[Dspforum] IIR audio output question

[O'Connor, Sean R.]

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