Figure 6.1: The Sound:To TextGid... form.

Transcription

1 Annotation of sounds means adding meta data to a sound. The meta data in Praat can be any text you like. This meta data is either coupled to an interval or to a point in time and is stored in a TextGrid. To annotate you simply edit the sound and the TextGrid together in the TextGridEditor. We start with a simple example from scratch. Select a sound that you want to annotate and then choose Annotation ->To TextGrid... from the dynamic menu. Form 6.1 pops up and you can choose your tiers. 1 Praat distinguishes two types of tiers: 1. Interval tier. An interval tier represents a series of contiguous marked intervals in time. You typically use an intervaltier when you want to mark phenomena that extend in time. For the labeling of words or phonemes in a speech sound you would use an interval tier. 2. Text tier. A text tier represents a series of marked points sorted in time. 2 You will typically use a text tier for labeling points in time where something interesting happens, like for example the moments of glottal closure. Figure 6.1: The Sound:To TextGid... form. The textgrid is the container for any number of interval tiers and text tiers. In the top field in form 6.1 you fill out names for all tiers you intend to use. In the bottom field you fill out which of the names in the top field are text tiers. The label of this field explicitly mentions point tier. If you do not intend to use text tiers or just don t know yet, you can leave the field blank. Default all tiers will be interval tiers unless you select one or more point tiers. Praat isn t too picky about how you fill out this form, the only thing that matters is that at least one name in the top field exists. In the textgrideditor you can always add, remove or rename tiers. 1 The meaning of tier that is used here is layer. A tier can be seen as an extra layer to a sound. Therefore, in Praat a tier is a function on a domain. 2 A text tier is a marked point process (see chapter 8). 43

2 Figure 6.2: The Sound:To TextGrid... form for two interval tiers. Because the example in the next section will show you how to annotate a sound at the word and at the phoneme level, we will choose a textgrid that consist of two interval tiers named phoneme and word. Of course the names you can choose are free. Figure 6.2 shows how to fill out the form. Once the textgrid exist, you will select it together with the sound, choose Edit and a textgrideditor appears. Figure 6.3 shows the textgrideditor. The top of the editor window shows the identification number of the textgrid (here 15) fol- Figure 6.3: The textgrideditor for an existing sound with a newly created textgrid that has two interval tiers. lowed by the text TextGrid de-vrouw-loopt-met-noisy which happens to be the name given to the textgrid object. The next line shows all the menu choices of the textgrideditor. More on these menus will follow in the rest of this chapter. Below these menus is a white field extending over the complete width of the editor. This text window will show the text from the selected interval. It is empty now because we have not done any annotation so far. Next, in the middle panel, the sound is displayed above the two interval tiers. The top and bottom numbers at the left of the sound panel indicate the maximum and the minimum amplitude of the displayed sound, in the figure for this particular sound these numbers happen to be and , respectively. The blue coloured number indicates the value of the amplitude at the position of the cursor. The cursor is positioned in the middle of the display 44

3 6.1 Making the first annotations. area and is shown with a dotted red line at position s which is also the midpoint of the sound with duration s. In a newly created textgrid any interval tier consists of only one interval, stretching the entire time domain. The two interval tiers are shown below the sound. The interval tier number is shown on the left of the pane and the assigned name on the right. The first interval tier named phoneme is selected and this is indicated the red pointing finger at the left. The selected interval is yellow. In the (1/1) text on the right, below the name of the selected tier, the number after the slash signals the total number of intervals in the tier while the number before the slash signals the index of the selected interval. 3 Since we have only one interval at the start and it is also selected, these numbers are both equal to one. The number between parentheses below the name of the second interval tier only shows the total number of intervals in this tier. Once you select an interval in the second tier, the pointing finger will move to the front of this tier and below the name the index of the selected interval will also be indicated. Below the interval tiers, the gray rectangles show what parts of the sound can be played. These rectangles have the same functionality as they have in the soundeditor. The four small boxes at the bottom-left position also behave identically as the ones in the soundeditor. 6.1 Making the rst annotations. We will now show you how to make annotations at the word and phoneme level with the textgrideditor. We start with the easy part which is marking the start and end times of all the words in a sentence. We use the following Dutch example sentence d@ VrOu lo:pt met ha:r dyr@ le:r@ sxun@ Op d@ nat@ dør viz@ EiX@ hœys (Dutch: De vrouw loopt met haar dure leren schoenen op de natte deur in het vieze eigen huis. Eng: The women walks in her expensive leather shoes on the wet door in the dirty house of her own). It contains the twelve Dutch vowels and the three Dutch diphthongs. 4 The example was recorded with the voice of the author on the computer with a simple microphone and a sampling frequency of Hz. This sound was displayed in the textgrideditor in figure 6.3. As one can easily see from the noisy parts at the start and the end of the sound, the quality of the recording is not very optimal. This was deliberately done because many of the recordings that people use for annotations are not of studio quality either. In this way we hope to show and be able to explain many of the problems that show up during annotation and during subsequent analysis. Because these types of microphones always show a considerable offset we have used the Sound>Modify ->Subtract mean command to compensate for this offset. Because annotating sound is very time consuming and difficult, we like to use every bit of information that we can during this process. We will therefore also include spectral information in the editor. Before we can use this the additional information we have to make some adjustments to the textgrideditor. Choose Spectrum>Spectrogram settings... and form 6.4 pops up. The settings in the figure are the default settings for a broad band spectrogram. The window length of s guarantees enough time resolution to be helpful during annotation. 3 This is the default behavior. With the File>Preferences... form you can modify this default. 4 The Dutch vowel Y is missing in the sentence. However, the in the first word d@ can be taken as a substitute. 45

