JP2007214633A - Pop noise reduction circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pop noise reduction circuit for reducing pop noise generated when a reproduction amplifier for a voice signal or an audio signal is started and stopped. <P>SOLUTION: A signal amplifier circuit 1 for amplifying the voice signal or the audio signal and driving a speaker includes a speaker driving amplifier 5, an input resistor 6, and a feedback resistor 7. A reference bias circuit 2 applies a reference DC voltage Vref with a waveform including only frequency components below an audible band which cannot be reproduced from the speaker to a noninverting input terminal of the speaker driving amplifier 5 at the start/stop of the reproduction amplifier. A switch control section 9 controls a switch 8 to switch the speaker drive amplifier 5 from an inverting amplifier into a noninverting amplifier and short-circuits the feedback resistor 7 for switching a signal output and muting during signal reproduction. Thus, a coupling capacitor 3 is charged and discharged under the same trajectory as the waveform at the start/stop of the reference bias circuit 2. Further, a signal and noise at the input terminal side are not outputted from the speaker driving amplifier 5 and not reproduced at the speaker 4. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a pop noise reduction circuit that reduces pop noise that occurs when a playback amplifier that plays back an audio signal such as a human voice signal or music is activated or stopped.

  In recent years, many portable information devices are compatible with not only voice but also music playback functions, and the signal playback section is required to have higher sound quality and lower noise characteristics. In particular, when playing music with high-performance earphones connected, the sound at the moment of starting or stopping the music playback application or the sound of starting or stopping the audio playback amplifier is audible. "Is often felt as. This “harsh sound” is generally called “pop noise”.

  FIG. 9 is a diagram showing the configuration of the pop noise reduction circuit of Conventional Example 1, and FIG. 10 is a diagram showing the configuration of the pop noise reduction circuit of Conventional Example 2 (see Patent Document 1). Hereinafter, the configuration and operation of the pop noise reduction circuit of Conventional Example 1 will be described with reference to FIG.

  The pop noise reduction circuit shown in FIG. 9 includes a signal amplification circuit 1 that amplifies an audio signal and drives a speaker 4, a reference bias circuit 2 that supplies a reference DC voltage Vref to the signal amplification circuit 1, and a signal amplification circuit 1. An output coupling capacitor 3 (Cout) connected to the output terminal, and a speaker 4 connected between the coupling capacitor 3 and the ground potential, and the signal amplification circuit 1 further includes a speaker driving amplifier. 5 and an input resistor 6 (R1) and a feedback resistor 7 (R2) for determining the amplification factor of the output signal Vout.

  The reference bias circuit 2 includes a resistor 16 (R3) and a resistor 17 (R4) that determine the reference DC voltage Vref of the speaker driving amplifier 5, and a smoothing capacitor 18 (C1) for smoothing the output reference voltage. It consists of

  When the power supply (VDD) of the reference bias circuit 2 is turned on at the start-up timing, the pop noise reduction circuit of Conventional Example 1 configured as shown in FIG. 9 has constants for the resistor 16, the resistor 17, and the smoothing capacitor 18. The reference DC voltage Vref rises according to the time constant determined by ## EQU3 ## and the output voltage Vout of the speaker drive amplifier 5 rises slowly and is activated.

  When the power supply (VDD) of the reference bias circuit 2 is stopped at the stop timing, the reference DC voltage Vref rises according to the time constants determined by the constants of the resistor 16, the resistor 17, and the smoothing capacitor 18 as in the start-up. The output voltage Vout of the speaker driving amplifier 5 is also slowly lowered and stopped. Here, the output voltage Vout of the speaker driving amplifier 5 is slowly increased or decreased, thereby suppressing the charge / discharge current to the output coupling capacitor 3 and suppressing the occurrence of pop noise.

  Next, the configuration and operation of the pop noise reduction circuit of Conventional Example 2 shown in FIG. 10 will be described. As shown in FIG. 10, a signal amplifier circuit 1 that amplifies an audio signal and drives a speaker 4, a reference bias circuit 2 that supplies a reference DC voltage Vref to the signal amplifier circuit 1, and an output terminal of the signal amplifier circuit 1 The output coupling capacitor 3 (Cout) is connected to the speaker 4 connected between the coupling capacitor 3 and the ground potential. The signal amplification circuit 1 further includes a speaker driving amplifier 5, The input resistor 6 (R1) and the feedback resistor 7 (R2) are used to determine the amplification factor of the output signal.

  The reference bias circuit 2 includes an A / D converter 19 for analog / digital (hereinafter referred to as A / D) conversion of the output signal of the speaker driving amplifier 5 and output data from the A / D converter 19. Based on the above, a reference DC voltage Vref is set to the speaker driving amplifier 5 so that the output signal during the start or stop mode of the speaker driving amplifier 5 becomes a signal having a frequency component below the audible band that cannot be reproduced by the speaker 4 during reproduction. A control unit 20 that generates digital data to be supplied, a memory 21 that holds in advance digital data that is a reference parameter for generating the digital data, and digital data that is calculated and generated by the control unit 20 The digital / analog (hereinafter referred to as D / A) conversion is received and supplied to the speaker driving amplifier 5 as a reference DC voltage Vref. It is composed of a D / A converter 22 to.

