JP2007060469A - Residual noise reduction circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a residual noise reduction circuit in which manufacturing costs are reduced, residual noises are sufficiently reduced, and sound quality is excellent. <P>SOLUTION: An audio circuit drives a speaker by amplifying and/or attenuating an audio signal outputted from a volume control mean (DSP) by means of an amplifier and/or an attenuator. With a predetermined threshold value (a) as a border in the middle of continuously changing a volume control signal (VR position) for controlling the volume control means from a maximum volume to a minimum volume, a gain of the volume control means is discontinuously increased. Within ranges where the volume control signal is higher and lower than the threshold value (a), the gain of the volume control signal is continuously decreased. At the threshold value (a), a gain of the amplifier and/or attenuator is changed so as to cancel the discontinuous gain changes of the volume control means. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to an audio circuit, and more particularly to a residual noise reduction circuit thereof.

  Conventionally, an electronic device having an audio circuit such as a wireless device has a problem that the S / N ratio is deteriorated when the volume is lowered due to residual noise. The residual noise will be described with reference to an example of an audio circuit of a conventional wireless device shown in FIG.

  A DSP (Digital Signal Processor) 20 in the audio circuit of the radio shown in FIG. 4 is inputted with an audio signal from a detection circuit (not shown), processed as a digital signal, converted into an analog signal, and configured by a resistor. The signal is output to the amplifier 21 via the attenuator 6. The amplifier 21 amplifies the input audio signal and drives the speaker 12 selected by the changeover switch 11 or an external speaker connected to the external speaker terminal 13.

  The DSP 20 performs volume control by a signal from a control circuit (not shown), and the gain of the amplifier 21 is constant. The sound output from the amplifier 21 when the volume is reduced to the minimum by the DSP 20 is referred to as residual noise. In this example, the volume control is performed by the DSP 20, but when the volume control is performed by an electronic volume or a mechanical volume, noise generated between these volume control circuits and the audio output unit is referred to as residual noise.

  That is, the residual noise is noise generated in the noise output from the volume control circuit, the amplified power line noise of the amplifier after the volume control circuit, the amplifier after the volume control circuit, or the like.

  In general, in the case of a wireless device, since it is assumed that the wireless device is used outdoors outdoors, the maximum level of the audio output cannot be set low. Therefore, when the volume is lowered and used when the room is used indoors quietly or when headphones are used, the S / N ratio is significantly deteriorated due to residual noise.

  When voice processing is performed by the DSP, the volume control is usually performed by the normal DSP as described with reference to FIG. In general wireless devices, the required maximum volume is determined, and the gain after the DSP (volume control circuit) is determined by the maximum possible output of the DSP.

  When an external electronic volume is used, the voltage amplitude of the full swing up to the power supply voltage can be controlled. However, if an electronic volume is used in addition to the DSP, the manufacturing cost increases. Since the voltage amplitude that can be handled by the DSP is lower than the voltage amplitude of the full swing, the gain after the volume control circuit is increased, and as a result, the residual noise level is increased. In order to reduce the residual noise, it may be possible to use an expensive low-noise component, but this is not practical.

  If the residual noise contains many frequency components with high audio frequency, it will sound noisy, so it is possible to suppress the influence of the residual noise by cutting the high range. For example, high frequency cut is effective for CW reception. Become. However, if the high frequency is cut in FM and AM modes with all mode radios (all radio format radios), the sound will be muffled. Therefore, in order to ensure the sound quality of FM and AM modes, the effect of residual noise will be affected by using a low-pass filter. I can't hold it down.

The band varying device proposed in Japanese Patent Laid-Open No. 10-53330 sets the reproduction bandwidth by LPF (low pass filter) and HPF (high pass filter) according to the received radio wave format. However, the one proposed in Japanese Patent Application Laid-Open No. 10-51330 cannot sufficiently reduce the influence of residual noise because the gain after the volume control circuit is constant.
JP-A-10-51330, paragraphs 0012 to 0015, FIGS. 1 and 2

  The present invention has been made in view of the above points, and its object is that the manufacturing cost is low, the residual noise is sufficiently reduced, and the sound quality in FM and AM modes in a radio is good. It is to provide a residual noise reduction circuit.

