JP2008118248A - D-class amplifier drive method, d-class amplifier drive circuit, electrostatic transducer, ultrasonic speaker, display device, and directional acoustic system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of driving a D-class amplifier capable of reducing spurious noise generated in PWM modulation and capable of reducing audible sound noise generated from an ultrasonic speaker, and to provide a drive circuit of the D-class amplifier. <P>SOLUTION: The method is used for driving a D-class amplifier. The D-class amplifier has at least a set of circuits composed of totem pole type output circuits in each of which a high-side switching element connected to a first power supply is connected to a low-side switching element connected to a second power supply or the ground and a low-pass filter connected to the output end side of the output circuit. In this case, a PWM signal in which an input signal is subjected to PWM modulation turns on or off each switching element in the output circuit for power amplification. When the input signal includes a carrier at a prescribed frequency, the basic frequency of the PWM signal is set to be an integral multiple of the carrier frequency. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a driving method and circuit configuration of a class D (digital) amplifier. In particular, it is effective when driving a class D amplifier with a modulated wave obtained by modulating a carrier wave of a predetermined frequency with a baseband signal (for example, an acoustic signal in an audible band), and the modulated wave is sharpened by outputting it from an ultrasonic transducer. The present invention relates to a driving method of a class D amplifier suitable for driving an ultrasonic speaker that reproduces sound having directivity, and a driving circuit of the class D amplifier. Furthermore, the present invention relates to an electrostatic transducer driven by a class D amplifier, an ultrasonic speaker, a display device including the ultrasonic speaker, and a directional acoustic system.

  The ultrasonic speaker can reproduce a sound having sharp directivity by outputting a modulated wave obtained by modulating a carrier wave in an ultrasonic band with an acoustic signal in an audible band. In order to produce sharp directivity, it is necessary to output a carrier wave in the ultrasonic band with a large amplitude, and thus an ultrasonic speaker generally requires a large input power. Further, a piezoelectric or electrostatic transducer is generally used as a transducer (transmitter) used in an ultrasonic speaker. Since these transducers are capacitive loads, unlike ordinary loudspeakers, the impedance decreases as the drive frequency increases, and a larger input power is required. Therefore, when the ultrasonic speaker is driven by an analog power amplifier, an amplifier having a large output is required, and there is a problem that the apparatus becomes large.

On the other hand, class D amplifiers that perform switching operation of output stage transistors are also widely used in audio power amplifiers (see, for example, Patent Document 1).
The class D amplifier is characterized in that a power MOSFET having a low on-resistance is used as an output stage element, and the loss in the output stage element can be reduced by switching the power MOSFET. As described above, since the loss in the output stage element is small compared with the analog amplifier, the class D amplifier can omit or reduce the size of the radiator that is essential in the analog power amplifier. Therefore, a small and high output amplifier can be realized. For this reason, class D amplifiers are increasingly used in in-vehicle amplifiers, portable terminal amplifiers, AV amplifiers with a large number of output channels, and the like that require miniaturization and low loss. Thus, since the class D amplifier is more efficient than the analog power amplifier, the size of the power amplifier can be reduced by driving the ultrasonic speaker with the class D amplifier.

  The class D amplifier switches and drives the class D output stage by a high frequency digital signal obtained by PWM modulation (pulse width modulation) of the input signal (baseband signal). On the other hand, an ultrasonic speaker reproduces sound having sharp directivity by outputting a modulated wave obtained by modulating a carrier wave in an ultrasonic band with an acoustic signal in an audible band. In order to obtain a sharp directivity, it is necessary to output a carrier wave in an ultrasonic band (large amplitude) having a large energy. Hereinafter, specific problems that occur when an ultrasonic speaker is driven by a class D amplifier will be described.

Here, noise generated when PWM modulation is further performed in order to output a signal subjected to amplitude modulation by the SSB-WC (Single Side Band with Carrier) method from the class D amplifier will be described. Here, a 60 kHz carrier wave (sine wave) is used, and the PWM modulation frequency is 625 kHz. An example of the PWM modulation waveform and the output waveform after passing through the LPF (low pass filter) at this time is shown in FIG. 6A, and an example of the frequency spectrum of the output waveform after passing through the LPF is shown in FIG. 6B.
In FIG. 6, it can be seen that there is a lot of spurious noise in addition to the carrier peak at 60 kHz. This is due to the fact that the phase of the modulated signal and the phase of the PWM signal differ (shift) for each period of the modulated signal. In other words, if the phase of the modulated signal (carrier wave) and the phase of the PWM signal do not always match, fluctuation (so-called jitter) occurs in the amplitude of the waveform after LPF, so this is a distortion component (spurious component) of the carrier sideband. Appears as noise).

  In an ultrasonic speaker using the parametric array effect, an audible sound with directivity is generated by generating a differential sound component between a carrier wave and its sidebands by the nonlinearity of air when the output sound wave propagates through the air. Self-demodulating.

  In a modulation method generally used in an ultrasonic speaker, a carrier wave is superimposed and output, so that the carrier wave is output even when the input signal is 0, that is, in the unmodulated state. In the example of FIG. 6, although the input signal (modulated signal) is 0, that is, only the carrier wave (no modulation), many spurious noise components are generated in the sideband of the carrier wave. I understand that. When the ultrasonic speaker is driven in such a state, the difference sound component between the carrier wave and the spurious noise is output as audible sound noise even though the input signal is zero. This not only deteriorates the S / N ratio of the speaker, but the generated audible noise tends to be annoying sound with spectrum peaks concentrated at a specific frequency, which may cause discomfort to the listener. There is a problem that there is. Therefore, audible noise will be output from the ultrasonic speaker even if no signal is input unless control is performed to reduce the carrier wave level to 0 when there is no input signal. .

As described above, when an ultrasonic speaker is driven by a class D amplifier, in addition to the amplitude modulation of the ultrasonic speaker, the PWM modulation of the class D amplifier is added, and therefore the process through two different modulation methods. This increases the possibility of generating a large amount of spurious noise. Since an ultrasonic speaker always outputs a strong carrier wave component, if there is spurious noise, the difference sound component between the carrier wave and the spurious noise tends to be audible noise. This can be said to be a problem peculiar when driving an ultrasonic speaker with a class D amplifier.
JP 2002-158550 A

  As described above, when driving an ultrasonic speaker with a class D amplifier, in addition to amplitude modulation of the ultrasonic speaker, PWM modulation of the class D amplifier is added. There is a high possibility that a large amount of spurious noise will occur. Since the ultrasonic speaker always outputs a strong carrier wave component, there is a problem that if there is spurious noise, the difference sound component between the carrier wave and the spurious noise easily becomes audible noise.

  The present invention has been made to solve such a problem, and an object thereof is to reduce spurious noise generated during PWM modulation and to reduce audible sound noise generated from an ultrasonic speaker or the like. Possible class D amplifier drive method, class D amplifier drive circuit, electrostatic transducer including the class D amplifier drive circuit, ultrasonic speaker using the electrostatic ultrasonic transducer, and ultrasonic speaker It is in providing the used display apparatus and a directional sound system.

The present invention has been made to solve the above problems, and the driving method of the class D amplifier of the present invention is connected to a high-side switching element connected to a first power supply and a second power supply or ground. And one or more sets of circuits consisting of a totem pole type output circuit connected to a low-side switching element and a low-pass filter connected to the output end of the output circuit, and the input signal is PWM modulated A method of driving a class D amplifier that performs power amplification by controlling on / off of each switching element of the output circuit by the PWM signal, and the PWM signal is included when the input signal includes a carrier wave of a predetermined frequency. Is set to an integral multiple of the carrier frequency.
By such a method, the PWM frequency of the class D amplifier is set to an integer multiple of the carrier frequency of the input signal, and the phase of the PWM signal and the phase of the carrier are synchronized.
Thereby, spurious noise generated during PWM modulation can be reduced. For this reason, for example, when the drive circuit of the class D amplifier of the present invention is used for an ultrasonic speaker, audible noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

Further, the driving method of the class D amplifier according to the present invention is a totem pole type output in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected. Including at least one set of a circuit and a low pass filter connected to the output end of the output circuit, and turning on each switching element of the output circuit by a PWM signal obtained by PWM-modulating the input signal, A method of driving a class D amplifier that performs power amplification by controlling off, wherein the amplitude modulation signal is generated by amplitude modulating a carrier wave of a predetermined frequency by the input signal, and the amplitude modulation signal is converted to the carrier wave A procedure for generating a PWM signal by PWM modulation at a frequency that is an integer multiple of the Characterized in that it comprises a step of switching a quenching element.
With such a procedure, the class D amplifier amplitude-modulates a carrier wave having a predetermined frequency by the input signal, and PWM modulates the amplitude-modulated signal at a frequency that is an integral multiple of the carrier wave frequency. The switching element is switched by this PWM signal.
As a result, switching can be performed in a state in which the phase of the PWM signal and the phase of the carrier wave are synchronized, so that spurious noise generated during PWM modulation can be reduced. For this reason, for example, when the drive circuit of the class D amplifier of the present invention is used for an ultrasonic speaker, audible noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

Further, the class D amplifier driving method of the present invention is characterized in that the amplitude modulation method is a single sideband amplitude modulation (SSB) method.
Thereby, when driving an ultrasonic speaker or the like, it is possible to use a single sideband amplitude modulation (SSB) system in which the distortion of the demodulated sound is small. That is, in the case of the double sideband amplitude modulation (DSB) method, the greater the modulation degree of the modulated wave that drives the ultrasonic speaker, the greater the distortion rate of the demodulated signal. Regardless of the modulation degree of the modulation wave to be driven, the distortion rate of the demodulated signal is substantially constant and lower than that in the DSB system.

