is driven between on and off states, inductor 328 generates a stepped-up voltage from input power supply VPS to a desired output voltage level for driving speaker 308. Similar to FET 326, as FET 329 is driven between on and off states; FET 329 operates as a Class-D amplifier for audio input signal 304. Active pass device 330, which is configured to operate as a diode, in combination with capacitor 310 low pass filters the output of ringing inductor 328 and FET 329. HV drive signal 306b controls the operation of active pass device 327.
HV drive signals 305 and 306 are modulated so that they are complements of each other and so that the signals appearing on capacitors 309 and 310 are compliments of each other. That is, drive signals 305 and 306 produce a differential output signal at the outputs of first and second modulated boost regulators 302 and 303 that is coupled to speaker 308. In one exemplary embodiment, HV drive signals are modulated using a pulse-width modulation (PWM) technique. In another exemplary embodiment, HV drive signals 305 and 306 are modulated using another modulation technique, such as, but not limited to, a pulse-density modulation (PDM) technique. In another exemplary embodiment, the HV signals 305 and 306 may 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 signals 205a and 305b—and likewise between signals 306a and 306b to 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 regulators 202 and 203 (302 and 303) are characterized as “boost” regulators because for the exemplary embodiments depicted in
Referring again to
The second feedback network comprises a resistor 336, a resistor 337, a switch 338, and a sampling capacitor 339. Resistors 336 and 337 form a resistor divider network that appropriately scale the second feedback signal. Switch 338, under control from timing and control 343, passes the scaled feedback signal 340 to capacitor 339, which is input to a second input to MUX 341.
A microphone 311, which is placed in proximity to speaker 308, provides an acoustic feedback signal 342 by sampling the acoustic backwave of speaker 308. In an alternative exemplary embodiment, microphone 311 could be configured to sample the acoustic frontwave of speaker 308. Acoustic feedback signal 342 is input to a third input to MUX 341. In one exemplary embodiment, microphone 311 is placed in proximity to speaker 308, such as within the enclosure for speaker 308, to sense the actual speaker acoustic output. The acoustic feedback signal is then fed back through MUX 341 and DSP 321 to improve the sound quality of speaker 308. That is, DSP 321 applies corrections to drive signals 305 and 306 that account for the speaker enclosure cabinet impulse response, the impulse response of microphone 311, the spectral and dynamic speaker errors, or any desired spatialization or equalization, or a combination thereof. Acoustic feedback signal 342 can be calibrated to ensure that the signal is a faithful representation of the frontwave acoustic signal radiated from the front of speaker 308. 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 microphone 311, and a characterization of the frequency response of the backwave enclosure so that sources of error between the frontwave of speaker 308 and the signal picked up by microphone 321 are characterized and calibrated.
The specific feedback circuit details may vary depending on the specific system requirements for getting the signals at the drains of FET 327 and FET 330 fed back to the system DSP (or microprocessor). In some exemplary embodiments, the signals output from FETs 327 and 330 may need to be scaled (such as by, for example, resistors 331, 332, 336 and 337.) In some exemplary embodiments, MUX 341 may or may not be used. In some exemplary embodiments, a Nyquist analog-to-digital (ADC) may be used and samplers 333 and 338 may 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 control 343 outputs a sampling signal 344 and a MUX selection signal 345. Sampling signal 344 controls the timing of the sampling of feedback signals 335 and 340. MUX selection signal 345 controls which input to MUX 341 is passed through to Analog-to-Digital Converter (ADC) 346. ADC 346 generates a digital signal representation of the selected feedback signal in a well-known manner, which is input to DSP 321. DSP processes the various feedback signals in combination with audio source input 304 in a well-known manner to generate drive signals 323 and 324 to HV logic and drive circuit 322. In one exemplary embodiment, DSP 321 determines 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 DSP 321 is 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, DSP 321 provides auto-calibration functionality, such as, but not limited to, Common Mode Rejection Ratio (CMRR) calibration.
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.
