Re: PWM -> Audio



MooseFET wrote:

PWM is the nonlinear operation, and this makes the things complicated.
How do you conclude it must be non-linear?

PCM(x) + PCM(y) == PCM(x+y)
PWM(x) + PWM(y) != PWM(x+y)

That is not enough to make it non-linear from our point of view. You
need some of the differences to be within the band of interest.

This is enough. The order of the difference is infinite.

Imagine a DC + small sinewave on the top of it. At the PWM output, the
phase shift of the sinewave will be dependent on the DC level.

This depends on how the PWM is done. If only one edge of the PWM is
modulated, it is true but if both edges of the PWM are modulated, it
need not be.

With modulation of both edges in PWM, the phase will be constant as
the DC level is changed.

Yes, the double sided PWM has lower distortion, then the single sided PWM. However the double sided PWM is not free from distortion either.

With two
sinewaves you will get all sorts of harmonics and intermods.
This is voice not music, so it matters a lot less.

The PWM artifacts should be of the same order of magnitude as all other artifacts (quantization, nonlinearity and such). This requires the PWM rate to be at least at x10 of the max. frequency of interest, unless the nonlinear correction is employed.

....but so much for the boring PWM stuff. Once you mentioned that you made an invertor using the switching rectifier on the secondary side. How did you control the cross conduction? For the proper operation, the current should be switched from one side to another precisely. The actual switching time depends on temperature, voltage and current. Did you just use snubbers or did you have to adjust the dead time dynamically?


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
.

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