The driving circuit for the class D amplifier according to the present invention is a totem pole type output in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected. Including at least one set of a circuit and a low pass filter connected to the output end of the output circuit, and turning on each switching element of the output circuit by a PWM signal obtained by PWM-modulating the input signal, A drive circuit for a class D amplifier that amplifies power by controlling off, and sets the fundamental frequency of the PWM signal to an integral multiple of the carrier frequency when the input signal includes a carrier wave of a predetermined frequency Means are provided.
With such a configuration, the PWM frequency of the class D amplifier is set to an integer multiple of the carrier frequency of the input signal, and the phase of the PWM signal and the phase of the carrier are synchronized.
Thereby, spurious noise generated during PWM modulation can be reduced. For this reason, for example, when the drive circuit of the class D amplifier of the present invention is used for an ultrasonic speaker, audible noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

The driving circuit for the class D amplifier according to the present invention is a totem pole type output in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected. Including at least one set of a circuit and a low pass filter connected to the output end of the output circuit, and turning on each switching element of the output circuit by a PWM signal obtained by PWM-modulating the input signal, A drive circuit for a class D amplifier that performs power amplification by controlling off, and generates a carrier wave having a predetermined frequency f_ca and also generates a PWM clock signal UP / DOWN having an integer multiple of the carrier frequency f_ca Circuit and amplitude modulation for modulating the carrier wave with the input signal to generate an amplitude modulated signal AM And road, based on the PWM clock signal UP / DOWN, characterized in that it comprises a PWM circuit for PWM modulating the amplitude-modulated signal AM.
With this configuration, the carrier wave generation circuit generates a carrier wave having a frequency f_ca based on the clock signal CLK generated by the clock oscillator, and a PWM clock signal having a frequency N times (N is an integer) the carrier frequency f_ca. Generate UP / DOWN. The amplitude modulation circuit amplitude-modulates the carrier wave with the input signal to generate an amplitude modulation signal AM. The PWM modulation circuit compares the amplitude-modulated amplitude signal AM with a triangular wave generated based on the PWM clock signal UP / DOWN and performs PWM modulation.
As a result, the PWM frequency of the class D amplifier (the frequency of the UP / DOWN signal) becomes an integral multiple of the carrier frequency, and the phase of the PWM signal and the phase of the carrier can be synchronized. As a result, spurious noise generated during PWM modulation can be reduced. For this reason, for example, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

The driving circuit for the class D amplifier according to the present invention is a totem pole type output in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected. Including at least one set of a circuit and a low pass filter connected to the output end of the output circuit, and turning on each switching element of the output circuit by a PWM signal obtained by PWM-modulating the input signal, A drive circuit of a class D amplifier that performs power amplification by controlling off, a clock oscillator that generates a clock signal CLK having a predetermined frequency f_CLK, and the clock signal CLK is divided into 1 / M (M is an integer) A frequency generator for generating a PWM clock signal UP / DOWN of frequency f_PWM (= f_CLK / M), and the PWM clock A carrier wave generator for generating a carrier wave having a frequency f_ca (= f_PWM / N) of 1 / N (N is an integer) of the frequency f_PWM of the clock signal UP / DOWN, and amplitude-modulating the carrier wave by the input signal, An amplitude modulator that generates AM; an up / down counter that counts up or down the clock signal CLK according to the signal level of the clock signal UP / DOWN; a level value of the amplitude modulation signal AM; A comparator that compares the count value of the up / down counter, converts the magnitude relationship into a binary level, and outputs the binary level.
With this configuration, the frequency divider divides the frequency f_CLK of the clock signal CLK (frequency f_CLK) by 1 / M (M is an integer), and generates an UP / DOWN signal of frequency f_PWM (= f_CLK / M). To do. The carrier wave generator generates a carrier waveform having a frequency f_ca (= f_PWM / N). The amplitude modulator AM modulates a carrier wave with an input signal. The up / down counter counts up or down the clock signal CLK according to the level of the UP / DOWN signal, and outputs the count value as a triangular waveform REF (frequency f_PWM). The comparator compares the level value of the amplitude modulation signal AM output from the amplitude modulator with the triangular waveform REF (up / down count value) to generate a PWM signal.
As a result, the amplitude modulation signal AM obtained by AM-modulating the carrier wave with the input signal can be PWM-modulated with a triangular wave having a frequency N times that of the carrier wave, so that the PWM frequency of the class D amplifier (the frequency of the UP / DOWN signal) is increased. It becomes an integral multiple of the carrier frequency, and the phase of the PWM signal and the phase of the carrier can be synchronized. As a result, spurious noise generated during PWM modulation can be reduced. For this reason, for example, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

The class D amplifier driving circuit of the present invention includes a first register (1) for holding a value N of a ratio between the frequency f_ca of the carrier wave and the frequency f_PWM of the PWM clock signal UP / DOWN as a set value; And a second register (2) for holding a frequency division ratio value M of the frequency divider as a set value. The carrier wave generator includes a first register (1) and a second register ( 2) with reference to the value of 2), a waveform calculator that samples a carrier waveform for one period into M × N values, a data memory that stores waveform data sampled by the waveform calculator, The data stored in the data memory is output while being sequentially read out in synchronization with the clock signal CLK, and M × N (one period of carrier wave) data is repeatedly output to thereby generate a carrier wave continuously. And a memory controller.
With such a configuration, the value of N representing the ratio of the frequency f_PWM of the UP / DOWN signal to the carrier frequency f_ca is stored in the first register (1), and the frequency f_PWM of the UP / DOWN signal relative to the reference clock frequency f_CLK is stored. A value corresponding to the circumference ratio M is stored in the second register (2).
The carrier wave generator refers to the values of the register (1) and the register (2), and the waveform calculator pre-calculates the amplitude value of the carrier waveform so as to have one cycle of M × N, and M × N Is stored in the data memory. In addition, the memory controller sequentially reads and outputs data stored in the data memory in synchronization with the clock signal CLK, and continuously outputs M × N (one carrier wave period) data repeatedly.
As a result, the PWM frequency of the class D amplifier (the frequency of the UP / DOWN signal) becomes an integral multiple of the carrier frequency, and the phase of the PWM signal and the phase of the carrier can be synchronized. As a result, spurious noise generated during PWM modulation can be reduced. For this reason, for example, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component). Further, the amplitude value data of the carrier waveform can be calculated in advance and stored in the data memory, so that the arithmetic processing required when the carrier waveform is output can be omitted.

The drive circuit of the class D amplifier according to the present invention is characterized in that the amplitude modulation system is a single sideband amplitude modulation (SSB) system.
Thereby, when driving an ultrasonic speaker or the like, it is possible to use a single sideband amplitude modulation (SSB) system in which the distortion of the demodulated sound is small. That is, in the case of the double sideband amplitude modulation (DSB) method, the greater the modulation degree of the modulated wave that drives the ultrasonic speaker, the greater the distortion rate of the demodulated signal. Regardless of the modulation degree of the modulation wave to be driven, the distortion rate of the demodulated signal is substantially constant and lower than that in the DSB system.

