connecting pwm output to low pass filter

[siliconchip]

hi all
im generating a pwm output using a pic 12f1840 there are 3 pwm values with which i want to create 3 different tones when connecting to a common collector amp but im using a low pass filter between the 2 to convert the pwm to an anolgue voltage, im under the impression that the pwm needs to be 10 - 100 times greater frequency than that of the low pass cut off frequency that's fine but i want to select 3 frequencys within that band 1.2kHz, 1.6KHz AND 2KHz which means my pwm out put needs to be at least 20KHz constantly ? to avoid possible distortion ?, so my question is how can i produce a 20khz pwm signal but pick out the 3 desired frequency's, ive added a program where i select the frequency's i want in succession but max is 2000Hz, not sure if this all make sense or if ive got this all completely wrong but any advice would be appreciated
cheers bob

[BenR]

Hello,

For a single tone output at a time the easiest way to do this is to change the PWM frequency by altering the prescalers, rollover period and duty. There is I beleive a ChangeFrequency function that should make this fairly easy. The PWM frequency itself would be changed to 1, 1.5, 2KHz. This is a square wave so you will likely be able to hear the harmonics in the audio output.

For multiple tones at once or for a higher fidelity signal you can keep the PWM frequency high say 20KHz and output frequencies by varying the duty at a corresponding rate. The easy way to do this would be to use the DSP components with one or more frequency generators connected through to a PWM output. You would use a timer interrupt to drive the timing of the DSP processing and keep in sync with the PWM.

For a really good output, audio quality I would recommend a DAC or an I2S driver. I've made relatively good audio using a 10-bit R2R DAC, much better than 10-bit PWM.