JP2010087939A - Digital amplifier system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce accuracy deterioration in a small signal region that is generated when mounting an electronic volume in a system using a PWM signal input type feedback digital amplifier. <P>SOLUTION: A digital amplifier system (100) is configured by including a digital signal processing unit (10), an electronic volume device (20) and a feedback digital amplifier (30). The electronic volume device is provided on a post-stage of the digital signal processing unit and on a pre-stage of the feedback digital amplifier, thereby maintaining sufficient signal reproducibility even in a small signal region. Furthermore, since an amplitude voltage of a PWM wave can be directly controlled in an analog manner at the electronic volume device, volume characteristics corresponding to an electronic volume of analog processing can be obtained. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is to reduce accuracy degradation in a small signal region that occurs when an electronic volume is mounted in a system using a feedback amplifier digital technology for digital amplifier control technology, particularly a PWM (Pulse Width Modulation) signal input. Regarding technology.

  As a classification of digital amplifier technology, from the input / output waveform, input analog input and output digital signal (PWM wave) (referred to as “analog input digital amplifier”) and digital signal (PWM wave) input Then, it is roughly classified into a type that outputs a digital signal (PWM wave) (referred to as “PWM signal input digital amplifier”).

  In addition, the type of signal processing includes a type having a feedback loop in the amplifier (referred to as “feedback digital amplifier”) and a type having no feedback loop in the amplifier (referred to as “non-feedback digital amplifier”). Sometimes it is divided roughly.

  As for the conventional digital amplifier, an analog input feedback type digital amplifier and a PWM signal input non-feedback type digital amplifier have occupied a large proportion.

  The advantage of the analog input feedback system digital amplifier is that it is possible to output sound with high accuracy without depending on the performance variation of LSI (Large Scale Integration) and the performance variation of external parts due to the feedback effect in the amplifier. In addition, since it is an analog input, it can be easily replaced from a conventional analog amplifier. On the other hand, the disadvantage is that an analog signal needs to be once passed from the digital signal source to the audio output, and the system is more complicated than the direct connection from digital to digital.

  On the other hand, the advantage of the PWM signal input non-feedback digital amplifier is that there is no need to pass an analog signal from the digital signal source to the audio output, so a digital signal processing LSI such as an audio DSP (Digital Signal Processor) and a digital amplifier The direct connection for digital is possible. Disadvantages are that it does not have a feedback loop, so it is vulnerable to variations in LSI performance and external component performance, and it is extremely difficult to guarantee stable and highly accurate audio output. It is difficult to replace from.

  At present, digital amplifier technology has entered a mature period, and a PWM signal input feedback digital amplifier, which is one step ahead of the conventional technology, is becoming mainstream. The PWM signal input feedback digital amplifier combines the advantages of the analog input feedback digital amplifier and the PWM signal input non-feedback digital amplifier to ensure stable and highly accurate audio output due to the feedback effect. In addition, the system can be simplified by providing a digital compatible direct connection between a digital signal processing LSI such as an audio DSP and a digital amplifier.

  By the way, in a system using a feedback digital amplifier with PWM signal input, it is not possible to implement a high-precision volume adjustment function like an electronic volume for analog processing. Have been implemented in the system. For example, Patent Document 1 describes an attenuation control technique for realizing volume control of an audio amplification system.

JP 2005-513901 A

  As described above, in a system using a feedback digital amplifier with PWM signal input, it is not possible to implement a high-accuracy volume adjustment function like an electronic volume for analog processing. It has been implemented in the system as an alternative. However, the digital volume of digital processing can easily adjust the fine volume peculiar to digital processing, but the signal reproduction is smaller in the small signal area than the analog volume of electronic processing due to restrictions such as the digital signal bit length. It is generally known that the performance is very inferior (it is greatly influenced by the quantization error peculiar to digital signal processing).

  Therefore, in a system using a feedback digital amplifier with PWM signal input, countermeasures against accuracy deterioration in a small signal region that occurs when an electronic volume is mounted is a major issue.