The electrostatic transducer according to the present invention includes a totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected. And at least one set of circuits composed of a low-pass filter connected to the output end side of the output circuit, and on / off control of each switching element of the output circuit by a PWM signal obtained by PWM-modulating the input signal An electrostatic transducer driven by a class D amplifier that amplifies power by generating a carrier wave of a predetermined frequency f_ca in the circuit for driving the class D amplifier, and an integer multiple of the carrier frequency f_ca A carrier wave generating circuit for generating a PWM clock signal UP / DOWN having a frequency of An amplitude modulation circuit that performs amplitude modulation with the input signal and generates an amplitude modulation signal AM, and a PWM modulation circuit that PWM modulates the amplitude modulation signal AM based on the PWM clock signal UP / DOWN. Features.
With such a configuration, the class D amplifier that drives the electrostatic transducer generates a carrier wave having the frequency f_ca based on the clock signal CLK generated by the clock oscillator by the carrier wave generation circuit, and N of the carrier frequency f_ca. A PWM clock signal UP / DOWN having a double frequency (N is an integer) is generated. The amplitude modulation circuit amplitude-modulates the carrier wave with the input signal to generate an amplitude modulation signal AM. The PWM modulation circuit PWM modulates the amplitude-modulated amplitude signal AM with a triangular wave generated based on the PWM clock signal UP / DOWN.
As a result, the PWM frequency of the class D amplifier (the frequency of the UP / DOWN signal) becomes an integral multiple of the carrier frequency, and the phase of the PWM signal and the phase of the carrier can be synchronized. As a result, spurious noise generated during PWM modulation can be reduced. For this reason, the audible noise generated from the electrostatic transducer can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

In the electrostatic transducer according to the present invention, the electrostatic transducer includes a plurality of pairs of a fixed electrode on the first surface side in which a plurality of holes are formed and a fixed electrode on the first surface side. A fixed electrode on the second surface side in which a hole is formed, and a vibration film sandwiched between the pair of fixed electrodes and having a conductive layer, and a DC bias voltage is applied to the conductive layer. It is characterized by.
With such a configuration, for example, a push-pull type electrostatic transducer shown in FIG. 7 is used as the electrostatic transducer driven by the drive circuit of the class D amplifier of the present invention.
The push-pull electrostatic transducer can cause electrostatic force to act symmetrically with respect to the diaphragm by the first (front side) fixed electrode and the second (back side) fixed electrode that sandwich the diaphragm. Therefore, the vibration waveform (output waveform) distortion itself can be first reduced. Furthermore, when a push-pull electrostatic transducer is driven by a class D amplifier, the PWM frequency of the class D amplifier (the frequency of the UP / DOWN signal) is an integer multiple of the carrier frequency, and the phase of the PWM signal and the phase of the carrier And the spurious noise generated during the PWM modulation can be reduced. For this reason, the audible noise generated from the electrostatic transducer can be reduced.

Further, the ultrasonic speaker of the present invention is a totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected; At least one set of circuits composed of a low-pass filter connected to the output end side of the output circuit is provided, and each switching element of the output circuit is controlled to be turned on / off by a PWM signal obtained by PWM-modulating the input signal. An ultrasonic speaker configured by connecting an electrostatic ultrasonic transducer between output terminals of a class D amplifier that amplifies the power, and the carrier for driving the class D amplifier receives a carrier wave having a predetermined frequency f_ca. A carrier that generates and generates a PWM clock signal UP / DOWN having a frequency that is an integral multiple of the carrier frequency f_ca. A generation circuit; an amplitude modulation circuit that modulates the carrier wave with the input signal to generate an amplitude modulation signal AM; and a PWM modulation circuit that PWM modulates the amplitude modulation signal AM based on the PWM clock signal UP / DOWN It is characterized by providing these.
With such a configuration, in the drive circuit of the class D amplifier that drives the electrostatic ultrasonic transducer used in the ultrasonic speaker, the carrier wave generation circuit generates a carrier wave having the frequency f_ca and N of the carrier wave frequency f_ca. A PWM clock signal UP / DOWN having a double frequency (N is an integer) is generated. The amplitude modulation circuit amplitude-modulates the carrier wave with the input signal to generate an amplitude-modulated amplitude modulated signal AM. The PWM modulation circuit PWM modulates the amplitude-modulated amplitude signal AM with a triangular wave generated based on the PWM clock signal UP / DOWN.
As a result, the PWM frequency of the class D amplifier (the frequency of the UP / DOWN signal) becomes an integral multiple of the carrier frequency, and the phase of the PWM signal and the phase of the carrier can be synchronized. As a result, spurious noise generated during PWM modulation can be reduced. For this reason, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) at the time of output 0 (only the carrier wave component).

The display device of the present invention is a display device including an ultrasonic speaker that reproduces an audio signal supplied from an acoustic source and reproduces an audio frequency band signal sound, and a projection optical system that projects an image on a projection surface. The ultrasonic speaker includes a totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected, and One or more sets of circuits composed of a low-pass filter connected to the output end side of the output circuit are provided, and each switching element of the output circuit is controlled to be turned on / off by a PWM signal obtained by PWM modulating the input signal The electrostatic ultrasonic transducer is connected between the output terminals of the class D amplifier that amplifies the power by using the class D amplifier. A circuit that generates a carrier wave having a predetermined frequency f_ca, a carrier wave generation circuit that generates a PWM clock signal UP / DOWN having a frequency that is an integer multiple of the carrier wave frequency f_ca, and amplitude-modulating the carrier wave by the input signal. An amplitude modulation circuit that generates an amplitude modulation signal AM, and a PWM modulation circuit that PWM modulates the amplitude modulation signal AM based on the PWM clock signal UP / DOWN.
With such a configuration, an ultrasonic speaker used in a display device including a projection optical system that projects an image is configured by an electrostatic ultrasonic transducer driven by a class D amplifier. In the driving circuit of the class D amplifier that drives the electrostatic ultrasonic transducer, the carrier wave generation circuit generates a carrier wave having the frequency f_ca, and PWM having a frequency N times (N is an integer) the carrier frequency f_ca. Clock signal UP / DOWN is generated. The PWM modulation circuit amplitude-modulates the carrier wave with the input signal to generate an amplitude-modulated amplitude signal AM. The PWM modulation circuit PWM modulates the amplitude-modulated amplitude signal AM with a triangular wave generated based on the PWM clock signal UP / DOWN.
As a result, the PWM frequency of the class D amplifier (the frequency of the UP / DOWN signal) becomes an integral multiple of the carrier frequency, and the phase of the PWM signal and the phase of the carrier can be synchronized. As a result, it is possible to reduce spurious noise generated during PWM modulation in the class D amplifier. For this reason, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

In addition, the directional acoustic system of the present invention includes an ultrasonic speaker that reproduces a signal in a first sound range among audio signals supplied from an acoustic source, and the first audio signal supplied from the acoustic source. A low-frequency sound reproduction speaker that reproduces a signal in a second sound range lower than the sound range, wherein the ultrasonic speaker includes a high-side switching element connected to a first power source and a first sound source. One or more sets of circuits composed of a totem pole type output circuit connected to a second power source or a low-side switching element connected to the ground, and a low-pass filter connected to the output terminal side of the output circuit And amplifying power by controlling on / off of each switching element of the output circuit by a PWM signal obtained by PWM-modulating an input signal. An electrostatic ultrasonic transducer is connected between the output terminals of the amplifier, and a circuit that drives the class D amplifier generates a carrier wave having a predetermined frequency f_ca and a frequency that is an integral multiple of the carrier frequency f_ca. Based on the carrier wave generating circuit for generating the PWM clock signal UP / DOWN, the amplitude modulating circuit for amplitude-modulating the carrier wave by the input signal and generating the amplitude modulation signal AM, and the PWM clock signal UP / DOWN, And a PWM modulation circuit that PWM modulates the amplitude modulation signal AM.
With such a configuration, an ultrasonic speaker that reproduces a signal in the first range among audio signals supplied from an acoustic source, and a speaker for low-frequency reproduction that reproduces a signal in the second range lower than the first range. The ultrasonic speaker is composed of an electrostatic ultrasonic transducer driven by the class D amplifier of the present invention. In the driving circuit of the class D amplifier that drives the electrostatic ultrasonic transducer, the carrier wave generation circuit generates a carrier wave having the frequency f_ca, and PWM having a frequency N times (N is an integer) the carrier frequency f_ca. Clock signal UP / DOWN is generated. The amplitude modulation circuit amplitude-modulates the carrier wave with the input signal to generate an amplitude-modulated amplitude modulated signal AM. The PWM modulation circuit PWM modulates the amplitude-modulated amplitude signal AM with a triangular wave generated based on the PWM clock signal UP / DOWN.
As a result, the reproduced sound quality can be further improved by supplementing the low frequency reproduction speaker with the low frequency band sound that is difficult to reproduce with the ultrasonic speaker. Further, the PWM frequency of the class D amplifier that drives the ultrasonic speaker (the frequency of the UP / DOWN signal) is set to an integral multiple of the carrier frequency, and the PWM signal and the phase of the carrier are synchronized, so that the PWM in the class D amplifier is synchronized. Spurious noise generated during modulation can be reduced. For this reason, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

Next, the best mode for carrying out the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a basic circuit configuration of the present invention. The function of each block will be described below.

