duce energy at 0Hz
        (carrier-phase-incr (hz-&gt;radians freq))
        (modulator-phase 0.0)
        (modulator-phase-incr (hz-&gt;radians (* mc-ratio freq))))
    (do ((i beg (+ i 1)))
	((= i end))
      (let* ((modulation (* index (sin modulator-phase)))
	     (pm-val (* amp (sin (+ carrier-phase modulation))))) 
	     ;; no integration in phase modulation
	(set! carrier-phase (+ carrier-phase carrier-phase-incr))
	(set! modulator-phase (+ modulator-phase modulator-phase-incr))
	(outa i pm-val)))))