A less mathematical way to show why this doesn't quite work: If you have a resonant filter and you want to modulate it from frequency A to frequency B, you would expect that resonant peak to "sweep" from one frequency to the next. If you instead use two filters/streams and crossfade between them, you don't get a sweep of the resonant peak. You have two separate (fixed frequency) resonant peaks. One of them will decrease in amplitude until it reaches 0, one will increase in amplitude from 0 to the target amount.
On Wed, Apr 26, 2023 at 11:09 AM Yisheng Jiang <[email protected]> wrote: > Sorry I was unclear in my original email. This is separate from the > amplitude envelope.. I’m talking about a modulating envelope that’s only > connected to the resonant frequency of the filter.. it shifts it by about > 1-semitone over the course of 1 second or so.. > > Sent from my iPhone > > > On Apr 26, 2023, at 1:07 AM, Andy Farnell <[email protected]> > wrote: > > > > Without time to listen to your example Yisheng I'm just voicing a > > theoretical/academic take; When it comes to LTI (linear time > > invariant) systems order doesn't matter. With time variant systems it > > does. > > > > As a very concrete practical example, I made some "rain" weather FX > > patches that employ short envelopes into resonant bandpass filters. > > They are IIR filters and meant to "ring". Placing the filters before > > the envelopes totally changes the effect, A time invariant FIR filter > > version would not behave differently, but the desired effect actually > > comes from the ringing, hence the DSP ordering. > > > > When things move very slowly, it tends to matter less. > > > > regards, > > andy > > > > > > > >> On Tue, Apr 25, 2023 at 09:19:02PM -0700, Ariadne Lewis-Towbes wrote: > >> Hi, > >> > >> You won't be able to fade between two filtered blocks of complex > >> (non-sinusoidal) input to the same effect as modulating an arbitrary > single > >> filter's frequency. That said, when modulating an IIR filter such as a > >> biquad (as shown in your second email's link), you should not need to > >> compute the z-transform. You may simply modulate the biquad's parameters > >> repeatedly, and unless you're doing something unusual with respect to > >> calculating those parameters, the cost will be relatively low. > >> > >> Best, > >> Ariadne Lewis-Towbes > >> > >>> On 2023-04-25 20:37, Yisheng Jiang wrote: > >>> I’m trying to render a note that feed into a IIR filter (2-3 poles) > >>> whose cutoff frequency following an envelope generator, and it’s not > >>> possible for me to compute the z-transform parameters every rendering > >>> block.. > >>> > >>> Is it approximately the same to generate two pcm streams with the > >>> starting and ending cutoff frequencies, then cross fade them to make > >>> the resultant sound? >