  In FIG. 1, a clock oscillator 10 generates an operation reference clock signal CLK. The carrier wave generation circuit 11 generates a carrier wave (sine wave) having a predetermined frequency and a clock signal UP / DOWN (PWM clock) having a frequency that is an integral multiple of the carrier wave. The amplitude modulation circuit 12 modulates the amplitude of the carrier wave (ultrasound band) generated by the carrier wave generation circuit 11 with an input signal (a signal in the audible band), and outputs it as an amplitude modulation signal AM. The PWM modulation circuit 13 PWM modulates the amplitude modulation signal AM output from the amplitude modulation circuit 12 based on the clock signal UP / DOWN generated by the carrier wave generation circuit 11 to generate a PWM signal. The gate drive circuit 14 generates a gate signal for turning on and off the switching elements M1 and M2 according to the PWM signal, and alternately turns on the switching elements M1 and M2 in the class D output stage 15 by the gate signal. Drive off.

  The class D output stage 15 includes a high-side switching element M1 connected to the positive power supply side and a low-side switching element M2 connected to the negative power supply side, and a totem pole type output circuit including M1 and M2 Is configured. The switching elements M1 and M2 are each composed of a power MOSFET having a small on-resistance, and the gate driving circuit 14 causes each power MOSFET to perform a switching operation. Since the output of the class D output stage 15 has a switching waveform (switching voltage waveform by the power supplies E1 and E2), the switching carrier component is removed by the low-pass filter 16 and then supplied to the load 17. For the low-pass filter 16, LC filters (L1, C1) with small power loss are used. The circuit configuration shown in FIG. 1 is a half-bridge type composed of two switching elements M1 and M2, but may of course be a full-bridge type composed of four switching elements.

  FIG. 2 is a block diagram showing a circuit configuration example embodying a main part of the drive circuit of the class D amplifier of the present invention, and FIG. 4 shows an example of the operation timing chart of FIG. Each component block will be described below.

  In FIG. 2, the clock oscillator 10 generates an operation reference clock signal CLK for the carrier wave generator 21, the frequency divider 22, and the UP / DOWN counter 23. (The clock signal CLK is not shown in FIG. 4)

The carrier wave generation circuit 11 includes a carrier wave generator 21 and a frequency divider 22. The carrier wave generator 21 generates and outputs a carrier wave having a predetermined frequency f_ca based on the carrier frequency setting value. The frequency divider 22 divides the clock signal CLK based on the PWM frequency setting value and outputs the clock signal UP / DOWN having the frequency f_PWM.
Here, when the frequency of the reference clock signal CLK is f_CLK (Hz), M and N are natural numbers.
f_PWM = f_CLK / M,
f_ca = f_PWM / N,
Thus, the value of the frequency f_PWM (Hz) of the UP / DOWN signal (carrier frequency setting value) and the value of the carrier frequency f_ca (Hz) (carrier frequency setting value) are set. Note that M is a frequency division ratio in the frequency divider 22, and N is a frequency ratio between the carrier wave and the UP / DOWN signal (PWM clock).

  The UP / DOWN counter 23 counts up or down the clock signal CLK output from the clock oscillator 10 in accordance with the level of the UP / DOWN signal (PWM clock) output from the frequency divider 22. Is output as a signal REF. Here, it is configured to count up when the UP / DOWN signal is High level, and to count down when it is Low level. In the example shown in FIGS. 2 and 4, the count value is scaled so that the value of REF changes (counts) from −1 to 1 with 0 as the center. The waveform of the output signal REF from the UP / DOWN counter is a triangular wave having a frequency synchronized with the UP / DOWN signal.

  The amplitude modulation circuit 24 amplitude-modulates the carrier wave generated by the carrier wave generator 21 with the input signal, and outputs it as an amplitude modulation signal AM.

The comparator 25 compares the amplitude value of the signal AM output from the amplitude modulation circuit with the amplitude value of the triangular waveform REF output from the UP / DOWN counter, and outputs the comparison result as a PWM signal. Here, if the amplitude value of the amplitude modulation signal AM is larger than the triangular waveform REF, the High level is outputted, and if it is smaller, the Low level is outputted. As a result, a PWM modulation signal is generated.
Here, the UP / DOWN counter 23 and the comparator 25 constitute the PWM modulation circuit 13.

FIG. 3 is a block diagram showing a circuit configuration example in which the essential parts of the present invention are extracted and more specifically shown, and FIG. 4 shows an example of the operation timing chart of FIG.
The circuit configuration example of FIG. 3 includes a first register (1) 31 and a second register (2) 32. The register (1) 31 has a PWM frequency (frequency of the UP / DOWN signal) with respect to the carrier frequency f_ca. ) The value of N representing the ratio of f_PWM is stored, and the value of M representing the frequency division ratio of the PWM frequency f_PWM with respect to the reference clock frequency f_CLK is stored in the register (2) 32.

  The clock oscillator 10 generates an operation reference clock signal CLK for the memory controller 35, the frequency divider 22, and the UP / DOWN counter 23. (The clock signal CLK is not shown in FIG. 4)

  The waveform calculator 33 refers to the values of the register (1) 31 and the register (2) 32, and samples the waveform of the carrier wave for one cycle into M × N values. That is, the amplitude value of the carrier waveform is calculated in advance so that M × N carrier waveforms for one cycle are formed. M × N pieces of waveform data (for one cycle) calculated by the waveform calculator 33 are stored in the data memory 34. The memory controller 35 outputs the carrier waveform data stored in the data memory 34 while sequentially reading out the data of the carrier waveform in synchronization with the clock signal CLK output from the clock oscillator 10, and M × N (one carrier cycle). The carrier wave is continuously generated by repeatedly outputting the data.

  The frequency divider 22 divides the frequency f_CLK of the clock signal CLK by 1 / M and outputs it as an UP / DOWN signal. The frequency of this UP / DOWN signal is the PWM modulation reference frequency f_PWM. The frequency divider 22 includes, for example, a counter, a reset register, and a flip-flop (not shown). The counter is configured to count the clock signal CLK, and when the count value overflows, resets the value of the reset register to the counter and simultaneously inverts the output of the flip-flop. In a reset register (not shown) referred to by the counter, an initial value for resetting the counter is calculated and set based on the value of the frequency division ratio M stored in the register (2) 32. For example, when the number of bits of the counter is 8 bits and the clock signal is to be divided by 1/10, that is, when the value of the division ratio M stored in the register (2) 32 is 10, the reset register Is set to 251 (= 255−10 / 2 + 1). As a result, the counter inverts the output of the flip-flop, that is, the level of the UP / DOWN signal every time the clock signal CLK is counted five times, so that the UP / DOWN signal output from the frequency divider 22 The clock signal is divided by 1/10.

  The UP / DOWN counter 23 counts up or down the clock signal CLK output from the clock oscillator 10 in accordance with the level of the UP / DOWN signal (PWM clock) output from the frequency divider 22. Is output as a signal REF. Here, it is configured to count up when the UP / DOWN signal is High level, and to count down when it is Low level. In the example shown in FIGS. 3 and 4, the count value is scaled so that the value of REF changes (counts) between −1 and 1 around 0. The waveform of the output signal REF from the UP / DOWN counter is a triangular wave having a frequency synchronized with the UP / DOWN signal.

  The amplitude modulation circuit 24 modulates the amplitude of the carrier wave (digital data) output from the memory controller 35 with the digitized input signal, and outputs it as an amplitude modulation signal AM.

The comparator 25 compares the amplitude value of the signal AM output from the amplitude modulation circuit with the amplitude value of the triangular waveform REF output from the UP / DOWN counter, and outputs the comparison result as a PWM signal. Here, if the amplitude value of the amplitude modulation signal AM is larger than the triangular waveform REF, the High level is outputted, and if it is smaller, the Low level is outputted. As a result, a PWM modulation signal is generated.
Here, the waveform calculator 33, the data memory 34, and the memory controller 35 constitute the carrier wave generator 21 of FIG.

  In the above configuration, the frequency f_PWM of the UP / DOWN signal (PWM clock) is an integral multiple (N times) of the carrier frequency f_ca. Therefore, it is possible to drive in a state where the phase of the UP / DOWN signal (PWM clock) is synchronized with the phase of the carrier wave.

  For example, when the clock frequency f_CLK is 100 MHz, when it is desired to drive the PWM clock frequency f_PWM at 500 kHz (M = 200) and the carrier frequency f_ca at 50 kHz (N = 10), the size of the data area of the carrier wave generator 21 is The amplitude data obtained by dividing one cycle of the carrier wave (sine wave) into 2000 is stored in the data area (data memory 35). In the frequency divider 22, an UP / DOWN signal is generated by dividing the frequency f_CLK of the clock signal by 1/200.

  FIG. 4 is a diagram showing an example of a timing chart in the drive circuit of the class D amplifier of the present invention, where the input signal level is 0 (zero), the carrier frequency is 50 kHz, and the frequency of the UP / DOWN signal (PWM clock). Shows an example in which is 500 kHz.

