The problem you are experiencing is caused by the fact that after changing
the filter coefficients, the state of the filter produces something
different to the current output. There are several ways to solve the
problem:
- The time varying bilinear transform:
http://www.aes.org/e-lib/browse.cfm?elib=18490
- Every time you modify the filter coefficients, modify the state of the
filter so that it will produce the output you are expecting. Easy to do.

I will also add that some filter structures are less prone to problems like
this. I used a lattice filter structure to allow audio rate modulation of a
biquad without any amplitude problems. I have no idea how it would work for
using the filter as an oscillator.

Best,
Ian

On Wed, Feb 20, 2019 at 9:07 AM Dario Sanfilippo <sanfilippo.da...@gmail.com>
wrote:

> Hello, list.
>
> I'm currently working with digital resonators for sinusoidal oscillators
> and I was wondering if you have worked with some design which allows for
> frequency variations without discontinuities in phase or amplitude.
>
> Thank you so much for your help.
>
> Dario
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