e of period doubling), or an error in the computation.
Our largest safe index is increment/sin(increment) which is just over 1.0.  If we change
the code to make sure the carrier phase doesn't back up, the bursts go away until the
index reaches about 1.4, then we start to zigzag at the zero crossing.
The take-home message is: "keep the index below 1.0!".
</p>

<!--
(define (feedbk beg dur freq amp index)
  (let* ((start (seconds->samples beg))
	 (end (+ start (seconds->samples dur)))
	 (y 0.0)
	 (x-incr (hz-