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From: gbell@sdcc13.ucsd.edu (Greg Bell)
Newsgroups: comp.dsp
Subject: Re: Autocorrelation Pitch Tracker
Message-ID: <18056@sdcc6.ucsd.edu>
Date: 7 Apr 91 07:33:34 GMT
References: <17823@sdcc6.ucsd.edu> <78395@bu.edu.bu.edu> <1991Apr6.062906.11886@cs.cmu.edu>
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Organization: University of California, San Diego
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In article <1991Apr6.062906.11886@cs.cmu.edu> dandb+@cs.cmu.edu (Dean Rubine) writes:
>
>    I may be wrong, but I think Greg Bell is not referring to the
>autocorrelation method for doing LPC (as Pankaj Tyagi seems to imply),
>but something much simpler.

Exactly... I wasn't sure what PT was talking about!  This is
actually part of an LPC analysis package, but you need to determine
whether a signal is periodic or not, and if so, its period in order
to feed this info to one of TI's LPC synthesizers.
>interpolate for a more accurate period.

>    There's also a comb-filter-based method that's kind of a poor man's
>autocorrelation.  Here, for various delays, the signal is subtracted from

Also called, according to my book, AMDF for Average Magnitude
Difference Function.

>	1 It seems to me that windowing will make it harder to determine
>	  periodicity so should be avoided.

It does make it harder to determine the periodicity, but apparently
it is necessary.  There are methods that do not require windowing,
but the autocorr. method is not one of them.

>	 
>	2 For a valid determination of the extrema the same number of terms
>	  needs to be present in the sum for each m.
>

Hmm... good point.  My book says each sum goes from n=1 to N where N
is the length of the sample.  This didn't make sense since one of
the terms is s[n+k] so you go out of the range of samples.  So, I
made my sum to from n=1 to N-k.  This doesn't follow what you are
saying, but it did work well for a larger segment.  I'll try your
guidelines for my troublesome short segments.

I'll have to read you guidelines for the number of sum terms later
when I can look 'em over a little more carefully.

Thanks for the input!


-- 
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  Who:  Greg Bell                            Address:  gbell@ucsd.edu
 What:  EE hobbyist and major                  Where:  UC San Diego
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