[PD] Debugging sound

Derek Holzer derek at umatic.nl
Sun Oct 19 00:13:58 CEST 2008

Hi David,

David F. Place wrote:

> I heard
> some acoustical artifacts that I hadn't noticed when I was working in
> front of my computer listening through speakers.   This extra "buzzing"
> sound is quite annoying.  It's hard to imagine how my patches are making
> it.   Does anyone have any wisdom about how to track down and fix these
> kind of problems?  Are there any usual things you check first?

I know you said you checked what happens when you increase the sampling 
rate, but I would double check and make sure that what you are hearing 
isn't aliasing. I've found the difference between using an oversampled 
or otherwise bandwidth-limited oscillator and the normal [phasor~] or a 
square wave derived from [phasor~] and [expr~ $v1 > 0.5] to be the 
difference between day and night (or Stockhausen and Donkey Kong). Check 
3.audio.examples/J07.oversampling.pd for one anti-aliasing strategy, or 
better yet compare a normal, ugly, aliased [phasor~] with the 
anti-aliased one in that example. List archives will turn up loads when 
you search for "bandwidth limited" as well.

Remember that once aliasing gets into a digital signal, no amount of 
filtering can remove it!

If aliasing isn't the problem, then you'd do best to tap a [dac~] into 
each section of your patch one by one and see where the problem is. I 
don't think anyone on this list is so psychic they can figure out it 
long-distance ;-)

Other things I would check, however, from experience:

1) Any Amplitude Modulation, Frequency Modulation or other changes in 
frequency or gain are done with audio signals, or via [pack 0 0] and 
[line~] with enough interpolation time set on the [pack] to get rid of 
any "stepping" or "zipper noise".
2) If you are using samples, that you haven't loaded such a large file 
into a table that the precision drops. You might have to go fishing in 
the archives to find out exactly how many samples it takes to do this, I 
can't remember offhand...
3) That you aren't clipping your final levels. Open the file up in a 
wave editor and see if the peaks hit 0dB. For some CD players even this 
is too hot, and you might want to aim for something more like -1dB or 
-3dB. Scout around on the internets for tips on mixing and mastering for 
CD to get an idea of where people tend to peak their levels.


derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 156:
"The tape is now the music"

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