Re: [Audacity-devel] Classic Filters status

[Federico M. <fm...@fc...>]

Martyn,

> Incidentally, can you hear the differences in these (440Hz and 880Hz 
> tones), I can't. https://dl.dropboxusercontent.com/u/1327769/phases.wav

Except for the last one, nobody can, according to Ohm's law for mono 
signals (you cannot hear any phase difference):

http://en.wikipedia.org/wiki/Ohm%27s_acoustic_law

In the last one, if you listen attentively, you will notice a second 
order effect that is explained in the book by Roederer, "The Physics and 
Psychophyisics of Music". It is a case where the second harmonic is not 
exactly in tune. The phase modulation is similar either to frequency 
modulation or a mistuned second harmonic, for instance 440 Hz and 880.44 Hz.

As regards phase differences that depart from linear phase with 
frequency, it is generally accepted that they can be noted in stereo 
signals since they represent different delays for different frequencies. 
This affects the sharpness of the stereo image. Probably most people 
cannot tell the difference, but experienced audiophiles can.

But the subtle difference probably cannot be perceived at all in the 
presence of fan noise from the computer if not acoustically isolated.

In the case of the antialiasing filters used in legacy A/D converters, 
I'm not sure if the audibility of this phase problem is not really due 
to the channel mismatch.

A 1 kHz order 10 lowpass Chebyshev has at 1 kHz a phase delay of 720º, 
about 2 ms, while at 100 Hz it has a delay of 0.13 ms. That means that 
there is a difference of about 1.9 ms, which corresponds to a distance 
of about 2 ft, that's why the source may, in theory, seem blurred.

Regards,

Federico