  An object of the present invention is to provide a technique for reducing accuracy degradation in a small signal region that occurs when an electronic volume is mounted in a system using a feedback digital amplifier with PWM signal input.

  The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

  A representative one of the inventions disclosed in the present application will be briefly described as follows.

  That is, a digital amplifier system is configured including a digital signal processing unit, an electronic volume device, and a feedback digital amplifier. By providing the electronic volume device after the digital signal processing unit and before the feedback digital amplifier, sufficient signal reproducibility can be secured even in a small signal region. In addition, since the electronic volume device can directly control the amplitude voltage of the PWM wave in an analog manner, a volume characteristic corresponding to an electronic volume for analog processing can be obtained.

  The effects obtained by the representative ones of the inventions disclosed in the present application will be briefly described as follows.

  That is, it is possible to reduce deterioration in accuracy in a small signal region that occurs when an electronic volume is mounted on a system using a feedback digital amplifier with PWM signal input.

1. Representative Embodiment First, an outline of a typical embodiment of the invention disclosed in the present application will be described. The reference numerals of the drawings referred to with parentheses in the outline description of the representative embodiments merely exemplify what are included in the concept of the components to which the reference numerals are attached.

  [1] A digital amplifier system (100) according to a typical embodiment of the present invention includes a digital signal processing unit (10), an electronic volume device (20), and a feedback system digital amplifier (30). The digital signal processing unit (10) performs predetermined signal processing on the input digital audio signal and outputs a PWM signal. The electronic volume device (20) is arranged after the digital signal processing unit and before the feedback digital amplifier, and controls the amplitude of the PWM signal output from the digital signal processing unit. The feedback digital amplifier (30) drives the load based on the PWM signal output from the digital signal processing unit.

  According to the electronic volume device having the above configuration, sufficient signal reproducibility can be secured even in a small signal region. In addition, since the electronic volume device 20 can directly control the amplitude voltage of the PWM wave in an analog manner, a volume characteristic corresponding to an electronic volume for analog processing can be obtained. Further, since the electronic volume device 20 is provided at the subsequent stage of the digital signal processing unit 10, the ΔΣ modulator (14) has a great influence on the system performance when the feedback type digital amplifier system with PWM signal input is configured. It is possible to reduce the influence of the requantization error that occurs.

  [2] In the above [1], the digital signal processing unit includes an audio signal processing circuit (13) that exhibits an electronic volume function (13A) by digital processing for an input audio signal, The electronic volume function in the audio signal processing circuit can be configured to share the signal attenuation. For example, the signal attenuation due to the electronic volume in the audio signal processing circuit is kept within the range where the influence of the quantization error does not become large, and by realizing the remaining attenuation with the electronic volume device, sufficient signal reproducibility is ensured even in a small signal region. It will be possible.

  [3] In the above [1], the electronic volume device includes a volume control signal generation circuit (22) for generating a volume control signal based on a digital signal having a plurality of bits transmitted from the audio signal processing circuit; A PWM amplitude control type electronic volume (21) that can control the amplitude of the PWM signal transmitted from the audio signal processing circuit based on the volume control signal can be configured.

  [4] To perform bias correction according to the feedback digital amplifier on the output signal of the PWM amplitude control type electronic volume based on the volume control signal in the electronic volume device in [3]. The bias correction circuit (23) can be provided.

  [5] In the above [1], the digital signal processing unit can be provided with a first volume control signal generator (22A) that generates a 1-bit PWM wave based on a digital signal having a plurality of bits. The electronic volume device includes a second volume control signal generator (22B) that generates a second volume control signal based on a 1-bit PWM wave generated by the first volume control signal generator; A PWM amplitude control type electronic volume (21) capable of controlling the amplitude of the PWM signal transmitted from the audio signal processing circuit based on the second volume control signal can be provided.

  [6] In the above [5], the electronic volume device has a bias correction according to the feedback type digital amplifier for the output signal of the PWM amplitude control type electronic volume based on the second volume control signal. A bias correction circuit (23) can be provided.

