Abstract:
A circuit for handling pulse width modulation (PWM) first signal and PWM second signal for outputting to an amplifier, the PWM first signal having one of a same phase and an opposite phase relationship with the PWM second signal, the circuit comprising a power detector for detecting power turn on to the amplifier and outputting a power on signal and detecting a power turn off to the amplifier and outputting a power off signal; a pulse generator having: a duty cycle generator for generating a first pulse signal corresponding to the PWM first signal and a second pulse signal corresponding to the PWM second signal, and a reduced-width generator for generating at least one of a reduced-width first pulse and a reduced-width second pulse; a controller for selecting one of the reduced-width first pulse and the reduced-width second pulse for outputting to the amplifier upon receipt of the power on signal and for selecting the first pulse signal and the second pulse signal for outputting to the amplifier thereafter; and a counter for counting the duration of the pulse width of the PWM first signal, the counter being activated upon detection of the power off signal and a select circuit for outputting an off select signal upon reaching a predetermined reduced-width time count to cause an output of one of a reduced-width PWM first signal and a reduced-width PWM second signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is directed to a sound signal generating apparatus and method for reducing pop noise in a class-D amplifier.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Audio signal amplifiers used for amplifying audio signals are generally classified into classes A, AB, B, C, and D depending on their operating state during power amplifying stage.  
           [0005]    Among the various classes of audio signal amplifiers, the class-D amplifiers are more popular because their efficiency is higher than that of class AB amplifiers and their linearity is also superior due to low cross-over.  
           [0006]    Class-D amplifiers are also referred to as switch-mode amplifiers because they resemble switch-mode voltage regulators. A class-D amplifier uses Pulse Width Modulation (PWM) method on an input analog signal or digital PCM signal. This means an input analog signal is modulated by a higher frequency modulation or carrier signal, usually a saw-tooth triangular wave or an input digital PCM signal is converted to a related PWM signal. Upon pulse width modulation, the analog input signal or digital PCM signal becomes discreet or digital with pulse widths used to represent signal strengths of the input original.  
           [0007]    The PWM signal presented to the amplifier is a high frequency digital signal with varying widths. A low band pass filter is used to filter the high frequency component to extract the input signal and reduce switching noise.  
           [0008]    [0008]FIG. 1 shows a waveform of input signals PWMA and PWMB being applied to a conventional class-D amplifier, wherein pulse width modulating signals PWMA, PWMB have substantially the same pulse width but are opposite to each other in phase. FIG. 2 shows a waveform of a speaker voltage VC 1  when PWMA, PWMB and power DET 1  are applied (Here, DET 1  is transited from “Low” to “High” when voltage is turned on). It can be seen from FIG. 2 that a severe overshoot occurs at or near the time when power is applied. The overshoot is caused by the same pulse width of the pulse width modulating signals PWMA, PWMB. This overshoot voltage VC 1  at the speaker results in ‘click or pop’ noise.  
           [0009]    Therefore, a need exists for an apparatus and a method for generating PWM signals which operate a class-D amplifier but without the pop noise upon application of power or when power is removed from the amplifier.  
         SUMMARY OF THE INVENTION  
         [0010]    A circuit is provided for handling pulse width modulation (PWM) first signal and PWM second signal for outputting to an amplifier, the PWM first signal having one of a same phase and an opposite phase relationship with the PWM second signal, the circuit comprising: a power detector for detecting power turn on to the amplifier and outputting a power on signal; and a pulse generator having: a duty cycle generator for generating a first pulse signal corresponding to the PWM first signal and a second pulse signal corresponding to the PWM second signal; and a pulse reducing generator for generating a reduced-width first pulse or a reduced-width second pulse for outputting to the amplifier upon receipt of the power on signal.  
           [0011]    Preferably, the pulse generator further includes a controller for outputting the reduced-width first pulse or the reduced-width second pulse upon receipt of the power on signal and for outputting the first pulse signal and the second pulse signal to the amplifier thereafter.  
           [0012]    According to this embodiment, the circuit further preferably includes a select circuit for generating a select signal for selecting between the PWM first and second signals during one selection mode and the signals from the pulse generator during another selection mode, and a counter for counting time upon receipt of the power on signal and to output the select signal in the another selection mode upon reaching a predetermined count for outputting the PWM first and second signals to the amplifier, wherein the reduced-width first pulse is about one-half the pulse width of the first pulse signal.  
