1 CS 551/651: Structure of Spoken Language Lecture 6: Phonological Processes John-Paul Hosom Fall 2008

2 Phonological Processes Phonemes undergo systematic variation depending on their context For example, forming the past tense: cause/k aa z/  caused/k aa z d/ talk/t aa k/  talked/t aa k t/ /d/ vs. /t/ is predictable based on voicing of word-final phoneme Allophones can be viewed as systematic variations of phonemes that are a result of cultural and physiological processes, but do not distinguish meaning of utterance For example, /p/ and /p h / in English is predictable: word or syllable initial voiceless stops are aspirated pit  [p h ih t [h] ]tip  [t h ih p [h] ]kin  [k h ih n] spit  [s p ih t [h] ]stick  [s t ih k [h] ]skin  [s k ih n]

3 Phonological Processes /p h ih t h t h ih p h k h ih n/ /s p ih t h s t ih k h s k ih n/

4 Phonological Processes Other types of phonetic processes: Assimilation, Deletion, Reduction, Insertion, Substitution, Me'tathesis (switching order of two phonemes) Assimilation “A feature of one segment is shared by a neighboring segment” Examples of Assimilation  Nasalization of vowels before nasal consonants  in- (negative prefix) becomes im- in words beginning with bilabial consonant (imbalance, imperfect, indifferent, intolerance)

5 Phonological Processes Assimilation may be due to coarticulation, or it may be language-specific, “arbitrary”: “word-final alveolar obstruent may take on place of articulation of following word-initial segment if word-initial segment is palato-alveoar” this /dh ih s/shop /sh aa p h /  this shop /dh ih sh sh aa p h / this /dh ih s/fish/f ih sh/  this fish /dh ih s f ih sh/ this /dh ih s/thing /th ih ng/  this thing /dh ih s th ih ng/ also, depending on dialect, not within-word: misshapen /m ih s sh ei p en/

6 Phonological Processes Example of assimilation of /s/ with /sh/ but not /f/: /dh ih sh sh aa pcl p h dh ih s f ih sh/

7 Phonological Processes Substitution: common in foreign accents or speaking impairments: welcome/v eh l k ah m/ McDonald/m a k uw d ow n aa r uw d ow/ Roger/w aa jh er/ Metathesis: changing order of two phonemes within a word (dialect variation) pretty/p er dx iy/ ask/ae k s/

8 Phonological Processes Deletion: Barbara/b aa r b ax r ah/  /b aa r b r ah/ Memory/m eh m ax r iy/  /m eh m r iy/ Reduction: unstressed vowels become /ax/ conduct (verb)/k ax n d ah k t/ conduct (noun)/k aa n d ax k t/ Insertion: voiceless stop inserted between nasal and voiceless consonant; voiceless stop always has same place of articulation as nasal fancy/f ae n t s iy/ Chomsky/ch aa m p s k iy/ schwa inserted after word-final nasal nine/n ay n ax/ dictionary pronunciation=

9 Phonological Processes Deletion: /m eh m r iy/

10 Phonological Processes Insertion: /f ae n t s iy ch aa m p s k iy/

11 Phonological Processes: Ladefoged Rules [–voiced, +stop]  [+aspirated] when syllable initial pit vs. spit [ax]  [–voiced] after syllable-initial [–voiced, +stop] and before [–voiced, +stop] potato [+consonantal]  longer at end of phrase bib, did, don, nod [–voiced, +stop]  [–aspirated] after syllable-initial /s/ spew, stew, skew [+vowel]  shorter before unvoiced phonemes in same syllable cap vs. cab, back vs. bag

12 Phonological Processes: Ladefoged Rules Devoicing, End-of-Phrase Length: /p h ax tcl t h ey dx ow/ /d aa n n aa dcl d/

