[MUSIC PLAYING]
All right, so in
this video we're
going to understand how
information travels wirelessly.
And what I mean by
this is that we're
going to understand how devices
like Wi-Fi, wireless internet
work.
How things like radios,
say, in cars work.
And how things like cellphones,
which definitely use
wireless communication,
how they work.
And the reason that we
can understand all these
in one pretty short
video is that they all
use the same principles
communication.
And they all use
communication via waves.
So waves is something we're
all intimately familiar with.
We know from childhood
that if we have say a lake,
and someone throws a
rock into this lake,
then the rock is going to cause
ripples to form in the lake.
And the key point
about these ripples--
what makes them called
waves-- is that they move.
They propagate.
And because they propagate,
they can carry information out
about the initial disturbance
that created them.
For example, the way that you're
listening to my voice right
now is that your
speaker-- this is
a very poor drawing
of a speaker--
but your speaker is moving.
And your speaker is
creating sound waves.
And what sound wave is just
regions of high and low density
in the air.
These are supposed
to be air molecules.
And so these sound
waves reach your year.
And your ear does some
complicated process
to transcribe that into sound.
But again, these waves--
this information transition
is possible because
the waves propagate.
Now, to understand how
all these devices work
we're going to have to
talk about something
called electromagnetic,
or EM, waves.
And the exact nature
of these waves
is too complicated
to go into now,
and it's not important
for understanding this.
But let's just give
you some examples
of waves you already know about
that are, in fact, EM waves.
So light, for example, is
an electromagnetic wave.
Microwaves use electromagnetic
waves to heat up food.
X-rays, whenever
you go to the doctor
and you get an
x-ray image taken,
the waves that form
that image are x-rays.
And lastly, of
course, radio waves
are another special type
of electromagnetic waves.
So how do we use waves
to transmit information?
That's what we're going
to try to understand now.
And before we do that,
we need to understand
two characteristics
of waves, two ways
that we can quantify what makes
one wave different from other.
So if I consider this
way of that I have here,
there are two things
that I can ask about it.
One, I can ask how
large is this way?
And to answer such a
question what I might do
is I might simply
measure the difference
between the maximum value
and the central value
or the equilibrium point.
And that is something we
call the amplitude in wave
terminology.
So let's say that the amplitude
is A. So this distance is A.
And obviously,
this distance is A.
And the difference between
the peak and trough,
called the peak
to peak distance,
is equal to, obviously, 2
times the amplitude or 2A.
So amplitude is a very
easy, very intuitive concept
about how to characterize
how big a wave is.
So the other concept that's
important is called frequency.
And frequency is just the
answer to the question
how fast do cycles
of the wave go by.
So by cycle I mean a
region like this of a wave.
The basic unit of the way that
repeats, each one of these
is a cycle.
And basically frequency
is equal to the number
of cycles that occur in a unit
time, let's say, a second.
So a wave that looks like this
has a much higher frequency
than a wave that looks
like this, even though they
have the same amplitude.
So now that we understand what
frequency and amplitude mean,
we can use these concepts
to transmit information.
And the first way that
I'm going to tell you
about how to do this is
called amplitude modulation.
And this is what the AM
stands for in AM radio.
So the basic idea is best
to see by this example.
So say I have some signal.
Maybe it's a sound
signal or something else.
Say I have some signal
that I want to send you,
and that signal looks like this.
So how do I send
you this signal?
Well, just for
convenience-- you'll
see why I do this
in a second-- but I
want to draw the
negative of the signal.
So this is the signal
that I want to send you.
And I want to send it to you
via an electromagnetic wave.
So what I do is I send you a
way of a particular frequency.
And I tell you to look for
waves of that frequency.
And what I'm going
to do is I'm going
to send you this wave where
I change the wave based
on the signal that
I want to send you.
So whatever the
signal is low, I make
the amplitude of my wave low.
And wherever the signals
I make the amplitude
of my way of high.
So here we have two
distinct pieces.
This is signal.
And this wave that
I'm sending you
is called the carrier wave,
because it carries the signal.
So to decode this
all you have to do
is you have to look at how big
the wave is at any given point.
And then you can
get to the signal
that l I was trying to send you.
So this scheme's called
amplitude modulation.
And it's the one that's
used in radios most often,
AM radios specifically, to get
you the songs that you here,
say, in a car when
you're driving
and listen to the radio.
Now there's AM and also FM.
And you've probably
guessed by now
that FM means
frequency modulation.
So frequency modulation
is another way
I can send you information.
And of course, we can
use frequency modulation
in the same way that I used
amplitude modulation to send
you some like sound file.
But what I want
to talk about now
is how things like cellphones
use frequency modulation
to send digital information.
And what I mean by that is if
cellphones want to send to each
other, or to wireless towers
that communicate with them,
they want to send information
in the forms of 1's and 0's.
And later they transmit--
some hardware in the cellphone
translate those 1's and 0's
into text or sounds or whatever.
But to do that what
we can simply do is we
can agree that if your
cellphone and you receive
a frequency that looks like
this, then you call that a 0.
But if you receive a frequency
that looks like this,
then you call that a 1.
So if I want to send
someone a message, 1 0 1 1.
All I do I send them
this signal in which I
change the frequency.
So I want to send a 1 first.
So here's a 1.
Then I want to send a 0.
So then I'll send another 1.
[INAUDIBLE] low frequency.
Then I want to send another 1.
So to decode this message,
all you have to do
is you have to look
at different points,
and you have to see
what's the frequency here.
Is it a 0?
Or is it a 1?
And so this idea, FM, is the
basis how digital communication
takes place wirelessly.
It's actually called
something more complex,
frequency shift keying.
So these two concepts
that I've outlined
for you guys, FM and AM,
constitute the majority
of how information is traveled
via electromagnetic waves
between wireless devices.
