Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!apple!malcolm
From: malcolm@Apple.COM (Malcolm Slaney)
Newsgroups: comp.dsp
Subject: Re: Autocorrelation Pitch Tracker
Summary: Pitch is not the same as periodicity
Message-ID: <51258@apple.Apple.COM>
Date: 6 Apr 91 17:30:36 GMT
References: <17823@sdcc6.ucsd.edu> <78395@bu.edu.bu.edu> <1991Apr6.062906.11886@cs.cmu.edu>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 48

>>>Does anybody out there have experience implementing the
>>>autocorrelation method of pitch detection? 
>
>    I've played around with these, but not for really short signals.
>

Everybody here has really been talking about periodicity detectors and NOT
pitch detection.

It is important to realize that pitch is a perceptual quality.  An official
definition of pitch (ANSI?) defines pitch as that quantity that humans would
perceive and try to sing to.  It is not defined in terms of the periodicities
of the signal....although if the signal is periodic then the pitch is
usually the same as the fundamental.  Once you add a bit of noise then your
signal is not periodic.

That said, you need more than two cycles of a signal to perceive its pitch!
I just tried it.  I synthesized (with MatLab) a signal that looks like
	1 second of silence 
	a small number of cycles of 220Hz sine wave
	1 second of silence
If there is only one or two cycles then all one hears is a click.  There is
nothing in this sound that would let me hear a pitch.  Not until I get to 
four or five cycles does it start to sound musical so I can assign a pitch
to it.  Try it, if you don't believe me.

It is also important to realize that pitch is not a unique quantity.  There
are many examples where it is possible to perceive more than one pitch.  
Shepard tones (on the ASA Auditory Demonstrations CD) and creaky voice are
two common examples.  The engineering world likes to reduce pitch to a 
single number but that just isn't realistic.  If your system outputs a 
single number then it is probably a periodicity detector and not a pitch
detector.

If you want to know more about pitch there is a book that reviews most of
the pre-1983 literature
	Hess, Wolfgang. PITCH DETERMINATION OF SPEECH SIGNALS (Berlin ;
	       Springer-Verlag, 1983)

Within the psychoacoustics world the Journal of the Acoustical Society
of America has a few articles a year talking about pitch.  There seems
to be two camps, the place based people led by Julius Goldstein and the
autocorrelation people.  I think both of these approaches are flawed and
a hybrid approach was described in my 1990 ICASSP paper and an upcoming
JASA article by Ray Meddis.

						Malcolm Slaney
						Apple Perception Group