           [0013]    According to an aspect of the invention, the circuit further includes a delay for delaying by a predetermined time the first pulse signal to output a delayed first pulse signal that transitions with a delay by the predetermined time to provide a time gap between the transition of the delayed first pulse signal and the transition of the second pulse signal, wherein the amplifier includes a serially connected pair of transistors for receiving at their gates the first pulse signal and the second pulse signal.  
           [0014]    According to another embodiment of the present invention, a circuit is provided for handling pulse width modulation (PWM) first signal and PWM second signal for outputting to an amplifier, the PWM first signal having one of a same and an opposite phase relationship with the PWM second signal, the circuit comprising: a power detector for detecting power turn off to the amplifier and outputting a power off signal; and a counter for counting the duration of the pulse width of the PWM first signal, the counter being activated upon detection of the power off signal, and for outputting a select signal upon reaching a predetermined reduced-width time count to cause an output of a reduced pulse width PWM first signal or reduced pulse width PWM second signal prior to complete power turn off from the amplifier. Preferably, the circuit further includes a synchronizing circuit for synchronizing the power off signal using a system clock, wherein the amplifier includes a pair of transistors for receiving at their gates the PWM first signal and the PWM second signal.  
           [0015]    The circuit further includes a mute circuit for outputting the select signal to cause an output of a reduced pulse width PWM first signal or reduced pulse width PWM second signal upon receipt of a mute signal, wherein the mute circuit is an AND gate, and wherein the reduced pulse width PWM first signal or reduced pulse width PWM second signal is the last pulse signal received by the amplifier prior to complete power turn off, and wherein the reduced-width is about one half of the width of the PWM first signal or the PWM second signal.  
           [0016]    According to still another embodiment of the invention, a circuit is provided for handling pulse width modulation (PWM) first signal and PWM second signal for outputting to an amplifier, the PWM first signal having one of a same phase and an opposite phase relationship with the PWM second signal, the circuit comprising: a power detector for detecting power turn on to the amplifier and outputting a power on signal and detecting a power turn off to the amplifier and outputting a power off signal; a pulse generator having: a duty cycle generator for generating a first pulse signal corresponding to the PWM first signal and a second pulse signal corresponding to the PWM second signal, and a reduced-width generator for generating at least one of a reduced-width first pulse and a reduced-width second pulse; a controller for selecting one of the reduced-width first pulse and the reduced-width second pulse for outputting to the amplifier upon receipt of the power on signal and for selecting the first pulse signal and the second pulse signal for outputting to the amplifier thereafter; and a counter for counting the duration of the pulse width of the PWM first signal, the counter being activated upon detection of the power off signal and a select circuit for outputting an off select signal upon reaching a predetermined reduced-width time count to cause an output of one of a reduced-width PWM first signal and a reduced-width PWM second signal, wherein the select circuit further includes a counter for counting time upon receipt of the power on signal and to output an on select signal for first outputting one of the reduced-width first pulse and the reduced-width second pulse and then the first pulse signal and the second pulse signal to the amplifier.  
           [0017]    The circuit according to this embodiment further includes a mute circuit for outputting the on select signal for outputting one of the reduced-width first pulse and the reduced-width second pulse to the amplifier upon receipt of a mute signal, wherein the mute circuit is synchronized using a system clock and upon mute inactivation, to cause an output of one of the reduced-width PWM first signal and the reduced-width PWM second signal, wherein the reduced-width first pulse signal has a pulse width about one half of the pulse width of the first pulse signal, and wherein the predetermined reduced-width time count is about one half the duration of the pulse width of the PWM first signal.  
           [0018]    The circuit according to this embodiment further includes a delay for delaying by a predetermined time the first pulse signal to output a delayed first pulse signal that transitions with a delay by the predetermined time to provide a time gap between the transition of the delayed first pulse signal and the transition of the second pulse signal, wherein the amplifier includes a pair of transistors for receiving at their gates the delayed first pulse signal and the second pulse signal, wherein the amplifier includes a pair of transistors for receiving at their gates the first pulse signal and the second pulse signal.  
           [0019]    According to another aspect of the invention, a method is provided for handling pulse width modulation (PWM) first signal and PWM second signal for outputting to an amplifier, the PWM first signal having one of a same phase and an opposite phase relationship with the PWM second signal, the method comprising: detecting power turn on to the amplifier and outputting a power on signal; and generating a first pulse signal corresponding to the PWM first signal and a second pulse signal corresponding to the PWM second signal; and generating a reduced-width first pulse or a reduced-width second pulse for outputting to the amplifier upon receipt of the power on signal.  
