Have a look at these
G. Stoyanov and M. Kawamata, “Variable digital filters,” J.Signal Processing,
vol. 1, no. 4, pp. 275–289, 1997.
J. Laroche, “On the stability of time-varying recursive filters,”Journal of the
Audio Engineering Society, vol. 55, no.6, pp. 460–471, 2007.
V. Välimäki and T.I. Laakso, “Suppression of transients intime-varying
recursive filters for audio signals,” in Proceedingsof the 1998 IEEE
International Conference on Acoustics,Speech and Signal Processing, 1998, May
1998, vol. 6,pp. 3569–3572 vol.6.
A. Wishnick, “Time-varying filters for musical applications,”in Proceedings of
the 17th International Conference on DigitalAudio Effects (DAFx-14), 2014, pp.
69–76.
From: Paul Stoffregen <p...@pjrc.com>
To: music-dsp@music.columbia.edu
Sent: Tuesday, 1 March 2016, 14:56
Subject: [music-dsp] Changing Biquad filter coefficients on-the-fly, how to
handle filter state?
Does anyone have any suggestions or publications or references to best
practices for what to do with the state variables of a biquad filter
when changing the coefficients?
For a bit of background, I implement a Biquad Direct Form 1 filter in
this audio library. It works well.
https://github.com/PaulStoffregen/Audio/blob/master/filter_biquad.cpp#L94
There's a function which allows the user to change the 5 coefficients.
Lines 94 & 95 set the 4 filter state variables (which are 16 bits,
packed into two 32 bit integers) to zero. I did this clear-to-zero out
of an abundance of caution, for concern (maybe paranoia) that a stable
filter might do something unexpected or unstable if the 4 state
variables are initialized with non-zero values.
The problem is people wish to change the coefficients in real time with
as little audible artifact as possible between the old and new filter
response. Clearing the state to zero usually results in a very
noticeable click or pop sound.
https://github.com/PaulStoffregen/Audio/issues/171
Am I just being overly paranoid by setting all 4 state variables to
zero? If "bad things" could happen, are there any guidelines about how
to manage the filter state safely, but with with as graceful a
transition as possible?
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