AN AMPLIFIER CIRCUIT FOR A PARAMETRIC TRANSDUCER AND A RELATED AUDIO DEVICE

Abstract

An amplifier circuit for a parametric transducer, comprising: a signal processor for processing an input signal into first and second signals; and at least a pair of output stages arranged to respectively receive the first and second signals for generating amplified first and second signals respectively, which are provided to operate the parametric transducer. The input, first and second signals are arranged with a substantially similar frequency to cause a switching frequency of the amplifier circuit to be matched to a carrier frequency of the parametric transducer. A related audio device is also disclosed.

Claims

1. An amplifier circuit for a parametric transducer, comprising: (i) a signal processor for processing an input signal into first and second signals; (ii) at least a pair of output stages arranged to respectively receive the first and second signals for generating amplified first and second signals respectively, which are provided to operate the parametric transducer, wherein the input, first and second signals are arranged with a substantially similar frequency to cause a switching frequency of the amplifier circuit to be matched to a carrier frequency of the parametric transducer.

2. The amplifier circuit according to claim 1, wherein the signal processor includes a signal separator.

3. The amplifier circuit according to claim 1, further comprising a resistive circuit element, and an inductive circuit element serially coupled to respective outputs of the pair of output stages.

4. The amplifier circuit according to claim 1, wherein the signal processor is further configured to receive the input signal.

5. The amplifier circuit according to claim 1, wherein the first and second signals are further generated with different phase-shifts.

6. The amplifier circuit according to claim 1, wherein the carrier frequency is 40 kHz.

7. The amplifier circuit according to claim 1, wherein the pair of output stages include being coupled to the parametric transducer.

8. The amplifier circuit according to claim 1, further including at least a pair of comparators arranged to respectively receive the first and second signals for generating corresponding first and second switching signals, wherein the first and second switching signals are provided to the output stages, and the amplified first and second signals are respectively amplified first and second switching signals.

9. The amplifier circuit according to claim 1, wherein the amplifier circuit is a Class D amplifier, and the output stages are Class D output stages.

10. An audio device comprising: (i) a signal processor for processing an input signal into first and second signals with different phase-shifts; (ii) at least a pair of parametric transducers; and (iii) at least two pairs of output stages arranged to respectively receive the first and second signals for generating two pairs of amplified signals, which are respectively provided to the parametric transducers for generating corresponding ultrasonic signals with the phase-shifts, wherein the signal processor, and output stages collectively form an amplifier circuit; wherein the input, first and second signals are arranged with a substantially similar frequency to cause a switching frequency of the amplifier circuit to be matched to carrier frequencies of the parametric transducers; and wherein the parametric transducers are arranged to direct the ultrasonic signals towards a common area in the air to cause interaction of the ultrasonic signals within the area to obtain an audible audio signal.

11. The device according to claim 10, wherein the signal processor includes a signal separator.

12. The device according to claim 10, further comprising a resistive circuit element, and an inductive circuit element coupled to outputs of the respective pairs of output stages.

13. The device according to claim 10, wherein the signal processor is further configured to receive the input signal.

14. The device according to claim 10, wherein the carrier frequency is 40 kHz.

15. The device according to claim 10, wherein each parametric transducer includes a bimorph transducer or at least one ultrasonic transducer.

16. The device according to claim 15, wherein the at least one ultrasonic transducer includes a plurality of ultrasonic transducers.

17. The device according to claim 10, further including at least a pair of comparators arranged to respectively receive the first and second signals for generating corresponding first and second switching signals, wherein the first and second switching signals are respectively provided to the two pairs of output stages, and the two pairs of amplified signals are amplified switching signals.

18. The device according to claim 10, wherein the amplifier circuit is a Class D amplifier, and the output stages are Class D output stages.

19-31. (canceled)

32. The device according to claim 11, further comprising a resistive circuit element, and an inductive circuit element coupled to outputs of the respective pairs of output stages.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Embodiments of the invention are disclosed hereinafter with reference to the accompanying drawings, in which:

[0051] FIG. 1 is a schematic diagram of a parametric loudspeaker system, according to the prior art;

[0052] FIG. 2 includes FIGS. 2a and 2b, which respectively is a schematic diagram of a plurality of ultrasonic transducers arranged to form a parametric emitter, and a is front view of said arrangement of FIG. 2a, according to the prior art;

[0053] FIG. 3 is a schematic diagram of a conventional Class D amplifier (CDA), according to the prior art;

