the present invention will now be described in detail with reference to the accompanied drawings . fig2 is a block diagram illustrating a parametric equalizer in accordance with the present invention . referring to fig2 , a parametric equalizer 100 comprises a first signal processing unit 110 , a second signal processing unit 120 and an adder 130 . the first signal processing unit 110 processes an input digital audio signal x ( pcm signal , for example ) according to its transfer function h 1 and outputs an output signal y 1 (= xh 1 ). the transfer function h 1 is defined in equation 2 below . where h ( z ) is the transfer function of the parametric equalizer 100 , and g is a preset gain of the parametric equalizer . fig3 a through 3c exemplify the transfer function h 1 . as shown in fig3 a through 3c , h 1 is obtained by shifting h ( z ) by − 1 along y axis . specifically , in case of h ( z ) shown in fig1 a , h 1 extends from −∞ to 0 , and then from 0 to −∞ (−∞→ 0 →−∞) in db ( decibel ) scale as shown in fig3 a . in case of h ( z ) shown in fig1 b , h 1 extends from 0 to −∞ ( 0 →−∞) in db scale as shown in fig3 b . in case of h ( z ) shown in fig1 c , h 1 extends from −∞ to 0 (−∞→ 0 ) in db scale as shown in fig3 c . however , h 1 is not limited to functions shown in fig3 a through 3c and may have various forms . since the transfer function h 1 of the first signal processing unit 110 is obtained by shifting h ( z ), the coefficients of the transfer function h 1 can be pre - calculated from equation 2 and are not required to be calculated in real time . still referring to fig2 , the second signal processing unit 120 processes the output signal y 1 of the first signal processing unit 110 by its transfer function h 2 according to a level gin of the output signal y 1 and outputs an output signal y 2 (= xh 1 h 2 ). the transfer function h 2 is defined in equation 3 below . the adder 130 adds the output signal y 2 of the second signal processing unit 120 to the input digital audio signal x to generate an output digital audio signal y . the output digital audio signal y according to the range of gin may be calculated as below . since h 2 = g − 1 from equation 3 , the output digital audio signal y is expressed as equation 4 . equation 5 means that the preset gain g of the parametric equalizer 100 in accordance with the present invention does not change . that is , when the level gin of the output signal y 1 of the first signal processing unit 110 is smaller than 1 / g , the parametric equalizer 100 in accordance with the present invention processes the input digital audio signal x according to the preset gain g without dynamically changing the preset gain g . from equation 3 , the output digital audio signal y is expressed as equation 6 below . y / x obtained from equation 6 can be expressed as equation 7 below . is set as g ′, y / x can be expressed as equation 8 below . as shown in fig4 , g ′ monotonically decreases when when g ′= 0 . as a result , the gain of the parametric equalizer 100 in accordance with the present invention monotonically decreases when that is , the gain of the parametric equalizer 100 dynamically varies in response to the level gin even when the coefficients are not calculated in real time according to the level gin . thus , the level of the input digital audio signal x is equal to that of the output signal y 1 (= gin ). further , in equation 7 , since the gain of y / x is gin and the level of the input digital audio signal x is gin , the level of the output digital audio signal y is 1 regardless of the level of the input digital audio signal x when since h 2 = 0 from equation 3 , the output digital audio signal y is expressed as equation 9 below . equation 9 means that the parametric equalizer in accordance with the present invention outputs the input digital audio signal x as the output digital audio signal y when 1 & lt ; gin . that is , when the level gin of the output signal y 1 of the first signal processing unit 110 is greater than 1 , the parametric equalizer in accordance with the present invention outputs the input digital audio signal x as the output digital audio signal y without amplifying the input digital audio signal x . fig5 is a flow diagram illustrating a method for adjusting a gain of a parametric equalizer in accordance with the present invention . referring to fig5 , an input digital audio signal x is inputted to a first signal processing unit having a transfer function h 1 and processed by the transfer function h 1 ( s 110 ). the transfer function h 1 is defined in equation 2 above . that is , the input digital audio signal x is inputted to the first signal processing unit having the transfer function h 1 , and the input digital audio signal x is processed by the first signal processing unit to generate an output signal y 1 (= xh 1 ). thereafter , the output signal y 1 of the first signal processing unit is inputted to a second signal processing unit having a transfer function h 2 and processed ( s 120 ). the transfer function h 2 is defined in equation 3 above . that is , the output signal y 1 of the first signal processing unit is inputted to the second signal processing unit having the transfer function h 2 and the second signal processing unit processes the output signal y 1 of the first signal processing unit according to the transfer function h 2 to generate an output signal y 2 (== xh 1 h 2 ). the step s 120 will be described in more detail below with reference to fig6 . the transfer function h 2 is expressed as h 2 = g − 1 as in equation 3 ( s 120 b ), and the second signal processing unit generates the output signal y 2 (= x [ h ( z )− 1 ]) ( s 120 