  In the pop noise reduction circuit of Conventional Example 2 configured as shown in FIG. 10, the control unit 20 takes in the output data of the speaker driving amplifier 5 supplied from the A / D converter 19, and the data and the speaker The DC voltage Vindc input to the speaker driving amplifier 5 is obtained by calculating digital data for generating the reference DC voltage Vref input to the driving amplifier 5.

From this data, the reference digital data is sequentially read out from the memory 21 and calculated so that the output signal during the start or stop mode of the speaker driving amplifier 5 has a frequency below the audible band that cannot be reproduced by the speaker 4. The noise is reproduced by the A converter 22 and supplied to the speaker driving amplifier 5 as the desired reference DC voltage Vref to reduce pop noise generated at the time of starting or stopping.
JP 2005-159871 A

  However, in the pop noise reduction circuit of the first conventional example shown in FIG. 9, at the moment when the speaker driving amplifier 5 is started or stopped, strictly speaking, the coupling capacitor 3 is always connected to the speaker end regardless of the magnitude of the time constant. As a result, a steep DC voltage difference differentiated in step (b) and an output waveform that smoothly returns from the start or stop state to the steady state are generated. For this reason, it will be heard as pop noise. In order to reduce the pop noise as much as possible, there is no means other than increasing the constants of the resistor 16, the resistor 17, and the smoothing capacitor 18 as much as possible. For this reason, the chip area or the mounting area of the external parts is increased. There is a problem that the time required for starting or stopping increases.

  Further, in the pop noise reduction circuit of Conventional Example 2 shown in FIG. 10, the circuit configuration is complicated and the control means is also complicated. Further, since the A / D converter 19, the D / A converter 22, and the memory 21 are mounted, there are problems such as an increase in chip size.

  The present invention is directed to solving the problems of the background art, and occurs when a playback amplifier that plays back an audio signal such as a human voice signal or music is started or stopped with a simple configuration and control method. An object of the present invention is to provide a pop noise reduction circuit that reduces pop noise.

  In order to achieve the above object, a pop noise reduction circuit according to the present invention includes an input resistor connected between an input terminal and an inverting input terminal of an operational amplifier, and feedback between the inverting input terminal and the output terminal of the operational amplifier. A signal amplifying circuit comprising resistors, a reference bias circuit for supplying a reference DC voltage connected to the non-inverting input terminal of the operational amplifier, an output coupling capacitor connected to the output terminal of the signal amplifying circuit, and a cup A speaker connected between the ring capacitor and the ground potential, a switch that short-circuits both ends of the feedback resistor, and a switch control unit that controls on / off of the switch are provided. Switch control for short-circuiting is performed.

  Further, the reference bias circuit includes a digital / analog converter that outputs a reference DC voltage, a digital / analog control unit that controls the digital / analog converter, and an audible band below which an operational amplifier cannot reproduce the reference DC voltage with a speaker. And a start data section for holding digital / analog conversion data having a start or stop waveform having only a frequency component of the reference, and when the start or stop is performed using the reference DC voltage as the start or stop waveform, the switch control section The switch control which short-circuits both ends is performed.

  According to the configuration, the operational amplifier is switched from the inverting amplifier to the normal amplifier by the switch and the switch control unit that short-circuits the feedback resistor connected to the operational amplifier included in the signal amplifier circuit that drives the speaker, and is started or stopped. Pop noise at the time can be reduced.

  Further, the pop noise reduction circuit has a signal amplifier circuit in which an input resistor is connected between the input terminal and the inverting input terminal of the operational amplifier, and a feedback resistor is connected between the inverting input terminal and the output terminal of the operational amplifier, A reference bias circuit for supplying a reference DC voltage connected to the non-inverting input terminal of the operational amplifier, an output coupling capacitor connected to the output terminal of the signal amplifier circuit, and between the output coupling capacitor and the ground potential A speaker connected to the input terminal, an input coupling capacitor for cutting off the DC component of the input signal connected to the input terminal of the signal amplifier circuit, a first switch for short-circuiting both ends of the feedback resistor, and the first switch turned on 1st switch control part which controls / off, 2nd switch which short-circuits both ends of input resistance, and 2nd switch on / off And a second switch control unit that controls the first switch control unit to short-circuit both ends of the feedback resistor and the second switch control unit to short-circuit both ends of the input resistor when starting or stopping. It is characterized by performing.

  Further, the reference bias circuit includes a digital / analog converter that outputs a reference DC voltage, a digital / analog control unit that controls the digital / analog converter, and an audible band below which an operational amplifier cannot reproduce the reference DC voltage with a speaker. And a start data section for holding digital / analog conversion data having a start or stop waveform having only a frequency component of the first switch control section at the time of start or stop using the reference DC voltage as the start or stop waveform. The feedback resistor is short-circuited at both ends, and the second switch control unit performs switch control to short-circuit both ends of the input resistor.

  According to the above configuration, the operational amplifier is switched from the inverting amplifier to the normal rotation amplifier by the switch and the switch control unit that short-circuits the feedback resistance and the input resistance connected to the operational amplifier included in the signal amplifier circuit that drives the speaker. Pop noise when starting or stopping can be reduced.

