Quote:
Originally Posted by nitsuj
The SVF filters are extremely good - much better than the RBJ cookbook ones that a lot of EQs used to use. Clean, stable, low noise and transparent.
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Could you expand a bit more on this, please?
My understanding was that the ZDF/digital-SVF filters were equivalent to a traditional biquad calculation for steady-state, and that the (rather cool) advantages only came either for fast modulation, or when modelling distortion in the analogue circuit.
It's possible I've made a mistake, but I think the maths comes out that for pure filtering (no modulation, no distortion) the ZDF is equivalent to a biquad structure.
(Discussions of ZDF stuff often bring up frequency-warping for the critical frequency, but the RBJ cookbook includes compensation for this aspect of the bilinear transform, which is standard. JSFX also uses 64-bit floating-point values, so I'd be very surprised if there were any difference in the noise levels.)
For illustration, here's an image comparing ReaEQ with ReEQ with the same frequency/bandwidth (oversampling turned off). Since both filters are minimum-phase, an identical amplitude response means an identical phase response as well:
So unless you're modulating the bands quite fast (or modelling non-linear components), they should be the same, right? Not that they aren't neat, just that I don't think we're getting the benefit from them in this situation.
Geraint