Abstract:
An anti-pop circuit is coupled with a sound outputting device to prevent a “pop” sound form being mixed into a sound signal. The anti-pop circuit includes a control signal generator and a fist diode. The control signal generator generates a control signal with a high level state and a low level state. The first diode couples with the sound outputting device. The sound signal is transferred to the first diode when said first diode is in a forward bias state, and the sound signal is outputted from an output end of the sound outputting device when the first diode is in a reverse bias state.

Description:
RELATED APPLICATIONS 
     The present application is based on, and claims priority from, Taiwan Application Serial Number 94114142, filed May 2, 2005, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention is about an anti-pop circuit, and more particularly, is about an anti-pop circuit including a diode. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  illustrates a typical audio output circuit  100 . An electrostatic protection circuit  102  is used to shunt harmful external static electricity away from the audio output circuit  100 . A sound signal from an audio IC or a speaker is outputted through a low-pass filter  104  and the electrostatic protection circuit  102 . 
     However, a “pop” sound is always intermixed with the outputted sound signal at the moment the power (Vcc) is turned on or off due to the voltage spiking. Typically, an anti-pop circuit  106  is installed in the audio output circuit  100  to eliminate the “pop” sound 
       Figure 1  illustrates a typical anti-pop circuit  106  including two NPN type bipolar junction transistors (BJTs)  108  and  110 . Before the power (Vcc) is turned on, a control signal triggers the anti-pop circuit  106  first to activate the BJT  110  for preventing the “pop” sound due to a voltage spike from mixing into the outputted voice signal. Specifically, a control signal with a low voltage level is outputted first to turn off the BJT  108  so that a high voltage level is generated to turn on the BJT  110 . 
     Typically, a special time period to keep BJT  110  turned on is set, such as 10 ms, after which the power is considered stable and not prone to cause “pop” sounds from spiking. After the set time period is reached, the control signal is transferred from a low voltage level to a high voltage level to turn on the BJT  108  so as to generate a low voltage level to turn off the BJT  110 . At this time, the anti-pop circuit  106  is turned off. 
     Because the control signal has an inverse polarity to that of the power (Vcc), a BJT  108  is required in a typical anti-pop circuit to act as an inverter, which requires an additional cost. Moreover, the control signal is required to cooperate with turning on and off the power (Vcc) to trigger the anti-pop circuit. Therefore, controlling the time sequence of the control signal and the power (Vcc) is very important. 
     Accordingly, an anti-pop circuit with simple structure and simple control is necessary. 
     SUMMARY OF THE INVENTION 
     Therefore, the main purpose of the present invention is to provide a simple structure anti-pop circuit. 
     Accordingly, the present invention provides an anti-pop circuit composed of diodes. A control signal is used to change the bias of the diodes to shunt the “pop” sound at the moment of turning on or off the power (Vcc). 
     In one embodiment, the anti-pop circuit of the present invention includes a first diode and a charge/discharge circuit. The charge/discharge circuit is composed of at least one resistor and at least one capacitor and is connected to the main power (Vcc). The resistor and the capacitor have a common contact. This common contact is connected to the first diode. The working time of the anti-pop circuit is determined by charging the capacitor. 
     In another embodiment, the anti-pop circuit of the present invention includes a first diode, a discharging route and a charge/discharge circuit. The charge/discharge circuit is composed of at least one resistor and at least one capacitor and is connected to a control power (Vcc). The resistor and the capacitor have a common contact. This common contact is connected to the first diode. The working time of the anti-pop circuit is determined by charging the capacitor. This discharging route can accelerate changing the diode bias. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated and better understood by referencing the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates a typical sound output apparatus with an anti-pop circuit; 
         FIG. 2  illustrates an anti-pop circuit according to the first embodiment of the present invention; 
         FIG. 3  illustrates an anti-pop circuit according to the second embodiment of the present invention; and 
         FIG. 4  illustrates an anti-pop circuit according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the anti-pop circuit are described in the following paragraphs to explain the present invention. It is noticed that the electrostatic protection circuit  102  illustrated in the figures can be removed in other embodiments. However, the anti-pop circuit of the present invention can be applied in any audio output apparatus no matter whether an electrostatic protection circuit is included or not. 
