Time-varying or tunable filters?

[soundmodel]

So far I've been studying only filters that are static, whose parameters are assumed to left untouched during the processing. However, most digital audio effects vary the filter parameters.

What are the basics of making filters with time-variable parameters?

[Mayae]

What are the basics of making filters with time-variable parameters?

The basics are really just to recalculate the coefficients at periodic intervals, leaving the state alone.

Different filters respond differently to this procedure, some might blow up and/or become unstable, others are designed specifically to or intrinsically withstands it. In the general case one usually applies smoothing to the parameters.

[soundmodel]

What are the basics of making filters with time-variable parameters?

The basics are really just to recalculate the coefficients at periodic intervals, leaving the state alone.

Different filters respond differently to this procedure, some might blow up and/or become unstable, others are designed specifically to or intrinsically withstands it. In the general case one usually applies smoothing to the parameters.

Periodically in the sense of time- or frequency domain? I.e. at periodic time intervals or frequency intervals?

What does smoothing the parameters mean (in relation to recalculating the coefficients)?

[matt42]

What are the basics of making filters with time-variable parameters?

Perhaps you could check the thread on sweepable filters that you started....

[soundmodel]

What are the basics of making filters with time-variable parameters?

Perhaps you could check the thread on sweepable filters that you started....

Good point, forgot that thread.

http://www.kvraudio.com/forum/viewtopic ... e#p6441717

But I also meant this more theory-focused rather than "where to get sweepable filter libraries". Specifically, because I'm yet to find a book chapter that goes in detail to how tunable filters are realized, everything I've read so far has been only about static filter designs.

[Mayae]

Periodically in the sense of time- or frequency domain? I.e. at periodic time intervals or frequency intervals?

In time. Often plugins have a "control rate" where coefficients will be updated, for instance every 32th sample or something. You can do it every sample to get audio rate modulation, obviously less efficient though.

What does smoothing the parameters mean (in relation to recalculating the coefficients)?

To avoid sudden/extreme changes in automation or control, that will make the filter blow up or in general not sound nice. Common choices are a first order lowpass or simply a linear interpolation to the next control value over some amount of time.

[soundmodel]

What does smoothing the parameters mean (in relation to recalculating the coefficients)?

To avoid sudden/extreme changes in automation or control, that will make the filter blow up or in general not sound nice. Common choices are a first order lowpass or simply a linear interpolation to the next control value over some amount of time.

So it's done, when the parameter change by the user has been too big in too small amount of time?

[Mayae]

What does smoothing the parameters mean (in relation to recalculating the coefficients)?

To avoid sudden/extreme changes in automation or control, that will make the filter blow up or in general not sound nice. Common choices are a first order lowpass or simply a linear interpolation to the next control value over some amount of time.

So it's done, when the parameter change by the user has been too big in too small amount of time?

If you consider the automation input data as a signal, a lowpass filter (be it linear interpolation or whatever) is applied to smooth transients and higher frequencies, just like with audio. This signal is then used to modulate the filter's parameters.

[logicalhippo]

A good introduction to this topic is http://www.dafx14.fau.de/papers/dafx14_ ... s_for_.pdf