2. Next, the embodiment will be described in more detail.

<Embodiment 1>
FIG. 1 shows a configuration example of a digital amplifier system according to the present invention.

  A digital amplifier system 100 shown in FIG. 1 is not particularly limited, but is mounted on an electronic device such as a home audio, a car audio, a television receiver, or a mobile phone, and a speaker or headphones based on an input digital audio signal. It has the function to drive. Such a digital amplifier system 100 includes a digital signal processing unit 10, an electronic volume device 20, and a feedback digital amplifier 30, and is not particularly limited, but may be one of a single crystal silicon substrate or the like by a known semiconductor integrated circuit manufacturing technique. Formed on a semiconductor substrate.

  The digital signal processing unit 10 has a function of performing various signal processing on a digital audio signal such as PCM (pulse code modulation) transmitted from a digital signal processing unit (not shown). The digital signal processing unit 10 is configured as follows.

  Reference numeral 11 denotes a digital audio signal such as PCM. This digital audio signal 11 is usually supplied from a digital signal source via a large-scale digital signal processing circuit such as an audio DSP. Reference numeral 12 denotes an oversampling filter, and 13 denotes an audio signal processing circuit including an electronic volume 13A by digital processing. The audio signal processing circuit 13 is a DSP that enables digital signal processing, and the processing result is generated by a signal A (digital code) and a subsequent ΔΣ (delta sigma) modulator and PWM (Pulse Width Modulation). Is transmitted to the device 14. Note that the audio signal processing circuit 13 may be positioned before the oversampling filter 12. The ΔΣ modulator and PWM generator 14 performs ΔΣ modulation and PWM wave formation. When the ΔΣ modulator output is a 1-bit PDM (Pulse Density Modulation) wave, the PWM generator can be omitted, and the PWM wave in the subsequent processing can be read as a PDM wave. Reference numeral 15 denotes a digital interface. The digital interface 15 is a block for controlling a register control signal from a microcomputer or the like in an electronic apparatus in which the digital amplifier system 100 is mounted, for example, an audio signal processing circuit 13 or an electronic volume device. 20 is provided. Although not particularly limited, the signal D supplied from the digital interface 15 to the electronic volume device 20 is a digital signal having a multi-bit configuration for volume adjustment.

  The electronic volume device 20 is arranged between the digital signal processing unit 10 and the feedback digital amplifier 30, that is, after the digital signal processing unit 10 and before the feedback digital amplifier 30. The electronic volume device 20 has a function of directly controlling the amplitude level of the output signal B (PWM wave) of the ΔΣ modulator and the PWM generator 14 in the digital signal processing unit 10. A volume operation is realized by receiving a PWM wave as a signal input and controlling its amplitude. The electronic volume device 20 will be described in detail later.

  The feedback digital amplifier 30 has a function of receiving an output signal C (PWM wave) from the electronic volume device 20 and driving a load (speaker or headphones) based on the output signal C (PWM wave). The feedback digital amplifier 30 includes a pulse feedback circuit 31 and a switching driver circuit 32. The pulse feedback circuit 31 constitutes a feedback loop by receiving the amplitude-controlled signal C (PWM wave) and feeding back the digital amplifier output signal to this circuit. The switching driver circuit 32 includes a power driver for driving a load, a switching control circuit, and the like. By combining the pulse feedback circuit 31 and the switching driver circuit 32, a feedback digital amplifier 30 with a PWM signal input corresponding to a single end output or a BTL output is configured. The output signal of the feedback digital amplifier 30 is transmitted to the LC filter, speaker, or headphones 40 in the subsequent stage. Here, the high frequency component is rapidly attenuated by the LC filter, and the speaker (or headphones) is driven by the filtered output.

  FIG. 2 shows a configuration example of the electronic volume device 20.

  As shown in FIG. 2, the electronic volume device 20 includes a PWM amplitude control type electronic volume 21, a volume control signal generation circuit 22, an automatic bias correction circuit 23, and an addition circuit 24.