       FIG. 2  is an anti-pop circuit according to the first embodiment of the present invention. The sound output apparatus  209  includes an input end  207 , a low-pass filter  203 , an electrostatic protection circuit  201  and an output end  210 . The anti-pop circuit  200  includes a first diode  202  that is triggered by an independent control signal generated by a control signal generator  211 . 
     When the power is turned on or turned off, the first diode  202  is maintained in a forward bias state to avoid the popping phenomenon. According to this embodiment, the control signal is set to a low voltage level to trigger the anti-pop circuit  200 . Then, the sound signal generated  209  at the moment of turning on or turning off the power drain through the low-pass filter  203 , the electrostatic protection circuit  201  and the first diode  202 . After a time frame, such as 8 ms, the first diode  202  is changed to reverse bias. According to this embodiment, the control signal is set to a high voltage level to close the anti-pop circuit  200 . At this time, the sound signal is outputted through the low-pass filter  203  and the electrostatic protection circuit  201 . Therefore, the high voltage level control signal generated by the control signal generator  211  is delayed for certain time frame behind the moment of turning on the power. In an embodiment, the control signal is generated by a general purpose input/output pin. When the power is turned off, the control signal changes the bias of the first diode  202  from reverse bias to forward bias so as to shunt the popping sound generated at the moment of turning off the power from the first diode  202 . 
     According to the first embodiment of the present invention, only one first diode  202  is required to form the anti-pop circuit, which simplifies the structure of the anti-pop circuit and reduces the cost to produce the circuit. Moreover, the phase of the control signal is the same as the phase of the power (Vcc). Therefore, when operating, after the time frame is passed, the control signal is switched to change the bias of the first diode from forward to reverse. The time sequence control is thus very simple. 
       FIG. 3  illustrates an anti-pop circuit according to the second embodiment of the present invention. In this embodiment, a charge/discharge circuit composed of one resistor and one capacitor is used to set a time frame. After the time frame elapsed, the bias of the first diode is switched from forward to reverse. The time frame can be adjusted by changing the values of the resistor and the capacitor. 
     According to the second embodiment, the sound output apparatus  309  includes an input end  307 , a low-pass filter  303 , an electrostatic protection circuit  301  and an output end  310 . The anti-pop circuit  300  includes a first diode  302 , a resistor  304  and a capacitor  306 . The first diode  302  is connected to a common contact  308  between the resistor  304  and the capacitor  306 . The resistor  304  and the capacitor  306  form a charge/discharge route. 
     When the power is turned on, the first diode  302  is maintained in a forward bias. At this time, the anti-pop circuit  300  is turned on to shunt the “pop” sound through the diode  302 . And, the power (Vcc) charges the capacitor  306  through the resistor  304  until the voltage across the capacitor  306 , the voltage of the common contact  308 , reaches the voltage needed to change the bias of the first diode  302 . The bias of the first diode  302  is then changed from forward to reverse to turn off the anti-pop circuit  300 . At this time, the sound signal is outputted through the low-pass filter  303  and the electrostatic protection circuit  301 . 
     On the other hand, when the power is turned off, the capacitor  306  is discharged through the resistor  304  to reduce the voltage of the common contact  308  so as to change the bias of the first diode  302  from reverse to forward. When the first diode  302  is in forward bias, the anti-pop circuit  300  is turned on again to shunt the popping sound generated when turning off the power through the first diode  302 . 
     According to the second embodiment of the present invention, the charge/discharge route composed of the resistor  304  and the capacitor  306  is connected to the power (Vcc). The time to charge the capacitor  306  to the voltage necessary to change the first diode  302  is defined as the working time of the anti-pop circuit  300 . Therefore, according to this embodiment, the working time of the anti-pop circuit  300  can be adjusted by modulating the values of the resistor  304  and the capacitor  306 . 
