Monday 3 February 2014

Comb filtering...

I've always been fascinated by the direct physical reference that you get with 'comb' filters. Other filter types tend to have names that are 'frequency response'-oriented, and so have names like: notch, low-pass and high-pass. But a series of notches is called a 'comb', and indeed the description is apt and correct - the response with frequency does look like a comb!

For such a shape of filter to have such an interesting effect on sound is a fascinating bonus. Resonant low-pass filters have that characteristic peak at the resonant frequency, whilst for comb filters it is the spacing between the notches that gives either a comb for linear spacing (as in a phase-shifter), or a coarse to fine spaced comb where the notches can match up to harmonics (as in a flanger). Because of the different ways that phasers and flangers typically implement comb filtering, flangers also introduce some time-modulation and time-delay effects (doppler shift and flutter echoes), but I really like either of the whooshy, resonant sounds produced by any modulated comb filter!

As you might have realised, I have been playing with comb filters recently, and I have posted the results of this to the MaxForLive web-site. Comber is a pair of modulated delay-based comb filters that can operate independently on two stereo channels, and then can be auto-panned into the final mix stage - a stereo flanger is one way of describing it.

The controls are in three sections. The LFO section modulates the delay time of the comb filter, and because this sounds like tuning the filter I have called it the 'Frequency'. The Feedback control is actually controlling the feedforward and feedback, and the Gain control allows extra tweakability (it controls the gain of the delay sections). The Pan section allows the two comb filters to be placed in the stereo image and moved around.

Because the Feedback and Gain controls in the filter can provide a wide variation in gain, there is a compensating 'Make Up' volume controller next to the Dry/Wet mix control.

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