[0054] FIG. 4 is a schematic diagram of a Class D amplifier circuit for a parametric transducer, based on a first embodiment;

[0055] FIG. 5 depicts various waveforms generated at different stages of the amplifier circuit of FIG. 4;

[0056] FIG. 6 is a schematic diagram of an audio device, based on a second embodiment;

[0057] FIG. 7 includes FIGS. 7a to 7c, which respectively is a schematic diagram of a pair of plurality of ultrasonic transducers which form a pair of parametric emitters of the audio device of FIG. 6, a front view of the arrangement of FIG. 7a, and an illustration of an arrangement of the pair of parametric emitters to cooperatively create an audio zone in the air;

[0058] FIG. 8 is a schematic diagram of a further audio device, based on a third embodiment; and

[0059] FIG. 9 illustrates how an audible audio signal generated by the audio device of FIG. 8 is heard only by a user of the audio device, and not by bystanders in the vicinity of the user.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0060] FIG. 4 shows schematics of a Class D amplifier circuit 400 for a parametric transducer 402, based on a first embodiment. It is to be appreciated that the term parametric transducer 402 is used throughout the description here to refer to a parametric emitter/loudspeaker, and hence not to be construed as limiting. Specifically, the amplifier circuit 400 comprises: a signal processor 404 (realised 1in the form of a signal separator in this case) for processing an input signal (V.sub.in) into first and second signals (V.sub.1, V.sub.2); at least a pair of (first and second) comparators 406a, 406b arranged to respectively receive the first and second signals (V.sub.1, V.sub.2) for generating corresponding (digital-like) first and second switching signals (V.sub.p1, V.sub.p2); and at least a pair of similar (first and second) Class D output stages 408a, 408b arranged to respectively receive the first and second switching signals (V.sub.p1, V.sub.p2) for generating amplified first and second switching signals (V.sub.out1, V.sub.out2) respectively, which are provided to operate the parametric transducer 402 for obtaining an audible audio signal. It is to be appreciated that any type of suitable Class D output stage may be used for the first and second Class D output stages 408a, 408b. The input signal (V.sub.in), which is an ultrasonic signal, is an electrical equivalent corresponding to the audible audio signal. An inductor 410 (L.sub.s) and a resistor 412 (R.sub.s), coupled to respective outputs of the two Class D output stages 408a, 408b, function similarly as per the case of conventional Class D amplifiers (as described afore in FIG. 3), and thus are also optional in this embodiment as well.

[0061] The electrical connections between the various components of the amplifier circuit 400 are now described. The signal processor 404 is configured to receive the input signal (V.sub.in), and generate the first and second signals (V.sub.1, V.sub.2) respectively at first and second output terminals. The first output terminal is electrically coupled to a positive input terminal of the first comparator 406a, while the second output terminal is electrically coupled to a negative input terminal of the second comparator 406b. On the other hand, a negative input terminal of the first comparator 406a and a positive input terminal of the second comparator 406b are coupled to an AC ground of the amplifier circuit 400. Respective output terminals of the first and second comparators 406a, 406b are electrically coupled to respective input terminals of the first and second Class D output stages 408a, 408b. An output terminal of the first Class D output stage 408a is serially coupled to the resistor 412 (R.sub.s), which in turn couples to a first input terminal of the parametric transducer 402. On other hand, an output terminal of the second Class D output stage 408b is serially coupled to the inductor 410 (L.sub.s), which then couples to a second input terminal of the parametric transducer 402.

[0062] It is to be appreciated that the magnitude and frequency of the first and second signals (V.sub.1, V.sub.2) are approximately the same as the input signal (V.sub.in), except that there is a slight phase-shift imparted. It is to be appreciated that the first and second signals (V.sub.1, V.sub.2) have different phase-shifts with respect to the input signal (V.sub.in), and are respectively expressed in accordance with equations (3)-(5) to be described below. Specifically, the input (V.sub.in), first and second signals (V.sub.1, V.sub.2) are arranged with a substantially similar frequency to cause a switching frequency of the amplifier circuit 400 to be matched to a carrier frequency of the parametric transducer 402. Preferably, the carrier frequency is arranged to be 40 kHz, but is not to be construed as limiting, since the amplifier circuit 400 is dynamically configurable to match the carrier frequency of any parametric transducer intended to be used with the amplifier circuit 400. To be elaborated later, this beneficially has an advantage of lowering EMI emitted by, and also significantly improving the power-efficiency of the amplifier circuit 400, achieved through reducing power dissipation of the amplifier circuit 400. This is to be contrasted with conventional Class D amplifiers, which are instead configured with switching frequencies significantly higher than carrier frequencies of parametric transducers used therewith.

