Download LSA.303 Introduction to Computational Linguistics

LSA 352:
Speech Recognition and Synthesis
Dan Jurafsky
Lecture 1:
1) Overview of Course
2) Refresher: Intro to Probability
3) Language Modeling
IP notice: some slides for today from: Josh Goodman, Dan Klein, Bonnie Dorr, Julia Hirschberg,
Sandiway Fong
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Outline
Overview of Course
Probability
Language Modeling
Language Modeling means “probabilistic grammar”
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Definitions
Speech Recognition
Speech-to-Text
– Input: a wavefile,
– Output: string of words
Speech Synthesis
Text-to-Speech
– Input: a string of words
– Output: a wavefile
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Automatic Speech Recognition (ASR)
Automatic Speech Understanding (ASU)
Applications
Dictation
Telephone-based Information (directions, air
travel, banking, etc)
Hands-free (in car)
Second language ('L2') (accent reduction)
Audio archive searching
Linguistic research
– Automatically computing word durations, etc
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Applications of Speech
Synthesis/Text-to-Speech (TTS)
Games
Telephone-based Information (directions, air travel,
banking, etc)
Eyes-free (in car)
Reading/speaking for disabled
Education: Reading tutors
Education: L2 learning
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Applications of Speaker/Lg
Recognition
Language recognition for call routing
Speaker Recognition:
Speaker verification (binary decision)
– Voice password, telephone assistant
Speaker identification (one of N)
– Criminal investigation
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History: foundational insights
1900s-1950s
Automaton:
Markov 1911
Turing 1936
McCulloch-Pitts neuron (1943)
– http://marr.bsee.swin.edu.au/~dtl/het704/lecture10/ann/node
1.html
– http://diwww.epfl.ch/mantra/tutorial/english/mcpits/html/
Shannon (1948) link between automata and Markov models
Human speech processing
Fletcher at Bell Labs (1920’s)
Probabilistic/Information-theoretic models
Shannon (1948)
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Synthesis precursors
Von Kempelen mechanical (bellows, reeds) speech
production simulacrum
1929 Channel vocoder (Dudley)
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History: Early Recognition
• 1920’s Radio Rex
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Celluloid dog with iron base
held within house by
electromagnet against force of
spring
Current to magnet flowed
through bridge which was
sensitive to energy at 500 Hz
500 Hz energy caused bridge to
vibrate, interrupting current,
making dog spring forward
The sound “e” (ARPAbet [eh])
in Rex has 500 Hz component
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History: early ASR systems
• 1950’s: Early Speech recognizers
1952: Bell Labs single-speaker digit recognizer
– Measured energy from two bands (formants)
– Built with analog electrical components
– 2% error rate for single speaker, isolated digits
1958: Dudley built classifier that used continuous spectrum
rather than just formants
1959: Denes ASR combining grammar and acoustic
probability
1960’s
FFT - Fast Fourier transform (Cooley and Tukey 1965)
LPC - linear prediction (1968)
1969 John Pierce letter “Whither Speech Recognition?”
– Random tuning of parameters,
– Lack of scientific rigor, no evaluation metrics
– Need to rely on higher level knowledge
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ASR: 1970’s and 1980’s
Hidden Markov Model 1972
Independent application of Baker (CMU) and Jelinek/Bahl/Mercer
lab (IBM) following work of Baum and colleagues at IDA
ARPA project 1971-1976
5-year speech understanding project: 1000 word vocab, continous
speech, multi-speaker
SDC, CMU, BBN
Only 1 CMU system achieved goal
1980’s+
Annual ARPA “Bakeoffs”
Large corpus collection
– TIMIT
– Resource Management
– Wall Street Journal
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State of the Art
ASR
speaker-independent, continuous, no noise, world’s
best research systems:
– Human-human speech: ~13-20% Word Error Rate
(WER)
– Human-machine speech: ~3-5% WER
TTS (demo next week)
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LVCSR Overview
Large Vocabulary Continuous (Speaker-Independent)
Speech Recognition
Build a statistical model of the speech-to-words
process
Collect lots of speech and transcribe all the words
Train the model on the labeled speech
Paradigm: Supervised Machine Learning + Search
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Unit Selection TTS Overview
Collect lots of speech (5-50 hours) from one speaker,
transcribe very carefully, all the syllables and phones
and whatnot
To synthesize a sentence, patch together syllables
and phones from the training data.
