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Explains the process used to pitch match words for use with the speak and sing.


Pitch correction of the human voice is a common activity, with applications in music, entertainment, and law. It can be used to alter pitch to produce a more accurate or more pleasing tone in music, as well as add distortion effects. Several programs for entertainment use a form of pitch correction to modulate and distort a user's voice, allowing one to sound like a different gender or emulate a celebrity or other well-known voice. Voice distortion is also often required to protect the anonymity of individuals in the criminal justice system. However, it is the first of these applications that we are most interested in - producing a pleasing, tone-accurate song from a human voice.


Pitch adjustment of a digitally-sampled audio file can be implemented simply using resampling. However, this completely alters the time scaling and cannot account for changes in the pitch and inflection of a voice over time, and thus cannot be considered. Instead, we shall use the more sophisticated Pitch-Synchronous Overlap Add algorithm, which allows us to modify pitch without compromised information or modifying the time scaling.

The pitch correction method involves the following basic steps:

  • Detection of original pitch
  • Parsing of desired pitch frequencies
  • Correction of pitch

Pitch detection

First, the pitch of the original signal is determined. This is done using the FAST-Autocorrelation algorithm. This algorithm makes use of the fact that for a signal to have pitch, it must have a somewhat periodic nature, even if it is not a strictly periodic wave. The signal is divided into several small windows, each only a few milliseconds long and containing thousands of samples - enough to detect at least two periods and thus to determine the window's frequency.

Finding periods

Each windowed segment is autocorrelated with itself to identify the length of the period. This is done by convolving the signal with itself with an increasing offset τ to obtain the autocorrelation function:

R(τ) = f(-τ) * f(τ)

For discrete, finite-length signals, it can be found as a sum of the product of the signal and its offset, in this form:

R(s) = Sum(x(n)x(n-s))

This autocorrelation acts as a match filter: the signal and its offset form will be the most alike when offset s is equal to one period. Thus, the autocorrelation function is at a minimum when the offset corresponds to the length of one period, in samples.

Making it fast

Autocorrelation in this fashion is very computationally expensive - one can expect that the algorithm will have to convolve two length-1000 signals several hundred times for each window to obtain the frequency from within the full possible range of frequencies for a human voice. To speed this up, we can make two assumptions:

  1. The frequency of a window should be relatively close to that of the window before it
  2. The first minimum corresponds to the period, so no further minima are needed

By starting at an offset relatively close to the previously found period length (perhaps 20 samples before where the period was found), we can eliminate a few hundred calculations per window. If a minimum is not found in this area, we simply broaden our range and try again. To reduce the computation time further, we also calculate the derivative dR(s)/ds to determine where the minimum occurs. Once we find the first minimum, we are finished with obtaining the frequency for this window, having shaved off up to 70% of our computation time.

Questions & Answers

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
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Akash Reply
it is a goid question and i want to know the answer as well
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for teaching engĺish at school how nano technology help us
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s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
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what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Speak and sing. OpenStax CNX. Dec 21, 2009 Download for free at http://cnx.org/content/col11151/1.1
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