[ntp:questions] measuring frequency

unruh unruh at invalid.ca
Sat Feb 9 17:18:25 UTC 2013

On 2013-02-08, Ivan Shmakov <oneingray at gmail.com> wrote:
>>>>>> David Woolley <david at ex.djwhome.demon.invalid> writes:
> 	[Cross-posting, and setting Followup-To:, to news:comp.dsp.]
> [...]
> > In Fourier terms, the spectral width of a piano note is going to be
> > more than 12ppm and depend on the exact interval over which you
> > measure it.  I would also suspect that the zero crossing interval
> > changes significantly during the note, particularly at the start.
> 	As per my experience with guitar signals, such a waveform is
> 	likely to cross zero more than once per period.  Hence, by
> 	counting such crossings one typically gets the frequency of one
> 	of the note's harmonics, and not that of the pure tone.
> 	BTW, is there a kind of overview of a method that could allow
> 	for frequency measurement for such a signal?  (Preferably one
> 	oriented to the uninitiated.  I can probably handle complex
> 	math, though.)

Use the fourier transform to get the rought freq. Then use zero
crossings at around that period to narrow in on the freq. Of course if
the harmonics are not exact (see piano) then those sero crossings will
not give the freq of the fundamantal. You could filter the intput around
that rough frequency of the fundamental, but that mightgive you very
little signal ( the ear hears the fundamental pitch even if there is no
fundamental i n the note )

> 	TIA.
> [...]

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