Muting circuit

A muting circuit comprising: a DC offset eliminating unit including an HPF (High Pass Filter) constituted by a first operational amplifier to eliminate a DC offset component of an AC input signal, the DC offset eliminating unit being configured to output an output signal obtained by eliminating the DC offset component of the AC input signal; a muting signal generating unit including a second operational amplifier configured to generate a second DC offset identical in level to a first DC offset at the first operational amplifier, the muting signal generating unit being configured to output a muting signal indicative of the second DC offset by fixing input to the second operational amplifier at a predetermined DC level; and a selecting unit configured to select and output the output signal when muting is not executed, and to select and output the muting signal when muting is executed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Japanese Patent Application No. 2006-256163, filed Sep. 21, 2006, of which full contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a muting circuit.

2. Description of the Related Art

An electronic volume that electronically controls the volume of speakers is typically provided with a muting circuit that produces no-sound condition (mute) by temporarily cutting off an audio signal when the noise superimposed upon the audio signal is detected or when a mute button is pressed. The pitch and the volume of an audio signal are known to have characteristics to be adjusted by frequency control and by amplitude control, respectively. With the application of such the characteristics of an audio signal, a muting circuit executes a process to produce no-sound condition (hereinafter, referred to as a muting process) by fixing a DC level at a predetermined level to eliminate the amplitude of the audio signal (seeFIG. 4). As the predetermined DC level, “AC-GND” in an electronic volume is generally employed. The AC-GND means a GND applied for an AC signal that indicates a constant DC level as the amplitude center of an AC signal waveform.

FIG. 5is a schematic illustrating the configuration of an electronic volume10equipped with a conventional muting circuit30. As shown in the figure, the electronic volume10is composed mainly of an attenuation amount control circuit20that controls the attenuation amount of an AC input signal IN (audio signal) for volume control, and a muting circuit30, that is equipped with analogue switches SW1and SW2that are switched on and off complimentarily by the electronic control of a microcomputer (not shown), that executes the muting process for the AC input signal IN after attenuation amount control (see, e.g., Japanese Patent Application Laid-Open Publication No. 1999-327579).

In normal state where muting process is not executed, the muting circuit30outputs the attenuation amount-controlled AC input signal IN as is as an AC output signal OUT by switching the analogue switches SW1on and SW2off. Here, the amplitude center of the AC input signal IN is to indicate a constant DC level Va. On the other hand, when muting process is executed, the muting circuit30outputs an AC-GND level Vb by switching the analogue switches SW1off and SW2on (see sections a and b ofFIG. 6). Consequently, no-sound condition can be produced since the DC level of the AC output signal OUT is fixed at the AC-GND level Vb.

In the case of a conventional muting circuit such as the muting circuit30shown inFIG. 5, an AC input signal IN without muting process contains a DC offset Vx, that is a difference voltage between the AC-GND level Vb, due to the internal circuit characteristics of the electronic volume10and/or due to the manufacturing variation of ICs that compose the attenuation amount control circuit20. The DC voltage of an AC output signal OUT is fluctuated by the DC offset Vx when the analogue switches SW1and SW2are switched-over between on and off, and noise is generated by such DC voltage fluctuations.

In the ideal state of an AC output signal OUT as shown in section c ofFIG. 6, where a DC voltage fluctuation is induced momentarily at the start or the end of muting, the noise generated by the DC voltage fluctuation becomes a sound out of the audible frequency range, and therefore, the sound is harmless in being inaudible to humans, and thus it is not problematic. However, as an AC output signal OUT fluctuates slowly at the start or the end of muting as shown in section d ofFIG. 6due to the internal circuit characteristics of the electronic volume10and/or due to the manufacturing variation of ICs, this has been problematic, generating abrasive noise within the audible frequency range.

SUMMARY OF THE INVENTION

A muting circuit according to an aspect of the present invention, comprises: a DC offset eliminating unit including an HPF (High Pass Filter) constituted by a first operational amplifier to eliminate a DC offset component of an AC input signal, the DC offset eliminating unit being configured to output an output signal obtained by eliminating the DC offset component of the AC input signal; a muting signal generating unit including a second operational amplifier configured to generate a second DC offset identical in level to a first DC offset at the first operational amplifier, the muting signal generating unit being configured to output a muting signal indicative of the second DC offset by fixing input to the second operational amplifier at a predetermined DC level; and a selecting unit configured to select and output the output signal when muting is not executed, and to select and output the muting signal when muting is executed.

Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

At least the following details will become apparent from descriptions of this specification and of the accompanying drawings.

<<Configuration/Operation of Audio Signal Processing System>>

FIG. 1is a schematic illustrating the configuration of an audio signal processing system according to an embodiment of the present invention. For audio signals according to the present invention, sound signals from various sound sources such as a television, an AM/FM radio, a VTR (Video Tape Recorder), optical discs like a CD and a DVD, AUX (AUXiliary), etc. can be considered.

The audio signal processing system is composed of an acoustic DSP (Digital Signal Processor)100that executes a digital signal process, such as equalization and sound field correction, on digital audio signals from various sound sources, an electronic volume200that controls sound volume by the attenuation amount control of an analogue audio signal, that is D/A converted from the output of the DSP100, a power amplifier300that drives a speaker500to output an analogue audio signal after volume-control by the electronic volume200, and a microcomputer400for the integrated control of the entire system.

Here, the electronic volume200is equipped with an attenuation amount control unit210and a muting circuit220, that are to be controlled by the microcomputer400, and is a single chip of analogue/digital system-on-a-chip LSI, that is independent from the DSP100.

The attenuation amount control unit210controls the attenuation amount of an AC input signal IN (analogue audio signal) for the purpose of sound volume control, that is a primary function of the electronic volume200. For example, with the parallel arrangement of a plurality of pairs of analogue switch and feedback resistance (both are not shown) in the negative-feedback path of a negative-feedback amplifier, attenuation amount by the negative-feedback amplifier is controlled based on the value of combined feedback resistance by appropriately switching on and off the multiple analogue switches according to the sound volume indicated by the microcomputer400.

The muting circuit220is a circuit to output an AC output signal that is obtained by muting an AC input signal IN′ (AC input signal IN after attenuation amount control) by fixing its amplitude level at a DC level (for example, AC-GND level).

In addition to the system configuration shown inFIG. 1, the electronic volume200may be disposed before the DSP100for the system configuration to receive an analogue audio signal. It is also possible to integrate the function(s) of the electronic volume200(for example, attenuation amount control function), other than the muting circuit220, as a part of the functions of the DSP100.

A muting circuit220according to an embodiment of the present invention is explained based onFIG. 2and referring toFIG. 3.FIG. 3is a schematic illustrating the principal state transition to explain the operation of a muting circuit220.

The muting circuit220according to the present invention is equipped with a DC offset eliminating unit221, a muting signal generating unit224, analogue switches SW1, SW2, and a voltage follower227.

The DC offset eliminating unit221is a circuit to eliminate DC offset components included in an AC input signal IN′, that are generated due to the internal circuit (e.g., the attenuation amount control unit210) characteristics of the electronic volume200and/or due to the manufacturing variation of ICs, by routing the DC offset components through an HPF (High Pass Filter) constituted by an operational amplifier222(a “first operational amplifier” according to the present invention).

In detail, a first feedback resistance R3is disposed in the negative-feedback path of the operational amplifier222; a first input resistance R1and a second input resistance R2are arranged at the inverting input side and the non-inverting input side of the operational amplifier222, respectively; the AC input signal IN′ is input in common to the inverting input and non-inverting input of the operational amplifier222through the first and second input resistances R1and R2, respectively; and a DC cut capacitor C1is disposed between the non-inverting input of the operational amplifier222and an AC-GND or an analogue GND in parallel with a first ground resistance R4. With this composition, the DC offset eliminating unit221constitutes the HPF.

While the DC offset components included in an AC input signal IN′, that is generated due to the internal circuit characteristics and/or due to the manufacturing variation of ICs, has been eliminated by the DC offset eliminating unit221, a DC offset Vy (a “first DC offset” according to the present invention) specific to the operational amplifier222will be included in the AC input signal IN′ since the operational amplifier222is employed in the DC offset eliminating unit221. This is because an ordinary operational amplifier has a potentiality to generate a slight difference in electric potential between an inverting and a non-inverting input even though its internal circuit is ideally designed. The DC offset Vy specific to the operational amplifier222is indicated as the DC voltage of an output voltage Vc referring to an AC-GND level.