[0063] The operation principle of the amplifier circuit 400 is set out theoretically with supporting equations below. In parametric-loudspeaker applications, the input of the amplifier circuit 400 may be expressed as:

V.sub.in=A.sub.0 cos(ω.sub.Ct+φ(t))   (1)

wherein A.sub.0 is the envelop signal and represents the input audio signal, cos(ω.sub.Ct+φ(t)) is the carrier signal of the parametric transducer 402, ω.sub.C is the carrier frequency in rad/s, and φ(t) is the phase-shift of the carrier signal.

[0064] Using straightforward trigonometry, the input signal (V.sub.in) may be expressed as a sum of the first and second signals (V.sub.1, V.sub.2) having equal magnitude V.sub.1, and V.sub.2, specifically set out as:

V.sub.in=V.sub.1+V.sub.2   (2)

and further wherein:

[00001]$\begin{array}{cc}{V}_1=V_0.Math.\cos \left({\omega }_C.Math.t+\varphi \left(t\right)+\theta \left(t\right)\right)& \left(3\right)\\ V_2=V_0.Math.\cos \left({\omega }_C.Math.t+\varphi \left(t\right)-\theta \left(t\right)\right)& \left(4\right)\\ \theta \left(t\right)={\cos }^{-1}\left(\frac{A_0}{2.Math.V_0}\right)& \left(5\right)\end{array}$

wherein V.sub.0 is the amplitude of the two separated signals, and θ(t) is the phase shift.

[0065] As mentioned, the first and second signals (V.sub.1, V.sub.2) are converted to the first and second switching signals (V.sub.p1, V.sub.p2) by comparison against the AC ground of the amplifier circuit 400. For reference, the associated waveforms 500 generated at different stages of the proposed amplifier circuit 400 are shown in FIG. 5. It is to be noted that the frequency of the first and second signals (V.sub.1, V.sub.2), as generated, is approximately the same as the input signal (V.sub.in) and hence the switching frequency of the amplifier circuit 400 is approximately the same as the carrier frequency of the parametric transducer 402. The higher order harmonics of the input signal (V.sub.in) are filtered by the inductor 410 and resistor 412, and the desired signal is recovered at the output of the pair of Class D output stages 408a, 408b, prior to being provided to the parametric transducer 402. Of interest, it may be seen from FIG. 5 that the voltage (V.sub.o1-V.sub.o2) across the parametric transducer 402 is fairly similar to the input signal (V.sub.in) (i.e. being the desired output signal also to be obtained at the parametric transducer 402). It is to be appreciated that the slight difference between V.sub.o1-V.sub.o2 and the input signal (V.sub.in) is due to the limited attenuation of the higher order harmonics of the input signal (V.sub.in), but is inconsequential due to its high frequency (e.g. about 3 times that of the carrier frequency).

[0066] Consequently, the significantly reduced switching frequency of the amplifier circuit 400 (i.e. reduced by about 10 times compared to the Class D amplifier 300 of FIG. 3) results in significantly reduced “wasted power” of the amplifier circuit 400, and hence substantially improves the power-efficiency of the amplifier circuit 400 from 80% to about 95%. So, due to the low power usage, the amplifier circuit 400 may be envisaged for suitable use in any mobile applications, where power resource is constrained. It is to be appreciated that the hardware of the proposed amplifier circuit 400 is uncompromised, i.e. the hardware is comparable to the Class D amplifier 300 of FIG. 3. To clarify the preceding statement, it is appreciated an IC area required for implementing the proposed Class D amplifier 400 is comparable to the conventional amplifier 300. This is because the Class D output stages 408a, 408b tend to occupy a large (i.e. more than 80%) portion of the total IC area. So since the output stages 408a, 408b of the proposed Class D amplifier 400 is largely same as the conventional amplifier 300, the corresponding IC areas of both amplifiers are thus of about the same size.

[0067] Besides the improved power-efficiency, EMI emission of the amplifier circuit 400 is also significantly lower due to its lower switching frequency. As a comparison, it is to be highlighted that due to the high supply voltage requirement for parametric-loudspeaker applications, the EMI emission of the conventional Class D amplifier 300 of FIG. 3 can be rather high, and so may undesirably affect the operating performance and functionality of other electronic circuits positioned in the vicinity.

