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Alien Autopsy Via Sample-Rate Reduction

Posted on | January 6, 2011 | 3 Comments

Here’s a cool sound-design trick. If you want to get a vocal-sounding ‘for­mant fil­ter’ effect out of a synth that only has a nor­mal low­pass fil­ter, you can take advan­tage of a quirk of sample-rate reduc­tion effects to gen­er­ate mul­ti­ple “mir­rored” fil­ter sweeps through the won­der of aliasing.

Here’s a sound clip from my machine­drum with a sim­ple saw­tooth note and a res­o­nant low­pass fil­ter being mod­u­lated down over a quick sweep. It’s played four times, each with increas­ing amounts of sample-rate reduc­tion applied:

increas­ing srr

This sam­ple looks like this in a sono­gram (I used the Sono­gram View plu­gin that Apple includes with XCode). Hor­i­zon­tal axis is time, ver­ti­cal is frequency:

Notice that as the alias­ing (reflected fre­quen­cies) increase with the sample-rate reduc­tion effect, you begin to see mul­ti­ple copies of the fil­ter sweep. This cre­ates the lovely, com­pli­cated “alien voice” sound. Here’s a short Machine­Drum loop I was play­ing around with when I real­ized what was going on here:


And for the Elektron-heads read­ing this, here’s the MD sysex for that pattern+kit:

PS: the wikipedia arti­cle on alias­ing has a good run­down on the details of this phenomenon.


3 Responses to “Alien Autopsy Via Sample-Rate Reduction”

  1. JC
    June 7th, 2011 @ 8:20 pm


    Look­ing for a frac­tal syn­the­sizer I found your pro­cess­ing exper­i­ment (and your juicy soundcloud!)

    Im try­ing to extract some syn­the­sis sound­scapes , aliens or what­ever could be inside from inter­ac­tive strange attrac­tors in pro­cess­ing too… lets see what happen.

    Greet­ings from Spain.

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