Opening a Sound File in Praat

Praat is a program that offers several different ways to process and manipulate sound files. When a sound file is opened in Praat, it is automatically converted into an “object,” which is the only type of data that the program can work with. To open a sound file, select “Read from file…” from the “Read” menu.

LPC Filters

Using Praat, it is possible to calculate the LPC filter coefficients of a sound object. To do this, select the sound object and use the function "To LPC (burg)" under "Formants and LPC." This function requires the following arguments:

Prediction order - The number of linear prediction coefficients.

Analysis window duration - The duration of each analysis frame, in seconds.

Time step - The time step between two consecutive analysis frames, in seconds.

Pre-emphasis frequency - A +6dB / octave filtering will be applied above this frequency (Hz). If you do not want pre-emphasis, choose a frequency greater than the Nyquist frequency.

This function will return an LPC object. To filter or inversely filter a sound object with an LPC object, simply select both of them simultaneously and choose the appropriate option. Inversely filtering a sound object with its associated LPC object will yield the excitation (or source) part of the sound. This excitation can be filtered with a different LPC object than the one it was created with to obtain the characteristics of another sound.

Changing Pitch

Praat can also be used to extract pitch information from a sound. To do this, a sound object must first be converted to a manipulation object through the “To Manipulation…” function. When a sound object is converted to a manipulation object, Praat automatically calculates the sound's pitch information using the PSOLA method. After selecting a manipulation object, the “Extract pitch tier” function can be used to obtain this pitch information.

A manipulation object’s pitch tier can be replaced with a separate pitch tier object. Selecting both objects and using the “Replace pitch tier” function will accomplish this. In order to get a sound object from a manipulation object, select “Resynthesize (LPC).”

Voice Conversion Algorithm

The preceding processes can be automated through a Praat script. The following block diagram illustrates one method of performing voice conversion in Praat:

Figure 1: The windowing for this process is done in Matlab. The rest of the system can be coded entirely in Praat.

A Voice Conversion Algorithm

References

Praat: Doing Phonetics by Computer. Paul Boersma and David Weenink of the University of Amsterdam. www.praat.org.