  4A shows the waveforms of the carrier wave and the input signal, FIG. 4B shows the UP / DOWN signal output from the frequency divider 22, and FIG. 4C shows the UP / DOWN counter. 23 shows the signal REF output from the output signal 23 and the amplitude modulation signal AM output from the amplitude modulation circuit 12 or the amplitude modulation circuit 24, and FIG. 4D shows the PWM modulation signal PWM output from the comparator 25. In FIG. 4, the point to be particularly noted is that the carrier wave (amplitude modulated wave when the modulation degree is 0) AM and the triangular wave REF are synchronized, that is, the phase of the carrier wave AM and the triangular wave REF, as shown in FIG. The relationship is fixed without being different for each period of the modulated signal AM. This is because the frequency f_PWM of the UP / DOWN signal (PWM clock) is set to an integral multiple (N times) of the carrier frequency f_ca.

  FIG. 5 is a diagram showing an example of a signal waveform and a frequency spectrum in the drive circuit of the class D amplifier of the present invention, showing an example in which the carrier wave signal is 62.5 kHz and the PWM clock frequency is 625 kHz. Yes. 5A shows an example of a PWM modulation waveform (switching waveform) and an output waveform after passing through the LPF, and FIG. 5B shows an example of a frequency spectrum of the output waveform after passing through the LPF. ing.

  In the conventional example, as shown in FIG. 6, when the PWM frequency is not an integer multiple of the modulated signal frequency, a distortion component (spurious noise) in the carrier sideband appears in the output waveform after LPF. In the present invention, as shown in FIG. 5, the PWM frequency is an integer multiple of the modulated signal frequency (carrier frequency), so that the distortion component (spurious noise) of the carrier sideband as seen in the example of FIG. You can see that it is not out. In the conventional example shown in FIG. 6, since the PWM frequency is not an integral multiple of the modulated signal frequency, the phase of the modulated signal and the phase of the PWM signal differ (shift) for each period of the modulated signal. This appears as a distortion component (spurious noise) in the carrier sideband.

  Considering the case where an ultrasonic speaker is driven as a load of a class D amplifier, when the PWM frequency is not an integral multiple of the carrier frequency (in the case of FIG. 6), even when the input signal is 0 (only the carrier wave state) Since spurious noise components stand in the wave band, the difference sound component between the carrier wave component and the spurious noise component is self-demodulated by the parametric array effect, and the difference sound component in the audible band may be heard as noise when there is no signal. . On the other hand, if the PWM frequency is set to an integral multiple of the carrier wave (in the case of FIG. 5), no spurious noise component is generated in the carrier sideband, so that no-signal noise such as the former can be suppressed.

  As described above, the S / N ratio of the reproduction output of the class D amplifier can be increased by setting the PWM frequency to an integer multiple of the carrier wave (modulated signal) frequency.

  As described above, by setting the PWM frequency of the class D amplifier to an integer multiple of the carrier frequency of the ultrasonic speaker, it is possible to reduce spurious noise generated during PWM modulation than when it is not an integral multiple (FIG. 6). Therefore, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

  Note that the load is not limited to the ultrasonic speaker, and can be applied if a modulated signal using a carrier wave of a specific frequency is output.

[Second Embodiment]
In the first embodiment, the configuration and operation of the drive circuit for the class D amplifier according to the present invention have been described. As a second embodiment, the electrostatic transducer is driven by the drive circuit for the class D amplifier according to the present invention. An example of an ultrasonic speaker will be described.

  FIG. 7 is a diagram showing an example of a push-pull type electrostatic transducer driven by the class D amplifier of the present invention, and has a structure particularly suitable for use as a transducer of an ultrasonic speaker. ing. An electrostatic transducer used for an ultrasonic speaker is also called an electrostatic ultrasonic transducer.

  FIG. 7A shows a cross section of the transducer. The vibrating membrane 112 having a conductive layer, the front surface (first surface) side fixed electrode 101A provided to face each surface of the vibrating membrane 112, and It has a pair of fixed electrodes composed of a back surface (second surface) side fixed electrode 101B (when both the front surface side fixed electrode 101A and the back surface side fixed electrode 101B are referred to as the fixed electrode 101). As shown in FIG. 7A, the vibration film 112 may be formed such that a conductive layer (vibration film electrode) 121 forming an electrode is sandwiched between insulating films 120, or the entire vibration film 112 is made of a conductive material. You may make it form.

  The front-side fixed electrode 101A sandwiching the vibrating membrane 112 is provided with a plurality of through-holes 114A, and the back-side fixed electrode 101B is opposed to each through-hole 114A provided in the front-side fixed electrode 101A. Are provided with through holes 114B having the same shape (referring to both the through holes 114A and the through holes 114B as the through holes 114). The front-side fixed electrode 101A and the back-side fixed electrode 101B are supported by a support member 111 with a predetermined gap from the vibration film 112, respectively. As shown in FIG. The support member 111 is formed so as to face partly through a gap.

  FIG. 7B shows a one-sided plan view of the transducer (a state in which a part of the fixed electrode 101 is cut away to expose the vibration film 112), and the plurality of through holes 114 are arranged in a honeycomb shape. Yes.

  Further, the DC power source 116 is a power source for applying a DC bias voltage to the vibrating membrane electrode 121, and the AC signals 118 </ b> A and 118 </ b> B are used to drive the vibrating membrane 112 and the front side fixed electrode 101 </ b> A and the back side fixed electrode It is a signal applied to 101B.

  FIG. 8 is a diagram showing a configuration example of an ultrasonic speaker using the drive circuit for the class D amplifier of the present invention. The ultrasonic speaker shown in FIG. 8 has an audio frequency wave signal source (audio signal source) 131 that generates a signal wave in the audio frequency band, and a class D amplifier 1 of the present invention (see FIGS. 1 and 2). is doing. The output of the class D amplifier 1 is applied to the electrostatic transducer 100 via the output transformer T. Note that the secondary winding of the output transformer T includes an intermediate tap, and a DC bias voltage E is applied to the diaphragm electrode 121 with reference to the intermediate tap.

  In the above configuration, an audible frequency signal (audio signal) output from the audible frequency wave signal source 131 is input to the class D amplifier 1. In the class D amplifier 1, the carrier wave (carrier wave) having a predetermined frequency is amplitude-modulated by the input signal, the amplitude-modulated signal is PWM-modulated by an integer multiple of the carrier frequency, and the class D output stage and the LC filter are used. Amplify and output. Then, the amplified amplitude modulation signal is applied to both ends of the primary winding of the output transformer T. As a result, the electrostatic transducer 100 connected to the secondary winding of the output transformer T is driven. Note that the PWM frequency of the class D amplifier 1 is set to be N times the carrier frequency.

  As a result, the modulated signal is converted into a sound wave of a finite amplitude level by the electrostatic transducer 100, and this sound wave is radiated into the medium (in the air) and is a signal in the original audible frequency band due to the nonlinear effect of the medium (air). The sound is self-playing. In other words, since sound waves are coarse and dense waves that are transmitted using air as a medium, the dense and sparse portions of air appear prominently in the process of propagation of the modulated ultrasonic waves, and the dense portions have high sound speed and sparseness. Since the speed of sound is slow in this part, the modulation wave itself is distorted. As a result, the waveform is separated into a carrier wave (ultrasonic wave) and an audible wave (original audio signal), and we humans have an audible sound (original audio signal) of 20 kHz or less. This is the principle of listening only to this, and is generally called the parametric array effect.

  In the example shown in FIG. 8, the push-pull type electrostatic transducer is driven by the class D amplifier drive circuit of the present invention. However, the load to be driven is push-pull. The present invention is not limited to the electrostatic transducer of the type, and other types of capacitive loads can be suitably driven. For example, it is also possible to drive an electrostatic transducer called a pull type having a structure in which a fixed electrode is disposed only on one side of a vibrating membrane and the vibrating membrane is sucked only on one side.

  FIG. 9 is a diagram illustrating a configuration example of a drive circuit of a pull type electrostatic transducer. A pull-type electrostatic transducer 200 shown in FIG. 9A has a dielectric 211 (insulator) such as PET (polyethylene terephthalate resin) having a thickness of about 3 to 10 μm as a vibrating body (vibrating film). Vibrating membrane). For the dielectric 211, an upper electrode 212 formed as a metal foil such as aluminum is integrally formed on the upper surface by a process such as vapor deposition, and a lower electrode 213 formed of brass is formed on the dielectric 211. It is provided in contact with the lower surface. The lower electrode 213 is connected to a lead 222 and is fixed to a base plate 215 made of bakelite or the like.