Paradigm: search
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Requirements and Grading
Readings:
Required Text:
Selected chapters on web from
– Jurafsky & Martin, 2000. Speech and Language Processing.
– Taylor, Paul. 2007. Text-to-Speech Synthesis.
Grading
Homework: 75% (3 homeworks, 25% each)
Participation: 25%
You may work in groups
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Overview of the course
http://nlp.stanford.edu/courses/lsa352/
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6. Introduction to Probability
Experiment (trial)
Repeatable procedure with well-defined possible outcomes
Sample Space (S)
– the set of all possible outcomes
– finite or infinite
Example
– coin toss experiment
– possible outcomes: S = {heads, tails}
Example
– die toss experiment
– possible outcomes: S = {1,2,3,4,5,6}
Quic kTime™ and a
TIFF (Unc ompres sed) dec ompres sor
are needed to see this pic ture.
Quic kTime™ and a
TIFF (Unc ompres sed) dec ompres sor
are needed to see this pic ture.
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Slides from Sandiway Fong
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Introduction to Probability
Definition of sample space depends on what we are asking
Sample Space (S): the set of all possible outcomes
Example
– die toss experiment for whether the number is even or odd
– possible outcomes: {even,odd}
– not {1,2,3,4,5,6}
Quic kTime™ and a
TIFF (Unc ompres sed) dec ompres sor
are needed to see this pic ture.
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More definitions
Events
an event is any subset of outcomes from the sample space
Example
die toss experiment
let A represent the event such that the outcome of the die toss
experiment is divisible by 3
A = {3,6}
A is a subset of the sample space S= {1,2,3,4,5,6}
Example
Draw a card from a deck
– suppose sample space S = {heart,spade,club,diamond} (four
suits)
let A represent the event of drawing a heart
let B represent the event of drawing a red card
A = {heart}
B = {heart,diamond}
Quic kT i me™ and a
T IFF (Unc ompres s ed) dec ompres s or
are needed t o s ee thi s pi c ture.
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Introduction to Probability
Some definitions
Counting
– suppose operation oi can be performed in ni ways, then
– a sequence of k operations o1o2...ok
– can be performed in n1  n2  ...  nk ways
Example
– die toss experiment, 6 possible outcomes
– two dice are thrown at the same time
– number of sample points in sample space = 6  6 = 36
Quic kTime™ and a
TIFF (Unc ompres sed) dec ompres sor
are needed to see this pic ture.
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Definition of Probability
The probability law assigns to an event a nonnegative
number
Called P(A)
Also called the probability A
That encodes our knowledge or belief about the
collective likelihood of all the elements of A
Probability law must satisfy certain properties
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Probability Axioms
Nonnegativity
P(A) >= 0, for every event A
Additivity
If A and B are two disjoint events, then the
probability of their union satisfies:
P(A U B) = P(A) + P(B)
Normalization
The probability of the entire sample space S is equal
to 1, I.e. P(S) = 1.
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An example
An experiment involving a single coin toss
There are two possible outcomes, H and T
Sample space S is {H,T}
If coin is fair, should assign equal probabilities to 2 outcomes
Since they have to sum to 1
P({H}) = 0.5
P({T}) = 0.5
P({H,T}) = P({H})+P({T}) = 1.0
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Another example
Experiment involving 3 coin tosses
Outcome is a 3-long string of H or T
S ={HHH,HHT,HTH,HTT,THH,THT,TTH,TTTT}
Assume each outcome is equiprobable
“Uniform distribution”
What is probability of the event that exactly 2 heads occur?
A = {HHT,HTH,THH}
P(A) = P({HHT})+P({HTH})+P({THH})
= 1/8 + 1/8 + 1/8
=3/8
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Probability definitions
In summary:
Probability of drawing a spade from 52 well-shuffled playing
cards:
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Probabilities of two events
If two events A and B are independent
Then
P(A and B) = P(A) x P(B)
If flip a fair coin twice
What is the probability that they are both heads?
If draw a card from a deck, then put it back, draw a card from
the deck again
What is the probability that both drawn cards are hearts?
A coin is flipped twice
What is the probability that it comes up heads both times?
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How about non-uniform
probabilities? An example
A biased coin,
twice as likely to come up tails as heads,
is tossed twice
What is the probability that at least one head occurs?
