Hi i had to build a LPF for a PWM audio signal (voice 4Khz, carrier
16Khz) to use with a Max4364My design is a passive filter (pi-greco style C100n-L8.2m-C100n). I heard a background noise and a scratch every time the sound starts.
I'd like to use a passive one.
Some hints ?
thank you
On a sunny day (Tue, 21 Oct 2008 23:23:20 -0700 (PDT)) it happened merco wrote in :
Maybe it is not the filter, but does the 'sound start' mean that the PWM signal is absent when no sound? In that case you have a 50% DC jump any time the sound starts or stops.
For voice work you don't need to worry at all about the phase shifts of the filter and you can stand a fair roll off. Look at placing a notch at the PWM frequency along with the normal lowpass. You can boost the high frequencies in the sound you are making to correct for the roll off of the filter.
Your PWM output should idle at the 50% duty cycle. The sound information should be centered around the same 50% point.
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