[sdiy] DC 'bounce' in second order filters

[Donald Tillman]

On Nov 23, 2011, at 6:06 AM, Harry Bissell wrote:

> I have a second order VCF  When I first apply a signal to it (starting from a zero condition) the filter
> have a DC bounce (of one polarity or another) which prevents the signal output from crossing zero for a couple of cycles.
> 
> Is the bounce an unavoidable artifact of a second (or higher) order filter, or can I do anything to reduce it. In my application missing a zero cross is an error condition I'd like to avoid

Harry,

If I'm understanding your description correctly...

Yes, the DC bounce is an artifact of the signal suddenly being turned on at the zero crossing point.  That turn-on, effectively multiplying the audio signal by a step function, does create a DC Fourier component.

For example, if the audio signal was a 1.0 V rms sine wave, and we turned it on for exactly a single positive half cycle, that would be a 0.5 V DC value.  For a full cycle: 0 V DC.  For 3 half cycles: 0.33 V DC.  4 half cycles: 0 V DC.  5 half cycles: 0.2 V DC.  And so on.

I suppose you could compensate for it.  I'm intrigued about the sort of application where the DC bounce would be an issue.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com