  The residual noise reduction circuit according to the present invention is a volume for controlling the volume control means in an audio circuit for driving a speaker by amplifying and / or attenuating an audio signal output from the volume control means with an amplifier and / or an attenuator. The gain of the volume control means is discontinuously increased at a predetermined threshold value in the middle of the control signal continuously changing from the maximum volume to the minimum volume, and the volume control signal is higher and lower than the threshold in the above range. The gain of the volume control signal is continuously decreased, and the gain of the amplifier and / or attenuator is changed at the threshold value so as to cancel out the discontinuous gain change of the volume control means.

  In the residual noise reduction circuit, a DSP is used as the volume control means, and an amplifier having a gain switching terminal is used as the amplifier.

  In each of the residual noise reduction circuits, an audio signal from a wireless device is input to the volume control unit, and the audio signal output from the volume control unit is configured to drive a speaker through a low-pass filter, and the radio The cut-off frequency of the low-pass filter is set to a high or low state according to the radio wave mode or squelch state received by the machine and the magnitude of the volume control signal.

  In the residual noise reduction circuit, a sound signal from a wireless device is input to the sound volume control unit, a sound signal from the wireless device is input to the sound volume control device, and a low pass filter is connected to the DSP as the sound volume control device. Is built in, and the cut-off frequency of the low-pass filter is set to a high state or a low state according to the mode of radio waves received by the radio or the state of the squelch and the magnitude of the volume control signal. It is.

  According to the residual noise reduction circuit of the present invention, the manufacturing cost is low, the residual noise is sufficiently reduced, and good sound quality is ensured in the FM and AM modes of the radio.

  The best mode for carrying out the present invention will be described below with reference to examples. FIG. 1 is a block diagram showing a radio apparatus to which a residual noise reduction circuit according to an embodiment of the present invention is applied. The received signals in the SSB, CW, FSK, FM, AM mode, etc. input from the antenna 1 shown in FIG. 1 are tuned / detected by the tuning / detection circuit 2 and are input to the DSP 5 as audio signals.

  The signal input to the DSP 5 is processed as a digital signal by the DSP 5, converted to an analog signal, and then output to the amplifier 8 via the attenuator 6 formed of a resistor. The amplifier 8 amplifies the input audio signal and drives the speaker 12 selected by the changeover switch 11 or an external speaker connected to the external speaker terminal 13.

  The control circuit 3 includes a microcomputer, and receives signals from the operation unit 4 including keys, rotary knobs, and the tuning / detection circuit 2, and controls the tuning / detection circuit 2, DSP 5, and analog switches 9, 10. . The operation unit 4 outputs a signal or the like for instructing a tuning frequency or a volume to the control circuit 3, and the control circuit 3 performs sound quality control and volume control according to the signal input from the operation unit 4.

  The analog switch 9 that is turned on / off by the control circuit 3 sets the cutoff frequency of the low-pass filter 7 composed of a capacitor to a high state or a low state. The amplifier 8 includes a gain switching terminal 8a, and the gain switching terminal 8a is connected to the ground via an analog switch 10 that is turned on / off by the control circuit 3.

  Next, the operation of the radio apparatus having the above configuration will be described with reference to FIGS. The rotation angle of the volume setting volume of the operation unit 4 is shown on the horizontal axis of the graph of FIG. A volume setting signal that monotonously decreases in accordance with the rotation angle of the volume setting volume is input from the operation unit 4 to the control circuit 3, and the control circuit 3 controls the gain of the DPS 5 and the analog switch 10 in accordance with the volume setting signal.

  That is, as shown in FIG. 2 (c), the gain of the DPS 5 is monotonously decreased until the volume setting signal when the rotation angle of the volume setting volume reaches the point a, and is increased discontinuously at the point a. When the point a is passed, the gain of the DPS 5 is monotonously decreased again.