     In this embodiment, the anti-pop circuit  300  is triggered to shunt the “pop” sound when the power is turned on, and the anti-pop circuit  300  is turned off when the capacitor  306  is charged to the voltage needed to change the bias of the first diode  302 . Therefore, it is not necessary to use an additional control signal to control the turning on and off of the anti-pop circuit in the second embodiment. 
       FIG. 4  illustrates an anti-pop circuit according to the third embodiment of the present invention. According to the third embodiment, the sound output apparatus  409  includes an input end  407 , a low-pass filter  403 , an electrostatic protection circuit  405  and an output end  410 . The anti-pop circuit  400  includes a first diode  402 , a second diode  401 , a resistor  404  and a capacitor  406 . The first diode  402  is connected to the common contact  408  between the resistor  404  and the capacitor  406 . The resistor  404  and the capacitor  406  form a charge/discharge route. The second diode  401  is used as an additional discharging route. A control signal generator  411  is connected to the resistor  404 . 
     When the power is turned on, the first diode  402  is maintained in forward bias. At this time, the anti-pop circuit  400  is turned on to shunt the “pop” sound through the diode  402 . Then, after a predetermined time period, the bias of the first diode  402  is changed from forward to reverse bias to turn off the anti-pop circuit  400 . At this time, the sound signal is outputted through the low-pass filter  403  and the electrostatic protection circuit  405 . According to this embodiment, the predetermined time frame is the time taken for the capacitor  406  to be charged to the voltage needed to change the bias of the first diode  402 . 
     In this embodiment, the control signal generator  411  is triggered when the power (Vcc) is turned on. The control signal from the control signal generator  411  may charge the capacitor  406  through the resistor  404 . While the capacitor  406  is charged, the first diode  402  is maintained in a forward bias to shunt the “pop” sound. In an embodiment, the control signal is generated by a general purpose input/output pin. When the voltage across the capacitor  406 , the voltage of the common contact  408 , reaches the voltage of changing the bias of the first diode  402 , the bias of the first diode  402  is changed from forward to reverse bias to turn off the anti-pop circuit  400 . At this time, the sound signal is outputted through the low-pass filter  403  and the electrostatic protection circuit  405 . 
     On the other hand, when the power is turned off, the capacitor  406  is discharged through the resistor  404  and the second diode  401  to reduce the voltage of the common contact  408  so as to change the bias of the first diode  402  from reverse to forward bias. When the first diode  402  is in forward bias, the anti-pop circuit  400  is turned on again to shunt the popping sound through the first diode  402 . 
     According to the third embodiment of the present invention, the charge/discharge route composed of the resistor  404  and the capacitor  406  may determine the changing time of the first diode  402 . Therefore, the changing time of the first diode  402  can be adjusted by modulating the values of the resistor  404  and the capacitor  406 . On the other hand, when the power (Vcc) is turned off, an additional discharging route, the second diode  401 , is provided in the present invention to enhance the capacitor  406  discharging. Therefore, the anti-pop circuit  400  can be turned on quickly to shunt the “pop” sound when the power (Vcc) is turned off. 
     Accordingly, the anti-pop circuit of the present invention includes a first diode. The working time of the anti-pop circuit is determined by the changing time of the diode. Moreover, a charge/discharge route composed of a resistor and a capacitor is used to trigger and turn off the anti-pop circuit. The working time of the anti-pop circuit can be adjusted by modulating the values of the resistor and the capacitor, which makes an additional control signal to control the turning on and off of the anti-pop circuit unnecessary in some embodiments. The circuit also can be simplified compared to the prior art. 
     As is understood by a person skilled in the art, the foregoing descriptions of the preferred embodiments of the present invention are an illustration of the present invention rather than a limitation thereof. Various modifications and similar arrangements are included within the spirit and scope of the appended claims. The scope of the claims should be accorded to the broadest interpretation so as to encompass all such modifications and similar structures. While preferred embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.