Transcript SPEECH RECOGNITION 1 DAY 14 – SEPT 27, 2013 Brain & Language LING 4110-4890-5110-7960 NSCI 4110-4891-6110 Harry Howard Tulane University.

SPEECH RECOGNITION 1
DAY 14 – SEPT 27, 2013
Brain & Language
LING 4110-4890-5110-7960
NSCI 4110-4891-6110
Harry Howard
Tulane University
9/27/13
Brain & Language, Harry Howard, Tulane University
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Course organization
• The syllabus, these slides and my recordings are
available at http://www.tulane.edu/~howard/LING4110/.
• If you want to learn more about EEG and neurolinguistics,
you are welcome to participate in my lab. This is also a
good way to get started on an honor's thesis.
• The grades are posted to Blackboard.
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REVIEW
Modularity
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Coltheart’s grouping & my explanation
1.
2.
3.
4.
5.
6.
7.
8.
9.
Specific to a domain
Information is
encapsulated
Fixed neural
structure
Matures in a specific
way
Fails in a specific
way
Limits central access
Operates mandatorily
Acts quickly
Analyzes ‘shallowly’
1.
by definition.
2.
by definition.
3.
in order to keep out all the other stuff.
4.
in order to build the fixed structure.
5.
because it was built in a specific way.
6.
in order to keep out other stuff.
7.
since there is no external access, it can’t
be turned on or off.
8.
because there is no other stuff to get in
the way of optimizing speed.
9.
because other stuff is necessary to
analyze deeply.
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SPEECH RECOGNITION
Ingram §5
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Three systems involved in speech
production
Supralaryngeal
Laryngeal
Respiratory
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Vocal folds and their location in the larynx
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Phonation
• Phonation,
or speech sound, is created by
turbulent oscillation between phases in which the
passage of air through the larynx is unconstricted
(the expiratory airflow has pushed the vocal folds
apart) and phases in which the passage of air is
blocked (the vocal folds snap back to their semiclosed position).
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Turbulent oscillation of vocal air
• The following figure depicts such a transition, in which increasing
darkness symbolizes increasing compression of the airflow.
• The heavy line represents the pressure of the airflow through the
vocal folds as a single quantity between a minimum and a maximum.
• as the vocal folds close, the outflow of air is compressed and its pressure
rises;
• as they open, the outflow of air is rarefied and its pressure falls.
• A single cycle of closing and opening is defined by the distance
between two peaks, marked by dotted white lines.
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Graph of turbulent oscillation of vocal air
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An example: "phonetician"
f
o
n
ə
t
ɪ
ʃ ə
n
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Frequency
• This cycling of airflow has a certain frequency
• the frequency of a phenomenon refers to the number of units that
occur during some fixed extent of measurement.
• The basic unit of frequency, the hertz (Hz), is defined as
one cycle per second.
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Two sine functions with different
frequencies
• A simple illustration can be found in the next diagram. It
consists of the graphs of two sine functions.
• The one marked with o’s, like beads on a necklace, completes an
entire cycle in 0.628 s, which gives it a frequency of 1.59 Hz.
• The other wave, marked with x’s so that it looks like barbed wire,
completes two cycles in this period. Thus, its frequency is twice as
much, 3.18 Hz.
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Graph of two sine functions with different frequencies
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Fundamental frequency
• The pitch of the human voice corresponds to the
frequency of vocal fold oscillation, called
fundamental frequency or F0.
• Fundamental frequency & gender
• The fundamental frequency of a man’s voice averages 125
Hz;
• the fundamental frequency of a woman’s voice averages
200 Hz.
• This 60% increase in the pitch of a woman’s voice can be
accounted for entirely by the fact that a man's vocal folds
are on average 60% longer than a woman’s.
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An example: "phonetician"
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The fundamental & higher frequencies
• This brief introduction to
the pitch of the human
voice leads one to believe
that the vocal folds vibrate
at a single frequency, that
of their fundamental
frequency, much as the
schematic string on the left
side is shown vibrating at
its fundamental frequency.
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Higher frequencies
• However, this is but a
idealization for the sake of
simplification of a rather
complex subject.
• In reality, the vocal folds vibrate
at a variety of frequencies that
are multiples of the
fundamental.
• The diagram depicts how this is
possible – a string can vibrate at
a frequency higher than its
fundamental because smaller
lengths of the string complete a
cycle in a shorter period of time.
• In the particular case of the
central diagram, each half of the
string completes a cycle in half
the time.
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Superposition of frequencies
• This figure displays the outcome of
superimposing both frequencies on
the string and the waveform.
• The result is that a pulse of vibration
created by the vocal folds projects
an abundance of different
frequencies in whole-number
multiples of the fundamental.
• If we could hear just this pulse, it
would sound, as Loritz (1999:93)
says, “more like a quick, dull thud
than a ringing bell”.
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An example: the spectrogram of
"phonetician"
f
o
n
ə
t
ɪ
ʃ ə
n
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Cavities & resonance
•
•
•
But the human voice does not sound like a quick, dull thud; it
sounds, well, it sounds like a human voice. This is because the
human vocal tract sits on top of the larynx, and the vocal tract
enhances the glottal pulse just like a trumpet enhances the shrill
tweet of its reed, as illustrated previously.
In particular, the buccal and nasal cavities resonate at certain
frequencies, thereby exaggerating some harmonics while muting
others.
The oral cavity itself sits in a channel between two smaller cavities
whose size varies according to the position of the tongue and lips.
The next diagram zooms in on the buccal cavity to distinguish the
other two. Counting from the back, there is
1. a pharyngeal cavity,
2. an oral cavity properly speaking, and
3. a labiodental cavity, between the teeth and the lips.
•
Notice how the difference in tongue position for [i], the vowel in
seed, and [a], the vowel in sod, changes the size of the oral and
pharyngeal cavities.
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The three buccal cavities, articulating [i]
and [a]
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Formants
• This difference produces a marked contrast in the frequencies
that resonate in these cavities, as shown by the schematic
plots of frequency over time in the next figure.
• Such enhanced frequencies, known as formants, carry the
acoustic information that allows us to distinguish [i] from [a], as
well as most other speech sounds. Roughly speaking,
• the resonance of all three cavities together produces the lowest or first
formant,
• the resonance of the pharyngeal & oral cavities produces the second
format,
• and the resonance of the labiodental cavity produces the third formant
(Loritz 1999:96).
• We hedge with “roughly” because the pharyngeal cavity can take on
special resonance properties, and the labiodental cavity can combine
with the oral cavity; see Ladefoged (1996:123ff) for more detailed
discussion.
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Schematic spectrograms of the lowest three resonant
frequencies (formants) of [i] and [a]
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What it really looks like
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NEXT TIME
Q4
Finish Ingram §5 & start §6.
☞ Go over questions at end of chapter.
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