CS 4705 Automatic Speech Recognition

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What is speech recognition? Transcribing words? Understanding meaning? Today: –Overview ASR issues –Building an ASR system –Using an ASR system –Future research

“It’s hard to... recognize speech/wreck a nice beach” Speaker variability: within and across Recording environment varies wrt noise Transcription task must handle all of this and produce a transcript of what was said, from limited, noisy information in the speech signal –Success: low word error rate (WER) WER = (S+I+D)/N * 100 –Thesis test vs. This is a test. 75% WER Understanding task must do more: from words to meaning

–Measure concept accuracy (CA) of string in terms of accuracy of recognition of domain concepts mentioned in string and their values I want to go from Boston to Baltimore on September 29 – Domain conceptsValues –source cityBoston –target cityBaltimore –travel dateSeptember 29 –Score recognized string “Go from Boston to Washington on December 29” (1/3 = 33% CA) –“Go to Boston from Baltimore on September 29”

Again, the Noisy Channel Model Input to channel: spoken sentence s – Output from channel: an observation O –Decoding task: find s’ = P(s|O) –Using Bayes Rule –And since P(O) doesn’t change for any hypothetical s’ –s’ = P(O|s) P(s) –P(O|s) is the observation likelihood, or Acoustic Model, and P(s) is the prior, or Language Model Source Noisy Channel Decoder

What do we need to build use an ASR system?ASR system Corpora for training and testing of components Feature extraction component Pronunciation Model Acoustic Model Language Model Algorithms to search hypothesis space efficiently

Training and Test Corpora Collect corpora appropriate for recognition task at hand –Small speech + phonetic transcription to associate sounds with symbols (Acoustic Model) –Large (>= 60 hrs) speech + orthographic transcription to associate words with sounds (Acoustic Model) –Very large text corpus to identify unigram and bigram probabilities (Language Model)

Representing the Signal What parameters (features) of the speech input –Can be extracted automatically –Will preserve phonetic identity and distinguish it from other phones –Will be independent of speaker variability and channel conditions –Will not take up too much space Speech representations (for [ae] in had): –Waveform: change in sound pressure over timeWaveform –LPC Spectrum: component frequencies of a waveformLPC Spectrum –Spectrogram: overall view of how frequencies change from phone to phoneSpectrogram

Speech captured by microphone and sampled (digitized) -- may not capture all vital information Signal divided into frames Power spectrum computed to represent energy in different bands of the signal –LPC spectrum, Cepstra, PLP –Each frame’s spectral features represented by small set of numbers Frames clustered into ‘phone-like’ groups (phones in context) -- Gaussian or other models

Why this works? –Different phonemes have different spectral characteristics Why it doesn’t work? –Phonemes can have different properties in different acoustic contexts, spoken by different people … –Nice white rice

Pronunciation Model Models likelihood of word given network of candidate phone hypotheses (weighted phone lattice) Allophones: butter vs. but Multiple pronunciations for each word Lexicon may be weighted automaton or simple dictionaryweighted automaton Words come from all corpora; pronunciations from pronouncing dictionary or TTS system

Acoustic Models Model likelihood of phones or subphones given spectral features and prior context Use pronunciation models Usually represented as HMMHMM –Set of states representing phones or other subword units –Transition probabilities on states: how likely is it to see one phone after seeing another? –Observation/output likelihoods: how likely is spectral feature vector to be observed from phone state i, given phone state i-1?

Initial estimates for Transition probabilities between phone states Observation probabilities associating phone states with acoustic examples Re-estimate both probabilities by feeding the HMM the transcribed speech training corpus (forced alignment) I.e., we tell the HMM the ‘right’ answers -- which words to associate with which sequences of sounds Iteratively retrain the transition and observation probabilities by running the training data through the model and scoring output until no improvement

Language Model Models likelihood of word given prior word and of entire sentence Ngram models: –Build the LM by calculating bigram or trigram probabilities from text training corpusLM –Smoothing issues very important for real systems Grammars –Finite state grammar or Context Free Grammar (CFG) or semantic grammar Out of Vocabulary (OOV) problem

Entropy H(X): the amount of information in a LM, grammar –How many bits will it take on average to encode a choice or a piece of information? –More likely things will take fewer bits to encode Perplexity 2 H : a measure of the weighted mean number of choice points in e.g. a language model

Search/Decoding Find the best hypothesis P(O|s) P(s) given –Lattice of subword units (Acoustic Model) –Segmentation of all paths into possible words (Pronunciation Model) –Probabilities of word sequences (Language Model) Produces a huge search space: How to reduce? –Lattice minimization and determinization –Forward algorithm: sum of all paths leading to a state –Viterbi algorithm: max of all paths leading to a state

–Forward-backward (Baum-Welch, Expectation- Maximization) algorithm: computes probability of sequence at any state in search space –Beam search: prune the lattice

Varieties of Speech Recognition Telephone, microphonesInput device > 30dB (high)  < 10dB (low) SNR 100 Perplexity Finite state, ngrams, CFGs, CSGsLanguage Model 60K -->~1M Vocabulary size Speaker-dependent or independentEnrollment Read, prepared, spontaneousStyle Isolated words  continuous Mode

Challenges for Transcription Robustness to channel characteristics and noise Portability to new applications Adapatation: to speakers, to environments LMs: simple ngrams need help Confidence measures OOV words New speaking styles/genres New applications

Challenges for Understanding Recognizing communicative ‘problems’ –ASR errors –User corrections –Disfluencies and self-repairsDisfluencies and self-repairs Possibilities: –Recognizing speaker emotion –Identifying speech acts: okay –Locating topic boundaries for topic tracking, audio browsing, speech data mining

An Unsuccessful Dialogue S: Hi, this is AT&T Amtrak schedule system. This is TOOT. How may I help you? U: I'd like to find a train from Ch- to Chicago from Baltimore. S: Sorry, I can't understand you. Please repeat your utterance. U: I'd like to find a train from Baltimore to Chicago. S: I heard you say “leave today around 5 thirty in the morning.“ Which city do you want to go to? U: I would like to leave Saturday at eight in the morning.

S: You can say the arrival city name, such as “New York City." U: Baltimore to Chicago. S: You can say the arrival city name, such as “New York City.” U: Baltimore to Chicago. (I hate it.) U: Train from Baltimore...S:...I heard you s...U: to Chicago.... Hello? S: You can say the name of your departure city, such as "New York City." U: Baltimore.

U: Bal-ti-more.... S: You can say... U: Baltimore. S: Do you want to continue and find a new set of trains? U: Yes. S: You can say "yes" or "I am done here" to find a new set of trains. U: Yes. S: Thank you for using AT&T Amtrak train time table system. See you next time. U: I nev-

Summary ASR technology relies upon a large number of phenomena and techniques we’ve already seen to convert sound into words –Phonetic/phonological, morphological, and lexical events –FSA’s, Ngrams, Dynamic programming algorithms Better modeling of linguistic phenomena will be needed to improve performance on transcription and especially on understanding For next class: we’ll start talking about larger structures in language above the word (Ch 8)

Disfluencies and Self-Repairs Disfluencies abound in spontaneous speech –every 4.6s in radio call-in (Blackmer & Mitton ‘91) hesitation: Ch- change strategy. filled pause: Um Baltimore. self-repair: Ba- uh Chicago. Hard to recognize Ch- change strategy. --> to D C D C today ten fifteen. Um Baltimore. --> From Baltimore ten. Ba- uh Chicago. --> For Boston Chicago.