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Mon Oct 9 14:31:06 CEST 2006

Dither, PWM, Sigma-Delta, Bresenham

What is dither[3] used for? It makes a digital system behave more like
an analog system, in that it can be used to make it exhibit _graceful
degradation_ instead of suddon failure.  Dither can increase
resolution on average, but adds noise to the immediate values. For
phenomena where the average is more important than the immediate
value, this can be an interesting trick.

A simple example of this is a first order difference equation in
finite precision.  Suppose I'm using 16-bit values to represent
oscillator periods.  In order to make logarithmic increments, I could
use a difference equation 

    y[k+1] = (1+a)y[k]

where a is a small constant. However, if ay is below hex 0.00008000
the value of y remains constant. In this case, average behaviour is
important, so using 0.0000rrrr as rounding term, where rrrr is a
16-bit uniformly distributed random number, gives an approximation of
the correct behaviour.

Random number generation (RNG) for this application is most
efficiently done using a finite field primitive polynomial.  However,
care needs to be taken to check the statistical properties in case the
same RNG is used to drive several dithering filters, although I can't
see any pathological cases at first glance.

If clear patterns in the output are not really a problem, a simpler
solution is to just use a triangle wave as dithering function, which
gives effectively a PWM output. What I'm really intereseted in is a
dithering scheme where the ripple is smaller after integration of the
output. This requires a higher frequency quantization step.

For example, instead of using a 01 ramp, a 81 ramp can be used. What
exactly is the relation to S-D modulation and the Bresenham[2] line
drawing algorithm? Let's make a list.


* uniform dither: Dither is from a uniform noise source.

* PWM: Dither from a square wave. Same average as uniform dither, but
  clear pattern. Large ripple after integration.

* nPWM: Like pwm, but using a triangle wave modulated to Nyquist
  frequency. Smaller ripple.

* S-D: Minimizes ripple after integration.

* Bresenham: Uses higher state resolution to generate a proper
  average. Ordinary rounding quantization.


The reason PWM is used over S-D in motor control (as I guess from PWM
modulators being part of uC hardware, but S-D not) could be switching
efficiency.  Larger ripple is tolerated if higher switching efficiency
(lower switch frequency) is attained. See Don Lancaster's magic
sinewaves[1] for explicit minimization of switching events for sine
wave generation as an alternative to PWM. When low ripple amplitude is
more important than efficiency, for example in audio applications, S-D
modulation can be used.

Question: can PWM hardware on uC be used to generate S-D signals?

[1] http://www.tinaja.com/glib/msinprop.pdf
[2] http://en.wikipedia.org/wiki/Bresenham's_line_algorithm
[3] http://en.wikipedia.org/wiki/Dither



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