           [0020]    Preferably, the method further includes outputting the reduced-width first pulse or the reduced-width second pulse to the amplifier upon receipt of the power on signal and outputting the first pulse signal and the second pulse signal to the amplifier thereafter, and generating a select signal for selecting between the PWM first and second signals during one selection mode and the first pulse signal and the second pulse signal during another selection mode for outputting to the amplifier, wherein the reduced-width first pulse is about one-half the pulse width of the first pulse signal.  
           [0021]    According to still another aspect of this embodiment, the method further includes delaying by a predetermined time the first pulse signal to output a delayed first pulse signal that transitions with a delay by the predetermined time to provide a time gap between the transition of the delayed first pulse signal and the transition of the second pulse signal, and the steps of: detecting power turn off to the amplifier and outputting a power off signal; counting the duration of the pulse width of the PWM first signal upon detection of the power off signal; and outputting a select signal upon reaching a predetermined reduced-width time count to cause an output of a reduced pulse width PWM first signal or reduced pulse width PWM second signal prior to complete power turn off from the amplifier, wherein the reduced pulse width PWM first signal or reduced pulse width PWM second signal is the last pulse signal received by the amplifier prior to complete power turn off, and wherein the reduced-width is about one half of the width of the PWM first signal or the PWM second signal.  
           [0022]    According to still another embodiment of the present invention, a circuit is provided for handling pulse width modulation (PWM) first signal and PWM second signal for outputting to an amplifier, the PWM first signal having one of a same phase and an opposite phase relationship with the PWM second signal, the circuit comprising: means for detecting power turn on to the amplifier and outputting a power on signal and detecting a power turn off to the amplifier and outputting a power off signal; means for generating a first pulse signal corresponding to the PWM first signal and a second pulse signal corresponding to the PWM second signal, and a reduced-width generator for generating at least one of a reduced-width first pulse and a reduced-width second pulse; means for selecting one of the reduced-width first pulse and the reduced-width second pulse for outputting to the amplifier upon receipt of the power on signal and for selecting the first pulse signal and the second pulse signal for outputting to the amplifier thereafter; and a counter for counting the duration of the pulse width of the PWM first signal, the counter being activated upon detection of the power off signal and a select circuit for outputting an off select signal upon reaching a predetermined reduced-width time count to cause an output of one of a reduced-width PWM first signal and a reduced-width PWM second signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The embodiments of the present invention will become more apparent when detailed description of embodiments are read with reference to the accompanying drawings in which:  
         [0024]    [0024]FIG. 1A shows a waveform of a PWM switching signal and FIG. 1B shows a class-D amplifier;  
         [0025]    [0025]FIG. 2 shows an overshoot response of a speaker voltage when used with the amplifier of FIG. 1;  
         [0026]    [0026]FIG. 3 shows a block diagram of a switching signal generating apparatus according to an embodiment of the present invention;  
         [0027]    [0027]FIG. 4 shows a circuit of a half-bridge typed class-D amplifier having one power source;  
         [0028]    [0028]FIG. 5A shows a circuit of a half-bridge typed class-D amplifier having two power sources;  
         [0029]    [0029]FIG. 5B is a circuit of a full-bridge typed class-D amplifier having one power source;  
         [0030]    [0030]FIG. 6 shows a detail block diagram of a pulse signal generating circuit of FIG. 3;  
         [0031]    [0031]FIG. 7 shows waveforms of a first detecting signal DET 1  and first and second pulse signals PUL 1 , PUL 2  from the pulse signal generating circuit according to an embodiment of the present invention.  
         [0032]    [0032]FIG. 8 shows switching waveforms at a time of power-on for the switching signal generating apparatus;  
         [0033]    [0033]FIG. 9 shows switching waveforms at a time of power-off for the switching signal generating apparatus;  
         [0034]    [0034]FIG. 10 shows a block diagram of a switching signal generating apparatus according to another embodiment of the present invention;  
         [0035]    [0035]FIG. 11 shows switching waveforms at a time of mute-on and mute-off for the switching signal generating apparatus;  
         [0036]    [0036]FIG. 12 is a graph showing a response of speaker voltage in an audio sound regenerating apparatus according to an embodiment of the present invention;  
         [0037]    [0037]FIG. 13 is a flowchart showing a method of generating a switching signal according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0038]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that like reference numerals are used for designation of like or equivalent parts or portion for simplicity of illustration and explanation.  
