New functionality was introduced in Praat 5.3.14 and is still under development. Future versions will allow the direct creation of Vocal Tract Tiers from LPC objects and LPC filtering of (source) sounds with sample-frequencies and durations that differ from those of the LPC analysis. Be aware that this code has not been tested extensively, so there will be bugs.
In Praat it is possible to calculate a vocal tract area function that is equivalent to a certain (vowel) sound. The sound can then be resynthesized using the calculated vocal tract area function as a filter. The vocal tract area functions can be manipulated and modified before resynthesis. In the list below, you find some example vocal tract area functions of sustained /a/, /i/, and /y/, and the resynthesized sounds.
Blend two vocal tracts: Paste the lips of an /y/ onto the vocal tract of an /i/. That is append the last four segments of /y/ to /i/ vocal tract function, adapt length etc.:
Blend two vocal tracts: Paste the lips of an /y/ onto the vocal tract of an /i/. That is append the last two segments of /y/ to /i/ vocal tract function, adapt length etc.:
Take measured areas from MRI slices of the lips, and attach them to an existing Vocal Tract Area function. Start with the recordings of /i/ and /y/ of the female speaker above. Areas for her lips were determined using an MRI image. Starting from the teeth (X=0) go outward. Only every third slice was used. Slice thickness was 1.4064 mm and the area value is positioned at slice midpoint. All values are recalculated to meters.
|X||/i/ (m2)||/y/ (m2)||/a/ (m2)|
Starting with the original recorded /i/
Starting with the original recorded /y/
The following table explains how to get from a Vocal Tract to a synthetic sound. For synthesis, a "Source" sound is needed that supplies the driver of the Vocal Tract filter. In normal speech, the source sound is produced by the glottal folds, or voice box. You can generate a source as specified below. Note that the sample frequency of the source sound has to be equal to the number of segments in the Vocal Tract in kHz. For instance, if you have 40 segments (tubes), you need a source sampled with 40kHz. Use the Praat Resample... function to perform the resampling. The length of the Vocal Tract Tier must be exactly the same as the length of the Source sound. Below, we take a duration of 3 seconds in the presented examples. The audio example is 5 seconds long.
Here is an example generated by determining the vocal tract area function at a point in a recorded /a/ and one at a corresponding point in a recorded /i/ from the same speaker. The voice source signal is entirely synthetic.
To test the synthesis, you can use the standard vocal tracts in Praat or create a Vocal Tract from recorded speech. The standard phone Vocal Tracts can be created in Praat from New->Articulatory synthesis->Create Vocal Tract from phone... . To create a Vocal Tract from recorded speech, simply read in the recording and convert it to LPC with the Formants & LPC -> LPC (autocorrelation)... options. Enter the number of segments you want in your Vocal Tract as the prediction order. Then use To VocalTract (slice)... to generate the Vocal Tract object. Save it with Save->Save as short text file... . Note that there is a rather convoluted relationship between the LPC prediction order, the sample frequency, the recorded sound and the quality of the resulting LPC model.
You can download Praat from www.praat.org
|Read VocalTract file||Open->Read from file...||Read from file... a.VocalTract|
|Convert to Vocal Tract Tier||To VocalTractTier...||To VocalTractTier... 0 3 0.5|
|Convert Tier to LPC||To LPC...||To LPC... 0.005|
|Select both LPC and Source audio file||Option/Control select source audio Sound||plus Sound Source|
|Filter Source with LPC||Filter...||Filter... no|
|Resample to 10kHz||Convert->Resample...||Resample... 10000 50|
|Generate Source sound|
|Create an empty PitchTier object||New->Tiers->Create PitchTier...||Create PitchTier... Source 0 3|
|Add a high starting point at 120Hz||Modify->Add point...||Add point... 0 120|
|Add a low end point at 100Hz||Modify->Add point...||Add point... duration 100|
|Convert it into a phonation sound||Synthesize->To Sound (phonation)...||To Sound (phonation)... 40000 1 0.05 0.7 0.03 3 4 no|
|Scale to a nice intensity||Modify->Scale intensity...||Scale intensity... 70|
|Create Vocal Tract|
|Read audio file||Open->Read from file...||Read from file... a.wav|
|Convert to LPC with predition order 40 for 40 tube segments||Formants & LPC -> LPC (autocorrelation)...||To LPC (autocorrelation)... 40 0.025 0.005 50|
|Convert LPC to Vocal tract, use slice at 2 seconds and a total vocal tract length of 20 cm||To VocalTract (slice)...||To VocalTract (slice)... 2 0.20|
The Vocal Tract area functions model the human vocal tract as a set of connected tubes with variable width. Determining the tube, or segment, areas with LPC is not very reliable. Below are presented tube models as determined with LPC (prediction order 40) and the "theoretical" models as given by Praat New->Articulatory synthesis->Create Vocal Tract from phone....
|Vocal Tract tube model of /a/ (example)||Vocal Tract tube model of /i/ (example)|
|Standard Vocal Tract tube model of /a/||Standard Vocal Tract tube model of /i/|
|Vocal Tract tube model of /y/ (example)||Standard Vocal Tract tube model of /y/|
The example VocalTract file below is created with Save as short text file... . There is a more descriptive (longer) format that is obtained with Save as text file... .
|File type = "ooTextFile"||The line by which Praat can recognize your file|
|Object class = "VocalTract 2"||The line that tells Praat about the contents|
|0||xmin: First segment (Glottis, meter)|
|0.2||xmax: Last segment (Lips, meter)|
|40||nx: Number of segments|
|0.005||dx: Segment length (m)|
|0.0025||x1: Position of first segment|
|0.00010813061971705616||Area in m2|
|0.00010390570341053334||Area in m2|
|8.903563828398031e-05||Area in m2|
|0.00010876151465927323||Area in m2|
|....||Many more values|
|0.008175693406171154||Area in m2|
|0.0013459947563683344||Area in m2|
|0.04293933951717365||Area in m2|
|0.000489118171677886||Area in m2|
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