Abstract:
An audio signal output circuit capable of decreasing pop noise includes an acoustic output device for generating sound, an audio signal generating circuit coupled to the acoustic output device for outputting audio signals, and a noise reduction circuit coupled between the acoustic output device and the audio signal generating circuit for smoothing dc level variation of audio signals outputted from the audio signal generating circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an audio signal output circuit capable of decreasing pop noise and a related method, and more particularly, to an audio signal output circuit capable of decreasing pop noises caused by an acoustic output device when suddenly being charged or discharged. 
         [0003]    2. Description of the Prior Art 
         [0004]    In this multimedia filled life, sound quality becomes more and more important. Conventional audio signal output circuits, however, generate irritating ‘pop’ noises caused by circuit switching during outputting of audio signals. 
         [0005]    Please refer to  FIG. 1 .  FIG. 1  is a diagram illustrating a conventional audio signal output circuit  100 . The audio signal output circuit  100  comprises an audio signal generating circuit  102 , an amplifier  104 , a resistor  106 , and an acoustic output device  108 . The audio signal generating circuit  102  comprises a voltage output circuit  110  and a signal generator  112 . The voltage output circuit  110  outputs a bias voltage Vb. The signal generator  112  outputs a sinusoidal signal centered around 0 volts (with an amplifier of AB class) according to a sound source. The acoustic output device  108  can be realized by an earphone or other acoustic output circuit and the equivalent circuit of the acoustic output device  108  can be an equivalent capacitor  114  and an equivalent resistor  116 . When the audio output circuit  100  outputs audio signals, the voltage output circuit  110  starts to output the bias voltage Vb so that the output sinusoidal signal of the signal generator  112  is biased around an appropriate voltage. When the audio signal output circuit  100  does not output audio signals, in order to save power, the voltage output circuit  110  will not output the bias voltage Vb. 
         [0006]    The acoustic output device  108  operates regularly with the sinusoidal signal of the signal generator  112  properly biased around the voltage Vb. In order to save power, however, the voltage output circuit  110  outputs the bias voltage Vb only when the acoustic output device  108  outputs sound. Thus, the acoustic output device  108  generates pop noises because of the switching of the voltage output circuit  110 . Please refer to  FIG. 2 .  FIG. 2  is a diagram illustrating the condition when the audio signal output circuit  100  in  FIG. 1  is turned on for outputting sound. When this occurs, the voltage output of the voltage output circuit  110  abruptly rises from 0 volts to the voltage Vb so that the equivalent capacitor  114  is suddenly charged, causing a current impulse +Ip into the acoustic output device  108  and generating the pop noise. Please refer to  FIG. 3 .  FIG. 3  is a diagram illustrating the condition when the audio signal output circuit  100  is turned off to stop outputting sound. In this situation, the voltage output of the voltage output circuit  100  abruptly falls from the voltage Vb to 0 volts so that the equivalent capacitor  114  is suddenly discharged, causing a current impulse −Ip sinking from the acoustic output device  108  and the generating the pop noise once again. 
         [0007]    In this way, the conventional audio signal output circuit  100  generates irritating pop noise when being turned on/off, thereby lowering the sound quality. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides an audio signal output circuit, capable of decreasing pop noise, comprising an acoustic output device for outputting sound; an audio signal generating circuit coupled to the acoustic output device for outputting audio signals; and a noise reduction circuit coupled between the audio signal generating circuit and the acoustic output device for smoothing dc level variation of audio signals outputted from the audio signal generating circuit. 
         [0009]    The present invention further provides a method for decreasing noise with an audio signal output circuit. The audio signal output circuit comprises an acoustic output device for outputting sound, an audio signal generating circuit coupled to the acoustic output device for outputting audio signals, and a noise reduction circuit coupled between the audio signal generating circuit and the acoustic output device for smoothing dc level variation of audio signals outputted from the audio signal generating circuit. The method comprises utilizing the noise reduction circuit to sink current when the audio signal generating circuit is not activated; activating the inactivated audio signal generating circuit; utilizing the noise reduction circuit to gradually stop sinking current; and pulling up an output voltage of the acoustic output device gradually. 
         [0010]    The present invention further provides a method for decreasing noise with an audio signal output circuit. The audio signal output circuit comprises an acoustic output device for outputting sound, an audio signal generating circuit coupled to the acoustic output device for outputting audio signals, and a noise reduction circuit coupled between the audio signal generating circuit and the acoustic output device for smoothing dc level variation of audio signals outputted from the audio signal generating circuit. The method comprises providing a equivalent resistance of the noise reduction circuit at a first predetermined value when the audio signal generating circuit is not activated; activating the inactivated audio signal generating circuit; gradually increasing the equivalent resistance of the noise reduction circuit from the first predetermined value to a second predetermined value; and pulling up an output voltage of the acoustic output device gradually. 
         [0011]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram illustrating a conventional audio signal output circuit  100 . 
           [0013]      FIG. 2  is a diagram illustrating the condition when the audio signal output circuit  100  in  FIG. 1  is turned on to output sound. 
           [0014]      FIG. 3  is a diagram illustrating the condition when the audio signal output circuit  100  is turned off to stop outputting sound. 
           [0015]      FIG. 4  is a diagram illustrating an audio signal output circuit  400  of an improved embodiment of the present invention. 
