125 u03bc seconds a signal representative of cos u03c0n 2 is supplied from cos u03c0n 2 generator 12 to multiplier 10 in turn multiplier 10 yields a n d n cos u03c0n 2 similarly a signal representative of sin u03c0n 2 is supplied from sin u03c0n 2 generator 13 to multiplier 11 in turn multiplier 11 yields b n d n sin u03c0n 2 signal a n is supplied to low pass filter 14 which yields a low pass version thereof namely u n similarly signal b n is supplied to low pass filter 15 which also yields a low pass version thereof namely v n in this example low pass filters 14 and 15 are each a second order recursive filter with a cutoff frequency at 2 khz both u n and v n are supplied to complex signal generator 16 which yields u03b3 n u n jv n u03b3 n is a complex low pass version of d n it is noted that the complex low pass version u03b3 n may be generated by other arrangements one example being a hilbert filter signal u03b3 n is supplied to multiplier 17 complex conjugate generator 18 and magnitude generator 19 the complex conjugate u03b3 n of the complex low pass version signal u03b3 n is supplied from complex.