Hi, I'm Carrie Anne, and welcome to Crash Course Computer Science!
Last episode we talked about computer vision – giving computers the ability to see and
understand visual information.
Today we're going to talk about how to give computers the ability to understand language.
You might argue they've always had this capability.
Back in Episodes 9 and 12, we talked about machine language instructions, as well as
higher-level programming languages.
While these certainly meet the definition of a language, they also tend to have small
vocabularies and follow highly structured conventions.
Code will only compile and run if it's 100 percent free of spelling and syntactic errors.
Of course, this is quite different from human languages – what are called natural languages
– containing large, diverse vocabularies, words with several different meanings, speakers
with different accents, and all sorts of interesting word play.
People also make linguistic faux pas when writing and speaking, like slurring words
together, leaving out key details so things are ambiguous, and mispronouncing things.
But, for the most part, humans can roll right through these challenges.
The skillful use of language is a major part of what makes us human.
And for this reason, the desire for computers to understand and speak our language has been
around since they were first conceived.
This led to the creation of Natural Language Processing, or NLP, an interdisciplinary field
combining computer science and linguistics.
INTRO
There's an essentially infinite number of ways to arrange words in a sentence.
We can't give computers a dictionary of all possible sentences to help them understand
what humans are blabbing on about.
So an early and fundamental NLP problem was deconstructing sentences into bite-sized pieces,
which could be more easily processed.
In school, you learned about nine fundamental types of English words: nouns, pronouns, articles,
verbs, adjectives, adverbs, prepositions, conjunctions, and interjections.
These are called parts of speech.
There are all sorts of subcategories too, like singular vs. plural nouns and superlative
vs. comparative adverbs, but we're not going to get into that.
Knowing a word's type is definitely useful, but unfortunately, there are a lot words that
have multiple meanings – like "rose" and "leaves", which can be used as nouns
or verbs.
A digital dictionary alone isn't enough to resolve this ambiguity, so computers also
need to know some grammar.
For this, phrase structure rules were developed, which encapsulate the grammar of a language.
For example, in English there's a rule that says a sentence can be comprised of a noun
phrase followed by a verb phrase.
Noun phrases can be an article, like "the", followed by a noun or they can be an adjective
followed by a noun.
And you can make rules like this for an entire language.
Then, using these rules, it's fairly easy to construct what's called a parse tree,
which not only tags every word with a likely part of speech, but also reveals how the sentence
is constructed.
We now know, for example, that the noun focus of this sentence is "the mongols", and
we know it's about them doing the action of "rising" from something, in this case,
"leaves".
These smaller chunks of data allow computers to more easily access, process and respond
to information.
Equivalent processes are happening every time you do a voice search, like: "where's
the nearest pizza".
The computer can recognize that this is a "where" question, knows you want the noun
"pizza", and the dimension you care about is "nearest".
The same process applies to "what is the biggest giraffe?" or "who sang thriller?"
By treating language almost like lego, computers can be quite adept at natural language tasks.
They can answer questions and also process commands, like "set an alarm for 2:20"
or "play T-Swizzle on spotify".
But, as you've probably experienced, they fail when you start getting too fancy, and
they can no longer parse the sentence correctly, or capture your intent.
Hey Siri... methinks the mongols doth roam too much, what think ye on this most gentle
mid-summer's day?
Siri: I'm not sure I got that.
I should also note that phrase structure rules, and similar methods that codify language,
can be used by computers to generate natural language text.
This works particularly well when data is stored in a web of semantic information, where
entities are linked to one another in meaningful relationships, providing all the ingredients
you need to craft informational sentences.
Siri: Thriller was released in 1983 and sung by Michael Jackson
Google's version of this is called Knowledge Graph.
At the end of 2016, it contained roughly seventy billion facts about, and relationships between,
different entities.
These two processes, parsing and generating text, are fundamental components of natural
language chatbots - computer programs that chat with you.
Early chatbots were primarily rule-based, where experts would encode hundreds of rules
mapping what a user might say, to how a program should reply.
Obviously this was unwieldy to maintain and limited the possible sophistication.
A famous early example was ELIZA, created in the mid-1960s at MIT.
This was a chatbot that took on the role of a therapist, and used basic syntactic rules
to identify content in written exchanges, which it would turn around and ask the user
about.
Sometimes, it felt very much like human-human communication, but other times it would make
simple and even comical mistakes.
Chatbots, and more advanced dialog systems, have come a long way in the last fifty years,
and can be quite convincing today!
Modern approaches are based on machine learning, where gigabytes of real human-to-human chats
are used to train chatbots.
Today, the technology is finding use in customer service applications, where there's already
heaps of example conversations to learn from.
People have also been getting chatbots to talk with one another, and in a Facebook experiment,
chatbots even started to evolve their own language.
This experiment got a bunch of scary-sounding press, but it was just the computers crafting
a simplified protocol to negotiate with one another.
It wasn't evil, it's was efficient.
But what about if something is spoken – how does a computer get words from the sound?
That's the domain of speech recognition, which has been the focus of research for many
decades.
Bell Labs debuted the first speech recognition system in 1952, nicknamed Audrey – the automatic
digit recognizer.
It could recognize all ten numerical digits, if you said them slowly enough.