  The control circuit 3 further closes the analog switch 10 at point a when the volume setting signal decreases monotonously. Therefore, as shown in FIG. 2 (b), the gain of the amplifier 8 becomes a binary value of high and low with respect to the point a. The absolute value of the decreasing gain of the DPS 5 and the increasing gain of the amplifier 8 at the point a is the same. Therefore, as shown in FIG. 2A, the audio output monotonously decreases according to the rotation angle of the volume setting volume. Although not shown in FIG. 2, the control circuit 3 monitors the state of the tuning / detection circuit 2 and closes the squelch, and in that case, decreases the gain of the DPS 5.

  The control circuit 3 also closes the analog switch 9 and sets the low-pass filter cutoff frequency low when the volume setting signal falls below a predetermined value when the received radio wave format is SSB, CW, FSK mode or the squelch is closed. To do. FIG. 3 shows the characteristics of the audio circuit gain with respect to the audio frequency when the analog switch 9 is open (OFF).

  The solid line in FIG. 3 shows the characteristics when the cutoff frequency of the low-pass filter is high, and the dotted line shows the characteristics when the cutoff frequency is low. In the SSB, CW, and FSK modes, when the volume is reduced by the volume setting volume, the high range is cut, but in these modes, the high range is not required, and the adverse effect of residual noise is further reduced.

  In the above embodiment, the residual noise can be greatly reduced without using an electronic volume or the like in addition to the DSP. Furthermore, since the high range is cut when the volume is low in the SSB, CW, and FSK modes, the difficulty of hearing residual noise at the low volume is further improved.

  The embodiment is configured as described above, but the invention is not limited to this. For example, instead of switching the gain of the amplifier after the volume control means, the gain of the attenuator provided after the volume control means may be switched.

  Furthermore, the effect of the present invention can be obtained without providing the low-pass filter 7. Further, the DSP 5 may have a low-pass filter function. Further, the effect of the present invention can be obtained even when an electronic volume is used as the volume control means instead of the DSP.

1 is a block diagram showing a radio device to which a residual noise reduction circuit according to an embodiment of the present invention is applied. FIG. It is a graph which shows the characteristic of each part of the radio. It is a graph which shows the frequency characteristic of the radio. It is a block diagram which shows the example of the audio circuit of the conventional radio | wireless machine.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Antenna 2 Tuning / detection circuit 3 Control circuit 4 Operation part 5 DSP
6 Attenuator 7 Low-pass filter 8 Amplifier, 8a Gain switching terminal 9, 10 Analog switch 11 Change-over switch 12 Speaker 13 External speaker output terminal 20 DSP
21 Amplifier

Claims (4)

In an audio circuit for driving a speaker by amplifying and / or attenuating an audio signal output from the volume control means with an amplifier and / or an attenuator, the volume control signal for controlling the volume control means is from a maximum volume to a minimum volume. The gain of the volume control means is increased discontinuously at a predetermined threshold value in the middle of continuous change, and the gain of the volume control signal is continuously increased in a range where the volume control signal is higher and lower than the threshold value. A residual noise reduction circuit characterized in that, at the threshold value, the gain of the amplifier and / or attenuator is changed so as to cancel out the discontinuous gain change of the volume control means. 2. The residual noise reduction circuit according to claim 1, wherein a DSP is used as the volume control means, and an amplifier having a gain switching terminal is used as the amplifier. An audio signal from a radio is input to the volume control means, and the audio signal output from the volume control means is configured to drive a speaker through a low-pass filter, and the radio wave mode or squelch received by the radio is received. The residual noise reduction circuit according to claim 1 or 2, wherein a cut-off frequency of the low-pass filter is set to a high state or a low state according to a state and a magnitude of the volume control signal. An audio signal from a radio is input to the volume control means, an audio signal from the radio is input to the volume control means, and a low-pass filter is incorporated in the DSP as the volume control means. The residual noise reduction circuit according to claim 1 or 2, wherein a cut-off frequency of the low-pass filter is set to a high state or a low state in accordance with a radio wave mode or squelch state received by the receiver and a magnitude of the volume control signal. .

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