[PD] a smooth crossfade for "equal" signals (freezing/unfreezing)

Mathieu Bouchard matju at artengine.ca
Mon Jul 18 19:39:48 CEST 2011


On Fri, 8 Jul 2011, Alexandre Torres Porres wrote:

> It seems now it's a matter of aligning the phases, but not sure how, good night

Phase alignment has to be done by a slight temporary pitch shift. You can 
shift up or shift down, but in any case, you need to shift.

If you do a plain linear cross-fade of signals (or a plain linear 
cross-fade of spectra, which is the same thing), it will automatically do 
this temporary pitch-shift, but will also do a temporary amplitude-drop at 
the same time.

This is because if you have two dots on a circle representing two phase 
alignments at the same amplitude, and you go from one to the other, you 
will go inside of the circle, where the amplitude is lower. The centre of 
the circle represents silence. In the extreme case of complete reversal of 
phase, the crossfade will quickly go through the silence point. But each 
frequency will have a different crossfade, so, overall, the amplitude drop 
will rarely be great, except if you tend to have few partials.

I'm thinking about phasor diagrams like 
http://en.wikipedia.org/wiki/Phasor , which may be considered as a use of 
Argand diagrams (complex numbers plane).

To keep constant amplitude, if you want to, is like rotating dots so that 
they keep the same amplitude (distance from centre). This is usually not 
easy to do, but you already have a components representation using 
sigmund~, so you have the opportunity to fudge the freqs a bit...

  _______________________________________________________________________
| Mathieu Bouchard ---- tél: +1.514.383.3801 ---- Villeray, Montréal, QC


More information about the Pd-list mailing list