         [0039]    [0039]FIG. 3 is a switching signal generating apparatus  600  according to a preferred embodiment of the present invention.  
         [0040]    Referring to FIG. 3, the switching signal generating apparatus  600  includes a power detecting circuit  610 , a pulse signal generating circuit  620 , a first selection circuit  630 , an audio signal processor  640 , a second selection circuit  650 , and a dead time controlling circuit  660 .  
         [0041]    The general operation of the switching signal apparatus  600  is herein described. The power detecting circuit  610  detects power turn-on and outputs a first detecting signal DET 1  to the pulse signal generating circuit  620 . The power detecting circuit  610  also detects a disconnection of the power supply or power off and outputs a second detecting signal DET 2 , which is output to the audio signal processor  640 . The power detecting circuit  610  receives a timing signal PPS for synchronizing timing signals of the device to a system clock. The period of the timing signal PPS corresponds to the period of the PWM signals generated from the audio signal processor  640 . Further, the power detecting circuit  610  generates and outputs a control signal SEL to the pulse signal generating circuit  620 , the first selection circuit  630 , and the second selection circuit  650 .  
         [0042]    The pulse signal generating circuit  620  generates a first pulse signal PUL 1  and a second pulse signal PUL 2  in response to the first detecting signal DET 1  and control signal SEL. A system clock signal can be used to synchronize the pulse signals PUL 1  and PUL 2 . According to at least one preferred embodiment according to the present invention, the pulse signals PUL 1  and PUL 2  are shaped by the pulse signal generating circuit  620  to output at opposite phase and to have initial smaller pulse widths upon application of the first detecting signals DET 1  to prevent simultaneous turn-on of the transistors on the audio sound regeneration circuit.  
         [0043]    The audio signal processor  640  receives an audio input signal AUDIO and modulates the audio input signal AUDIO with a pulse train to output a pulse width modulated audio signal APWM, wherein the width of the pulses represent the strengths of the input audio signal. The audio signal processor  640  outputs the pulse width modulation period PPS and is preferably inactivated when the second detecting signal DET 2  is active, signaling power off.  
         [0044]    The first selection circuit  630  selects either the first pulse signal PUL 1  or the pulse width modulated audio signal APWM in response to the control signal SEL and outputs a first selected signal MUXA. The second selection circuit  650  selects either the second pulse signal PUL 2  or the pulse width modulated audio signal APWM in response to the control signal SEL and outputs a second selected signal MUXB. Thus, each output signal from the first and second selection circuits  630 ,  650  is either the pulse signals PUL 1 , PUL 2  or the pulse width modulated audio signal APWM in opposite phase. The first and second selection circuits  630 ,  650  are preferably multiplexers.  
         [0045]    The dead time controlling circuit  660  receives the first and second selected signals MUXA, MUXB from the first and second selection circuits  630 ,  650 , respectively. According to an embodiment of the present invention, the dead time controlling circuit  660  includes delay elements (not shown) for delaying the switching signals by a predetermined dead time. Preferably, the delay is applied to the rising (transition from low to high) edges of the MUXA and MUXB signals. This dead time delay creates a time gap between the transition of the delayed first pulse signal and the transition of the second pulse signal to prevent simultaneously turning on or off the transistors in an audio sound regenerating apparatus. The dead time controlling circuit  660  outputs first and second switching signals PWMA, PWMB to the audio sound regenerating apparatus.  
         [0046]    [0046]FIG. 4 shows a half-bridge type audio sound regenerating apparatus  680 , which includes two MOS transistors  101 ,  103  connected in series, two diodes M 1 , M 2 , a band pass filter having an inductance L and a capacitor C, and a speaker  105 . One terminal of each of the diode M 1  and M 2  is connected to a source of the transistor and the other terminal of each of the diode is connected to a drain of the transistor.  
         [0047]    The first and second switching signals PWMA, PWMB generated from the switching signal generating circuit  600  are inputted into the gates of the MOS transistors  101 ,  103 , respectively, and are amplified by the MOS transistors  101 ,  103  corresponding to the first and second switching signals PWMA, PWMB. Since the first and second amplified switching signals PWMA and PWMB include a sound component and a switching frequency component, a low frequency band pass filter is used to filter the switching frequency component to recover the sound signal.  