  According to the present invention, it is possible to reduce pop noise at the time of starting or stopping, and to suppress a circuit mounting area and a chip area by a simple configuration and control method, thereby realizing an effect of reducing a cost increase.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing a circuit configuration of a pop noise reduction circuit according to Embodiment 1 of the present invention. Here, components having the same functions corresponding to the components described in FIGS. 9 and 10 showing the conventional example are denoted by the same reference numerals, and the same applies to the following drawings.

  As shown in FIG. 1, a signal amplifying circuit 1 that amplifies an audio signal to drive a speaker, a reference bias circuit 2 that supplies a reference DC voltage Vref to the signal amplifying circuit 1, and an output terminal of the signal amplifying circuit 1 are connected. The output coupling capacitor 3 (Cout) and the speaker 4 connected between the coupling capacitor 3 and the ground potential. The signal amplifier circuit 1 includes a speaker driving amplifier 5 and an operational amplifier. , An input resistor 6 (R1) and a feedback resistor 7 (R2) for determining a signal amplification factor, a mute switch 8 (SW1) for short-circuiting both ends of the feedback resistor 7 (R2), and a mute switch 8 It is a pop noise reduction circuit comprised with SW1 control part 9 which carries out on / off control.

  FIG. 2 is a timing chart when the pop noise reduction circuit according to the first embodiment is started and stopped. The operation of the pop noise reduction circuit according to the second embodiment will be described with reference to FIG. 2 and with reference to FIG.

  Now, assuming that the reference DC voltage Vref output from the reference bias circuit 2 at time t = 0 is Vref = 0 [V], the reference DC voltage Vref is a predetermined operating voltage level Voref of the speaker driving amplifier 5 at time t = t1. The reference bias circuit 2 is activated so that the signal is input to the non-inverting (+ side) input terminal of the speaker driving amplifier 5. At this time, the reference DC voltage Vref in the start mode is an operating voltage from 0 [V] while drawing a start waveform having only a frequency component below the audible band in which the output voltage Vout of the speaker driving amplifier 5 cannot be reproduced by the speaker 4. It shall rise to level Voref.

  During time t = 0 to t1, the mute switch 8 is turned on by the SW1 controller 9 to short-circuit both ends of the feedback resistor 7, thereby operating the speaker driving amplifier 5 as a normal amplifier. As a result, the reference DC voltage Vref input to the non-inverting (+ side) input terminal is buffered from the speaker driving amplifier 5, and the coupling capacitor 3 operates with the same locus as the starting waveform of the reference bias circuit 2. It becomes possible to charge up to the level Voref.

  In addition, even when a signal or noise is input from the input terminal (Vin) side during the time t = 0 to t1, it is input from the speaker driving amplifier 5 because the mute switch 8 is turned on. Signals and noise are not output and are not reproduced by the speaker 4.

  Thereafter, the speaker drive amplifier 5 is switched to the inverting amplifier by turning off the mute switch 8 during the time t = t1 to t2, that is, in the reproduction period of the audio signal or the audio signal. When an audio signal is input to the speaker driving amplifier 5, the signal is output at an amplification factor determined by the input resistor 6 and the feedback resistor 7 and reproduced by the speaker 4. When no audio signal is input, the silent output state is maintained.

  Subsequently, when the reproduction of the audio signal or the audio signal is finished and stopped after time t = t2, the mute switch 8 is turned on at time t = t2, and the speaker driving amplifier 5 is switched to the normal rotation amplifier configuration again. . Thereafter, the reference DC voltage Vref output from the reference bias circuit 2 is input to the non-inverting (+ side) input terminal of the speaker driving amplifier 5 while falling so that Vref = 0 [V] at time t = t3. To do.

  At that time, the reference DC voltage Vref in the stop mode output from the reference bias circuit 2 operates while drawing a stop waveform having only a frequency component below the audible band in which the output voltage Vout of the speaker driving amplifier 5 cannot be reproduced by the speaker 4. The voltage level Voref falls from 0 [V]. As a result, the speaker driving amplifier 5 can discharge the coupling capacitor 3 to 0 [V] along the same locus as the stop waveform of the reference bias circuit 2.

  On the other hand, if the mute switch 8 is turned on only for a certain period, for example, a period of time t = t4 to t5, during the time t = t1 to t2 and while the audio signal is input, the speaker is only for that period. The driving amplifier 5 can mute the input signal.

  As described above, when the mute switch 8 is turned on, the speaker driving amplifier 5 is operated as a normal rotation type amplifier in the start or stop mode, and the speaker drive amplifier 5 is in the start or stop mode from the reference bias circuit 2. By directly inputting a start or stop waveform having a frequency component below the audible band that cannot be reproduced by the speaker 4 to the non-inverted (+ side) input terminal of the speaker driving amplifier 3, a conventional audio signal or It is possible to reduce pop noise that has occurred when a playback amplifier that plays back an audio signal is started or stopped.

  The start time tr = t1 and the stop time tf = | t3-t2 | are also within the frequency range below the audible band where the speaker drive amplifier 5 cannot reproduce the start or stop waveform of the reference DC voltage Vref by the speaker 4. It becomes possible to speed up. Further, the mute switch 8 is ON / OFF controlled by the SW1 control unit 9 at the timing of time t = t1 and time t = t2, and the resistance value of the switch smoothly changes from the minimum to the maximum or from the maximum to the minimum. By controlling so that the switching noise can be reduced, the switching noise at the timing of time t = t1 and time t = t2 can also be reduced. As a result, it is possible to suppress “harsh sounds” including pop noises in all processes from startup to playback and further stop.