  The volume control signal generation circuit 22 forms a volume control signal F for volume adjustment according to the digital signal D having a plurality of bits transmitted from the digital interface 15 in the digital signal processing unit 10. The volume control signal F is a DC voltage corresponding to the digital signal D having a plurality of bits transmitted from the digital interface 15, and is supplied to the PWM amplitude control type electronic volume 21 and the automatic bias correction circuit 23. The PWM amplitude control type electronic volume 21 is an amplifier capable of amplifying the amplitude of the PWM signal by operating with the volume control signal F as a power supply voltage. With this configuration, the PWM amplitude control type electronic volume 21 performs amplitude control of the PWM signal according to the volume control signal F at that time. That is, according to the PWM amplitude control type electronic volume 21, it is possible to directly control the amplitude voltage of the PWM wave in an analog manner. The output signal of the PWM amplitude control type electronic volume 21 (amplitude-adjusted PWM signal) is transmitted to the adder 24 at the subsequent stage. The automatic bias correction circuit 23 biases according to the specifications of the feedback type digital amplifier 30 so that the PWM signal whose amplitude is controlled by the PWM amplitude control type electronic volume 21 is correctly recognized by the feedback type digital amplifier 30 in the subsequent stage. Make corrections. This bias correction is made possible by adjusting the signal level added by the adder 24 to the output signal of the PWM amplitude control type electronic volume 21 according to the volume control signal F at that time. Specifically, it can be performed as follows.

  The PWM signal whose amplitude is controlled by the amplitude control type volume 21 is controlled by setting the control signal F to the H (high) level. As a result, the waveform shown in FIG. 6A operates near the L (low) level. It becomes a signal like this. Therefore, for example, when the specification of the feedback digital amplifier 30 in the subsequent stage requires an operating point (bias point) corresponding to the midpoint potential between the power supply and the ground (GND), as shown in FIG. Input signal does not satisfy the input condition (H / L cannot be recognized correctly). Therefore, by adjusting the operating point (bias point) according to the control signal F by the automatic bias correction circuit 23, an amplitude level waveform is generated as shown in FIG. By using the input signal to 30, the input condition is satisfied. Such bias correction is not limited to the midpoint potential, and can be adjusted according to the specifications of the feedback digital amplifier 30 in the subsequent stage.

As described above, the bias correction is performed in conjunction with the PWM amplitude control in the PWM amplitude control type electronic volume 21, so that the feedback digital amplifier 30 can control the signal regardless of the amplitude control by the PWM amplitude control type electronic volume 21. C (PWM wave) can be accurately recognized.
(1) Since the electronic volume device 20 is provided, it is possible to obtain an effect of reducing the influence of the quantization error in the small signal region, which is a problem when the digital volume of digital processing is used. For example, as shown in FIG. 5, when the electronic volume device 20 (see FIG. 1) is not interposed between the digital signal processing unit 10 and the feedback system digital amplifier 30, for example, the volume attenuation is 0 dB to − When it is necessary in the range of 90 dB, it is necessary to realize all the control by a digital processing electronic volume. Therefore, when -80 dB or -90 dB is set, the signal is affected by the quantization error peculiar to digital signal processing. Will be greatly affected.

  On the other hand, as shown in FIG. 1, when the electronic volume device 20 is interposed between the digital signal processing unit 10 and the feedback digital amplifier 30, the volume setting in the system is set to 0 dB to -90 dB. In this case, it is not necessary to set the signal attenuation amount in the digital volume of digital processing to -90 dB. For example, it is possible to share the signal attenuation amount with -50 dB in the digital volume electronic volume 13A and -40 dB with the PWM amplitude control type electronic volume device 20. In other words, the signal attenuation amount by the electronic volume 13A in the audio signal processing circuit 13 is limited to a range in which the influence of the quantization error does not increase, and the remaining attenuation amount is realized by the electronic volume device 20, so that it is sufficient even in a small signal region. Signal reproducibility can be secured. In addition, since the PWM amplitude control type electronic volume device 20 can directly control the amplitude voltage of the PWM wave in an analog manner, a volume characteristic corresponding to an electronic volume for analog processing can be obtained. The attenuation amount in the electronic volume 13A and the attenuation amount in the electronic volume device 20 can be arbitrarily set depending on the application.

