There have been some interesting diversions in this thread.

  Back on topic however, the *purpose* of at least one of the two
original enquiries was a speed control for a stepper motor system, an
application in which jitter less than a modest fraction of a millisecond
is of no consequence whatever.

  For that application, a phase accumulator algorithm on a PIC, quite
possibly a PIC12C508, is *more* than adequate for the task to generate
three frequencies.  Actually that is a bit simplistic - in practice a
16C71 or 16F84 would be used to generate the *complete* drive waveforms
for three steppers.

  It certainly remains true that the Phase accumulator/ DDS procedure is
capable of generating extremely stable and accurate frequencies with any
resolution (reasonably) specified, but only ever within the accuracy of
its clock source.
--
  Cheers,
        Paul B.