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If the number of cascaded filters is sufficiently large to cover the majority of the audible spectrum and the transitions in the cyclical sweeping smooth enough, this “infinite sweeping” sensation can be perceived. In order to generate the glissando effect, each notch filter is cyclically swept from its original cut-off frequency to twice its value. Moreover, their cut-off gains respond to an inverted raised cosine envelope across the frequency spectrum to preserve the equivalence to the Shepard tone spectral structure.
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These conveniently placed notch filters have their cut-off frequency one octave apart from one another, so they have a constant separation in the logarithmic frequency scale. The goal of the authors was to come up with a flanging/phasing effect that generates the same “ infinite sweeping” illusion on an input signal by placing adequately gain-scaled cascaded notch filters on the frequencies where the Shepard tone harmonics are placed. The barberpole filter is based on a more or less well-studied effect known as the Shepard-Risset glissando. This paper turned out to be both interesting, current, and not overly complicated. On the other hand, it wouldn’t hurt if it ended up being part of my set of audio effects on my current production/performance Live set as well. Learning by doing is usually the best approach for getting used to a new environment, so I set out to search for a relatively simple -but interesting- problem to implement. Of course, it was a small price to pay for the joy of experimenting with audio in real time and focusing on the important aspects instead of endless debugging. Especially knowing that many times the same problem could be solved in a couple lines of good old code. I really like the visual programming aspect of the Max/MSP but, to be honest, having learned most of my DSP knowledge in traditional ways and seeing all these boxes and cables inside patches made me cringe every once in a while. This is something I’ve been meaning to do for some months now, as I was promised an idyllic world of sample-accurate processing and and a “procedural” or at least object-oriented programming environment (as opposed to visual) straight into my DAW of choice (that’s Ableton Live). I decided to make some space during the weekend to get acquainted with Max/MSP’s gen~ codebox environment.