19 Nov
2017
19 Nov
'17
1:10 p.m.
Fons Adriaensen <fons-dDzkXPnfpdzhj6bIAHgUAw(a)public.gmane.org> writes:
On Sun, Nov 19, 2017 at 01:23:44AM +0100, David
Kastrup wrote:
Correct.
Now the EQ section of the Onyx mixers has been
designed by Carl Perkins
and is liked by a few people. Since it has to get along with not too
many elements and uses opamps, the actual equations for the various
sections (shelved low-pass, parametric mid pass(es), shelved high-pass)
are comparatively simple and basically amount to comparatively simple
transfer functions yielding themselves reasonably well to bilinear
transforms and consequently low-order IIR filters.
Indeed, and that basically means there is no point in 'emulating' those
filters.
Take a for example a second order parametric section. If it has
controls for center frequency, bandwidth and gain then it can generate
*all* frequency responses of that type (within the range of its
controls). There or no more free parameters given the order and
the general shape
of the frequency response.
In other words, any such filter can do whatever any other can.
Correct. The problem is _exactly_ that it can do whatever any other can
and that you have far too many free parameters to get under control. A
few distinctly different good choices in practice lead to better
results, particularly given time constraints, than a full continuum of
every available choice.
The continuum is what tool builders can work best with. But the actual
use cases want ready-made tools.
What makes EQ sections of the same type different is
how control
settings map to actual parameter values or how they interact, for
example how the effective bandwidth changes as a function of
gain. This will give then a certain 'feel'. But it doesn't make the
actual EQ different.
I have no beef with stealing existing code/filters and just putting a
surface on them. The particular filters here have some overshoot in the
pass area, so they aren't straight Butterworth. Generic IIR filters
probably need some care in order to react gracefully to
automation/parameter changes.
--
David Kastrup