Signal processor

Abstract

To provide technical means for restoring flat low-frequency frequency characteristics attenuated by an HPF in a signal processor including an ADC having the HPF.SOLUTION: A signal processor 100 includes: a first signal processing unit 10 that performs A / D conversion and band limiting processing on an input signal; and a second signal processing unit 20 that performs signal restoration processing on an output signal of the first signal processing unit 10, based on an inverse function of a transfer function obtained by approximating the band limiting processing.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This invention relates to a signal processing device suitable for a voice input device.

Background Art

[0002] Voice input devices such as digital mixers and audio interfaces are required to have flat frequency characteristics in the audible band. However, in this type of voice input device, an ADC (Analogue to Digital Converter) having an HPF (High Pass Filter) is provided for DC offset removal and the like (see, for example, Non-Patent Document 1). For this reason, in voice input devices, a problem occurs in that the frequency characteristics in the low frequency range are attenuated.

Prior Art Documents

Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] To avoid the attenuation of the low-frequency characteristics, it is conceivable to use an ADC without an HPF. However, as a general trend, an ADC with an HPF as the hardware configuration of the ADC circuit is cheaper than an ADC without an HPF. In addition, since the ADC with an HPF has a higher degree of freedom in the design of the ADC circuit, the hardware designer can design the ADC circuit from a wide range of hardware configuration options when selecting the ADC. Therefore, adopting an ADC with an HPF can reduce the component price of the ADC and lower the cost. Thus, it is conceivable to use an ADC with an HPF and restore the attenuated low-frequency characteristics by the HPF to flat by means such as an EQ (Equalizer). However, with such means, the low frequency cannot be fully restored.

[0005] The present invention has been made in view of the above circumstances, and an object thereof is to provide a technical means for restoring the attenuated low-frequency characteristics by an HPF to flat in a signal processing device including an ADC having an HPF.

Means for Solving the Problems

[0006] The present invention provides a signal processing device having a first signal processing unit that performs A / D conversion and band-limiting processing on an input signal, and a second signal processing unit that performs signal restoration processing based on the inverse function of the transfer function obtained by approximating the band-limiting processing on the output signal of the first signal processing unit.

Brief Description of the Drawings

[0007]

Figure 1

Embodiments for Carrying Out the Invention

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0009] FIG. 1 is a block diagram showing the configuration of a signal processing apparatus 100 according to an embodiment of the present invention. The signal processing apparatus 100 is an apparatus that processes an analog signal generated by an analog signal generation unit 1, and includes a first signal processing unit 10 and a second signal processing unit 20.

[0010] The analog signal generation unit 1 is a sensor in an electroacoustic conversion device such as a microphone, for example, and outputs an analog signal indicating a received sound pressure waveform.

[0011] The first signal processing unit 10 includes an HPFcup and an ADC12 connected in series. Here, HPFcup is an analog HPF constituted by a coupling capacitor interposed between the analog signal generation unit 1 and the ADC12. The ADC12 is a circuit that converts an analog signal supplied from the analog signal generation unit 1 via the HPFcup into a digital signal. The ADC12 in the present embodiment includes an HPFint that attenuates the low frequency range. This HPFint is a digital HPF incorporated in the ADC12.

[0012] The second signal processing unit 20 includes an HPFa, an invHPFcup, and an invHPFint connected in series. Here, HPFa is a digital HPF that performs the band-limiting process (high-pass process) that should originally be performed on the analog signal generated by the analog signal generation unit 1 on the output signal of the first signal processing unit 10.

[0013] invHPFcup is a filter that cancels the action of the HPFcup of the first signal processing unit 10, and invHPFint is a filter that cancels the action of the HPFint of the first signal processing unit 10. More specifically, invHPFcup is a digital filter that performs a signal restoration process based on the inverse function of the transfer function of HPFcup on the output signal of HPFa. Also, invHPFint is a digital filter that performs a signal restoration process based on the inverse function of the transfer function of HPFint on the output signal of invHPFcup.