[0068] The remaining configurations will be described hereinafter. For the sake of brevity, description of like elements, functionalities and operations that are common between the different configurations are not repeated; reference will instead be made to similar parts of the relevant configuration(s).

[0069] According to a second embodiment, there is disclosed an audio device 600 (as shown in FIG. 6), which is largely based on the first embodiment. An object of the audio device 600 is to enable improvement in the general directivity of parametric loudspeakers, coupled with lower power dissipation, without increasing a physical size of the parametric loudspeakers or a carrier frequency thereof.

[0070] The audio device 600 may be incorporated into smart devices, and comprises: the signal processor 404 (of FIG. 4) for processing an input signal (V.sub.in) into first and second signals (V.sub.1, V.sub.2) having different phase-shifts, in which the input signal (V.sub.in) is an electrical equivalent of an audible audio signal; the first and second comparators 406a, 406b (of FIG. 4) arranged to respectively receive the first and second signals (V.sub.1, V.sub.2) for generating corresponding (digital-like) first and second switching signals (V .sub.1, V.sub.p2); at least a pair of (first and second) parametric transducers 602a, 602b; and at least two similar (first and second) pairs of Class D output stages 604a, 604b arranged to respectively receive the first and second switching signals (V.sub.p1, V.sub.p2) for generating two pairs of amplified switching signals (V.sub.out1, −V.sub.out1 and V.sub.out 2, −V.sub.out2), which are respectively provided to the parametric transducers 602a, 602b for generating corresponding ultrasonic signals having the different phase-shifts. The first pair of Class D output stages 604a includes first and second output stages 604a2, 604a4, while the second pair of Class D output stages 604b includes third and fourth output stages 604b2, 604b4. Each output stage 604a2, 604a4, 604b2, 604b4 is similar to any of the Class D output stages 408a, 408b of the first embodiment. Also, each parametric transducer 602a, 602b comprises an arrangement of equal plurality of ultrasonic transducers 700 (i.e. see FIGS. 7a and 7b), and each corresponding pair of amplified switching signals (V.sub.out1, −V.sub.out1, and V.sub.out2, −V.sub.out2) is provided to each parametric transducer 602a, 602b. In other cases, each parametric transducer 602a, 602b may also be formed from other suitable transducers, as necessary, such as a bimorph transducer.

[0071] Further, the two ultrasonic signals generated have a same magnitude and frequency. More specifically, the frequency of the two ultrasonic signals is the same as the input signal, and the magnitude of the two ultrasonic signal is the same to each other, but however may be different to the input signal (V.sub.in). It is to be appreciated that each ultrasonic signal on its own is not decipherable sensibly, and simply perceived as noise, if heard. The parametric transducers 602a, 602b are cooperatively arranged to direct the generated ultrasonic signals towards a common area/zone in the air to cause acoustic interaction between the ultrasonic signals within the area to obtain the audible audio signal. Specifically, the interaction involves demodulating and acoustically combining the two ultrasonic signals (with different phase-shifts) in the air to obtain a resultant ultrasonic signal, which is an amplitude-modulated ultrasonic signal. The amplitude-modulated ultrasonic signal demodulates in the air to provide the desired audible audio signal. The common area in the air where the audible audio signal is obtained may be termed as a private audio zone 750 (i.e. see FIG. 7c). The audible audio signal can only be heard by a listener positioned within the private audio zone 750 and not anywhere else, thus meaning that any other listeners outside of the private audio zone 750 are unable to hear the audio signal simply by listening to the two ultrasonic signals separately.

[0072] Moreover, it is to be appreciated that the signal processor 404, the comparators 406a, 406b, and the output stages 604a, 604b together form an amplifier circuit 606. From observing FIG. 4 and FIG. 6, it will be seen that the amplifier circuit 606 of FIG. 6 is in fact analogous to the configuration of the amplifier circuit 400 of FIG. 4, and thus the audio device 600 also enjoys all the advantages as set forth in the first embodiment. That is, the first pair of Class D output stages 604a collectively corresponds to the first Class D output stage 408a of FIG. 4, whereas the second pair of Class D output stages 604b collectively corresponds to the second Class D output stage 408b of FIG. 4. Similarly, the input (V.sub.in), first and second signals (V.sub.1, V.sub.2), with reference to the amplifier circuit 606, are arranged with a substantially similar frequency to cause a switching frequency of the amplifier circuit 606 to be matched to carrier frequencies of the parametric transducers 602a, 602b. It is to be appreciated that the carrier frequencies of the parametric transducers 602a, 602b are similar. Accordingly, the above described electrical connections between the different components of the amplifier circuit 400 of FIG. 4 therefore also apply, mutatis mutandis, to the components of the audio device 600, and hence will not be repeated for brevity. Further, the equations (1)-(5) may also be applied accordingly in the same manner to describe the amplifier circuit 606 of FIG. 6.