         [0048]    [0048]FIG. 5A shows an alternate audio sound regenerating apparatus, which is a half-bridge type having two transistors  101 ,  103 , two power sources ½ Vdc, ½ Vdc, a band pass filter having an inductance L and a capacitor C, and a speaker  105 . FIG. 5B shows another audio sound regenerating apparatus which is a full-bridge type having one power source Vdc, four transistors  101 ,  103 ,  301 ,  303 , and a band pass filter L 1 , L 2 , C. One skilled in the art can readily appreciate that the audio sound regenerating apparatus  680  applicable to the present invention is not limited to the apparatus shown in FIGS. 4, 5A and  5 B, but includes any audio sound regenerating apparatus having at least two transistors with respective gates for receiving switching signals PWMA, PWMB, and a low band pass filter.  
         [0049]    [0049]FIG. 6 is an exemplary circuit diagram of the pulse signal generating circuit  620  of FIG. 3. This circuit receives as inputs the power-on detect signal DET 1  and signal SEL from power detect circuit  610  to generate pulse signals PUL 1  and PUL 2 . Upon transition from ‘Low’ to ‘High’ of the DET 1  signal, the half-pulse generators  622  and  623  generate initial pulses at half width (quarter cycle) to output the initial pulses of PUL 1  and PUL 2  with a pulse width equal to one half (½) of the pulse width of modulated audio signal APWM. Then, MUX  626  and MUX  627  are selected to pass through signals output from the 50:50 pulse generators  624  and  625 , with duty cycle the same as system clock CLK and same period as pulse period signal PPS. According to an alternative embodiment, only one of the two half-pulse generators  622  and  623  is employed, depending on whether transistors  101 ,  301 ,  103 ,  301  are NMOS or PMOS type, to generate an initial half pulse at either PUL 1  or PUL 2 .  
         [0050]    [0050]FIG. 7( a ) shows waveforms of the first detecting signal DET 1  and the first and second pulse signals PUL 1 , PUL 2  in case that the MOS transistors  101 ,  303 ,  103 ,  301  of FIGS. 1 and 3 are NMOS transistors. FIG. 7( b ) shows waveforms of the first and second pulse signals PUL 1 , PUL 2  applicable when the MOS transistors  101 ,  303  of FIGS. 1 and 3 are PMOS transistors and the MOS transistor  103 ,  301  are NMOS transistors. It can be seen that signals PUL 1  and PUL 2  are opposite in phase at all times.  
         [0051]    [0051]FIG. 8 shows switching waveforms at a time of power-on for the switching signal generating apparatus  600  and switching signals PWMA, PWMB. With select signal SEL initially at logic level ‘Low’, signals PUL 1  and PUL 2  are output through first select circuit  630  and second select circuit  650  as PWMA and PWMB.  
         [0052]    Operation of the switching signal generating apparatus  600  is further described with reference to FIGS. 3, 6,  7 , and  8 . When power PW is supplied to the switching signal generating apparatus  600 , the power detecting circuit  610  outputs the first detecting signal DET 1  with transition from logic level ‘Low’ to logic level ‘High’ to the pulse signal generating circuit  620 . The power detecting circuit  610  also outputs signal SEL, initially having logic level ‘Low’, to the pulse signal generating circuit  620 , the first selection circuit  630 , and the second selection circuit  650 .  
         [0053]    The pulse signal generating circuit  620  generates the first pulse signal PUL 1  and the second pulse signal PUL 2  in response to the first detecting signal DET 1 , as shown in FIGS.  7 ( a ) and  7 ( b ). The audio signal processor  640  outputs the pulse width modulated audio signal APWM.  
         [0054]    The first selection circuit  630  outputs the first selected signal MUXA to the dead time controlling circuit  660  and the second selected signal MUXB to the dead time controlling circuit  660  in response to the ‘Low’ level of the control signal SEL.  
         [0055]    The dead time controlling circuit  660  outputs the first and second switching signals PWMA, PWMB to the MOS transistors of the audio sound regenerating apparatus  680  (FIGS. 4, SA and  5 B).  