  (2) A ΔΣ modulator (14) that greatly affects system performance when a feedback type digital amplifier system with PWM signal input is configured by providing the electronic volume device 20 at the subsequent stage of the digital signal processing unit 10. The influence of the requantization error generated in the above can be reduced.

  For example, in the system configuration shown in FIG. 5, noise caused by a requantization error at a level equal to or higher than that of the feedback digital amplifier is generated in the ΔΣ modulator and the PWM generator 14. Performance is limited.

  On the other hand, according to the system configuration shown in FIG. 1, the PWM amplitude control type electronic volume device 20 is provided at the subsequent stage of the ΔΣ modulator and the PWM generator 14, so that the attenuation amount of the electronic volume is set large. In this case, the noise caused by the requantization error is also attenuated in the same manner as the signal. As a result, the system performance (signal reproducibility) in the small signal region is improved.

  (3) Since the feedback digital amplifier has a feedback loop, it has a fluctuation elimination characteristic of the PWM wave amplitude level. Normally, the PWM pulse in the digital amplifier is internally processed with the H (high) level as the power supply voltage and the L (low) level as the ground level, but as a method of directly controlling the amplitude level in the same way as described above. When the power supply voltage level is varied, the intended volume characteristic cannot be obtained due to the fluctuation removal characteristic by the feedback loop. For example, even if the H level voltage of PWM is changed to half (attenuation amount -6 dB), the attenuation amount of the output PWM wave is corrected in the direction that the pulse width in the time axis direction is doubled (+6 dB) by the feedback loop. In spite of the -6 dB setting, the total attenuation is 0 dB, which is not the intended attenuation.

  On the other hand, in the system configuration shown in FIG. 1, the PWM amplitude control type electronic volume device 20 is arranged before the feedback digital amplifier, in other words, outside the feedback loop. Will not be affected. Thereby, volume adjustment can be performed smoothly.

  (4) As described above, this system configuration can solve various disadvantages that have conventionally occurred in a system using a feedback type digital amplifier with PWM signal input, and can reduce the influence of quantization error. Therefore, a feedback digital amplifier system with excellent signal reproducibility can be realized.

<Embodiment 2>
FIG. 3 shows another configuration example of the digital amplifier system 100. The digital amplifier system 100 shown in FIG. 3 is greatly different from that shown in FIG. 1 in that a first volume control signal generator 22A is provided in the digital signal processing unit 10. The first volume control signal generator 22A forms a PWM signal E as a first volume control signal corresponding to the digital signal having a multi-bit configuration for volume adjustment supplied from the digital interface 15. This PWM signal E is transmitted to the electronic volume device 20.

  FIG. 4 shows another configuration example of the electronic volume device 20. The electronic volume device 20 shown in FIG. 4 is greatly different from that shown in FIG. 2 in that a volume control signal generating circuit for generating a volume control signal F for volume adjustment according to a digital signal D having a plurality of bits. Instead of 22, a second volume control signal generator 22B that forms a second volume control signal G for volume adjustment in accordance with a 1-bit PWM signal E is provided. That is, the second volume control signal generator 22B forms the second volume control signal G in an analog manner in accordance with the first volume control signal (PWM signal E) generated by the first volume control signal generator 22A. To do. The second volume control signal G is a DC voltage corresponding to the first volume control signal E (PWM wave) transmitted from the first volume control signal generator 22A, and, similarly to the case shown in FIG. This is supplied to the amplitude control type electronic volume 21 and the automatic bias correction circuit 23. Therefore, in the system configuration shown in FIGS. 3 and 4, the same operation and effect as in the case of the system configuration shown in FIGS. 1 and 2 can be obtained. In particular, in the system configuration shown in FIGS. 3 and 4, the signal wiring from the first volume control signal generator 22A to the second volume control signal generator 22B is required to transmit a 1-bit PWM signal. Since only the book is sufficient, the number of wirings between the digital signal processing unit 10 and the electronic volume device 20 can be greatly reduced as compared with the system configuration shown in FIGS.