A muting signal generating unit224is a circuit constituted by an operational amplifier225(a “second operational amplifier” according to the present invention) that generates a DC offset Vy′ (a “second DC offset” according to the present invention) identical to the level of a DC offset Vy specific to the operational amplifier222, and outputs a muting signal Vd that indicates the DC offset Vy′ by fixing an input to the operational amplifier225at the AC-GND level that is common to the operational amplifier222. The DC offsets Vy and Vy′ can be set at an identical level by employing the operational amplifiers222and225produced from an identical wafer and thus with an identical performance.

Being provided with an identical peripheral circuit with the HPF constituted by the operational amplifier222, the operational amplifier225can set DC offsets Vy and Vy′ more accurately at an identical level. Its concrete configuration is as follows: a second feedback resistance R7identical to the first feedback resistance R3is disposed in the negative-feedback path of the operational amplifier225; a third input resistance R5identical to the first input resistance R1and a fourth input resistance R6identical to the second input resistance R2are arranged on the inverting input side and on the non-inverting input side of the operational amplifier225, respectively; and the third and fourth input resistances R5and R6are connected in common to the AC-GND, that is common to the operational amplifier222, through a second ground resistance R8identical to the first ground resistance R4.

A selecting unit226is a circuit that receives an AC input signal IN′ (hereinafter, referred to as an AC input signal Vc) after DC offset elimination, output by the DC offset eliminating unit221, and a muting signal Vd output by the muting signal generating unit224, and then selects the AC input signal Vc to output as an AC output signal OUT in a normal state without muting, or selects the muting signal Vd to output as an AC output signal OUT in a state muting is necessary.

The selecting unit226is provided with the analogue switches SW1and SW2with identical characteristics, that are complementarily switched on and off by the electronic control by the microcomputer400, to select and to output an AC input signal Vc or a muting signal Vd.

The analogue switch SW1is disposed between the output of the operation amplifier222and the non-inverting input of the voltage follower227. In the case that no muting command is given by the microcomputer400, the analogue switch SW1is switched on, and an AC input signal Vc is input to the non-inverting input of the voltage follower227. On the other hand, in the case that a muting command is given by the microcomputer400, the analogue switch SW1is switched off, and the input of the AC input signal Vc to the non-inverting input of the voltage follower227is shut off.

The analogue switch SW2is disposed between the output of the operation amplifier225and the non-inverting input of the voltage follower227. In the case that a muting command is given by the microcomputer400, the analogue switch SW2is switched on, and a muting signal Vd is input to the non-inverting input of the voltage follower227. On the other hand, in the case that no muting command is given by the microcomputer400, the analogue switch SW2is switched off, and the input of the muting signal Vd to the non-inverting input of the voltage follower227is shut off.

In the case that the analogue switch SW1is on and the analogue switch SW2is off, a muting process is not executed, and an AC output signal OUT from the muting circuit220, that is output through the voltage follower227, is identical to an AC input signal Vc with a DC level of the DC offset level Vy specific to the operational amplifier222(sections a and b ofFIG. 3).

On the other hand, in the case that the analogue switch SW1is off and the analogue switch SW2is on, a muting process is executed, and an AC output signal OUT from the muting circuit220, that is output through the voltage follower227, is a muting signal Vd that indicates a DC offset Vy′ identical to the level of the DC offset Vy specific to the operational amplifier222(sections a and b ofFIG. 3).

As the operational amplifiers222and225are produced from an identical wafer and thus have identical performances, and as their peripheral circuit designs and composing elements are also identical, the DC offsets Vy and Vy′ can be regarded as virtually identical. Therefore, as the level of the AC output signal OUT from the muting circuit220can be regarded to be constant without fluctuations (section c ofFIG. 3) at the timing of the switching over of the analogue switches SW1and SW2between on and off, i.e., at the timing of the start or the end of muting, it is possible to reduce not only noises out of the audible frequency range, as has been done, but also those within the audible frequency range.

The above embodiments of the present invention are simply for facilitating the understanding of the present invention and are not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompass equivalents thereof.