         [0056]    It can be seen from FIGS.  7 ( a ) and  8  that a width Tonf of a first pulse  701  of the first pulse signal PUL 1  is about Ton/ 2 , and is smaller than each width (Ton) of other pulses  702 ,  703 ,  704 ,  705 . The first pulse  701  represents a first generated pulse of the first pulse signal PUL 1  at the pulse signal generating circuit  620  when the power PW is supplied. The pulses  703 ,  705  represent second and third pulses, respectively, of the first pulse signal PUL 1 . The pulse  702  represents a first pulse of the second pulse signal PUL 2 , and the pulse  704  represents a second pulse of the second pulse signal PUL 2 . Preferably, the pulse width Tonf of the first pulse  701  of the first pulse signal PUL 1  is about one half the width of the other pulses  702 ,  703 ,  704 ,  705 . Further, the width Tonf of the first pulse  701  of the first pulse signal PUL 1  is about a quarter (¼) cycle of the other pulses  702 ,  703 ,  704 . Each cycle (Tsw) of the other pulses  702 ,  703 ,  704 ,  705  is the same as a cycle of the pulse width modulated audio signal APWM. A width of an (n)-th pulse is substantially the same as a width of an (n+1)-th pulse. The initial pulse width  701  applied to the transistor of a class-D amplifier (e.g.,  101  in FIG. 4) acts to turn on the transistor at reduced energy upon application of power, thus the pop-noise is minimized since an excess response is minimized. Although the initial pulse width of PUL 1  or PUL 2  is reduced to one half pulse width as described, other pulse width reductions, such as one quarter to one third pulse width can also be applicable to reduce pop noise.  
         [0057]    After a predetermined time is passed, the control signal SEL is changed from logic ‘Low’ to logic ‘High’, and the pulse signal generating circuit  620  is disabled in response to the active (logic ‘High’) control signal SEL. Then, the output of the audio signal processor  640 , the pulse width modulated audio signal APWM, is selected by the first and second selector circuits  630  and  650  to the dead time control circuit  660 . According to an embodiment of the present invention, the predetermined time is set by a manufacturer as a default value or by a user as any value. According to the embodiment as shown in FIG. 8, the predetermined time is about 2½ cycles.  
         [0058]    The dead time control circuit  660  outputs the first and second switching signals PWMA, PWMB to the MOS transistors of the class-D amplifiers  408  (FIGS. 4 and 5). According to an embodiment of the present invention, the predetermined dead time (DT) is a time for protecting simultaneously turning-on or off of the MOS transistors  101 ,  103 ,  301 ,  303 . The dead time control circuit  660  makes smaller the width of at least one the first and second switching signals PWMA, PWMB by the predetermined dead time (DT), thus preventing simultaneously turning-on or off the MOS transistors  101 ,  103 ,  301 ,  303 . The dead time controlling circuit  660  includes delay elements for implementing the delay (dead time) to make smaller one or both switching signals PWMA and PWMB shorter in pulse width at logic ‘High’. According to one embodiment, the delay is applied to the rising edges of both switching signals PWMA and PWMB.  
         [0059]    Referring again to FIG. 8, the switching signal generating apparatus  600  outputs the first and second switching signals PWMA, PWMB during the predetermined period “Tp” (or “Starting Mode”). During the “Tp” period, the first and second switching signals PWMA, PWMB are substantially the same as the first and second pulse signals PUL 1 , PUL 2 . After “Tp” period, the switching signal generating apparatus  600  outputs the first and second switching signals PWMA, PWMB during a “Ta” period (or “Sound PWM Mode”). During the “Ta” period, the first and second switching signals PWMA, PWMB are substantially the same as the pulse width modulated audio signal APWM. According to an embodiment of the present invention, the pulse widths  702 ,  704  of the second switching signal PWMB are smaller than the signal PWMA, preferably at an amount of 2*DT, as compared to switching signal PWMA, pulses  703  and  705 .  
         [0060]    Protective measures applicable to power-off from the amplifier is described with reference to FIG. 9, which shows switching waveforms at a time of power-off for the switching signal generating apparatus  600 .  
         [0061]    According to a preferred embodiment of the invention, the audio signal processor  640  estimates on-periods Ton 1 , Ton 2  of signal APWM and outputs an estimated value PPS to the power detecting circuit  610 . Preferably, the pulse period or duration of each cycle of the first and second switching signals PWMA, PWMB are substantially constant, and the on-periods Ton 1 , Ton 2  are variable according to an audio signal AUDIO. The cycle duration is preferably estimated by using a counter (not shown). This counter value can be stored in a buffer unit (not shown) and the buffer is updated with the estimated value PPS every cycle. The estimated value PPS is used for controlling a pulse width  903  to reduce pop-noise when the supplied power PW is turning off, that is, the power PW transitions from level ‘High’ to level ‘Low’.  