  The upper electrode 212 is connected to a lead 221, and the lead 221 is connected to a DC bias power supply 230. A DC bias voltage for attracting the upper electrode of about 50 to 150 V is always applied to the upper electrode 212 by the DC bias power source 230, and the upper electrode 212 is attracted to the lower electrode 213 side. Reference numeral 231 denotes a signal source.

  The dielectric 211, the upper electrode 212, and the base plate 215 are caulked by the case 201 together with the metal rings 216, 217, and 218 and the mesh 219.

  On the surface of the lower electrode 213 on the dielectric 211 side, a plurality of minute grooves (uneven portions) of about several tens to several hundreds μm having a non-uniform shape are formed. Since this minute groove becomes a gap between the lower electrode 213 and the dielectric 211, the distribution of capacitance between the upper electrode 212 and the lower electrode 213 changes minutely. The random minute grooves are formed by manually roughing the surface of the lower electrode 213 with a file. In the pull type electrostatic transducer, the frequency characteristics have a wide band by forming innumerable capacitors having different gap sizes and depths.

  The pull type electrostatic transducer shown in FIG. 9A can be driven by the drive circuit of the class D amplifier of the present invention. FIG. 9B is a diagram showing a circuit configuration for driving a pull-type electrostatic transducer with a class D amplifier 1 of the present invention (see FIGS. 1 and 2), and is a pull-type electrostatic transducer. The equivalent capacitance of the type transducer 200 is shown as Cpull.

  In FIG. 9B, the output from the class D amplifier 1 is boosted via the output transformer T and then applied to the pull type electrostatic transducer (Cpull) 200. One terminal of the secondary winding of the output transformer T is connected to the upper electrode 212 of the pull-type electrostatic transducer (Cpull) 200 via the DC bias power supply 230, and the other terminal is connected to the lower electrode 213. Each is connected.

  With the above configuration, an AC signal superimposed on a DC bias voltage is applied to the upper electrode 212 and the lower electrode 213 of the electrostatic transducer 200. In this way, by applying a DC bias voltage and an AC signal to the upper electrode 212, the attractive force with respect to the lower electrode 213 of the upper electrode 212 changes, so that the vibration film (dielectric material) 211 vibrates, and the vibration film A sound wave is emitted from 211.

  FIG. 10 is a diagram illustrating a configuration example of a drive circuit of the piezoelectric ultrasonic transducer. A configuration example of a piezoelectric ultrasonic transducer that performs conversion from an electric signal to ultrasonic waves using a piezoelectric ceramic as a vibration element is shown. FIG. 10A shows a bimorph piezoelectric transducer (ultrasonic transducer).

  A bimorph type piezoelectric transducer 301 shown in FIG. 10A includes two piezoelectric elements (piezoelectric ceramics) 311 and 312, a cone 313, a case 314, leads 315 and 316, and a screen 317. ing. The piezoelectric elements 311 and 312 are bonded to each other, and a lead 315 and a lead 316 are connected to a surface opposite to the bonded surface, respectively.

  Also in the piezoelectric transducer shown in FIG. 10A, the drive circuit of the class D amplifier of the present invention can be preferably used. FIG. 10B is a diagram showing a circuit configuration of a piezoelectric ultrasonic transducer, and shows an equivalent capacitance of the bimorph piezoelectric transducer 301 as Cbm.

  In FIG. 10B, the output from the class D amplifier 1 is boosted via the output transformer T and then applied to the piezoelectric transducer (Cbm) 301. One terminal of the secondary winding of the output transformer T is connected to one piezoelectric element 311 of the piezoelectric transducer (Cbm) 301, and the other terminal is connected to the other piezoelectric element 312.

  With the above configuration, an AC signal is applied to the piezoelectric elements 311 and 312 of the piezoelectric transducer 301. As a result, the piezoelectric elements 311 and 312 vibrate to emit sound waves.

  As described above, when the ultrasonic speaker is driven by a class D amplifier, the spurious noise generated during PWM modulation can be reduced by setting the PWM frequency to an integer multiple of the carrier frequency of the ultrasonic speaker. it can. For this reason, the audible sound noise generated from the ultrasonic speaker can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

[Third embodiment]
In the second embodiment, the example of the ultrasonic speaker using the drive circuit of the class D amplifier of the present invention has been described. However, the ultrasonic speaker of the present invention is used as the third embodiment of the present invention. An example of the display device will be described.

  FIG. 11 shows a projector as an example of a display device, and shows a use state thereof. As shown in the figure, a projector (display device) 401 is installed behind the viewer 403, projects an image on a screen 402 installed in front of the viewer 403, and is equipped with an ultrasonic wave mounted on the projector 401. A virtual sound source is formed on the projection surface of the screen 402 by a speaker to reproduce sound.

An external configuration of the projector 401 is shown in FIG. The projector 401 includes a projector main body 420 including a projection optical system that projects an image on a projection surface such as a screen, and an electrostatic ultrasonic transducer (hereinafter simply referred to as “ultrasonic transducer”) that can oscillate sound waves in an ultrasonic frequency band. ) 424A and 424B, and an ultrasonic speaker that reproduces an audible frequency band signal sound from an audio signal supplied from an acoustic source. In the present embodiment, in order to reproduce a stereo audio signal, ultrasonic transducers 424A and 424B constituting ultrasonic speakers are mounted on the left and right with a projector lens 431 constituting the projection optical system interposed therebetween.
Further, a low-pitched sound reproduction speaker 423 is provided on the bottom surface of the projector main body 420. Reference numeral 425 denotes a height adjusting screw for adjusting the height of the projector main body 420, and reference numeral 426 denotes an air cooling fan exhaust port.

  In the projector 401, an electrostatic ultrasonic transducer driven by a class D amplifier of the present invention is used as an ultrasonic transducer constituting the ultrasonic speaker.

  Next, the electrical configuration of the projector 401 is shown in FIG. The projector 401 includes an ultrasonic speaker including an operation input unit 410, a reproduction range setting unit 412, a reproduction range control processing unit 413, an audio / video signal reproduction unit 414, class D amplifiers 422A and 422B, and ultrasonic transducers 424A and 424B. , High pass filters 417A, 417B, low pass filter 419, mixer 421, power amplifier 422C, low sound reproduction speaker 423, and projector main body 420.

  The class D amplifiers 422A and 422B use the class D amplifier drive circuit of the present invention (see FIGS. 1 and 2). As shown in FIGS. A modulation circuit 12 and the like are provided, and as shown in FIG. 2, the frequency of the carrier wave (carrier wave) generated by the carrier wave generation circuit 11 can be set externally.

  The projector main body 420 includes a video generation unit 432 that generates a video and a projection optical system 433 that projects the generated video onto a projection surface. As described above, the projector 401 is configured by integrating the ultrasonic speaker and the bass reproduction speaker 423 and the projector main body 420.

  The operation input unit 410 includes various function keys including a numeric keypad, numeric keys, and a power key for turning on / off the power. The playback range setting unit 412 can input data for specifying the playback range of a playback signal (signal sound) by a user operating the operation input unit 410 with a key. The frequency of the carrier wave that defines the reproduction range of the signal is set and held. The reproduction range of the reproduction signal is set by designating the distance that the reproduction signal reaches in the radial axis direction from the sound wave emitting surfaces of the ultrasonic transducers 424A and 424B.

Further, the reproduction range setting unit 412 can set the frequency of the carrier wave by the control signal output according to the video content from the audio / video signal reproduction unit 414.
Also, the reproduction range control processing unit 413 refers to the setting contents of the reproduction range setting unit 412 and has a function of controlling the class D amplifiers 422A and 422B so as to change the frequency of the carrier wave so as to be within the set reproduction range. Have.
For example, when the above-mentioned distance corresponding to a carrier wave frequency of 50 kHz is set as internal information of the reproduction range setting unit 412, control is performed so that a 50 kHz carrier wave is generated for the class D amplifiers 422A and 422B.

The reproduction range control processing unit 413 stores in advance a table indicating the relationship between the distance that the reproduction signal reaches in the radial axis direction from the sound wave emitting surfaces of the ultrasonic transducers 424A and 424B that define the reproduction range and the frequency of the carrier wave. It has a storage part. The data in this table is obtained by actually measuring the relationship between the frequency of the carrier wave and the reach distance of the reproduction signal.
The reproduction range control processing unit 413 obtains the frequency of the carrier wave corresponding to the distance information set with reference to the table based on the setting content of the reproduction range setting unit 412 and class D amplifier so as to obtain the frequency. 422A and 422B are controlled.

