while the voltage level at a is l in u00d7 r 2 r 1 r 2 according to the circuit diagram thus l out l in u00d7 r 2 r 1 r 2 u00d7 r 3 r 4 r 3 l in u2218 when the control signal ctrl is high sw 1 will close and sw 2 will open the right channel audio r in will be inputted into the u201c u2212 u201d terminal of op 1 through resistor r 3 according to the principle of the operational amplifier the right channel audio r in will generate an output of r in u00d7 u2212 r 4 r 3 u2212 u00bdr in at b while the left channel audio l in will generate an output of l in at b as described above thus the composition voltage of l out at b is l in u2212 u00bdr in u2218 in the circuit of the audio processor stated above since it is designed by letting r 1 r 4 and r 2 r 3 the left channel audio l in can be reproduced at l out if we need to demonstrate the influence of the right channel audio r in it is only necessary to change the voltage level of the control signal ctrl and the user can clearly distinguish the effect of adding the right channel audio r in u2218 in the circuit of the audio processor stated above l out l in but if we change the ratio between r 1 and r 2 the coefficient before l in in l out can be changed and if we change the ratio between r 3 and r 4 the coefficient before r in in l out can be.