[Dspforum] IIR audio output question

[Wenstrand, Doug S.]

Sean, my guess is that this is perfectly fine.  Without seeing your designed filter, I can easily imagine that it approaches something much like a 697 or 770Hz resonator.  Assuming you have somewhat of a noisy input, the output of the filter is going to be the shape of the filter passband, which probably sounds like an eerie tone.  Pipe some white noise in at varying amplitudes using NCHtone to confirm.

Doug

p.s. can you email me your filter coefficients and I'll confirm for myself?


-----Original Message-----
From: dspforum-bounces at echelonembedded.com [mailto:dspforum-bounces at echelonembedded.com] On Behalf Of O'Connor, Sean R.
Sent: Sunday, November 08, 2009 4:44 PM
To: dspforum at echelonembedded.com
Subject: [Dspforum] IIR audio output question


Even with no audio input, I hear a faint eerie resonance on both channels.  The oscillations are of the correct frequency, namely ~697Hz on the left channel and ~770Hz on the right channel.  The design seems to work when I input test tones, but the resonance is always present (it is even quieter with only one biquad).  It seems like there is some instability in the filter, although the output does not 'run away' or saturate the output.  Any thoughts on this?

Also, is the float to integer conversion supposed to be done with DSPF_fltoq15()?  Currently I am just casting the float filter output to an integer, this is the approach taken in the book.

Thanks,
Sean
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