4 To be able to better visualize the first and second formant in the spectrogram we modify the Figure 6.4: TextGridEditor: Spectrogram settings. upper view range to 3000 Hz and then click OK. The spectrogram will become visible in a newly created pane below the sound once you have selected Spectrum>Show Spectrogram. With the spectrogram now visible in the editor we can start with marking the words in the sound. We zoom in to the first part of the sound as is shown in figure 6.5. Figure 6.5: Textgrideditor. Labeling words. The rectangle at the bottom of the textgrideditor that stretches from left border to the right border, shows that the sound s total duration is s. The rectangle above it of the same width as the sound panel, shows that only s of the sound is actually visible in the editor. The rectangle at the bottom that shows the number indicates that the initial part of the sound of this duration, is not displayed in the editor. In the same way the rectangle at the bottom right shows that the final s of the signal aren t displayed either. The first four words in the sound have been labeled already. The second interval has been selected 46

5 6.2 Annotate a new interval and its text is shown in the interval tier as d@ and in the text window as d\sw. The text combination \sw is Praat s internal representation of the schwa symbol and guarantees that textgrids written to a file will be interchangeable on all computers Annotate a new interval Before you can annotate an interval, you have to create one. Creating a new interval in Praat equals splitting an existing interval into two intervals at a newly created boundary. Clicking in the sound produces a cursor at that time position in the sound panel: the red dotted line in the figure at time At the same time position this cursor is extended into the tiers as potential interval boundaries and these are drawn with a fat gray line. At the top of each potential boundary you see a blue circle. After clicking the circle, the gray line changes to a red color and the circle disappears: you have created one new boundary and the number of intervals has increased by one. Now you can start typing text in the newly selected interval. 6.3 Finding zero crossings It is very important that every boundary in an interval tier is placed on a zero crossing. Failing to do so produces audible distortions when you play intervals because of discontinuities at the start and the end of sound intervals. This placing is so important that Praat has several commands for automatically finding the right positions in a sound. First of all we have three commands in the Select menu that apply only to the sound panel. Two of these commands apply to the selection and one command applies to the cursor of the sound panel. Move start of selection to nearest zero crossing and Move end of selection to nearest zero crossing find the exact position of the nearest zero crossing and move the corresponding marker to this position. The exact position of the zero crossing is found by a very precise interpolation technique. If no selection is active in the sound panel and only a cursor is present, the Move cursor to nearest zero crossing command is available. To move a boundary in the interval tier to a zero crossing the Boundary>Move to nearest zero crossing command suffices. 6.4 Remove an interval To remove one interval you have to remove one boundary. If you remove a boundary the text of the interval on the right is appended to the text of interval on the left. If you don t want this to happen you either have to correct the remaining interval afterwards or you have to take care that one of the two adjacent intervals is empty before you remove the boundary. To remove 5 Most of the phonetic signs for vowels and consonants and many mathematical symbols are represented in Praat in a computer independent way by three character representations that always start with a backslash. For example the representations for V, O and E are \vs, \ct and \ep, respectively. For more information about possible representations see the TextGridEditor>Help>About special symbols and Phonetic symbols. 47

6 a boundary you click on the boundary, the boundary changes color from blue to red and you choose Boundary>Remove boundary or simply press Alt-Backspace. Removing annotations is much harder than adding them. This is defendable considering the amount of work involved in positioning these boundaries. 48