Sample space = {hh, ht, th, tt} (h = heads, t = tails)
Sample points/probability for the event:
ht 1/3 x 2/3 = 2/9
th 2/3 x 1/3 = 2/9
hh 1/3 x 1/3= 1/9
tt 2/3 x 2/3 = 4/9
Answer: 5/9 = 0.56 (sum of weights in red)
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Moving toward language
What’s the probability of drawing a 2 from a
deck of 52 cards with four 2s?
4
1
P(drawing a two) 
  .077
52 13
What’s the probability of a random word (from
a random dictionary page) being a verb?

# of ways to get a verb
P(drawing a verb) 
all words
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Probability and part of speech tags
• What’s the probability of a random word (from a
random dictionary page) being a verb?
P(drawing a verb) 
# of ways to get a verb
all words
• How to compute each of these
• All words = just count all the words in the dictionary

• # of ways to get a verb: number of words which are
verbs!
• If a dictionary has 50,000 entries, and 10,000 are
verbs…. P(V) is 10000/50000 = 1/5 = .20
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Conditional Probability
A way to reason about the outcome of an experiment
based on partial information
In a word guessing game the first letter for the word
is a “t”. What is the likelihood that the second letter
is an “h”?
How likely is it that a person has a disease given that
a medical test was negative?
A spot shows up on a radar screen. How likely is it
that it corresponds to an aircraft?
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More precisely
Given an experiment, a corresponding sample space S, and a
probability law
Suppose we know that the outcome is within some given event B
We want to quantify the likelihood that the outcome also belongs
to some other given event A.
We need a new probability law that gives us the conditional
probability of A given B
P(A|B)
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An intuition
• A is “it’s raining now”.
• P(A) in dry California is .01
• B is “it was raining ten minutes ago”
• P(A|B) means “what is the probability of it raining now if it was
raining 10 minutes ago”
• P(A|B) is probably way higher than P(A)
• Perhaps P(A|B) is .10
• Intuition: The knowledge about B should change our estimate of
the probability of A.
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Conditional probability
One of the following 30 items is chosen at random
What is P(X), the probability that it is an X?
What is P(X|red), the probability that it is an X given
that it is red?
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Conditional Probability
let A and B be events
p(B|A) = the probability of event B occurring given event A occurs
definition: p(B|A) = p(A  B) / p(A)
S
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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Conditional probability
P(A|B) = P(A  B)/P(B)
Or
P( A | B) 
P( A, B)
P( B)
Note: P(A,B)=P(A|B) · P(B)
Also: P(A,B) = P(B,A)
A
A,B B
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Independence
What is P(A,B) if A and B are independent?
P(A,B)=P(A) · P(B) iff A,B independent.
P(heads,tails) = P(heads) · P(tails) = .5 · .5 = .25
Note: P(A|B)=P(A) iff A,B independent
Also: P(B|A)=P(B) iff A,B independent
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Bayes Theorem
P( A | B) P( B)
P( B | A) 
P( A)
• Swap the conditioning
• Sometimes easier to estimate one
kind of dependence than the other
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Deriving Bayes Rule
P(A  B) P(B | A)  P(A  B)
P(A | B) 
P(A)
P(B)
P(A | B)P(B)  P(A  B) P(B | A)P(A)  P(A  B)

P(A | B)P(B)  P(B | A)P(A)


P(B | A)P(A)
P(A | B) 
P(B)
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Summary
Probability
Conditional Probability
Independence
Bayes Rule
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How many words?
I do uh main- mainly business data processing
Fragments
Filled pauses
Are cat and cats the same word?
Some terminology
Lemma: a set of lexical forms having the same
stem, major part of speech, and rough word sense
– Cat and cats = same lemma
Wordform: the full inflected surface form.
– Cat and cats = different wordforms
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How many words?
they picnicked by the pool then lay back on the grass and looked at the
stars
16 tokens
14 types
SWBD:
~20,000 wordform types,
2.4 million wordform tokens
Brown et al (1992) large corpus
583 million wordform tokens
293,181 wordform types
Let N = number of tokens, V = vocabulary = number of types
General wisdom: V > O(sqrt(N))
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Language Modeling
We want to compute P(w1,w2,w3,w4,w5…wn), the
probability of a sequence
Alternatively we want to compute
P(w5|w1,w2,w3,w4,w5): the probability of a word
given some previous words
The model that computes P(W) or P(wn|w1,w2…wn1) is called the language model.
