in this embodiment , a single side band modulation system is used to describe the approach . other modulation systems such as vsb / atsc / ntsc / pal whose modulated rf signals do not contain complete baseband signals can be implemented in likewise manners and are also within the scope of the invention . the single side band modulation can be modeled by an analytic approach . from the fourier theory , if s ( t ) is a real number baseband signal , then its frequency domain signal s ( ω ) is hermitian symmetrical about the zero frequency as shown on fig1 . therefore to model the modulation process , we can use the analytic signal s a ( t ) of s ( t ). please refer to fig2 and the following equation : then we shift s a ( t ) signal to the modulated frequency as : z ( t )= s a ( t ) e jω t t = s ssb ( t )+ js ss { circumflex over ( b )} ( t ) equation ( 2 ) where z ( t ) is s a ( t ) to be shifted by e jω c t . which means all frequency components are shifted by + f c frequency , so there are no negative - frequency components . please refer to fig3 . note that z ( t ) shown on fig3 is still an analytic signal and has no negative frequency components . since z ( t ) is a complex number , it cannot be sent through a channel . the single side band modulation signal s ssb ( t ) shown on fig4 can be modeled by simply picking the real number of z ( t ). and s ssb ( t ) can be used as a model of single side band modulation signal or other modulation signal such as vsb / atsc / ntsc / pal . the equation of s ssb ( t ) can be derived as follows : please refer to fig5 . fig5 is a block diagram of a receiver 6 according to the present invention . the receiver 6 comprises an antenna 8 , an lna ( low noise amplifier ) 10 , a first mixer 12 , a second mixer 14 orthogonal to the first mixer 12 , a dsp ( digital signal processor ) 16 , and a low pass filter 18 . the detailed description of demodulation process is given below . please refer to fig6 . in order to recover the s ssb ( t ), we need to convert s ssb ( t ) into complex number base band s ssb0 ( t ) by shifting frequency e i (− ω c − w h ) t through the first mixer 12 and the second mixer 14 as the following equation ( 4 ) represents , and fig6 shows the shifting processing . where ω h is the half frequency bandwidth of base band . after the low pass filter 18 performed in the following equation ( 5 ), we get s ssb0 — lowpass ( t ) signal . s ssb0 — lowpass ( t )= ½ [( s ( t ) cos (− ω h ) t − ŝ ( t ) sin (− ω h ) t )+ j ( s ( t ) sin (− ω h ) t + ŝ ( t ) cos (− ω h ) t )] equation ( 5 ) for direct down , the i channel signal can be gotten by the following equation ( 6 ): and for q channel , the q channel signal can be gotten by the following equation ( 7 ): after digitizing the i signal which is equation ( 6 ) and q signal which is equation ( 7 ), the complex number of s ssb0 — lowpass signal which is the equation ( 5 ) can be formed by these two signals through the digital signal processor ( dsp ) 16 . s ssb0 — lowpass is derived by shifting s ssb0 — lowpass by e jω h t performed in the dsp 16 . the real part of s ssb0 — lowpass — shiftω h t is the recovered signal derived through the digital signal processing in the dsp 16 . please refer to fig7 . fig7 is a diagram of shifting and taking the real part of s ssb0 — lowpass — shiftω h t . the following equation ( 8 ) describes the shifting processing : by taking real part of s ssb0 — lowpass — shiftω h t through the digital signal processing in the dsp 16 , we can recover s ( t ) signal as performed in the following equation ( 9 ): s ( t )= 2 * re { s ssb0 — lowpass — shiftω h ( t )} equation ( 9 ) alternatively , in another embodiment of the present invention , the steps of shifting and taking the real part of s ssb0 — lowpass — shiftω h t can be implemented by performing only the real part calculation of the equation ( 8 ) in the dsp 16 instead of performing both the real part and imaginary part calculations and then taking the real part of the calculated result as the listed below equation ( 10 ): to sum up , in real implementation , after zero if conversion , the i channel base band signal which is the real part of s ssb0 — lowpass ( t ) is generated . and the q channel base band signal is the imaginary part of s ssb0 — lowpass ( t ). after passing through an analog to digital converter , a digital signal processing can be utilized to implement the s ssb0 ( t ) and s ssb0 — lowpass — shiftω h t ( t ). then take the real part of s ssb0 — lowpass — shiftω h t ( t ) signal , can be taken to retrieve the original base band signal back . compared with qam , qpsk , ofdm , gsm and cdma , the modulated signals of ssb / vsb / atsc / ntsc / pal only occupy a partial modulated bandwidth , therefore the demodulation methods of ssb / vsb / atsc / ntsc / pal are a lot more complicated and not straightforward . the present invention utilizing the zero - if and iq interface to recover the modulated signals of ssb / vsb / atsc / ntsc / pal not only uses fewer components and reduces power consumption but the entire system also can be much easily implemented in a single chip ( soc ). 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 .