         [0062]    During power-off, the power detecting circuit  610 , the pulse signal generating circuit  620 , and the audio signal processor  640  can be disabled. The power detecting circuit  610  detects the power-off and outputs a second detecting signal DET 2  to the audio signal processor  640 . The audio signal processor  640  stops the operation of pulse width modulation in response to the second detecting signal DET 2 . According to an embodiment of the present invention, the second detecting signal DET 2  is changed to a logic ‘Low’ (or inactivated) at a reduced pulse width, preferably at half period of the on-period Ton 2  or at quarter-cycle. Thus, the width of a pulse  903  is smaller than the width Ton 2  of the pulse  902 , pulse  903 , being the last pulse before the power is off. Preferably, the width of the pulse  903  is substantially a half-width of the pulse  902  or a quarter of a period of a normal cycle of the first or second switching signal PWMA or PWMB. The reduced pulse width is a function of the counter value stored in the buffer. For example, an one third pulse width in pulse  903  is obtained by transitioning DET 2  from ‘High’ to ‘Low’ at one third the time value stored in the buffer.  
         [0063]    [0063]FIG. 10 is a block diagram of a switching signal generating apparatus  1000  according to another embodiment of the present invention. The switching signal generating apparatus  1000  controls an on-period of first and second switching signals PWMA, PWMB in response to a mute signal/MUTE.  
         [0064]    Referring to FIG. 10, the switching signal generating apparatus  1000  includes a power detecting circuit  610 , a pulse signal generating circuit  620 , a first selection circuit  630 , a audio signal processor  641 , a second selection circuit  650 , a dead time controlling circuit  660 , and a logic gate  1010 .  
         [0065]    The power detecting circuit  610  detects a supplied power PW and generates a pre-control signal PSEL. The pre-control signal PSEL is input to one of two inputs of logic gate  1010 . The output of the logic gate  1010  is control signal SEL, which functions similarly as the SEL signal in FIG. 3 to select between PUL 1  and PUL 2  or APWM to output from selectors  630  and  650 , respectively.  
         [0066]    The audio signal processor  641  outputs a mute control signal CMUTE to the logic gate  1010  in response to mute signal/MUTE. The audio signal processor  641  controls the time when CMUTE is active or inactive. The operations and functions of the audio signal processor  641  of the present embodiment are substantially the same as the audio signal processor  640  in FIG. 3, with the exception of the ability to disable the control signal (SEL) by the audio signal processor  641  via the CMUTE signal. According to the present embodiment, the logic gate  1010  is preferably an AND gate or an equivalent gate of the AND gate. The logic gate  1010  receives the mute control signal CMUTE and the pre-control signal PSEL, and generates a control signal SEL.  
         [0067]    [0067]FIG. 11 shows switching waveforms at mute-on and mute-off for the switching signal generating apparatus  1000  in FIG. 10. Referring to FIGS. 10 and 11, when the switching signal generating apparatus  1000  is operated as a “sound pulse width modulating (PWM) mode”, the PSEL signal is at logic ‘High’ and mute signal/MUTE is inactive at logic ‘High’ (or “mute-off mode”). During the mute-off mode, the switching signal generating apparatus  1000  outputs first and second switching signals PWMA, PWMB substantially the same as pulse width modulated audio signal APWM outputted from the audio signal processor  641 .  
         [0068]    When the mute signal/MUTE is transited to a logic ‘Low’ (or “mute-on mode”), the audio signal processor  641  in turn outputs a mute control signal CMUTE to the logic gate  1010 . When the mute control signal CMUTE is transited to a logic ‘Low’, the control signal SEL is transited to a logic ‘Low’. During a sound PWM mode, the pre-control signal is at a ‘High’ level, the control signal SEL is transited to the same logic level as the logic level of the mute control signal CMUTE. As in the embodiment described above for power turn-off with reference to FIG. 9, the audio signal processor  641  estimates the pulse width of an “on-period” (Ton 1 ). The audio signal processor  641  can further generate the mute control signal CMUTE to control the “on-period” of the last pulse  1103  at a reduced pulse width prior to the mute controller.  