  FIG. 3 is a diagram showing a circuit configuration of the pop noise reduction circuit according to the second embodiment of the present invention. As shown in FIG. 3, a signal amplifier circuit 1 that amplifies an audio signal to drive a speaker, a reference bias circuit 2 that supplies a reference DC voltage Vref to the signal amplifier circuit 1, and an output terminal of the signal amplifier circuit 1 are connected. Coupling for output 3 (Cout), coupling between the coupling capacitor 3 and the ground potential, and coupling for input that cuts off the DC voltage of the signal input to the speaker 4 and the signal amplifying circuit 1 The signal amplifier circuit 1 includes a speaker driving amplifier 5 as an operational amplifier, an input resistor 6 (R1) and a feedback resistor 7 (R2) for determining a signal amplification factor. Furthermore, a mute switch 8 (SW1) that short-circuits both ends of the feedback resistor 7 (R2), and a SW1 system that controls the mute switch 8 on / off. And parts 9, a bypass switch 11 for short-circuiting both ends of the input resistor 6 (R1) (SW2), a pop noise reduction circuit constituted by the bypass switch 11 and the ON / OFF control to SW2 control unit 12.

  FIG. 4 is a timing chart when the pop noise reduction circuit according to the second embodiment is started and stopped. The operation of the pop noise reduction circuit according to the second embodiment will be described with reference to FIG. 4 and with reference to FIG.

  Now, assuming that the reference DC voltage Vref output from the reference bias circuit 2 at time t = 0 is Vref = 0 [V], the reference DC voltage Vref is a predetermined operating voltage level Voref of the speaker driving amplifier 5 at time t = t1. The reference bias circuit 2 is activated so that the signal is input to the non-inverting (+ side) input terminal of the speaker driving amplifier 5. At this time, the reference DC voltage Vref in the start mode is an operating voltage from 0 [V] while drawing a start waveform having only a frequency component below the audible band in which the output voltage Vout of the speaker driving amplifier 5 cannot be reproduced by the speaker 4. It shall rise to level Voref.

  During time t = 0 to t1, the mute switch 8 is turned on by the SW1 controller 9 to short-circuit both ends of the feedback resistor 7, thereby operating the speaker driving amplifier 5 as a normal amplifier. As a result, the reference DC voltage Vref input to the non-inverting (+ side) input terminal is buffered from the speaker driving amplifier 5, and the coupling capacitor 3 operates with the same locus as the starting waveform of the reference bias circuit 2. It becomes possible to charge up to the level Voref.

  At the same time, the bypass capacitor 11 is turned on by the SW2 control unit 12 to short-circuit the input resistor 6, so that the coupling capacitor 10 operates with the same locus as the starting waveform of the reference bias circuit 2 by the output voltage Vout of the speaker drive amplifier 5. It is possible to charge up to the voltage level Voref. Thereby, the DC voltage error between the input and output of the speaker driving amplifier 5 can be eliminated.

  In addition, even when a signal or noise is input from the input terminal (Vin) side during the time t = 0 to t1, it is input from the speaker driving amplifier 5 because the mute switch 8 is turned on. Signals and noise are not output and are not reproduced by the speaker 4.

  Thereafter, the speaker drive amplifier 5 is switched to the inverting amplifier by turning off the mute switch 8 and the bypass switch 11 during the time t = t1 to t2, that is, in the playback period of the audio signal or the audio signal. When an audio signal is input to the speaker driving amplifier 5, the signal is output at an amplification factor determined by the input resistor 6 and the feedback resistor 7 and reproduced by the speaker 4. When no audio signal is input, the silent output state is maintained.

  Here, as described above, the coupling capacitor 10 can be charged with the speaker driving amplifier 5 to eliminate a DC voltage error between the input and output of the speaker driving amplifier 5. For this reason, even when the mute switch 8 and the bypass switch 11 are turned off, inflow or outflow of charge / discharge current does not occur between the input coupling capacitor 10 and the output coupling capacitor 3 before and after that. Therefore, the occurrence of pop noise can be suppressed.

  Subsequently, when the reproduction of the audio signal or the audio signal is finished and stopped after the time t = t2, the mute switch 8 and the bypass switch 11 are turned on at the time t = t2, and the speaker driving amplifier 5 is again rotated in the normal rotation type. Switch to amplifier configuration. Thereafter, the reference DC voltage Vref output from the reference bias circuit 2 is input to the non-inverting (+ side) input terminal of the speaker driving amplifier 5 while falling so that Vref = 0 [V] at time t = t3. To do.

  At that time, the reference DC voltage Vref in the stop mode output from the reference bias circuit 2 operates while drawing a stop waveform having only a frequency component below the audible band in which the output voltage Vout of the speaker driving amplifier 5 cannot be reproduced by the speaker 4. The voltage level Voref falls from 0 [V]. As a result, the speaker driving amplifier 5 can discharge the coupling capacitor 3 from the operating voltage level Voref to 0 [V] along the same locus as the stop waveform of the reference bias circuit 2. At the same time, the electric charge of the coupling capacitor 10 can be discharged from the operating voltage level Voref to 0 [V] by the speaker drive amplifier 5 along the same locus as the stop waveform of the reference bias circuit 2.

