On 7/9/07, Russell McMahon <apptech@paradise.net.nz> wrote:
> >> now I am driving it from 2 cpu pin's with complimentary outputs
>
> Two pin drive allows you to do what you want.
> For full volume at a given frequency you flip pins simultaneously.
> For variable amplitude:
>
> Short answer:
>
> - Use complementary PWM on the two pins and low pass filter (or use
> PWM so high that the result is inaudible).

Switching a PWM for an inaudible signals equal creating EMI and noise
on the board for nothing while some power is used for this.


> - Alternate PWM mark space ratio at twice the desired output
> frequency.
>
> PWM near 50:50 produces zero output.
> Moving to more uneven mark/space ratios increases output amplitude.
>
> Amplitude = 2 x (50 - mark%*) % of full scale
>
>        * use larger of mark or space % here to avoid negative sign
>          which is meaningful but not useful here.
>
> By suitable amplitude shaping within each output half cycle you can
> produce a sine wave or any other desired waveform.
>
> PWM can be done in hardware (if complementary PWM hardware facilities
> available) or software.
>
> Other mark space schemes than raw N:1 will allow lower update
> overheads.
>
> Processing overhead may be severe for wide ranging volume variations.
>
>
>
> 2.    Another method will less processor load is to use one pin to
> produce tone out and PWM modulate the other at a somewhat higher
> frequency to chop the tone. Filtering needed to deal with nasty
> intermod sounds. Single pin output halves transducer drive voltage
> over toggled pair system and may reduce max sound level to 1/4
> depending on transducer.

This methode is often named one pin for frequency, one pin for amplitude
and works on both situation when the piezo has or not a transformer.
Is the best way to do it.


>
> 3.    A diode gate driven from a DC signal derived from control PWM
> would lessen the PWM load and remove intermodulation problems.
>
> Wazzat - they don't teach people about diode gates anymore ? :-)
>
>
>
> __________________
>
> Explanation of sorts:
>
> If you create a series of transitions at much greater rate than the
> fundamental output frequency you can produce a PWM signal of varying
> amplitude and with alternating polarity across the transducer at the
> fundamental frequency. You will probably need to low pass filter
> this - although if the transition rate is high enough this may be
> avoidable.
>
> As a very simple example - imagine you want a 500 Hz signal - 1 ms
> high then 1 ms low alternately at 10% of the amplitude of the full
> scale signal.
>
> Repeat 100 times
>    Output state 10 for 4.5 uS.
>    Output state 01 for 5.5 uS
> Repeat 100 times
>    Output state 01 for 4.5 uS.
>    Output state 10 for 5.5 uS
>
> First 100 steps produce a signal of 0.45 of full scale
> Second 100 steps produce 0.55 of full scale.
> Output signal is 0.1 of full scale.
>
>
>
>            Russell McMahon
>
>
>
>
>
>
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