[music-dsp] Fw: A little question about DSP performance

Charles Henry czhenry at gmail.com
Fri Oct 2 16:59:40 EDT 2009


Allow me to be the devil's advocate here, and present another side of
the discussion.

Do phase relationships between components influence the experience of
sound?  Maybe you could make a demonstration that is personally
convincing, but does it hold up for someone not expecting it?

It's been generally held that experience of sound is invariant under
transformations of phase--as far as I know, it's not been proven.  In
fact, there's a good deal of psychoacoustics that could be re-written.
 The phases of complex tones may have a lot of impact on the behavior
in the cochlea itself-which is both actively and passively nonlinear.
Furthermore, the inner hair cells produce phase locked spikes.  So,
all the information in your complex tone is represented in the nervous
system... but is that information represented cognitively or
perceptually?  It might be a feature of sound that has no purpose and
is not extracted from the raw amount of information.

Which one is the real sawtooth?
Any loudspeaker contains reactive elements (coils, compliances, and
acoustic loading) which causes frequency dependant phase shifts as a
continuous function.  Also, there's the room in which you're listening
and the head-related transfer function.  Any waveform we create
electronically has all its components phase shifted when recreating
the acoustic waves that our ears receive.  Even changing one's
position in a room may be enough to change the relative phases between
components in the sound.  It's possible that every sawtooth wave
you've ever heard was different, even if it sounds the same to you.

Then, there's other ways things can go wrong that you might not
anticipate.  Shifting the phase of one component may change the peaks
so that you clip the sound digitally.  Or perhaps the loudspeaker has
it's own material nonlinearity that generates new harmonics due to the
peaks.  I can't even begin to imagine all the things needed to control
every other possiblity. ><

Chuck


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