The audio / video signal reproduction unit 414 is, for example, a DVD player that uses a DVD as a video medium. Of the reproduced audio signals, the R channel audio signal is sent to the D-class amplifier 422A via the high-pass filter 417A. The signal is output to the class D amplifier 422B via the high-pass filter 417B, and the video signal is output to the video generation unit 432 of the projector main body 420.
The R channel audio signal and the L channel audio signal output from the audio / video signal reproduction unit 414 are combined by the mixer 421 and input to the power amplifier 422C via the low-pass filter 419. . The audio / video signal reproduction unit 414 corresponds to an acoustic source.

  The high-pass filters 417A and 417B have a characteristic of passing only the frequency components in the middle and high sound range (first sound range) in the R-channel and L-channel audio signals, respectively. Only the low frequency range (second range) frequency component of the audio signal of the channel is passed.

  Accordingly, among the R channel and L channel audio signals, the mid and high range audio signals are reproduced by the ultrasonic transducers 424A and 424B, respectively, and among the R channel and L channel audio signals, the low range audio signals are low frequencies. Playback is performed by the playback speaker 423.

  The audio / video signal playback unit 414 is not limited to a DVD player, and may be a playback device that plays back an externally input video signal. Also, the audio / video signal playback unit 414 instructs the playback range setting unit 412 to dynamically change the playback range of the playback sound in order to produce an acoustic effect according to the scene of the video to be played back. Has a function of outputting a control signal.

The video generation unit 432 includes a display such as a liquid crystal display and a plasma display panel (PDP), and a drive circuit that drives the display based on a video signal output from the audio / video signal reproduction unit 414. A video obtained from the video signal output from the audio / video signal reproduction unit 414 is generated.
The projection optical system 433 has a function of projecting an image displayed on the display onto a projection surface such as a screen installed in front of the projector main body 420.

  Next, the operation of the projector 401 having the above configuration will be described. First, data (distance information) for instructing the reproduction range of the reproduction signal is set in the reproduction range setting unit 412 from the operation input unit 410 by the user's key operation, and the audio / video signal reproduction unit 414 is instructed to reproduce.

  As a result, distance information that defines the reproduction range is set in the reproduction range setting unit 412, and the reproduction range control processing unit 413 takes in the distance information set in the reproduction range setting unit 412 and stores it in the built-in storage unit. The carrier wave generation circuit 11 in the class D amplifiers 422A and 422B (refer to FIG. 1 and FIG. 1) so as to obtain the carrier wave frequency corresponding to the set distance information, 2).

  As a result, the carrier wave generation circuit 11 (see FIGS. 1 and 2) in the class D amplifiers 422A and 422B generates a carrier wave wave having a frequency corresponding to the distance information set in the reproduction range setting unit 412 and performs amplitude modulation. It outputs to the circuit 24 (refer FIG.1 and FIG.2).

  On the other hand, the audio / video signal reproduction unit 414 outputs the R channel audio signal to the class D amplifier 422A via the high pass filter 417A and the L channel audio signal to the class D via the high pass filter 417B. The amplifier 422B outputs an R channel audio signal and an L channel audio signal to the mixer 421, and outputs a video signal to the video generation unit 432 of the projector main body 420, respectively.

Therefore, the high-pass filter 417A inputs the mid-high range audio signal of the R channel audio signal to the class D amplifier 422A, and the high-pass filter 417B converts the mid-high range audio signal of the L channel audio signal to the class D. Input to the amplifier 422B.
The R channel audio signal and the L channel audio signal are synthesized by the mixer 421, and the low frequency audio signal of the R channel audio signal and the L channel audio signal is input to the power amplifier 422C by the low-pass filter 419. Is done.

  The video generation unit 432 generates and displays a video by driving the display based on the input video signal. The image displayed on the display is projected onto a projection surface, for example, the screen 402 shown in FIG. 11 by the projection optical system 433.

The modulated signals of the carrier waves amplified by the class D amplifiers 422A and 422B are the front side fixed electrode (upper electrode) 101A and the back side fixed electrode (lower electrode) 101B of the ultrasonic transducers 424A and 424B, respectively (see FIG. 7). The modulation signal is converted into a sound wave (acoustic signal) having a finite amplitude level and radiated to a medium (in the air), and the ultrasonic transducer 424A receives a medium and high tone in the R channel audio signal. A sound signal in the high frequency range is reproduced from the ultrasonic transducer 424B.
In addition, the low frequency sound signal in the R channel and the L channel amplified by the power amplifier 422C is reproduced by the low sound reproduction speaker 423.

  As described above, in the propagation of ultrasonic waves radiated into the medium (in the air) by the ultrasonic transducer, the sound speed increases at a portion where the sound pressure is high and the sound speed is slow at a portion where the sound pressure is low. Become. As a result, waveform distortion occurs.

  When a signal (carrier wave) in the radiated ultrasonic band is modulated (AM modulation) with a signal in the audible frequency band, the signal wave in the audible frequency band used for modulation is super It is formed so as to be self-demodulated separately from the carrier wave in the sonic frequency band. At this time, the spread of the reproduction signal becomes a beam shape due to the characteristics of ultrasonic waves, and the sound is reproduced only in a specific direction completely different from that of a normal speaker.

  Beam-like reproduction signals output from the ultrasonic transducers 424A and 424B constituting the ultrasonic speaker are radiated toward the projection plane (screen) on which the image is projected by the projection optical system 433, reflected by the projection plane, and diffused. To do. In this case, the reproduction signal is separated from the carrier wave in the radial axis direction (normal direction) from the sound wave emission surface of the ultrasonic transducers 424A and 424B according to the frequency of the carrier wave set in the reproduction range setting unit 412. The reproduction range changes because the distance to the distance and the beam width of the carrier wave (beam divergence angle) are different.

  FIG. 14 shows a state when a reproduction signal is reproduced by an ultrasonic speaker including the ultrasonic transducers 424A and 424B in the projector 401. In the projector 401, when the ultrasonic transducer is driven by the modulation signal obtained by modulating the carrier wave with the audio signal, if the carrier frequency set by the reproduction range setting unit 412 is low, the sound wave emission surface of the ultrasonic transducer 424 The distance until the reproduction signal is separated from the carrier wave in the radial axis direction (normal direction of the sound wave emission surface), that is, the distance to the reproduction point becomes long.

  Accordingly, the reproduced beam of the reproduced signal in the audible frequency band reaches the projection surface (screen) 402 without being relatively expanded, and is reflected on the projection surface 402 in this state. Therefore, the reproduction range is as shown in FIG. Becomes a audible range A indicated by a dotted arrow, and a reproduction signal (reproduction sound) can be heard only in a relatively far and narrow range from the projection plane 402.

  On the other hand, when the carrier frequency set by the reproduction range setting unit 412 is higher than the case described above, the sound wave radiated from the sound wave emission surface of the ultrasonic transducer 424 is narrowed compared to the case where the carrier frequency is low. However, the distance from the sound wave emission surface of the ultrasonic transducer 424 to the separation of the reproduction signal from the carrier wave in the radial axis direction (normal direction of the sound wave emission surface), that is, the distance to the reproduction point is shortened.

  Therefore, the reproduced reproduction signal beam in the audible frequency band spreads before reaching the projection surface 402 and reaches the projection surface 402, and is reflected at the projection surface 402 in this state. 14, the audible range B is indicated by a solid arrow, and a playback signal (playback sound) can be heard only in a relatively close and wide range from the projection surface 402.

  As described above, the display device (projector or the like) of the present invention includes the electrostatic ultrasonic transducer having the drive circuit for the class D amplifier of the present invention. Spurious noise that occurs during PWM modulation can be reduced by setting the PWM frequency to an integral multiple of the carrier frequency of the ultrasonic speaker when driving the circuit, so it is generated from the electrostatic ultrasonic transducer. Audible noise can be reduced. In particular, it is possible to suppress the audible sound noise component generated (feeled) when the output is 0 (only the carrier wave component).

  Although the embodiments of the present invention have been described above, the drive circuit, the ultrasonic speaker, and the display device of the class D amplifier according to the present invention are not limited to the above-described illustrated examples, but the gist of the present invention. Of course, various changes can be made without departing from the scope of the invention.

