--- S Webster <s.webster@STUDENT.UNSW.EDU.AU> wrote:
A.K.A. Paul

> Nikolai Golovchenko wrote:
>
> > Is this the same as phase accumulators?
>
>   I beg to differ from Tracy.  I think it is.  Phase
> accumulators are
> based on the principle of - as the name suggests -
> adding up little
> pieces of the cycle and "rolling over" when the sum
> of these amounts to
> or in fact exceeds, a full cycle.

True, but that's not what Nikolai described (I
thought). OTOH, I'm hardly the Phase Accumulator
expert. My understanding of the phase accumulator is:

You start off with a single cycle of something you
wish to generate. This may be a square wave, sine
wave, or a hyperbolic paraboloid wave (what in the
hell is that?). The phase accumulator simply
determines which portion of the cycle you wish to
access.

In its simplest form, the single cycle of the wave
form is in an array and the phase accumulator is the
index. The array is accessed at a constant rate,
however the index is advanced according to the
frequency of the wave form. Here's a simple C example:

#define TABLE_SIZE  256

char wave_table[TABLE_SIZE];

float phase,delta_phase;

void phase_accumulator(void)
{
  int i;

  while(1) {

    phase += delta_phase;

    if(phase > TABLE_SIZE)
      phase -= TABLE_SIZE

    i = phase;

    output(wave_table[i]);

    routine_that_updates_delta_phase(&delta_phase);
  }

}

void main(void)
{

  delta_phase = ....
  init_wave_table(wave_table);

}


I'd definite do this in assembly instead of C (but
that's another thread). In each application, there are
always ways to optimize this approach. For example,
for square waves you don't need a table at all nor do
you need floating point arithmetic for the phase
accumulator.  Fixed point arithmetic works fine and
the most significant bit of the phase accumulator is
the output. For most pics, you're forced to optimize
because 256 element arrays (in ram aren't possible).
256 element arrays in code are possible, but are
always a pain in the ass (because of page boundaries).



Who knows, perhaps it's only semantic and S. and I are
on the same page. But like I said, I'm not the
expert...

>   I would think that even a 3-bit (six resistor) ADC
> would considerably
> improve performance, and perhaps allow output of
> both tones via the one
> channel (though Dave Van Horn's "twist" would still
> be a difficulty).

Borrowing a little from the best of both worlds is
often the best solution.

.lo

=====

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