  Here, as described above, the coupling capacitor 10 can be discharged by the speaker driving amplifier 5 to eliminate the DC voltage error between the input and output of the speaker driving amplifier 5, and the mute switch 8 and the bypass switch 11 can be connected. Even when the switch is turned off, the charging / discharging current does not flow in or out between the coupling capacitor 10 and the coupling capacitor 3 before and after that, so that the occurrence of pop noise can be suppressed.

  On the other hand, if the mute switch 8 is turned on for a certain period, for example, the time t = t4 to t5, during the time t = t1 to t2 and while the audio signal is being input, the speaker driving amplifier only for that period. 5 can mute the input signal.

  Further, at the timing before the start operation or after the stop operation, the control of the bypass switch 11 is in the on state (SW2 control unit (A) in FIG. 4) or the off state (SW2 control unit (B) in FIG. 4). Is not restricted.

  As described above, the mute switch 8 is turned on, and at the same time, the bypass switch 11 is turned on to operate the speaker driving amplifier 5 as a normal rotation type amplifier in the start or stop mode. A startup or stop waveform having only a frequency component below the audible band in which the output voltage Vout during the startup or stop mode of the amplifier 5 cannot be reproduced by the reproduction speaker 4 is applied to the non-inverted (+ side) input terminal of the speaker driving amplifier 3 By directly inputting, it becomes possible to reduce the pop noise that has occurred at the time of starting or stopping the reproduction amplifier that reproduces the conventional audio signal or audio signal.

  The start time tr = t1 and the stop time tf = | t3-t2 | are also within the frequency range below the audible band where the speaker drive amplifier 5 cannot reproduce the start or stop waveform of the reference DC voltage Vref by the speaker 4. It becomes possible to speed up. Further, the mute switch 8 and the bypass switch 11 are ON / OFF controlled at the timing of time t = t1 and time t = t2 by the SW1 control unit 9 and the SW2 control unit 12, but the resistance value of the switch is from minimum to maximum. Alternatively, by controlling so as to smoothly change from the maximum to the minimum, it is possible to reduce the switch switching noise at the timings of time t = t1 and time t = t2. As a result, it is possible to suppress “harsh sounds” including pop noises in all processes from startup to playback and further stop.

  FIG. 5 is a diagram showing a circuit configuration of a pop noise reduction circuit according to Embodiment 3 of the present invention. As shown in FIG. 5, the configuration is the same as that of the first embodiment described above, and the reference bias circuit 2 controls the D / A converter 13 that outputs the reference DC voltage Vref and the D / A converter 13. The D / A control unit 14 that performs the operation and the activation data unit 15 that outputs the activation data input to the D / A control unit 14 are configured. The start-up data is D / A conversion data having a start-up or stop waveform having only a frequency component below the audible band in which the speaker drive amplifier 5 cannot reproduce the reference DC voltage Vref by the speaker 4.

  FIG. 6 is a timing chart when the pop noise reduction circuit according to the third embodiment is started and stopped. The operation of the pop noise reduction circuit according to the third embodiment will be described with reference to FIG. 6 and with reference to FIG.

  Now, assuming that the reference DC voltage Vref output from the reference bias circuit 2 at time t = 0 is Vref = 0 [V], the reference DC voltage Vref is a predetermined operating voltage level Voref of the speaker driving amplifier 5 at time t = t1. The reference bias circuit 2 is activated so that the signal is input to the non-inverting (+ side) input terminal of the speaker driving amplifier 5. However, the reference bias circuit 2 is used for D / A conversion in which the waveform of the reference DC voltage Vref from the activation data unit 15 by the D / A control unit 14 becomes an activation waveform having only a frequency component below the audible band that cannot be reproduced by the speaker 4. It is assumed that data is read out and reproduced by the D / A converter 13, and the reference DC voltage Vref rises from 0 [V] to the operating voltage level Voref.

  By passing a low-pass filter between the output of the D / A converter 13 and the non-inverted (+ side) input terminal of the speaker drive amplifier 5 with a frequency band that does not affect the start or stop time as a stop band. It is desirable to suppress the noise generated by the conversion operation noise of the D / A converter 13 and the discontinuity of the reference DC voltage Vref before and after the start of the D / A conversion operation, thereby eliminating the sense of discomfort in the sense of hearing.

  During time t = 0 to t1, the mute switch 8 is turned on by the SW1 controller 9 to short-circuit both ends of the feedback resistor 7, thereby operating the speaker driving amplifier 5 as a normal amplifier. As a result, the reference DC voltage Vref input from the reference bias circuit 2 to the non-inverted (+ side) input terminal is buffered from the speaker driving amplifier 5, and the coupling capacitor 3 has the same startup waveform as the reference bias circuit 2. It is possible to charge up to the operating voltage level Voref along the locus.

  In addition, even when a signal or noise is input from the input terminal (Vin) side during the time t = 0 to t1, it is input from the speaker driving amplifier 5 because the mute switch 8 is turned on. Signals and noise are not output and are not reproduced by the speaker 4.