A better term for this would be “The Grammar”
But “Language model” or LM is standard
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Computing P(W)
How to compute this joint probability:
P(“the”,”other”,”day”,”I”,”was”,”walking”,”along”,”and”,”
saw”,”a”,”lizard”)
Intuition: let’s rely on the Chain Rule of Probability
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The Chain Rule of Probability
Recall the definition of conditional probabilities
Rewriting:
P( A^ B)
P( A | B) 
P( B)
P( A^ B)  P( A | B) P( B)
More generally
P(A,B,C,D) = P(A)P(B|A)P(C|A,B)P(D|A,B,C)
In general
P(x1,x2,x3,…xn) = P(x1)P(x2|x1)P(x3|x1,x2)…P(xn|x1…xn-1)
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The Chain Rule Applied to joint
probability of words in sentence
P(“the big red dog was”)=
P(the)*P(big|the)*P(red|the big)*P(dog|the big red)*P(was|the
big red dog)
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Very easy estimate:
How to estimate?
P(the|its water is so transparent that)
P(the|its water is so transparent that)
=
C(its water is so transparent that the)
_______________________________
C(its water is so transparent that)
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Unfortunately
There are a lot of possible sentences
We’ll never be able to get enough data to compute
the statistics for those long prefixes
P(lizard|the,other,day,I,was,walking,along,and,saw,a)
Or
P(the|its water is so transparent that)
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Markov Assumption
Make the simplifying assumption
P(lizard|the,other,day,I,was,walking,along,and,saw,a)
= P(lizard|a)
Or maybe
P(lizard|the,other,day,I,was,walking,along,and,saw,a)
= P(lizard|saw,a)
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Markov Assumption
So for each component in the product replace with
the approximation (assuming a prefix of N)
n1
1
P(wn | w
)  P(wn | w
n1
nN 1
)
Bigram version
n1
1
P(w n | w
)  P(w n | w n1 )
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Estimating bigram probabilities
The Maximum Likelihood Estimate
count(wi1,wi )
P(wi | wi1) 
count(wi1 )
c(wi1,wi )
P(wi | wi1) 
c(wi1)
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An example
<s> I am Sam </s>
<s> Sam I am </s>
<s> I do not like green eggs and ham </s>
This is the Maximum Likelihood Estimate, because it is the one
which maximizes P(Training set|Model)
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Maximum Likelihood Estimates
The maximum likelihood estimate of some parameter of a model
M from a training set T
Is the estimate
that maximizes the likelihood of the training set T given the
model M
Suppose the word Chinese occurs 400 times in a corpus of a
million words (Brown corpus)
What is the probability that a random word from some other text
will be “Chinese”
MLE estimate is 400/1000000 = .004
This may be a bad estimate for some other corpus
But it is the estimate that makes it most likely that “Chinese”
will occur 400 times in a million word corpus.
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More examples: Berkeley
Restaurant Project sentences
can you tell me about any good cantonese
restaurants close by
mid priced thai food is what i’m looking for
tell me about chez panisse
can you give me a listing of the kinds of food that are
available
i’m looking for a good place to eat breakfast
when is caffe venezia open during the day
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Raw bigram counts
Out of 9222 sentences
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Raw bigram probabilities
Normalize by unigrams:
Result:
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Bigram estimates of sentence
probabilities
P(<s> I want english food </s>) =
p(i|<s>) x p(want|I) x p(english|want)
x p(food|english) x p(</s>|food)
= .24 x .33 x .0011 x 0.5 x 0.68
=.000031
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What kinds of knowledge?
P(english|want) = .0011
P(chinese|want) = .0065
P(to|want) = .66
P(eat | to) = .28
P(food | to) = 0
P(want | spend) = 0
P (i | <s>) = .25
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The Shannon Visualization
Method
Generate random sentences:
Choose a random bigram <s>, w according to its probability
Now choose a random bigram (w, x) according to its probability
And so on until we choose </s>
Then string the words together
<s> I
I want
want to
to eat
eat Chinese
Chinese food
food </s>
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Shakespeare as corpus
N=884,647 tokens, V=29,066
Shakespeare produced 300,000 bigram types out of
V2= 844 million possible bigrams: so, 99.96% of the
possible bigrams were never seen (have zero entries
in the table)
Quadrigrams worse: What's coming out looks like
Shakespeare because it is Shakespeare
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The wall street journal is not
shakespeare (no offense)
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Evaluation
We train parameters of our model on a training set.