13 Phonological Processes: Ladefoged Rules Length before Voiceless: /k h ae pc p h k h ae bc b b ae kc k h b ae gc g/

14 Phonological Processes: Ladefoged Rules [–voiced]  longer when at end of syllable sass, shook vs. push [+stop]  unreleased before [+stop] apt, act (often see some mark in spectrogram) [–voiced, +alveolar, +stop]  [+glottal stop] when before an alveolar nasal in same word beaten  /b iy q en/ [+nasal]  [+syllabic] at word end when following [+obstruent] chasm  /k ae z em/ NOT film (obstruent = complete closure of airway; /l/ is not) [+liquid]  [+syllabic] at word end and following [+consonant] paddle, whistle, kennel NOT snarl unless classify /r/ as [+vowel, –syllabic]

15 Phonological Processes: Ladefoged Rules /ae pcl tcl t h ae kcl tcl t h / /bcl b iy q tcl en ax_h/

16 Phonological Processes: Ladefoged Rules [+alveolar, +stop]  [+voiced, +flap] when between two vowels, second of which is unstressed This rule has speaker-dependent variations [+alveolar, +stop]  omitted between two consonants most people, sandpaper, grand master [+consonant]  shortened before identical [+consonant]   [–voice, +stop] between [+nasal] and [–voice, +fricative] when following vowel absent or unstressed prince vs. prints (e'penthesis)   [&] following word-final [+nasal, +consonantal] nine come sang(e'penthesis)

17 Phonological Processes: Ladefoged Rules “most people and grand masters use sandpaper” /m ow s pc p h iy pc p h el n gc g r ae n m ae s tc t h er z yu z s ae n pc p h ey pc p h er/

18 Phonological Processes: Ladefoged Rules “nine come sang” /n ay n ax kcl kh ah m ax s ae ng ax/

19 Phonological Processes: Ladefoged Rules [+vowel]  longer in open syllables sea vs. seed vs. seat sigh vs. side vs. sight (equalize length of syllables with differing numbers of segments) [+vowel]  longer in stressed syllable below vs. billow (stressed syllables are longer in duration than unstressed) [+vowel]  [+nasal] before [+nasal] consonant [+vowel, –stressed]  schwa (vowel reduction) able vs. ability Canada vs. Canadian photograph vs. photography

20 Phonological Processes: Ladefoged Rules “sigh side sight” /s ay s ay dcl d s a tcl t h /

21 Phonological Processes: Ladefoged Rules “below billow” /b ax l ow b ih l ow/

22 Phonological Processes Why is this useful? (a) Providing models of known phenomenon is better than having classifier learn the phenomenon from data (b)Provides humans with appropriate cues for understanding, naturalness (c)Accurate phonetic modeling improves ability of classifier to discriminate between classes Example for Text-to-Speech (case (b)):  Create a TTS system  Don’t shorten vowels before voiceless plosives  Creates, by default, acoustic cue for voiced plosives  Decrease intelligibility or at least naturalness of system

23 Phonological Processes Example for Automatic Speech Recognition (case (c)):  Train a speech recognizer using “dictionary” pronunciation  Then, in all cases where [–voice, +stop] between [+nasal] and [–voice, +fricative] such as “fancy” (in CMU dictionary as /f ae n s iy/), acoustics show alveolar stop, but trained as either nasal /n/ or fricative /s/.  Decreases ability of model to discriminate classes  Decreases performance of system Difficulty is in providing comprehensive, accurate rules that are not inappropriately “forced” on a system

24 Stops/Plosives There are six plosives (oral stops) in American English:.bilabialalveolarvelar unvoiced | /p/ /t/ /k/ voiced | /b/ /d/ /g/ plus the flap /dx/ which is a very short /t/ or /d/ Plosives can be difficult to identify and discriminate; contextual cues can be varied Cue (1) is the formant transitions of neighboring vowels: for bilabials, F2 drops at CV boundary for alveolars, F2 goes toward 1800 Hz at CV boundary for velars, F2 may meet F3 (velar pinch) or be fairly flat Cue (2) is that voiced plosives may have pre-voicing; more likely when plosive is between two vowels