           [0016]      FIG. 5  is a diagram illustrating the audio signal output circuit  400  being turned on to output sound. 
           [0017]      FIG. 6  is a diagram illustrating the audio signal output circuit  400  being turned off to stop outputting sound. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Please refer to  FIG. 4 .  FIG. 4  is a diagram illustrating an audio signal output circuit  400  of an improved embodiment of the present invention. The audio signal output circuit  400  comprises a power supply circuit  416 , an acoustic output device  418 , an audio signal generating circuit  404 , and a noise reduction circuit  401 . The power supply circuit  416  outputs bias voltages V 1  and V 2 . The acoustic output device  418  comprises a driving circuit and a loading circuit. The driving circuit comprises an amplifier  402 . The loading circuit can be an earphone or other acoustic output circuit and the equivalent circuit of the loading circuit can be an equivalent capacitor  436  and an equivalent resistor  438 . The amplifier  402  comprises a first input end  420 , a second input end  422 , and an output end  424 . The output end  424  is coupled to the equivalent capacitor  436 . The second input end  422  is coupled between the output end  424  and the equivalent capacitor  436 . The audio signal generating circuit  404  comprises a voltage output circuit  426 , a signal generator  428 , and a first resistor  406 . The voltage output circuit  426  outputs a bias voltage Vbias for biasing the sinusoidal signal output from the signal generator  428  around the voltage Vbias. The noise reduction circuit  401  comprises a transistor  408 , a control signal generator  410 , a capacitor  412 , and a second resistor  414 . The transistor  408  comprises a gate  430 , a drain  432 , and a source  434 . The drain  432  is coupled to the first input end  420  of the amplifier  402 . The source  434  is coupled to the ground end. In the present embodiment, the transistor  408  is an N-type metal oxygen semiconductor (NMOS) transistor. The control signal generating circuit  410  outputs a control signal Vpop_ctrl according to the status of the audio signal output circuit  400 . The detailed description is as follows. 
         [0019]    Those skilled in the art appreciate that when the voltage of the gate of the NMOS transistor falls from a high level, the impedance of the drain to the source of the NMOS transistor rises. The present invention utilizes such a characteristic with the transistor  408 , the capacitor  412 , and the second resistor  414  to avoid the pop noise occurring. In the default status, the audio signal generating circuit  404  is not turned on and the control signal Vpop_ctrl of the control signal generator  410  stays at a high level so that the transistor  408  is turned on by the high voltage of the gate  430  and the voltage Vc on the first input end  420  of the amplifier  402  is pulled down to the ground. In other words, the equivalent resistance of the noise reduction circuit  401  between the first input end  420  and the ground is 0 ohm. In such a condition, the output end  424  of the amplifier  402  remains at 0 volts despite the output signal of the audio signal generator  404 . Thus the acoustic output device  418  does not output sound. When the audio signal output circuit  400  is turned on to output sound, the output voltage of the voltage output circuit  426  becomes the voltage Vbias. Meanwhile, the control signal Vpop_ctrl falls to a low level, which discharges the capacitor  412 . In such a situation, the voltage of the gate  430  of the transistor  408  gradually falls and the impedance of the drain  432  to the source  434  rises. In other words, the equivalent resistance of the noise reduction circuit  401  between the first input end  420  and the ground gradually becomes infinity. Thus the voltage Vc of the first input end  420  of the amplifier  402  is gradually pulled up from the ground. Consequently, the voltage of the output end  424  of the amplifier  402  is gradually pulled up as well. Therefore, the acoustic output device  418  does not generate the pop noise because no current impulse is generated. On the other hand, when the audio signal output circuit  400  is turned off to stop outputting sound, the control signal Vpop_ctrl rises to the high level, which charges the capacitor  412 . In such a situation, the voltage of the gate  430  of the transistor  408  gradually rises and the impedance of the drain  432  to the source  434  falls. Thus the voltage Vc of the first input end  420  of the amplifier  402  is gradually pulled down to the ground. Thus the equivalent capacitor  436  is gradually discharged which also avoids generating the pop noise. 
         [0020]    Please continue referring to  FIG. 5  and  FIG. 6 .  FIG. 5  is a diagram illustrating the audio signal output circuit  400  being turned on to output sound.  FIG. 6  is a diagram illustrating the audio signal output circuit  400  being turned off to stop outputting sound. In  FIG. 5  and  FIG. 6 , the waveforms Wvc and Wve respectively represent the voltage Vc of the first input end  420  of the amplifier  402  and the voltage Ve between the equivalent capacitor  436  and the equivalent resistor  438 . As shown in  FIG. 5 , the audio signal output circuit  400  is turned on to output sound at time t 1  and the dc level of the signal on the first input end rises to the voltage Vbias gradually instead of abruptly, which avoids generating the pop noise. As shown in  FIG. 6 , when the audio signal output circuit  400  is turned off to stop outputting sound at time t 1 ′, the control signal Vpop_ctrl rises to the high level, and the dc level of the signal on the first input end falls to 0 gradually instead of abruptly, which also avoids generating the pop noise. 
         [0021]    The present invention controls the transistor  408  with the control signal generator  410 , capacitor  412 , and the second resistor  414  to avoid suddenly charging or discharging the equivalent capacitor  436  so that the acoustic output device  418  does not generate the pop noise. Thus, the present invention not only saves power but also raises the sound quality, thereby providing significant benefits to users. 
         [0022]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.