text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
shrink the font size or
increase the margins. | 4,511.72 | 1.95 |
Or maybe more realistically you
write a five page paper that's | 4,513.67 | 2.732 |
supposed to be a 10 page paper,
and so you increase the font size | 4,516.402 | 2.708 |
or increase the margins so as to
expand or decompress the essay. | 4,519.11 | 5.86 |
So, similarly here, what if
we wanted to compress text, | 4,524.97 | 4.57 |
but we want to do it
losslessly in a way that we | 4,529.54 | 2.9 |
don't lose any information
by just throwing away | 4,532.44 | 2.21 |
characters, or paragraphs,
or pages, but we | 4,534.65 | 4.214 |
want to use the system with which
we're familiar from week zero. | 4,538.864 | 2.666 |
So ASCII, again, is just this code,
this mapping of letters to numbers. | 4,541.53 | 3.38 |
And so, A is-- capital A is 65
and that's some pattern of bits, | 4,544.91 | 4.07 |
but it's some pattern of
8 bits-- 7 historically, | 4,548.98 | 3.13 |
but really 8 bits in
practice So every one | 4,552.11 | 2.94 |
of the characters in the
English alphabet, at least here, | 4,555.05 | 3.68 |
takes up 8 bits. | 4,558.73 | 2.16 |
Now, that sounds fine. | 4,560.89 | 1.1 |
That allows us to express as many
as 256 possible characters, which | 4,561.99 | 3.25 |
is more than enough for English
characters, plus some punctuation | 4,565.24 | 2.74 |
and so forth. | 4,567.98 | 1.18 |
But it seems wasteful. | 4,569.16 | 2.23 |
I type A, E, and I, maybe
O and U pretty often. | 4,571.39 | 3.24 |
I use the values often--
the vowels often. | 4,574.63 | 3.778 |
B and D, I feel like I use those a lot. | 4,578.408 | 2.852 |
I don't really type Q all
that much, Z all that much. | 4,581.26 | 4.31 |
So, there are certain
letters that I just | 4,585.57 | 1.735 |
feel like I don't type them
that often, and indeed, | 4,587.305 | 2.125 |
probably if we analyzed a dictionary,
we wouldn't see them as frequently | 4,589.43 | 3.24 |
as other letters. | 4,592.67 | 0.82 |
Indeed, if you've ever played
or watched Wheel of Fortune, | 4,593.49 | 2.5 |
certainly all the
contestants on that show | 4,595.99 | 2.04 |
know which are the most popular
letters in English words. | 4,598.03 | 3.48 |
And it seems silly and
perhaps inefficient-- | 4,601.51 | 3.33 |
certainly for a computer
scientist-- that we are not somehow | 4,604.84 | 2.65 |
embracing the fact that some letters
are more commonly used than others, | 4,607.49 | 4.37 |
and yet we are just blindly using
8 bits, the same amount of memory, | 4,611.86 | 3.92 |
for every darn letter in our alphabet. | 4,615.78 | 1.71 |
Why? | 4,617.49 | 0.9 |
If you keep writing a certain
letter again and again, | 4,618.39 | 2.29 |
why not use fewer bits for
the more popular letters, | 4,620.68 | 3.12 |
and more bits for the
less popular letters | 4,623.8 | 2.24 |
so that at least you're optimizing
for the common case, so to speak? | 4,626.04 | 4.61 |
Well, it turns out that someone
named Huffman years ago did | 4,630.65 | 3.31 |
figure this out and introduced what's
generally known as Huffman coding. | 4,633.96 | 3.61 |
And, at first glance, it's a little
similar in spirit to something | 4,637.57 | 2.84 |
some of you might have grown up learning
a little something about called Morse | 4,640.41 | 3.249 |
code, but it's better
in a couple of ways. | 4,643.659 | 2.141 |
Morse code typically transmitted with
electrical signals or audible signals. | 4,645.8 | 5.52 |
It has dots and dashes
where a dot is a quick beep | 4,651.32 | 3.09 |
and a dash is a slightly
longer beep, and you | 4,654.41 | 2.43 |
can use those series of dots and
dashes, as per this chart here, | 4,656.84 | 4.13 |
