embodiments of the invention will be described with reference to the attached drawings . these drawings illustrate the invention but do not restrict its scope , which should be determined solely from the appended claims . referring to fig1 all embodiments receive a left - channel input signal ( lin ) and a right - channel input signal ( rin ), and generate a left - channel output signal ( lout ) and a right - channel output signal ( rout ). all embodiments comprise a first summing circuit 11 for adding the left - channel input signal lin and right - channel input signal rin , a differencing circuit 12 for taking the difference between the left - channel input signal lin and right - channel input signal rin , a low - pass filler ( lpf ) 13 coupled to filter the sum signal output by the first summing circuit 11 , thereby producing a low - frequency signal , a second summing circuit 14 for adding this low - frequency signal to the difference signal produced by the differencing circuit 12 , and a phase - shifting circuit 15 that receives the signal output by the second summing circuit 14 . in the first embodiment , as shown in fig1 the phase - shifting circuit 15 comprises a first phase shifter 16 that shifts the output of the second summing circuit 14 by a first phase angle - φ , and a second phase shifter 17 that shifts the output of the second summing circuit 14 by a second phase angle + φ . the two phase angles - φ and + φ are equal in magnitude and opposite in sign . the output of the first phase shifter 16 is the left - channel output signal lout . the output of the second phase shifter 17 is the right - channel output signal rout . the summing , differencing , and phase - shifting circuits in fig1 can be constructed by using , for example , well - known operational amplifier circuits . specific circuit descriptions will be omitted to avoid obscuring the invention with needless detail . in a vocal musical broadcast , the singer &# 39 ; s voice is normally picked up by a single microphone located directly in front of the singer , often a hand - held microphone , while the musical accompaniment is picked up by , for example , two microphones disposed on either side of the singer . the singer &# 39 ; s voice signal is accordingly identical in the left - and right - channel input signals lin and rin , while the musical accompaniment signals differ between the two channels . in the difference signal output by the differencing circuit 12 , the singer &# 39 ; s voice signal is therefore completely canceled . the musical accompaniment is attenuated to various degrees , depending on the placement of different instruments in relation to the microphones and the specific frequencies involved . the output of the summing circuit 11 comprises all frequencies of both channels . the cut - off frequency of the low - pass filter 13 is set at or below the bottom of the human vocal range , so that voice frequencies are removed from the output of the low - pass filter 13 , which consists of a mixture of the lower frequencies of the musical accompaniment from both channels . the output of the low - pass filter 13 compensates for the attenuation of these lower frequencies in the output of the differencing circuit 12 . the output of the second summing circuit 14 is accordingly a monaural karaoke signal consisting of the musical accompaniment with some attenuation of higher instrumental frequencies , and with the singer &# 39 ; s voice completely removed . the first phase shifter 16 shifts this monaural karaoke signal by a phase angle of - φ degrees to produce the left - channel output signal lout , while the second phase shifter 17 shifts the monaural karaoke signal by a phase angle of + φ degrees to produce the right - channel output signal rout . these phase shifts make different frequencies seem to come from different locations , thereby restoring spatial spread , creating a stereo illusion , and giving a sense of presence in the audio output . fig2 shows a second embodiment , comprising the same summing circuits 11 and 14 , differencing circuit 12 , and low - pass filter 13 as the first embodiment , but differing in the structure of the phase - shifting circuit 15 . the phase - shifting circuit 15 in fig2 comprises a phase shifter 21 for phase - shifting the monaural karaoke signal output by the second summing circuit 14 to produce a phase - shifted signal , and a further pair of summing and differencing circuits 22 and 23 . the third summing circuit : 22 adds the phase - shifted signal output by the phase shifter 21 to the monaural karaoke signal thereby producing the left - channel output signal lout . the second differencing circuit 23 subtracts the phase - shifted signal output by the phase shifter 21 from the monaural karaoke signal , thereby producing the right - channel output signal rout . the left - and right - channel output signals lout and rout have phase shifts that depend on the output amplitude and phase shift φ of the phase shifter 21 . for example , if φ = π / 2 ( ninety degrees ) and the input and output amplitudes of the phase shifter 21 are equal , the phase - shifting circuit 15 in fig2 functions like the phase - shifting circuit 15 in fig1 shifting the phase of the monaural karaoke signal by π / 4 ( forty - five degrees ) in opposite directions in the left and right channels . the phase angle φ and the amplification factors of the phase shifter 21 , third summing circuit 22 , and second differencing circuit 23 can be adjusted to provide other combinations of phase shifts in the left and right output channels , as desired . the first and second embodiments provide satisfactory output signals from stereo input , but the sound signals of many television broadcasts are monaural , the left - and right - channel input signals being identical , and some broadcasts are bilingual , the left and right channels being used to carry sound tracks in different languages . for a monaural broadcast , the output of the differencing circuit 12 is completely mute , and the user hears only the low frequencies passed by the low - pass filter 