[0073] It will be understood by now that unlike conventional parametric loudspeakers (e.g. see FIG. 3) where only one parametric transducer is employed, the proposed audio device 600 includes at least two parametric transducers 602a, 602b, in which each parametric transducer 602a, 602b is electrically driven by an individual Class D amplifier. Comparing to the first embodiment, where the first and second signals (V.sub.1, V.sub.2) are electrically combined in the proposed amplifier circuit 400, the current set of first and second signals (V.sub.1, V.sub.2) in the second embodiment are instead acoustically combined in the air, as explained above. It is also to be highlighted that although at least two parametric transducers 602a, 602b are employed for this embodiment, a total number of ultrasonic transducers 700 used is however the same as the parametric loudspeaker system 100 depicted in FIGS. 1 and 2. In other words, an overall combined size of the two parametric transducers 602a, 602b is the same as the size of the single parametric loudspeaker 108 of FIG. 1.

[0074] It is also to be appreciated that if any additional parametric transducers are to be added to the configuration set out in FIG. 6, then an accompanying comparator together with an associated pair of Class D output stages need accordingly be required for each additional parametric transducer included in the new configuration. Needless to say, the signal processor 404 is configured to adaptively process the input signal (V.sub.in) into a corresponding number of secondary signals, depending on a number of comparators arranged in the new configuration. But the remaining operation of the audio device 600 will remain the same as above described.

[0075] Based on a third embodiment, in FIG.8, there is disclosed another audio device 800 comprising: a parametric transducer 802 arranged to generate a first audio signal based on a first signal; and a low frequency transducer 804 arranged to generate a second audio signal based on a second signal, wherein the first audio signal has a frequency higher than a predetermined frequency (e.g. 800 Hz), and the second audio signal has a frequency lower than the predetermined frequency, and wherein the parametric and low frequency transducers 802, 804 are integrally formed to have a common central axis. The first signal has the same frequency as the first audio signal, and the second signal correspondingly has the same frequency as the second audio signal. The parametric and low frequency transducers 802, 804 are shown separately in FIG. 8 simply for ease of illustration and explanation, and hence not to be construed as such. So, after generation, the propagating first and second audio signals moving outwardly away from the audio device 800 share a common central axis. It is to be appreciated that the first audio signal is generated as a directional audio beam, while the second audio signal is generated as an omnidirectional audio signal. More specifically, the audio device 800 is a loudspeaker in this case. The parametric transducer 802 may include a bimorph transducer, or at least one ultrasonic transducer (which can also be a plurality of ultrasonic transducers). The low frequency transducer 804 may be any conventional low frequency loudspeaker, such as a woofer.

[0076] The audio device 800 also includes a signal processor component 806, which collectively comprises a low pass filter 808, a high pass filter 810, a signal processor 812, a carrier generator 814, and first and second power amplifiers 816a, 816b. The signal processor component 806 is arranged to receive an input signal 818 (which is the electrical equivalent of an audible audio signal to be played by the parametric and low frequency transducers 802, 804) for processing into the first and second signals. First, the input signal 818 is separated into two signals of different frequency bands: a low frequency band signal (e.g. lower than 800 Hz) and a high frequency band signal (e.g. greater than 800 Hz) by the low pass and high pass filters 808, 810 respectively. The low frequency band signal is then amplified by the first power amplifier 816a into the second signal and provided to the low frequency transducer 804 for generating the second audio signal. On the other hand, the high frequency band signal is provided to the signal processor 812, which is modulated with a carrier signal from the carrier generator 814 to produce a modulated high frequency band signal. This modulated high frequency band signal is consequently amplified by the second power amplifier 816b into the first signal and provided to the parametric transducer 802 for generating the first audio signal. It is also to be appreciated that in certain variant embodiments, the signal processor component 806 may not be included as part of the audio device 800.