c ). as in equation 3 ( s 120 e ), and the second signal processing unit generates the output signal when 1 & lt ; gin , the transfer function h 2 is expressed as h 2 = 0 as in equation 3 ( s 120 g ), and the second signal processing unit generates the output signal y 2 (= 0 ) ( s 120 h ). referring back to fig5 , the output signal y 2 is then added to the input digital audio signal x to generate an output digital audio signal y ( s 130 ). is expressed as equation 4 above . the gain g of the parametric equalizer is not varied when and the input digital audio signal is processed according to the preset gain g . is expressed as equation 6 above . when the level gin is equal to or greater than 1 / g and equal to or less than 1 , the gain of the parametric equalizer monotonically decreases . that is , the gain of the parametric equalizer may be dynamically adjusted according to the level gin without calculating the coefficients of h ( z ) in real time . the output digital audio signal y for 1 & lt ; gin is expressed as equation 9 above . when 1 & lt ; gin , the input digital audio signal x is equal to the output digital audio signal y . that is , when the level gin of the output signal y 1 is greater than 1 , the input digital audio signal x is outputted as the output digital audio signal y without amplification . fig7 is a block diagram illustrating a parametric equalizer system in accordance with the present invention . referring to fig7 , the parametric equalizer system 200 comprises a volume controller 210 and a parametric equalizer 100 including a first signal processing unit 110 , a second signal processing unit 120 , an adder 130 and a level detector 140 . the volume controller 210 adjusts a level of an input digital audio signal x and outputs the adjusted input digital audio signal x as an output signal x 1 . the volume controller 210 increases or decreases the level of the input digital audio signal x according to a selection of a user . the input digital audio signal x may include a pcm audio signal . the first signal processing unit 110 processes the output signal x 1 according to its transfer function h 1 and outputs an output signal y 1 (= x 1 h 1 ). the level detector 140 detects a level gin of the output signal y 1 of the first signal processing unit 110 and provides the output signal y 1 to the second signal processing unit 120 . the gain g may be dynamically adjusted in real time according to the level gin of the output signal y 1 of the first signal processing unit 110 . that is , the output signal x 1 is amplified or bypassed with out any amplification or the gain g is adjusted to be inversely proportional to an amplitude of the level gin of the output signal y 1 of the first signal processing unit 110 . the second signal processing unit 120 processes the output signal y 1 of the first signal processing unit 110 by its transfer function h 2 according to the level gin of the output signal y 1 and outputs the processed output signal y 1 as an output signal y 2 (= x 1 h 1 h 2 ). the transfer function h 2 is defined in equation 3 above and a graph thereof is shown in fig4 . the adder 130 adds the output signal y 2 of the second signal processing unit 120 to the output signal x 1 of the volume controller 210 to generate an output digital audio signal y . the output digital audio signal y according to the range of gin may be calculated as below . since h 2 = g − 1 from equation 3 , the output digital audio signal y is expressed as equation 10 below . y / x 1 obtained from equation 10 can be expressed as equation 5 below . equation 11 means that the preset gain g of the parametric equalizer 100 in accordance with the present invention does not change . that is , when the level gin of the output signal y 1 of the first signal processing unit 110 is smaller than 1 / g , the parametric equalizer 100 in accordance with the present invention processes the input digital audio signal x according to the preset gain g without dynamically changing the preset gain g . from equation 3 , the output digital audio signal y is expressed as equation 12 below . y / x 1 obtained from equation 12 can be expressed as equation 13 below . is set as g ′, y / x 1 can be expressed as equation 14 below . as shown in fig4 , g ′ monotonically decreases when when g ′= 0 . as a result , the gain of the parametric equalizer 100 in accordance with the present invention monotonically decreases when that is , the gain of the parametric equalizer 100 dynamically varies in response to the level gin even when the coefficients are not calculated in real time according to the level gin . thus , the level of the output signal x 1 is equal to that of the output signal y 1 (= gin ). further , in equation 13 , since the gain of y / x , is gin and the level of the output signal x 1 is gin , the level of the output digital audio signal y is 1 regardless of the level of the output signal x 1 when since h 2 = 0 from equation 3 , the output digital audio signal y is expressed as equation 15 below . equation 15 means that the parametric equalizer in accordance with the present invention outputs the output signal x 1 as the output digital audio signal y when 1 & lt ; gin . that is , when the level gin of the output signal y 1 of the first signal processing unit 110 is greater than 1 , the parametric equalizer in accordance with the present invention outputs the output signal x 1 as the output digital audio signal y without amplifying the output signal x 1 . fig8 is a flow diagram illustrating a method for adjusting a gain of a parametric equalizer system in accordance with the present invention . the method is performed in the parametric