Filter theory

I am feeling ready to plunge into my first Reaktor project, having poked about in others for a number of weeks.

It is common to hear people express that the 'soul' of a synth is in its filters. This may be overstating things a bit, but there is no doubt that the idiosyncracies of various synth filters are responsible for a good deal of their individual characters.

How can I go about learning how to create filters with new characteristics in Reaktor? I am reasonably quick to understand new bits of synth theory, but I am by no means a mathmatician (I dropped the subject when I was 16). Does anyone know of any online resources that could help me, and indeed would anyone like to throw some scraps of knowledge my way?

Wuntun

 

I'd suggest that if you're not into math, don't bother with from-scratch filter design. Reaktor isn't really set up for it anyway.

Reaktor already has a good set of filters included. You can build new kinds of filters by just combining the existing ones in parallel or serial, or by adding waveshapers.

For example, a handful of bandpass filters in parallel is a cool way to mimic the spectral response of an acoustic instrument.

Chet

 

using multiple 1pole and 2 pole filters in series but tapped and mixed, with phase inversion on some taps, a little sampledelay on others can help creating a new filter-sound..
or try a notch behind a lpf/hpf at the approximate cutoff frq to eliminate the resonance amplitude bump while dialing in a lot of res; this way you colour the sound without the res becoming too strong. you can create a sound similar to the lovely emaxII filters this way.

 

I have trouble with waveshapers in general... how would you do this for a filter effect?

 

I didn't mean anything particularly clever. Just that I like to use waveshapers (saturation, clipping, rectification, and the like) in combination with filters.

Chet

 

> using multiple 1pole and 2 pole filters in series but tapped and mixed


what exactly does tapped and mixed mean?
hope the question isn't too 'dumb'

thanks!
-b

 

"tapped and mixed" doesn´t mean anything esotheric....
i just meant:
connect multiple filters in series,
but connect all of their outputs to a mixer as well. these additional connections are the "taps". you might want to invert phase at various points in the signal flow, and/or add sample-/modulation-delays, possibly following the cuttoff frq./the resonance amt. (the mod-delays that is).
you can assemble some really smooth filters in this way.

 

It is possible to do some math-heavy filtering in Reaktor. I have a Chebychev filter in my JoeMakros collection in the user lib. This is probably the opposite of what you're looking for if you want heavy coloration though - it's ruler flat, then has a brick wall cutoff (up to 12th order). It's good for positively defining bands. The only math problem with Reaktor filters is a pole at 1/4 of the sample rate - tune your filter near this frequency with significant res, and you get very bad behavior. Only cure is to use a higher sample rate (or for NI to fix the code).

Now, for serious coloration you need distortion in the middle of the filter. NI has at least one example of this somewhere in the library (no doubt there are hundreds). Basically make a chain of 2nd-order filters with distortion modules (choose your favorite kind) in between. Real analog filters usually include components (diodes or transistors) which provide a variable resistance for tuning (acting as VCF). These components invariably add some distortion too - originally seen as a problem, now appreciated as a vital part of the character of the synth. Depending on the circuit layout, you get various types of distortion, thus distinguishing different synth brands. If you really want to get fancy, you can add feedback loops to your filter topology (remember to add unit delays). With this you can more closely approximate real-world filters, which have a distortion component for each 1st-order section, and a pair of 1st orders with feedback is a resonant 2nd order. Or put feedback around bigger sections for wild and crazy results.