Acoustic / Lexical Model Derk Geene. Speech recognition P(words|signal)= P(signal|words) P(words) /...
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Transcript of Acoustic / Lexical Model Derk Geene. Speech recognition P(words|signal)= P(signal|words) P(words) /...
Speech recognition P(words|signal)=
P(signal|words) P(words) / P(signal)
P(signal|words): Acoustic model P(words): Language model
Idea: Maximize P(signal|words) P(words) Today: Acoustic model
Variability Variation
Speaker Pronunciation Environmental Context
Static acoustic model will not work in real applications.
Dynamically adapt P(signal|words) while using the system.
Measuring errors (1) 500 sentences of 6 – 10 words each from 5
to 10 different speakers. 10% relative error reduction
Training set / Development set
First decide optimal parameter settings.
Measuring errors (2) Word recognition errors:
Substitution Deletion Insertion
Correct: Did mob mission area of the Copeland ever go to m4 in nineteen eighty one?
Recognized: Did mob mission area ** the copy land ever go to m4 in nineteen east one?
Measuring errors (3)Correct: The effect is clearRecognised: Effect is not clear
Error RateOne by one: 75%
Subs + Dels + Ins#words in correct sentence
Word error rate=100% x
Word error rate
Units of speech (1) Modeling is language dependent.fixme
Modeling unit Accurate Trainable Generalizable
Units of speech (2) Whole-word models
Only suitable for small vocabulary recognition
Phone models Suitable for large vocabulary recognition Problem: over-generalize less accurate
Syllable models
Context dependency (1) Recognition accuricy can be improved by
using context-dependent parameters.
Important in fast / spontanious speech.
Example: the phoneme /ee/
Context dependency (2) Triphone model: phonetic model that takes into
consideration both the left and the right neightbouring phones.
If two phones have the same identity, but different left or right contexts, there are considered different triphones.
Interword context-dependent phones. Place in the word:
Beginning Middle End
Context dependency (3) Stress
Longer duration Higher pitch More intensity
Word-level stress Import – Import Italy – Italian
Sentence-level stress I did have dinner. I did have dinner.
Context dependency (4) Vary much triphones.
503 = 125.000 Many phonemes have the same effects
/b/ & /p/ labial (pronounces by using lips) /r/ & /w/ liquids
Clustered acoustic-phonetic unitsIs the left-context phone a fricative?Is the right-context phone a front vowel?
Acoustic model After feature extraction, we have a
sequence of feature vectors, such as the MFCC vector, as input data.
Feature stream
Phonemes / units
Words
Segmentation and labeling
Lexical access problem
Acoustic model Signal Phonemes
Problem: phonemes can be pronounced differently Speaker differences Speaker rate Microphone
Acoustic model Phonemes Words
The three major ways to do this: Vector Quantization Hidden Markov Models Neural Networks
Acoustic model Problem: Multiple pronunciations:
owt
aa
eyt ow
t
ow
ax
m
aa
ey
t ow
0,5
0,5
0,8
m
Dialect variation
Coarticulation
0,5
0,5
0,2