5…
9…
7?
The project didn't go anywhere because it was much faster to enter telephone numbers
with a finger.
Ten years later, at the 1962 World's Fair, IBM demonstrated a shoebox-sized machine capable
of recognizing sixteen words.
To boost research in the area, DARPA kicked off an ambitious five-year funding initiative
in 1971, which led to the development of Harpy at Carnegie Mellon University.
Harpy was the first system to recognize over a thousand words.
But, on computers of the era, transcription was often ten or more times slower than the
rate of natural speech.
Fortunately, thanks to huge advances in computing performance in the 1980s and 90s, continuous,
real-time speech recognition became practical.
There was simultaneous innovation in the algorithms for processing natural language, moving from
hand-crafted rules, to machine learning techniques that could learn automatically from existing
datasets of human language.
Today, the speech recognition systems with the best accuracy are using deep neural networks,
which we touched on in Episode 34.
To get a sense of how these techniques work, let's look at some speech, specifically,
the acoustic signal.
Let's start by looking at vowel sounds, like aaaaa…and Eeeeeee.
These are the waveforms of those two sounds, as captured by a computer's microphone.
As we discussed in Episode 21 – on Files and File Formats – this signal is the magnitude
of displacement, of a diaphragm inside of a microphone, as sound waves cause it to oscillate.
In this view of sound data, the horizontal axis is time, and the vertical axis is the
magnitude of displacement, or amplitude.
Although we can see there are differences between the waveforms, it's not super obvious
what you would point at to say, "ah ha! this is definitely an eeee sound".
To really make this pop out, we need to view the data in a totally different way: a spectrogram.
In this view of the data, we still have time along the horizontal axis, but now instead
of amplitude on the vertical axis, we plot the magnitude of the different frequencies
that make up each sound.
The brighter the color, the louder that frequency component.
This conversion from waveform to frequencies is done with a very cool algorithm called
a Fast Fourier Transform.
If you've ever stared at a stereo system's EQ visualizer, it's pretty much the same
thing.
A spectrogram is plotting that information over time.
You might have noticed that the signals have a sort of ribbed pattern to them – that's
all the resonances of my vocal tract.
To make different sounds, I squeeze my vocal chords, mouth and tongue into different shapes,
which amplifies or dampens different resonances.
We can see this in the signal, with areas that are brighter, and areas that are darker.
If we work our way up from the bottom, labeling where we see peaks in the spectrum – what
are called formants – we can see the two sounds have quite different arrangements.
And this is true for all vowel sounds.
It's exactly this type of information that lets computers recognize spoken vowels, and
indeed, whole words.
Let's see a more complicated example, like when I say: "she.. was.. happy"
We can see our "eee" sound here, and "aaa" sound here.
We can also see a bunch of other distinctive sounds, like the "shh" sound in "she",
the "wah" and "sss" in "was", and so on.
These sound pieces, that make up words, are called phonemes.
Speech recognition software knows what all these phonemes look like.
In English, there are roughly forty-four, so it mostly boils down to fancy pattern matching.
Then you have to separate words from one another, figure out when sentences begin and end...
and ultimately, you end up with speech converted into text, allowing for techniques like we
discussed at the beginning of the episode.
Because people say words in slightly different ways, due to things like accents and mispronunciations,
transcription accuracy is greatly improved when combined with a language model, which
contains statistics about sequences of words.
For example "she was" is most likely to be followed by an adjective, like "happy".
It's uncommon for "she was" to be followed immediately by a noun.
So if the speech recognizer was unsure between, "happy" and "harpy", it'd pick "happy",
since the language model would report that as a more likely choice.
Finally, we need to talk about Speech Synthesis, that is, giving computers the ability to output
speech.
This is very much like speech recognition, but in reverse.
We can take a sentence of text, and break it down into its phonetic components, and
then play those sounds back to back, out of a computer speaker.
You can hear this chaining of phonemes very clearly with older speech synthesis technologies,
like this 1937, hand-operated machine from Bell Labs.
Say, "she saw me" with no expression.
She saw me.
Now say it in answer to these questions.
Who saw you?
She saw me.
Who did she see?
She saw me.
Did she see you or hear you?
She saw me.
By the 1980s, this had improved a lot, but that discontinuous and awkward blending of
phonemes still created that signature, robotic sound.
Thriller was released in 1983 and sung by Michael Jackson.
Today, synthesized computer voices, like Siri, Cortana and Alexa, have gotten much better,
but they're still not quite human.
But we're soo soo close, and it's likely to be a solved problem pretty soon.
Especially because we're now seeing an explosion of voice user interfaces on our phones, in
our cars and homes, and maybe soon, plugged right into our ears.
This ubiquity is creating a positive feedback loop, where people are using voice interaction
more often, which in turn, is giving companies like Google, Amazon and Microsoft more data
to train their systems on...
Which is enabling better accuracy, which is leading to people using voice more, which
is enabling even better accuracy… and the loop continues!
Many predict that speech technologies will become as common a form of interaction as
screens, keyboards, trackpads and other physical input-output devices that we use today.
That's particularly good news for robots, who don't want to have to walk around with
keyboards in order to communicate with humans.
But, we'll talk more about them next week.
See you then.
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