On Wed, Jun 18, 2003 at 06:52:05PM +0300, Kimmo Sundqvist wrote: I have an old csound instrument i wrote that does this, in a similar way to the old analog "octave" pedals. there are more sophisicated techniques, i guess involving analysis of the signal and generating sine waves based on the analysis... but mine can be quite useful. I would love it if somebody made a LADSPA plugin out of this algorithm. my basic technique was this: steep lowpass filter (reduces the number of zero crossings) count upward-going zero crossings in the sine wave. if count % N == 1, set the output to 1. if count % N == 0, set the output to -1. N determines the frequency of the subharmonic. If N == 2, you get an octave down. If N == 4, you get two octaves down. You now have a square wave with no dynamics, whose fundamental frequency is a subharmonic of the input signal. It has no dynamics, meaning it's likely to produce horrible noise if the input is low with a non-zero noise floor :) We'll fix that later. Note that, like its analog counterparts, this algorithm gives somewhat odd results when the input signal is complex, e.g. a dissonant chord. Next, apply a steep lowpass filter on the output unless you like the square wave :) Now apply gain balancing so that the output volume tracks the input volume. I don't know how you'd do that efficiently in C, i just used a csound opcode that's provided for this purpose. The csound instrument is attached, if that's useful... -- Paul Winkler http://www.slinkp.com Look! Up in the sky! It's OCTO BARON-MAN! (random hero from isometric.spaceninja.com)