[music-dsp] On time-varying filters

[Jon Watte]

I've done this with reasonable success by using a single set of interpolated 
coefficients per 4 frames, and calculating new targets every 128 frames. 
This was for a 4-th order IIR. Really fast movement caused some temporary 
resonance spike like glitches, but that sounded kind-of cool so I left it in 
there :-)

If you're doing it on a bi-quad, it works best on the un-transposed form 
because its coefficients are the least sensitive, I'm told.

Cheers,

				/ h+



>On Wed, 23 Aug 2000, Alexandre F. Bouchet wrote:
>
>>    How do you implement time-varying filters in real time?
>
>How about linear ramps between old and new coefficient values and precise
>calculation of coefficients from the parameters (cutoff etc) at the
>startpoint of each ramp. The frequency of the ramps would need to be
>high at times when there are large changes in parameters. Linear ramping
>is cheap, just one addition per coefficient per sample.
>
>The hard part is that you need to know "what's coming next" and "when".
>
>-olli
>
>
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