[0077] FIG. 9 is an example illustration 900 of how the audible audio signal generated by the audio device 800 can only be heard by a user 902 of the audio device 800 (included in a smart device), and not by bystanders 904a, 904b in the vicinity of the user 902. The user 902 is positioned to directly face the audio device 800, so that the first audio signal is directionally beamed and transmitted to him. It may be seen that the user 902 hears the complete audio spectrum signals (including both the low frequency components (i.e. the low frequency components of vowels) and the high frequency components (i.e. the high frequency components of the vowels and full spectrum of the consonants)), whilst the bystanders 904a, 904b only hear the low frequency components of vowels. As a result, the intelligibility of the audible audio signal received by the bystanders 904a, 904b is significantly impaired, and hence the audio privacy of the user 902 is largely preserved and achieved. In other words, the user 902 has a private audio zone, as a result of using the audio device 800.

[0078] Hence, the audio device 800 (of FIG. 8) is configured to use the low frequency transducer 804 to generate most of the low frequency components of vowels (encompassing the first formant of speech), and use the parametric transducer 802 to generate higher frequency components of the vowels and the full spectrum of consonants (and fricatives). In this manner, bystanders besides a listener (to whom the audible audio signal is intended for) may only hear audio sounds below the said predetermined frequency (i.e. the low frequency components of vowels but not the consonants), while the intelligibility of the audible audio signal heard by the bystanders is significantly impaired/unintelligible. In this respect, it is well-established in the field of phonetics that if the low frequency (i.e. most vowels are of low frequencies), or high frequency (i.e. most consonants are of high frequencies) components of an audio signal are removed, the speech intelligibility is significantly impaired. Accordingly, a user of a smart device, which incorporates the proposed audio device 800 of FIG. 8, who is positioned in the private audio zone, would then hear the full speech spectrum sounds of the audible audio signal (i.e. intelligible audio). So, usage of the audio device 800 helps to beneficially improve audio privacy for users of personal smart devices, such as smart phones or tablet-computers (tablets). Use of earphones with the smart devices is then no longer necessary with deployment of the proposed audio device 800.

[0079] In all, the foregoing disclosed embodiments have advantages in improving the perception of added loudness (including increased low frequency and higher intelligibility) and significantly reducing power dissipation of the amplifier circuit 400 and audio devices 6000, 800 compared to conventional parametric loudspeakers, whilst also enabling a user to retain his own audio privacy when listening to audio output from his personal smart device.

[0080] Based on a fourth embodiment, there is disclosed a speaker apparatus (not shown) comprising a plurality of the audio device 800 of the third embodiment, wherein the apparatus has a substantially uniform thickness, and the thickness is substantially similar to a printed film, or other printed medium. So the speaker apparatus may also be termed a thin film speaker. Importantly, speaker apparatus may inconspicuously and cheaply be incorporated in wallpapers for rooms, due to its relatively thin thickness, to realise part of the functionalities for a smart home, but it will nonetheless be understood that other suitable envisaged applications for the speaker apparatus area are also broadly possible.

[0081] It is to be appreciated that the thickness of a printed film may generally range from anywhere between less than 1 μm to several mm (as will be understood by a skilled person), but in this case, a thickness of the thin film speaker is purposefully devised to be less than 1 mm. More specifically, the thickness of the thin film speaker may broadly be about 20 μm to 200 μm.

[0082] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary, and not restrictive; the invention is not limited to the disclosed embodiments. For example, the proposed amplifier circuit 400 may be fabricated using CMOS processes, or any other suitable non-CMOS fabrication processes (e.g. III-V integration to further enhance power efficiency) as well. Also, the proposed amplifier circuit 400 may also be configured as other types of suitable amplifiers such as a linear amplifier, instead of being a Class D amplifier. Accordingly, other suitable output stages may then be used in place of the first and second Class D output stages 408a, 408b. Further, the first and second comparators 406a, 406b are considered optional (i.e. may be omitted), since they are not required, if the signal processor 404 is implemented using a Digital Signal Processor, or the like. For good order, it is to be appreciated that the preceding statement also applies, mutatis mutandis, to the audio device 600 of FIG. 6. Moreover, it is to be appreciated that the input signal (V.sub.in) may be processed by the signal processor 404 into more than two signals (e.g. five signals), and as a result, a number of required output stages 408a, 408b (and also comparators 406a, 406b, if not deemed optional in that new configuration) needs to be increased accordingly (e.g. five output stages are required if the input signal (V.sub.in) is divided into 5 secondary signals).

[0083] For the audio device 800 of FIG. 8, the parametric and low frequency transducers 802, 804 need not always be integrally formed to have a common central axis. Indeed, the parametric and low frequency transducers 802, 804 may be arranged (and positioned) on different respective axes, since the collective position of said transducers 802, 804 is not very critical due to the low frequency transducer 804 not being configured to emit a directional audio signal.