25 Stops/Plosives Cue (3) is that voiced plosives usually have VOT of < 30 msec, but unvoiced plosives usually have VOT of > 50 msec Cue (4) is that the VOT is shortest for bilabials, longer for alveolars, and longest for velars. (VOT /p/ < /t/ < /k/ and /b/ < /d/ < /g/) Cue (5) is that aspirated (unvoiced) plosives show evidence of F2 and F3 during aspiration; voiced plosives usually don’t Cue (6) is the spectral shape; in theory, the shape of the spectrum at burst release can be used to distinguish plosives: /p/ and /b/ have energy low in frequency or weakly spread throughout spectrum, /t/ and /d/ have more energy above 4KHz (related to alveolar fricatives /s/ and /z/), /k/ and /g/ tend to have more well-defined peaks in the spectrum (near formant locations).

26 Stops/Plosives Other cues related to spectral shape: Cue (7a): In the context of front vowels, /k/ and /g/ have spectral peak just above F2 of adjacent vowel, making them confusable with /t/ and /d/; but front vowels show more “velar pinch” Cue (7b): In the context of back vowels, /k/ and /g/ have one spectral peak between 1000 and 1500 Hz, a second peak between 3000 and 4500 Hz. Cue (8): Velar bursts also sometimes display “double burst”, or a second burst during the frication Cue (9): Post-vocalic consonants are often unreleased; they can be identified by (a) glottalization, (b) sudden drop in vowel energy, or (c) formant movement at end of vowel

27 Stops/Plosives Cue (10): When the plosive is unreleased, the voicing distinction is based more on length of preceding vowel; voiced plosives are associated with longer vowels, unvoiced plosives with shorter vowels Cue (11): In V 1 C 1 C 2 V 2 patterns, where both C are plosives, the existence of two plosives is in the different formant transitions in V 1 and V 2, the longer duration of closure, and sometimes in a brief “click” in spectrum indicating a change in place of articulation Cue (12): Plosives have different characteristics in stressed vs. unstressed environments. VOT for unvoiced plosives before unstressed vowels is shorter than VOT for unvoiced plosives before stressed vowels; plosives in an unstressed-vowel environment are less spectrally clear; in unstressed syllables, /t/ and /d/ may be realized as a flap /dx/.

28 Stops/Plosives Cue (13): Flaps have short duration (< 30 msec), dip in energy levels between two vowels, weak F2 and F3, and F2 tends toward 1800 Hz Cue (14): Consonant clusters can provide restrictions; for 3-consonant clusters (beginning with /s/-plosive), the only valid combinations are: /s p l/, /s p r/, /s p y/ /s t r/, /s t y/ /s k l/, /s k r/, /s k y/, /s k w/ Cue (15): In /s/−plosive−vowel combinations, VOT tends to be shorter and duration of /s/ shorter than normal

29 Plosives: Unvoiced Initial in Front-Vowel Context /p iy t iy k iy/

30 Plosives: Voiced Initial in Front-Vowel Context /b iy d iy g iy/

31 Plosives: Unvoiced Initial in Mid-Vowel Context /p ah t ah k ah/

32 Plosives: Voiced Initial in Mid-Vowel Context /b ah d ah g ah/

33 Plosives: Unvoiced Initial in Back-Vowel Context /p aa t aa k aa/

34 Plosives: Voiced Initial in Back-Vowel Context /b aa d aa g aa/

35 Plosives: Unvoiced Final in Front-Vowel Context /iy p iy t iy k/

36 Plosives: Voiced Final in Front-Vowel Context /iy b iy d iy g/

37 Plosives: Unvoiced Final in Mid-Vowel Context /ah p ah t ah k/

38 Plosives: Voiced Final in Mid-Vowel Context /ah b ah d ah g/

39 Plosives: Unvoiced Final in Back-Vowel Context /aa p aa t aa k/

40 Plosives: Voiced Final in Back-Vowel Context /aa b aa d aa g/