How do we evaluate how well our model works?
We look at the models performance on some new data
This is what happens in the real world; we want to
know how our model performs on data we haven’t
seen
So a test set. A dataset which is different than our
training set
Then we need an evaluation metric to tell us how
well our model is doing on the test set.
One such metric is perplexity (to be introduced
below)
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Unknown words: Open versus
closed vocabulary tasks
If we know all the words in advanced
Vocabulary V is fixed
Closed vocabulary task
Often we don’t know this
Out Of Vocabulary = OOV words
Open vocabulary task
Instead: create an unknown word token <UNK>
Training of <UNK> probabilities
– Create a fixed lexicon L of size V
– At text normalization phase, any training word not in L changed to
<UNK>
– Now we train its probabilities like a normal word
At decoding time
– If text input: Use UNK probabilities for any word not in training
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Evaluating N-gram models
Best evaluation for an N-gram
Put model A in a speech recognizer
Run recognition, get word error rate (WER)
for A
Put model B in speech recognition, get word
error rate for B
Compare WER for A and B
In-vivo evaluation
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Difficulty of in-vivo evaluation of
N-gram models
In-vivo evaluation
This is really time-consuming
Can take days to run an experiment
So
As a temporary solution, in order to run experiments
To evaluate N-grams we often use an approximation
called perplexity
But perplexity is a poor approximation unless the test
data looks just like the training data
So is generally only useful in pilot experiments
(generally is not sufficient to publish)
But is helpful to think about.
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Perplexity
Perplexity is the probability of the test set
(assigned by the language model),
normalized by the number of words:
Chain rule:
For bigrams:
Minimizing perplexity is the same as maximizing probability
The best language model is one that best predicts an
unseen test set
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A totally different perplexity
Intuition
How hard is the task of recognizing digits ‘0,1,2,3,4,5,6,7,8,9,oh’:
easy, perplexity 11 (or if we ignore ‘oh’, perplexity 10)
How hard is recognizing (30,000) names at Microsoft. Hard:
perplexity = 30,000
If a system has to recognize
Operator (1 in 4)
Sales (1 in 4)
Technical Support (1 in 4)
30,000 names (1 in 120,000 each)
Perplexity is 54
Perplexity is weighted equivalent branching factor
Slide from Josh Goodman
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Perplexity as branching factor
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Lower perplexity = better model
Training 38 million words, test 1.5 million words, WSJ
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Lesson 1: the perils of overfitting
N-grams only work well for word prediction if the test
corpus looks like the training corpus
In real life, it often doesn’t
We need to train robust models, adapt to test set, etc
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Lesson 2: zeros or not?
Zipf’s Law:
A small number of events occur with high frequency
A large number of events occur with low frequency
You can quickly collect statistics on the high frequency events
You might have to wait an arbitrarily long time to get valid
statistics on low frequency events
Result:
Our estimates are sparse! no counts at all for the vast bulk of
things we want to estimate!
Some of the zeroes in the table are really zeros But others are
simply low frequency events you haven't seen yet. After all,
ANYTHING CAN HAPPEN!
How to address?
Answer:
Estimate the likelihood of unseen N-grams!
Slide adapted from Bonnie Dorr and Julia Hirschberg
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Smoothing is like Robin Hood:
Steal from the rich and give to the poor (in
probability mass)
Slide from Dan Klein
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Laplace smoothing
Also called add-one smoothing
Just add one to all the counts!
Very simple
MLE estimate:
Laplace estimate:
Reconstructed counts:
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Laplace smoothed bigram counts
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Laplace-smoothed bigrams
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Reconstituted counts
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Note big change to counts
C(count to) went from 608 to 238!
P(to|want) from .66 to .26!
Discount d= c*/c
d for “chinese food” =.10!!! A 10x reduction
So in general, Laplace is a blunt instrument
Could use more fine-grained method (add-k)
But Laplace smoothing not used for N-grams, as we have much
better methods
Despite its flaws Laplace (add-k) is however still used to smooth
other probabilistic models in NLP, especially
For pilot studies
in domains where the number of zeros isn’t so huge.