to represent letters of the
alphabet and some numbers. | 4,660.97 | 3.18 |
The one problem, though, as efficient
as this seems-- and then by efficient | 4,664.15 | 4.4 |
I mean look at E. Mr. Morse realized
that is super popular, so he | 4,668.55 | 4.14 |
used literally the shortest symbol
for it, just a dot, a simple blip, | 4,672.69 | 3.41 |
to represent an E. And,
meanwhile, as I kind of imagined, | 4,676.1 | 2.99 |
Z is not that common,
so dash, dash, dot, | 4,679.09 | 3.47 |
dot is longer than just a single dot. | 4,682.56 | 2.33 |
So Z is probably less popular,
and that's why we did this. | 4,684.89 | 2.59 |
And Y may be even less
popular-- dash, dot, dash-- | 4,687.48 | 3.437 |
I don't know why I'm using this voice. | 4,690.917 | 1.583 |
But it's longer than E, so we
optimized for the shorter characters. | 4,692.5 | 5.056 |
Unfortunately, suppose that you receive
the message dot, dot, dot, dot, dot, | 4,697.556 | 10.064 |
dot, so six dots in a row, and I
technically paused in between them. | 4,707.62 | 4.82 |
Six dots, what message
did I just send you? | 4,712.44 | 2.052 |
4,714.492 | 3.698 | |
Six dots. | 4,718.19 | 0.76 |
4,718.95 | 5.232 | |
So, I wanted to say hi, so I said
dot, dot, dot, dot, which is H, | 4,724.182 | 4.068 |
and then dot, dot which is I. I
should not have paused between them, | 4,728.25 | 3.23 |
because the whole point of Morse code
is to do this as quickly as possible, | 4,731.48 | 3.125 |
even though you probably do want
to pause to resolve ambiguity, | 4,734.605 | 3.045 |
and indeed, that's the problem. | 4,737.65 | 1.53 |
I wanted to send you
hi, H-I, but maybe I | 4,739.18 | 4.03 |
just sent you E, E, E,
E, E, E, six Es in a row, | 4,743.21 | 4.59 |
because those two were just dots. | 4,747.8 | 1.69 |
So, in other words, Morse code
is not immediately decodable | 4,749.49 | 2.87 |
when you're reading, or hearing,
or seeing the dots and dashes come | 4,752.36 | 3.097 |
over the wire, so to speak,
because there's these ambiguities, | 4,755.457 | 2.583 |
unless this transmitter
does indeed pause, | 4,758.04 | 3.47 |
as I accidentally did there, to give
you a moment to take your breath | 4,761.51 | 3.54 |
and realize, oh, that was an H. That's
an I. As opposed to E, E, E, E, E, E. | 4,765.05 | 4.95 |
So, it's not necessarily the best
system in so far as some letters | 4,770 | 4.02 |
share prefixes with other letters. | 4,774.02 | 3.03 |
In other words, I, dot dot, has a
common prefix with E. Both of them | 4,777.05 | 4.93 |
start with a single dot. | 4,781.98 | 1.485 |
It just so happens that
I is a little longer, | 4,783.465 | 1.875 |
and that can lead
potentially to ambiguity, | 4,785.34 | 1.91 |
and it certainly means that the
transmitter should probably slow down. | 4,787.25 | 4.86 |
So, the whole system is meant to
be super fast, super efficient, | 4,792.11 | 2.71 |
but you probably should
pause between certain letters | 4,794.82 | 3.04 |
so that the recipient
doesn't get confused as | 4,797.86 | 1.98 |
to the message you're actually sending. | 4,799.84 | 2.11 |
Well, thankfully Huffman coding--
which as we'll see in a moment | 4,801.95 | 3.25 |
is based on trees-- does
not have that ambiguity. | 4,805.2 | 2.6 |
It is a immediately decodable. | 4,807.8 | 1.76 |
And suppose for the sake of
discussion, as per this example here, | 4,809.56 | 3.06 |
you just have a whole bunch of
text that you want to transmit. | 4,812.62 | 1.87 |
This is meaningless. | 4,814.49 | 0.833 |
There's no pattern in these
As, and E, B, C, Ds, and Es, | 4,815.323 | 2.787 |
but if you go through and count them
up, each these letters-- A, B, C, D, E-- | 4,818.11 | 4.343 |
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