1 is a block diagram showing a basic circuit configuration of the present invention. The figure which shows the circuit structural example of the principal part of the drive circuit of the class D amplifier of this invention. The figure which shows the circuit structural example which showed the principal part shown in FIG. 2 more concretely. 4 is an operation timing chart of the circuit shown in FIG. The figure which shows the example of the signal waveform and frequency spectrum in the class D amplifier of this invention. The figure which shows the example of the signal waveform and frequency spectrum in the conventional class D amplifier. The figure which shows an example of the electrostatic transducer driven with a class D amplifier. The figure which shows the structural example of an ultrasonic speaker. The figure which shows the structural example of the drive circuit of a pull type electrostatic transducer. The figure which shows the structural example of the drive circuit of a piezoelectric type ultrasonic transducer. FIG. 3 is a diagram illustrating a configuration example of a projector. FIG. 12 is a diagram showing an overview configuration of the projector shown in FIG. 11. FIG. 13 is a diagram showing an electrical configuration example of the projector shown in FIG. 12. The figure which shows the reproduction | regeneration state of the reproduction signal by an ultrasonic transducer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Class D amplifier, 10 ... Clock oscillator, 11 ... Carrier wave generation circuit, 12 ... Amplitude modulation circuit, 13 ... PWM modulation circuit, 14 ... Gate drive circuit, 15 ... Class D output stage, 16 ... Low-pass filter, 17 ... Load, 21 ... Carrier wave generator, 22 ... Frequency divider, 23 ... UP / DOWN counter, 24 ... Amplitude modulation circuit, 25 ... Comparator, 31 ... Register (1), 32 ... Register (2), 33 ... Waveform calculation 34 ... Data memory, 35 ... Memory controller, 100 ... Push-pull type electrostatic transducer, 101 ... Fixed electrode, 101A ... Front side fixed electrode, 101B ... Rear side fixed electrode, 111 ... Support member, 112 ... Vibration Membrane, 114A, 114B ... through hole, 116 ... DC power supply, 118A, 118B ... AC signal, 120 ... insulating film, 121 ... vibrating membrane electrode, 131 ... audible Wave number wave signal source, 200 ... pull type electrostatic transducer, 211 ... dielectric (vibration film), 212 ... upper electrode, 213 ... lower electrode, 230 ... DC bias power supply, 231 ... signal source, 301 ... piezoelectric type Transducer, 311, 312 ... Piezoelectric element, 401 ... Projector, 402 ... Screen, 402 ... Projection surface (screen), 410 ... Operation input unit, 412 ... Playback range setting unit, 413 ... Playback range control processing unit, 414 ... Audio / Video signal reproduction unit, 417A, 417B ... high pass filter, 419 ... low pass filter, 420 ... projector main body, 421 ... mixer, 422A, 422B ... class D amplifier, 422C ... power amplifier, 423 ... bass reproduction speaker, 424A, 424B ... Ultrasonic transducer, 431 ... projector lens, 432 ... video Generating unit, 433 ... projection optical system

Claims (13)

A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. A driving method comprising:
When the input signal includes a carrier wave having a predetermined frequency, the fundamental frequency of the PWM signal is set to an integer multiple of the carrier wave frequency. A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. A driving method comprising:
Generating an amplitude modulated signal by amplitude modulating a carrier wave of a predetermined frequency with the input signal;
Generating a PWM signal by PWM modulating the amplitude modulated signal at a frequency that is an integer multiple of the carrier;
A procedure for switching the switching element by the PWM signal;
A method for driving a class D amplifier, comprising: The method of driving a class D amplifier according to claim 2, wherein the amplitude modulation method is a single sideband amplitude modulation (SSB) method. A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. A drive circuit,
A drive circuit for a class D amplifier, comprising means for setting the fundamental frequency of the PWM signal to an integral multiple of the carrier frequency when the input signal includes a carrier wave of a predetermined frequency. A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. A drive circuit,
A carrier wave generating circuit for generating a carrier wave having a predetermined frequency f_ca and generating a PWM clock signal UP / DOWN having a frequency that is an integral multiple of the carrier frequency f_ca;
An amplitude modulation circuit for amplitude-modulating the carrier wave with the input signal to generate an amplitude-modulated signal AM;
A PWM modulation circuit for PWM modulating the amplitude modulation signal AM based on the PWM clock signal UP / DOWN;
A drive circuit for a class D amplifier, comprising: A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. A drive circuit,
A clock oscillator that generates a clock signal CLK having a predetermined frequency f_CLK;
A frequency divider that divides the clock signal CLK by 1 / M (M is an integer) to generate a PWM clock signal UP / DOWN of frequency f_PWM (= f_CLK / M);
A carrier wave generator for generating a carrier wave having a frequency f_ca (= f_PWM / N) of 1 / N (N is an integer) of the frequency f_PWM of the PWM clock signal UP / DOWN;
An amplitude modulator that amplitude-modulates the carrier wave with the input signal to generate an amplitude-modulated signal AM;
An up / down counter that counts up or down the clock signal CLK according to the signal level of the clock signal UP / DOWN;
A comparator that compares the level value of the amplitude modulation signal AM with the count value of the up / down counter, converts the magnitude relationship into a binary level, and outputs the binary level;
A drive circuit for a class D amplifier, comprising: A first register (1) that holds a value N of a ratio between the frequency f_ca of the carrier wave and the frequency f_PWM of the PWM clock signal UP / DOWN as a set value;
A second register (2) for holding a division ratio value M of the frequency divider as a set value;
Further comprising
The carrier generator includes
A waveform calculator that references the values of the first register (1) and the second register (2) and samples the waveform of the carrier wave for one period into M × N values;
A data memory for storing waveform data sampled by the waveform calculator;
A memory for continuously generating a carrier wave by outputting data stored in the data memory while sequentially reading in synchronization with the clock signal CLK and repeatedly outputting M × N (one carrier wave period) data. A controller,
The drive circuit of the class D amplifier according to claim 6, comprising: The drive circuit of the class D amplifier according to any one of claims 4 to 7, wherein the amplitude modulation system is configured to be a single sideband amplitude modulation (SSB) system. A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. A driven electrostatic transducer comprising:
In the circuit for driving the class D amplifier,
A carrier wave generating circuit for generating a carrier wave having a predetermined frequency f_ca and generating a PWM clock signal UP / DOWN having a frequency that is an integral multiple of the carrier frequency f_ca;
An amplitude modulation circuit for amplitude-modulating the carrier wave with the input signal to generate an amplitude-modulated signal AM;
A PWM modulation circuit that PWM modulates the amplitude modulation signal AM based on the PWM clock signal UP / DOWN.
An electrostatic transducer characterized by the above. The electrostatic transducer is
A fixed electrode on the first surface side in which a plurality of holes are formed;
A fixed electrode on the second surface side in which a plurality of holes paired with the fixed electrode on the first surface side are formed;
The electrostatic transducer according to claim 9, wherein the electrostatic transducer includes a conductive layer sandwiched between the pair of fixed electrodes, and a vibration film to which a DC bias voltage is applied to the conductive layer. A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. An ultrasonic speaker configured by connecting an electrostatic ultrasonic transducer between output terminals,
In the circuit for driving the class D amplifier,
A carrier wave generating circuit for generating a carrier wave having a predetermined frequency f_ca and generating a PWM clock signal UP / DOWN having a frequency that is an integral multiple of the carrier frequency f_ca;
An amplitude modulation circuit for amplitude-modulating the carrier wave with the input signal to generate an amplitude-modulated signal AM;
A PWM modulation circuit that PWM modulates the amplitude modulation signal AM based on the PWM clock signal UP / DOWN.
Ultrasonic speaker characterized by. A display device comprising: an ultrasonic speaker that reproduces an audio signal supplied from an acoustic source and reproduces an audio frequency band signal sound; and a projection optical system that projects an image on a projection surface,
The ultrasonic speaker is
A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. Construct and connect an electrostatic ultrasonic transducer between the output terminals,
In the circuit for driving the class D amplifier,
A carrier wave generating circuit for generating a carrier wave having a predetermined frequency f_ca and generating a PWM clock signal UP / DOWN having a frequency that is an integral multiple of the carrier frequency f_ca;
An amplitude modulation circuit for amplitude-modulating the carrier wave with the input signal to generate an amplitude-modulated signal AM;
A PWM modulation circuit that PWM modulates the amplitude modulation signal AM based on the PWM clock signal UP / DOWN.
A display device. An ultrasonic speaker that reproduces a signal in the first range among audio signals supplied from an acoustic source, and a signal in a second range that is lower than the first range among audio signals supplied from the acoustic source. A directional acoustic system having a bass reproduction speaker for reproduction,
The ultrasonic speaker is
A totem pole type output circuit in which a high-side switching element connected to a first power supply and a low-side switching element connected to a second power supply or ground are connected to the output end side of the output circuit. A class-D amplifier that performs power amplification by turning on and off each switching element of the output circuit with a PWM signal obtained by PWM-modulating an input signal. Construct and connect an electrostatic ultrasonic transducer between the output terminals,
In the circuit for driving the class D amplifier,
A carrier wave generating circuit for generating a carrier wave having a predetermined frequency f_ca and generating a PWM clock signal UP / DOWN having a frequency that is an integral multiple of the carrier frequency f_ca;
An amplitude modulation circuit for amplitude-modulating the carrier wave with the input signal to generate an amplitude-modulated signal AM;
A PWM modulation circuit that PWM modulates the amplitude modulation signal AM based on the PWM clock signal UP / DOWN.
A directional acoustic system characterized by

Images