equalizer system including a parametric equalizer having a transfer function h ( z ) with a preset gain g as a parameter . referring to fig8 , an output signal x 1 is generated by adjusting a level of an input digital audio signal x ( s 210 ). thereafter , the output signal x 1 is inputted to a first signal processing unit having a transfer function h 1 and processed by the transfer function h 1 ( s 220 ). the transfer function h 1 is defined in equation 2 above . that is , the output signal x 1 is inputted to the first signal processing unit having the transfer function h 1 , and the output signal x 1 is processed by the first signal processing unit to generate an output signal y 1 (= x 1 h 1 ). thereafter , a level gin of the output signal y 1 is detected ( s 230 ). thereafter , the output signal y 1 of the first signal processing unit is inputted to a second signal processing unit having a transfer function h 2 and processed ( s 240 ). the transfer function h 2 is defined in equation 3 above . that is , the output signal y 1 of the first signal processing unit is inputted to the second signal processing unit having the transfer function h 2 and the second signal processing unit processes the output signal y 1 of the first signal processing unit according to the transfer function h 2 to generate an output signal y 2 (== x 1 h 1 h 2 ). the step s 140 will be described in more detail below with reference to fig9 . when the transfer function h 2 is expressed as h 2 = g − 1 as in equation 3 ( s 240 b ), and the second signal processing unit generates the output signal y 2 (=[ h ( z )− 1 ]) ( s 240 c ). ( s 240 d ), the transfer function h 2 is expressed as as in equation 3 ( s 240 e ), and the second signal processing unit generates the output signal when 1 & lt ; gin , the transfer function h 2 is expressed as h 2 = 0 as in equation 3 ( s 240 g ), and the second signal processing unit generates the output signal y 2 (= 0 ) ( s 240 h ). referring back to fig8 , the output signal y 2 is then added to the signal x 1 to generate an output digital audio signal y ( s 250 ). is expressed as equation 10 above . the gain g of the parametric equalizer is not varied when and the input digital audio signal is processed according to the preset gain g . is expressed as equation 12 above . when the level gin is equal to or greater than 1 / g and equal to or less than 1 , the gain of the parametric equalizer monotonically decreases . that is , the gain of the parametric equalizer may be dynamically adjusted according to the level gin without calculating the coefficients of h ( z ) in real time . the output digital audio signal y for 1 & lt ; gin is expressed as equation 15 . when 1 & lt ; gin , the output signal x 1 is equal to the output digital audio signal y . that is , when the level gin of the output signal y 1 is greater than 1 , the input digital audio signal x 1 is outputted as the output digital audio signal y without amplification . the parametric equalizer and the parametric equalizer system in accordance with the present invention differ from conventional ones in that : ( i ) the parametric equalizer and the parametric equalizer in accordance with the present invention detects the level of the input signal and process the input signal according to the level while conventional the parametric equalizers process the input signal by the transfer function thereof regardless of the level of the input signal once the transfer function is defined by various parameters ; and ( ii ) the parametric equalizer and the parametric equalizer in accordance with the present invention detects the level of the input signal and maintain or vary the gain thereof according to the level while conventional the parametric equalizers process the input signal by the transfer function thereof without varying the various parameters . while the parametric equalizer and the parametric equalizer in accordance with the present invention are similar to the conventional ones in that the input signal is processed according to preset transfer functions , the parametric equalizer and the parametric equalizer in accordance with the present invention change the processing of the input signal according to the level of the input signal contrary to the conventional ones which process the input signal regardless of the level of the input signal . the method , the parametric equalizer and the parametric equalizer system in accordance with the present invention has following advantages over conventional ones . ( i ) the parametric equalizer and the parametric equalizer system capable of dynamically varying the gain thereof according to the level of the digital audio signal can be embodied without calculating the coefficients of the filter constituting the parametric equalizer in real time . ( ii ) the parametric equalizer and the parametric equalizer system prevents the clopping and the distortion of the digital audio signal by varying the gain thereof according to the level of the digital audio signal . ( iii ) the parametric equalizer and the parametric equalizer system can be embodied with a simpler hardware compared to the parametric equalizers which calculate the coefficients of the filter included therein in real time . ( iv ) low power and low cost parametric equalizer and the parametric equalizer system can be embodied compared to the parametric equalizers that calculate the coefficients of the filter included therein in real time . although a few embodiments of the present invention have been shown and described , it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention , the scope of which is defined in the claims and their equivalents .