generating means for producing a second sine-wave signal C???SIN y and a second cosine-wave signal D???COS y, where y is a variable and C and D are constants, (d) arithmetic means for processing the first sine- and cosine-wave signals A???SIN x and B???COS x and the second sine- and cosine-wave signals C???SIN y and D???COS y to produce a sine-wave signal E???SIN (y-x), where E is a constant, and