[0014] According to this embodiment, the signal restoration process based on the inverse function of the transfer function of HPFcup of the first signal processing unit 10 is executed by invHPFcup of the second signal processing unit 20, and the signal restoration process based on the inverse function of the transfer function of HPFint of the first signal processing unit 10 is executed by invHPFint of the second signal processing unit 20. Therefore, the low-frequency characteristics of the analog signal attenuated by HPFcup and HPFint of the first signal processing unit 10 can be restored to the original frequency characteristics. In this embodiment, each process is executed in the order of HPFa, invHPFcup, and invHPFint for the output signal of the first signal processing unit 10, but the order of each process is arbitrary.

[0015] Next, a specific design example of this embodiment will be described. In this design example, HPFcup and HPFint of the first signal processing unit 10 are each approximated by a first-order IIR filter. The transfer function H(z) of this first-order IIR filter is as shown in the following equation.

Equation

[0016] In this design example, for each of HPFcup and HPFint, the transfer function H(z) of the above equation (1) is obtained. More specifically, based on the frequency characteristics of each HPF, the coefficients a1, b0, and b1 of the transfer function H(z) are obtained.

[0017] Next, in this design example, the transfer function invH(z) of each of invHPFcup and invHPFint of the second signal processing unit 20 is obtained. This transfer function invH(z) is the inverse function of the transfer function H(z) of the above equation (1) and is given by the following equation.

Equation

[0018] And the coefficients a'1, b'0, and b'1 in the transfer function invH(z) are given by the following equation.

Equation

[0019] In this design example, for each of invHPFcup and invHPFint in the second signal processing unit 20, the coefficients a'1, b'0, and b'1 in the above formulas (3·1) to (3·3) are calculated using the transfer functions of HPFcup and HPFint in the first signal processing unit 10.

[0020] By the way, in the transfer function invH(z) of the above formula (2), since the coefficient a'1 is 1, z = 1 in the z-plane becomes a pole. When a pole occurs on the unit circle in the z-plane or when a pole occurs outside the unit circle in this way, the IIR filter diverges.

[0021] Therefore, in this design example, for example, by making the coefficient a'1 slightly smaller than the value obtained by the formula (3·1), the pole is moved inside the unit circle, and the IIR filter of the transfer function formula (2) is converged. Then, using the coefficients thus obtained, invHPFcup and invHPFint in the second signal processing unit 20 are configured.

[0022] According to the present embodiment, since invHPFcup and invHPFint that cancel the actions of HPFcup and HPFint in the first signal processing unit 10 are provided in the second signal processing unit 20, the low-frequency characteristics of the analog signal attenuated by HPFcup and HPFint can be restored to the original frequency characteristics. Further, in the present embodiment, since the transfer functions of invHPFcup and invHPFint are transfer functions in which some coefficients of the inverse functions of the transfer functions of HPFcup and HPFint are adjusted, divergence can be avoided and stable signal processing can be performed. Further, by performing such coefficient adjustment, the group delay of invHPFcup and invHPFint can also be shortened.

[0023] As described above, one embodiment of the present invention has been described, but other embodiments are also conceivable for the present invention. For example, as follows.

[0024] (1) In the above embodiment, HPFcup and HPFint are approximated by a first-order IIR filter, but they may be approximated by filters other than the first-order IIR filter.

[0025] (2) In the above embodiment, the present invention is applied to a signal processing device that processes a single-channel analog signal, but the present invention is also applicable to a signal processing device that processes a multi-channel analog signal.

[0026] (3) The analog signal generation unit 1 and the signal processing device 100 may be integrated to form, for example, a microphone with a signal processing device.

Description of Reference Numerals

[0027] 1... Analog signal generation unit, 100... Signal processing device, 10... First signal processing unit, 20... Second signal processing unit, 12... ADC.

Claims

1. A first signal processing unit that performs A / D conversion and band-limiting processing on an input signal; A second signal processing unit that performs signal restoration processing based on the inverse function of the transfer function obtained by approximation of the band-limiting processing on the output signal of the first signal processing unit; A signal processing apparatus comprising the above.

2. The signal processing apparatus according to claim 1, wherein the inverse function has poles on the unit circle in the z-plane, and the signal restoration processing is a process of moving the poles inside the unit circle by adjusting coefficients constituting the transfer function of the inverse function.

3. The signal processing apparatus according to claim 1, wherein the first signal processing unit is connected to an electroacoustic conversion device.

4. The signal processing apparatus according to claim 1, wherein the signal restoration processing is a first-order IIR filter.

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