  Thereafter, the speaker drive amplifier 5 is switched to the inverting amplifier by turning off the mute switch 8 during the time t = t1 to t2, that is, in the playback period of the audio signal or audio signal, and the audio signal is sent to the speaker drive amplifier 5. When input, a signal is output at an amplification factor determined by the input resistor 6 and the feedback resistor 7 and reproduced by the speaker 4. When no audio signal is input, the silent output state is maintained.

  Subsequently, when the reproduction of the audio signal or the audio signal is finished and stopped after time t = t2, the mute switch 8 is turned on at time t = t2, and the speaker driving amplifier 5 is switched to the normal rotation amplifier configuration again. . Thereafter, the D / A control unit 14 reads out the D / A conversion data from the start-up data unit 15 so that the waveform of the reference DC voltage Vref is a stop waveform having only a frequency component below the audible band that cannot be reproduced by the speaker 4. The signal is lowered from the operating voltage level Voref to 0 [V] by the / A converter 13 and is input from the reference bias circuit 2 to the non-inverting (+ side) input terminal of the speaker driving amplifier 5.

  As a result, the speaker driving amplifier 5 can discharge the coupling capacitor 3 from the operating voltage level Voref to 0 [V] along the same locus as the stop waveform of the reference bias circuit 2.

  On the other hand, if the mute switch 8 is turned on only for a certain period, for example, a period of time t = t4 to t5, during the time t = t1 to t2 and while the audio signal is input, the speaker is only for that period. The driving amplifier 5 can mute the input signal.

  As the data held in the startup data unit 15 described above, for example, a waveform represented by a cosine function shown in FIG. 6 is preferably the reference DC voltage Vref output from the reference bias circuit 2. In this case, when the speaker 4 is driven while the coupling capacitor 3 is charged / discharged by the speaker driving amplifier 5, a waveform represented by a smooth sine function at Vhpout in FIG. 6 is output, which is representative of pop noise. A steep differential waveform that is a complex waveform is not output.

  Further, since the startup waveform and the stop waveform described above are symmetrical when the startup time tr = t1 and the stop time tf = | t3-t2 | are the same, the startup data portion 15 is shown in FIG. As shown in the figure, it is possible to hold data only at the time of start-up, reverse the data reading order at the time of stop (switching between count-up and count-down in the internal counter, etc.), and keep the count value during normal reproduction Therefore, easy control is possible, leading to reduction of the chip size.

  Further, as described above, by turning on the mute switch 8, the speaker drive amplifier 5 is operated as a normal rotation type amplifier in the start or stop mode, and the start or stop mode of the speaker drive amplifier 5 is started from the reference bias circuit 2. By directly inputting a start or stop waveform having only a frequency component below the audible band that cannot be reproduced by the speaker 4 to the non-inverted (+ side) terminal of the speaker driving amplifier 3, a conventional audio signal or It is possible to reduce pop noise that has occurred when a playback amplifier that plays back an audio signal is started or stopped.

  The start time tr = t1 and the stop time tf = | t3-t2 | are also within the frequency range below the audible band where the speaker drive amplifier 5 cannot reproduce the start or stop waveform of the reference DC voltage Vref by the speaker 4. It becomes possible to speed up. Further, the mute switch 8 is ON / OFF controlled by the SW1 control unit 9 at the timing of time t = t1 and time t = t2, and the resistance value of the switch smoothly changes from the minimum to the maximum or from the maximum to the minimum. By controlling so that the switching noise can be reduced, the switching noise at the timing of time t = t1 and time t = t2 can also be reduced. As a result, it is possible to suppress “harsh sounds” including pop noises in all processes from startup to playback and further stop.

  FIG. 7 is a diagram showing a circuit configuration of a pop noise reduction circuit according to Embodiment 4 of the present invention. As shown in FIG. 7, the configuration is the same as that of the above-described second embodiment, and the reference bias circuit 2 is the same configuration as the reference bias circuit 2 of the third embodiment.

  FIG. 8 is a timing chart when the pop noise reduction circuit according to the fourth embodiment is started and stopped. The operation of the pop noise reduction circuit according to the fourth embodiment will be described based on FIG. 8 and with reference to FIG.

  Now, assuming that the reference DC voltage Vref output from the reference bias circuit 2 at time t = 0 is Vref = 0 [V], the reference DC voltage Vref is a predetermined operating voltage level Voref of the speaker driving amplifier 5 at time t = t1. The reference bias circuit 2 is activated so that the signal is input to the non-inverting (+ side) input terminal of the speaker driving amplifier 5. However, the reference bias circuit 2 uses the D / A control unit 14 to generate D / A conversion data having a startup waveform having only a frequency component below the audible band in which the reference DC voltage Vref cannot be reproduced by the speaker 4 from the startup data unit 15. It is assumed that the reference DC voltage Vref rises from 0 [V] to the operating voltage level Voref after being read and reproduced by the D / A converter 13.

  In addition, a low-pass filter having a frequency band that does not affect the starting or stopping time between the output of the D / A converter 13 and the non-inverting (+ side) input terminal of the speaker drive amplifier 5 is passed. Therefore, it is desirable to suppress the conversion operation noise of the D / A converter 13 and the noise generated from the discontinuity of the reference DC voltage Vref before and after the start of the D / A conversion operation, thereby eliminating the sense of incongruity in hearing.

