[PD] [PD]spatialization : filter problems

julien.breval at tremplin-utc.net julien.breval at tremplin-utc.net
Tue Oct 28 17:35:23 CET 2003


I am currently trying to transfer some quadraphonic works performed with Pure
Data to stereo (for CD burning)

The idea is to program a dual-outlet~ abstraction that performs fake quadra
spatialization as a psychoacoustic illusion, using tunable doppler, pan and
filtering effects.
(btw is there such external for Pd ? for example under Max there is ircam's Spat
or GMEM's holo-spat)

Unfortunately I have some problems with the filtering section :

*1* Q factor problem
How is it possible to control the Q factor (slope) of the hip~ and lop~ objects ? 
The only solution I found was to use a series of filters (more filters -- >
better Q). There must be a reason why there is no "Q" parameter like the 3rd
inlet of "bp~".

*2* Spectrum of hip~
The spectrum of a hip~-processed signal (noise for example) looks strange : it
seems impossible to cut ALL the frequencies, even if the cutoff is close to
20000 Hz (if you try this, turn your monitors down) : indeed, the energies of
high frequencies are getting higher and higher if the cutoff increases, and only
a few low frequencies are actually cut.   
Is hip~ really different to lop~ (the spectrum of lop~ looks "normal") ? 

*About (classic) low/high shelving
I need this for my patch
I have programmed low and high shelving abstractions, using hip~ and lop~, but
these are quite complicated (difficult to add or remove some frequencies with
the SAME parameter) and don't sound very well (due to the filter problems I
mentionned above).  

It's easy to use an FFT-filter (multiplication between the spectrum table and a
drwan cutoff table), and the quality is far better, but there must be a simpler
solution using only classic low and highpass filters. Any idea ? 

About Internet externals, I found something called "lowshelf", but I don't know
at all how it works. Please tell me if there is a help file or a readme
elsewhere, or at least what the inlets and outlets are...  

Thanks a lot 


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