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Better discounting algorithms
Intuition used by many smoothing algorithms
Good-Turing
Kneser-Ney
Witten-Bell
Is to use the count of things we’ve seen once to help
estimate the count of things we’ve never seen
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Good-Turing: Josh Goodman
intuition
Imagine you are fishing
There are 8 species: carp, perch, whitefish, trout,
salmon, eel, catfish, bass
You have caught
10 carp, 3 perch, 2 whitefish, 1 trout, 1 salmon, 1 eel
= 18 fish
How likely is it that next species is new (i.e. catfish or
bass)
3/18
Assuming so, how likely is it that next species is
trout?
Must be less than 1/18
Slide adapted from Josh Goodman
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Good-Turing Intuition
Notation: Nx is the frequency-of-frequency-x
So N10=1, N1=3, etc
To estimate total number of unseen species
Use number of species (words) we’ve seen once
c0* =c1
p0 = N1/N
All other estimates are adjusted (down) to give
probabilities for unseen
Slide from Josh Goodman
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Good-Turing Intuition
Notation: Nx is the frequency-of-frequency-x
So N10=1, N1=3, etc
To estimate total number of unseen species
Use number of species (words) we’ve seen once
c0* =c1
p0 = N1/N p0=N1/N=3/18
All other estimates are adjusted (down) to give
probabilities for unseen
P(eel) = c*(1) = (1+1) 1/ 3 = 2/3
Slide from Josh Goodman
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Bigram frequencies of
frequencies and GT re-estimates
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Complications
In practice, assume large counts (c>k for some k) are reliable:
That complicates c*, making it:
Also: we assume singleton counts c=1 are unreliable, so treat N-grams
with count of 1 as if they were count=0
Also, need the Nk to be non-zero, so we need to smooth (interpolate)
the Nk counts before computing c* from them
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Backoff and Interpolation
Another really useful source of knowledge
If we are estimating:
trigram p(z|xy)
but c(xyz) is zero
Use info from:
Bigram p(z|y)
Or even:
Unigram p(z)
How to combine the trigram/bigram/unigram info?
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Backoff versus interpolation
Backoff: use trigram if you have it, otherwise
bigram, otherwise unigram
Interpolation: mix all three
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Interpolation
Simple interpolation
Lambdas conditional on context:
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How to set the lambdas?
Use a held-out corpus
Choose lambdas which maximize the probability of
some held-out data
I.e. fix the N-gram probabilities
Then search for lambda values
That when plugged into previous equation
Give largest probability for held-out set
Can use EM to do this search
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Katz Backoff
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Why discounts P* and alpha?
MLE probabilities sum to 1
So if we used MLE probabilities but backed off to
lower order model when MLE prob is zero
We would be adding extra probability mass
And total probability would be greater than 1
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GT smoothed bigram probs
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Intuition of backoff+discounting
How much probability to assign to all the zero
trigrams?
Use GT or other discounting algorithm to tell us
How to divide that probability mass among different
contexts?
Use the N-1 gram estimates to tell us
What do we do for the unigram words not seen in
training?
Out Of Vocabulary = OOV words
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OOV words: <UNK> word
Out Of Vocabulary = OOV words
We don’t use GT smoothing for these
Because GT assumes we know the number of unseen events
Instead: create an unknown word token <UNK>
Training of <UNK> probabilities
– Create a fixed lexicon L of size V
– At text normalization phase, any training word not in L changed to
<UNK>
– Now we train its probabilities like a normal word
At decoding time
– If text input: Use UNK probabilities for any word not in training
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Practical Issues
We do everything in log space
Avoid underflow
(also adding is faster than multiplying)
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ARPA format
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Language Modeling Toolkits
SRILM
CMU-Cambridge LM Toolkit
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Google N-Gram Release
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Google N-Gram Release
serve
serve
serve
serve
serve
serve
serve
serve
serve
serve
as
as
as
as
as
as
as
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incoming 92
incubator 99
independent 794
index 223
indication 72
indicator 120
indicators 45
indispensable 111
indispensible 40
individual 234
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Advanced LM stuff
Current best smoothing algorithm
Kneser-Ney smoothing
Other stuff
Variable-length n-grams
Class-based n-grams
– Clustering
– Hand-built classes
Cache LMs
Topic-based LMs
Sentence mixture models
Skipping LMs
Parser-based LMs
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Summary
LM
N-grams
Discounting: Good-Turing
Katz backoff with Good-Turing discounting
Interpolation
Unknown words
Evaluation:
– Entropy, Entropy Rate, Cross Entropy
– Perplexity
Advanced LM algorithms
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