  During time t = 0 to t1, the mute switch 8 is turned on by the SW1 controller 9 to short-circuit both ends of the feedback resistor 7, thereby operating the speaker driving amplifier 5 as a normal amplifier. As a result, the reference DC voltage Vref input from the reference bias circuit 2 to the non-inverted (+ side) input terminal is buffered from the speaker driving amplifier 5, and the coupling capacitor 3 has the same startup waveform as the reference bias circuit 2. It is possible to charge up to the operating voltage level Voref along the locus.

  At the same time, the SW2 control unit 12 turns on the input resistance bias switch 11 to short-circuit the input resistor 6, so that the coupling capacitor 10 has the same start waveform as the reference bias circuit 2 by the output voltage Vout of the speaker drive amplifier 5. It is possible to charge up to the operating voltage level Voref on the locus. Thereby, the DC voltage error between the input and output of the speaker driving amplifier 5 can be eliminated.

  In addition, even when a signal or noise is input from the input terminal (Vin) side during the time t = 0 to t1, it is input from the speaker driving amplifier 5 because the mute switch 8 is turned on. Signals and noise are not output and are not reproduced by the speaker 4.

  Thereafter, the speaker drive amplifier 5 is switched to the inverting amplifier by turning off the mute switch 8 and the bypass switch 11 during the time t = t1 to t2, that is, in the playback period of the audio signal or the audio signal. When an audio signal is input to the speaker driving amplifier 5, the signal is output at an amplification factor determined by the input resistor 6 and the feedback resistor 7 and reproduced by the speaker 4. When no audio signal is input, the silent output state is maintained.

  Here, as described above, the coupling capacitor 10 can be charged by the speaker driving amplifier 5 to eliminate the DC voltage error between the input and output of the speaker driving amplifier 5, and the mute switch 8 and the bypass switch 11. Even when the switch is turned off, charging / discharging current does not flow in or out between the input coupling capacitor 10 and the output coupling capacitor 3 before and after that, so that the occurrence of pop noise can be suppressed.

  Subsequently, when the reproduction of the audio signal or the audio signal is finished and stopped after the time t = t2, the mute switch 8 and the bypass switch 11 are turned on at the time t = t2, and the speaker driving amplifier 5 is again rotated in the normal rotation type. After switching to the amplifier configuration, the D / A control unit 14 reads out D / A conversion data having a stop waveform having only a frequency component below the audible band that cannot be reproduced by the speaker 4 from the start-up data unit 15, and performs D / A conversion. The voltage is lowered from the operating voltage level Voref to 0 [V] by the device 13 and is input from the reference bias circuit 2 to the non-inverting (+ side) input terminal of the speaker driving amplifier 5.

  As a result, the speaker driving amplifier 5 can discharge the coupling capacitor 3 from the operating voltage level Voref to 0 [V] along the same locus as the stop waveform of the reference bias circuit 2. At the same time, the electric charge of the coupling capacitor 10 can be discharged from the operating voltage level Voref to 0 [V] by the speaker drive amplifier 5 along the same locus as the stop waveform of the reference bias circuit 2.

  Here, as described above, the input coupling capacitor 10 can be discharged by the speaker driving amplifier 5 to eliminate the DC voltage error between the input and output of the speaker driving amplifier 5, and the mute switch 8 and the bypass switch. Even when 11 is turned off, the charging / discharging current does not flow in or out between the coupling capacitor 10 and the coupling capacitor 3 before and after that, so that the occurrence of pop noise can be suppressed.

  On the other hand, if the mute switch 8 is turned on only for a certain period, for example, for the time t = t4 to t5, during the time t = t1 to t2 and while the audio signal is being input, the speaker is only for that period. The driving amplifier 5 can mute the input signal.

  Further, at the timing before the start operation or after the stop operation, the control of the bypass switch 11 is switched to either the on state (SW2 control unit (A) in FIG. 8) or the off state (SW2 control unit (B) in FIG. 8). Is not restricted.

  As the data held in the activation data unit 15 described above, it is desirable that the waveform represented by the cosine function shown in FIG. 8 is the reference DC voltage Vref output from the reference bias circuit 2, for example. In this case, when the speaker 4 is driven while the coupling capacitor 3 is charged or discharged by the speaker driving amplifier 5, a waveform represented by a smooth sine function at Vhpout in FIG. 8 is output, which is a typical pop noise. A steep differential waveform that is a complex waveform is not output.

  Furthermore, since the start waveform and the stop waveform described above are symmetrical when the start time tr = t1 and the stop time tf = | t3−t2 | are the same, the start data portion 15 is shown in FIG. In this way, it is possible to hold data only at startup, and reverse the data reading order when stopping (such as switching between count-up and count-down in the internal counter), and keep the count value during normal playback. Easy control is possible, leading to a reduction in chip size.

  As described above, the mute switch 8 is turned on, and at the same time, the bypass switch 11 is turned on to operate the speaker driving amplifier 5 as a normal rotation type amplifier in the start or stop mode. By directly inputting and controlling the same waveform as the start or stop waveform with the output voltage Vout during the start or stop mode of the amplifier 5 having only a frequency component below the audible band that cannot be reproduced by the reproduction speaker 4, It is possible to reduce pop noise that has occurred when a reproduction amplifier that reproduces a signal or audio signal is started or stopped.