13 . for a bilingual broadcast , both channels pass through the differencing circuit 12 and the user hears both languages at once . in neither case is the output satisfactory . the third embodiment provides a solution to this problem by allowing the vocal canceler to operate only when the input is stereo . referring to fig3 the third embodiment comprises the summing circuits 11 and 14 , differencing circuit 12 , anti low - pass filter 13 of the first and second embodiments but now has a first switch 25 that furnishes separate left - and right - channel inputs to the phase - shifting circuit 15 . this switch 25 is controlled by a stereo discriminator 27 that determines whether the left - and right - channel input signals constitute a stereo signal or not . the phase - shifting circuit 15 is similar to the phase - shifting circuit 15 in fig1 but has a second switch 29 that selects whether or not to route the input signals through the phase shifters 16 and 17 . when the stereo discriminator 27 detects stereo input , it sets the first switch 25 to the position marked by the letter b . the second switch 29 is also set to the b position . the monaural karaoke signal output by the second summing circuit 14 is supplied as both the left - and right - channel inputs to the phase - shifting circuit 15 , and is sent through the phase shifters 16 and 17 , providing the same vocal - canceled , simulated stereo output as in the first embodiment . when the stereo discriminator 27 detects that the input is not stereo , it sets the first switch to the position marked by the letter a , so that the unaltered left - and right - channel input signals lin and rin are sent to the phase - shifting circuit 15 . this avoids unwanted voice canceling of monaural and bilingual input signals . if the second switch 29 is also set to the a position , these unaltered signals become the left - and right - channel output signals lout and rout . the first and second switches 25 and 29 can be linked so that they always operate together as described above , either both being set , to the a position , or both to the b position . alternatively , they can be controlled independently , providing the user with further options . one option is monaural output of a vocal - canceled stereo signal , as in a conventional vocal canceler , by setting the second switch 29 to the a position when the first switch 25 is set to the b position . another option is simulated stereo output of a monaural input signal , by setting the second switch 29 to the b position when the first switch 25 is set to the a position . fig4 shows one possible configuration of the stereo discriminator 27 , designed for a television system in which stereo and bilingual broadcasts are identified by cue signals superimposed on the sound signal . stereo broadcasts have a cue frequency of 952 hz : bilingual broadcasts have a cue frequency of 922 hz ; monaual broadcasts have no cue signal . this stereo discriminator comprises a first bandpass filter 31 , an amplitude - modulation ( am ) detector 32 , a second bandpass filter 33 , a wave shaper 34 , and a frequency counter 35 , coupled in series . the first bandpass filter 31 passes an intermediate frequency band containing the stereo cue signal . the am detector 32 demodulates the resulting signal to a baseband sound signal . the second bandpass filter 33 passes the stereo cue frequency of 952 hz . the wave shaper 34 reshapes the resulting output signal to remove distortion and restore lost amplitude . the frequency counter 25 counts the frequency of the reshaped signal for positive identification of the stereo cue . the output of the frequency counter 25 is furnished to a circuit such as a microcontroller , not shown in the drawing , which controls the first switch 25 in fig3 . fig5 shows a fourth embodiment , which is identical to the first embodiment in fig1 except for the addition of a high - pass filter ( hpf ) 37 and a fourth summing circuit 39 . the high - pass filter 37 receives the output of the first summing circuit 11 . the fourth summing circuit 39 adds the output of the high - pass filter 37 to the monaural karaoke signal output by the second summing circuit 14 . the phase - shifting circuit 15 receives the output of the fourth summing circuit 39 instead of the output of the second summing circuit 14 . the high - pass filter 37 has a cutoff frequency equal to or higher than the top of the human vocal range . together , the low - pass filter 13 and high - pass filter 37 form a band - stop filter that eliminates vocal frequencies from the output of the first summing circuit 11 while passing higher and lower frequencies . if the lin and rin inputs are stereo , the fourth embodiment provides essentially the same simulated stereo karaoke output as the first embodiment , but with greater timbre , as the high - pass filter 37 allows more overtones and other high instrumental frequencies to pass . if the lin and rin inputs are monaural , the fourth embodiment cancels voice frequencies while allowing both higher and lower frequencies to pass . although the midrange of the musical accompaniment is removed , the remaining instrumental frequencies are still usable for karaoke purposes , and the phase - shifting circuit 15 produces a simulated stereo effect . although the third and fourth embodiments in fig3 and 5 employed the phase - shifting circuit 15 of the first embodiment in fig1 they could just as well use the phase - shifting circuit of the second embodiment in fig2 . other phase - shifting circuit configurations are also possible : for example , analog delay lines can be used instead of phase shifters . the resulting phase shifts will be frequency - dependent , and will provide a type of echo effect that also simulates a stereo output signal . if the left and right channel signals are digital , memory circuits such as first - in - first - out ( fifo ) circuits can be used instead of phase shiffters in a similar way . those skilled in the art will recognize that still further modifications can be made without departing from the scope claimed below .