         [0069]    The first selection circuit  630  and the second selection circuit  650  are switched in response to the control signal SEL. Preferably, the first selection circuit  630  and the second selection circuit  650  are switched when the pulse width Ton 1 f (or on-period) of the pulse  1103  is less than the pulse width Toni (or on-period) of the pulse  1101 , preferably, Ton 1 f is one half of Ton 1 . The audio signal processor  641  outputs the pulse width modulated audio signal APWM. Thus, pop-noise generated the class-D amplifier is reduced in response to the pulse  1103 .  
         [0070]    Further, when the mute-on mode is transited to the mute-off mode, the mute signal/MUTE is transited to a logic ‘High’. The audio signal processor  641  outputs the mute control signal CMUTE to the AND gate  1010  for transiting the control signal to a logic ‘High’ in response to the mute signal/MUTE. The audio signal processor  641  can thus generate the mute control signal CMUTE to control “on-period” of pulse  1105 .  
         [0071]    The AND gate  1010  outputs the control signal SEL having a logic ‘High’ in response to the mute control signal CMUTE having a logic ‘High’ and the pre-control signal PSEL having a logic ‘High’.  
         [0072]    The first selection circuit  630  and the second selection circuit  650  are switched in response to the control signal SEL having a logic ‘High’. The first selection circuit  630  and the second selection circuit  650  is switched when pulse width Ton 3 f (or on-period) of the pulse  1105  is less than the pulse width Ton 3  (or on-period) of the pulse  1107 . According to a preferred embodiment of the present invention, the on-period of the pulse  1105  is about a half of the on-period of the pulse  1107 . Thus, any pop-noise that may be generated by the pulse  1105  would be less as compared to pop-noise that may be generated by the pulse  1107 .  
         [0073]    [0073]FIG. 12 is a graph showing a response of speaker voltage in an audio sound regenerating apparatus according to an embodiment of the present invention. Voltage Vc 1  in FIG. 12 is smaller than voltage Vc 1  in FIG. 2 and the overshoot response is considerably reduced compared to the conventional overshoot response in FIG. 2. Therefore, pop-noise for the sound signal generating apparatus (class-D amplifier) is reduced by using the switching signal generating apparatus  600  or  1000 .  
         [0074]    [0074]FIG. 13 is a flowchart showing a method of generating a switching signal according to an embodiment of the present invention. Referring to FIGS. 3, 10, and  13 , first, power is on (Step  1400 ). The power detecting circuit  610  detects whether the power is on (Step  1401 ). When the power is on, the power detecting circuit  610  outputs a first detecting signal DET 1  (Step  1403 ). The pulse signal generating circuit  620  generates pulse signals PUL 1 , PUL 2  (Step  1405 ). An initial first pulse  701  has a half width as compared to pulses  702 ,  703 ,  704 .  
         [0075]    The control signal SEL generated from the power detecting circuit  610  transits from a logic ‘Low’ to a logic ‘High’ after passing a predetermined period. The predetermined period is a period for stabilizing the pulse width modulated audio signal APWM generated from the audio signal processor  640  or  641 . Regular pulse signals PUL 1  and PUL 2  are output after the initial reduced width signals (Step  1407 ).  
         [0076]    Upon stabilization of the pulse width modulated audio signal APWM, the first selection circuit  630  and the second selection circuit  650  are selected to output PWM signals in sound PWM mode in response to the control signal SEL at logic ‘High’ (Step  1409 ).  
         [0077]    When power PW is disconnected or being burned-off, the power detecting circuit  610  detects power-off and outputs the second detecting signal DET 2  to the audio signal processor  640  (Step  1410 ). As shown in FIG. 9, the second detecting signal DET 2  transits to inactive level when the last pulse width  903  is about half width of the pulse width  902 . Then, the sound PWM mode is stopped when power is completely off (Step  1411 ).  
         [0078]    The audio signal processor  640  detects a state of the mute signal/MUTE and determines whether the mute signal/MUTE is in a state of mute-on. If the mute signal/MUTE is not in a state of mute-on, the switching signal generating apparatus  1000  performs a sound PWM mode. While the switching signal generating apparatus  1000  is in a state of mute-on, the audio signal processor  641  outputs a mute controlling signal CMUTE to the AND gate  1010 . The first selection circuit  630  and the second selection circuit  650  are switched in response to the control signal SEL.  
         [0079]    Although the present invention has been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those precise embodiments, and various other changes and modifications may be affected therein by one skilled in the art without departing from the scope and spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.