  The start time tr = t1 and the stop time tf = | t3-t2 | are also within the frequency range below the audible band where the speaker drive amplifier 5 cannot reproduce the start or stop waveform of the reference DC voltage Vref by the speaker 4. It becomes possible to speed up. Further, the mute switch 8 and the bypass switch 11 are ON / OFF controlled at the timing of time t = t1 and time t = t2 by the SW1 control unit 9 and the SW2 control unit 12, but the resistance value of the switch is from minimum to maximum. Alternatively, the switching noise at the timing of time t = t1 and time t = t2 can also be reduced by controlling to smoothly change from the maximum to the minimum. As a result, it is possible to suppress “harsh sounds” including pop noises in all processes from startup to playback and further stop.

  The pop noise reduction circuit according to the present invention reduces the pop noise at the time of starting or stopping, and can reduce the increase in cost by suppressing the mounting area and the chip area by a simple configuration and control method. It is useful as a noise reduction circuit for a reproduction amplifier that reproduces an audio signal such as the above.

The figure which shows the circuit structure of the pop noise reduction circuit in Embodiment 1 of this invention. Timing chart when starting and stopping the pop noise reduction circuit according to the first embodiment The figure which shows the circuit structure of the pop noise reduction circuit in Embodiment 2 of this invention. Timing chart when starting and stopping the pop noise reduction circuit according to the second embodiment The figure which shows the circuit structure of the pop noise reduction circuit in Embodiment 3 of this invention. Timing chart when starting and stopping the pop noise reduction circuit according to the third embodiment The figure which shows the circuit structure of the pop noise reduction circuit in Embodiment 4 of this invention. Timing chart when starting and stopping the pop noise reduction circuit according to the fourth embodiment The figure which shows the structure of the pop noise reduction circuit of the prior art example 1. The figure which shows the structure of the pop noise reduction circuit of the prior art example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Signal amplifier circuit 2 Reference | standard bias circuit 3,10 Coupling capacitor | condenser 4 Speaker 5 Speaker drive amplifier 6 Input resistance 7 Feedback resistance 8 Mute switch 9 SW1 control part 11 Bypass switch 12 SW2 control part 13 and 22 D / A converter 14 D / A control unit 15 Start-up data units 16, 17 Resistor 18 Smoothing capacitor 19 A / D converter 20 Control unit 21 Memory

Claims (4)

A signal amplifier circuit in which an input resistor is connected between the input terminal and the inverting input terminal of the operational amplifier, and a feedback resistor is connected between the inverting input terminal and the output terminal of the operational amplifier, and the non-inverting input of the operational amplifier A reference bias circuit for supplying a reference DC voltage connected to a terminal; an output coupling capacitor connected to an output terminal of the signal amplifier circuit; a speaker connected between the coupling capacitor and a ground potential; A switch for short-circuiting both ends of the feedback resistor, and a switch control unit for controlling on / off of the switch,
A pop noise reduction circuit, wherein the switch control unit performs switch control to short-circuit both ends of the feedback resistor at the time of starting or stopping. The reference bias circuit includes a digital / analog converter that outputs a reference DC voltage, a digital / analog control unit that controls the digital / analog converter, and an audible band below which an operational amplifier cannot reproduce the reference DC voltage by a speaker. A start-up data section for holding digital / analog conversion data for a start or stop waveform having only a frequency component of
2. The pop noise reduction circuit according to claim 1, wherein the switch control unit performs switch control for short-circuiting both ends of the feedback resistor at the time of start or stop with the reference DC voltage as the start or stop waveform. A signal amplifier circuit in which an input resistor is connected between the input terminal and the inverting input terminal of the operational amplifier, and a feedback resistor is connected between the inverting input terminal and the output terminal of the operational amplifier, and the non-inverting input of the operational amplifier A reference bias circuit for supplying a reference DC voltage connected to a terminal, an output coupling capacitor connected to an output terminal of the signal amplifier circuit, and a speaker connected between the output coupling capacitor and a ground potential An input coupling capacitor that cuts off the DC component of the input signal connected to the input terminal of the signal amplifier circuit, a first switch that short-circuits both ends of the feedback resistor, and the first switch is turned on / off. A first switch control unit for controlling off, a second switch for short-circuiting both ends of the input resistor, and turning on the second switch And a second switch control unit for turning off control,
Pop noise, wherein the first switch controller short-circuits both ends of the feedback resistor and the second switch controller short-circuits both ends of the input resistor when starting or stopping Reduction circuit. The reference bias circuit includes a digital / analog converter that outputs a reference DC voltage, a digital / analog control unit that controls the digital / analog converter, and an audible band below which an operational amplifier cannot reproduce the reference DC voltage by a speaker. A start-up data section for holding digital / analog conversion data for a start or stop waveform having only a frequency component of
When starting or stopping the reference DC voltage as the start or stop waveform, the first switch control unit short-circuits both ends of the feedback resistor, and the second switch control unit performs switch control to short-circuit both ends of the input resistor. 4. The pop noise reduction circuit according to claim 3, wherein the pop noise reduction circuit is performed.

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