  Although the invention made by the present inventor has been specifically described above, the present invention is not limited thereto, and it goes without saying that various changes can be made without departing from the scope of the invention.

1 is a block diagram illustrating a configuration example of a digital amplifier system according to the present invention. FIG. 2 is a block diagram illustrating a configuration example of an electronic volume device included in the digital amplifier system illustrated in FIG. 1. It is a block diagram of another example of the configuration of the digital amplifier system according to the present invention. FIG. 4 is a block diagram illustrating a configuration example of an electronic volume device included in the digital amplifier system illustrated in FIG. 3. FIG. 3 is a block diagram illustrating a configuration example of a system to be compared with the digital amplifier system illustrated in FIGS. 1 and 2. It is a wave form diagram for demonstrating the bias correction | amendment performed with the said electronic volume apparatus.

Explanation of symbols

10 Digital Signal Processing Unit 12 Oversampling Filter 13 Audio Signal Processing Circuit 14 ΔΣ Modulator and PWM Generator 15 Digital Interface 20 Electronic Volume Device 21 PWM Amplitude Control Type Electronic Volume 22 Volume Control Signal Generation Circuit 22A First Volume Control Signal Generator 22B Second volume control signal generator 23 Automatic bias correction circuit 24 Adder 30 Feedback type digital amplifier 31 Pulse feedback circuit 32 Switching driver circuit 40 LC filter, speaker or headphone 100 Digital amplifier system

Claims (6)

A digital signal processing unit that performs signal processing for outputting a PWM signal to an input digital audio signal and outputs a PWM signal;
A feedback digital amplifier for driving a load based on the PWM signal output from the digital signal processing unit;
And an electronic volume device that is arranged after the digital signal processing unit and before the feedback digital amplifier, and controls the amplitude of the PWM signal output from the digital signal processing unit. Amplifier system. The digital signal processing unit includes an audio signal processing circuit that exhibits an electronic volume function by digital processing for an input audio signal,
2. The digital amplifier system according to claim 1, wherein an attenuation amount of the signal is shared by the electronic volume device and a digital electronic volume function in the audio signal processing circuit. The electronic volume device includes a volume control signal generation circuit for generating a volume control signal for varying the amplitude of the PWM signal based on a digital signal having a plurality of bits transmitted from the audio signal processing circuit;
2. The digital amplifier system according to claim 1, further comprising: a PWM amplitude control type electronic volume capable of controlling the amplitude of the PWM signal transmitted from the audio signal processing circuit based on the volume control signal.   Based on the volume control signal, the electronic volume device is optimal for the PWM signal whose amplitude is varied in accordance with the input condition of the feedback digital amplifier with respect to the output signal of the PWM amplitude control type electronic volume. 4. The digital amplifier system according to claim 3, further comprising a bias correction circuit for adjusting the high-level and low-level voltages of the PWM signal without changing the amplitude so as to operate at various operating points. The digital signal processing unit includes a first volume control signal generating unit that generates a 1-bit PWM wave based on a digital signal having a plurality of bits.
The electronic volume device has a second volume control signal for varying the amplitude of the PWM signal transmitted from the audio signal processing circuit based on the 1-bit PWM wave generated by the first volume control signal generator. A second volume control signal generator for generating;
The digital amplifier system according to claim 1, further comprising: a PWM amplitude control type electronic volume capable of controlling the amplitude of the PWM signal transmitted from the audio signal processing circuit based on the second volume control signal.   The electronic volume device is configured such that, based on the second volume control signal, the amplitude of the output signal of the PWM amplitude control type electronic volume is varied according to the input condition of the feedback digital amplifier. 6. The digital circuit according to claim 5, further comprising a bias correction circuit for adjusting the high-level and low-level voltages of the second PWM signal without changing the amplitude so that the PWM signal operates at an optimum operating point. Amplifier system.

Images