Name: 最大帶寬是多少？- 60個符號 (What is the maximum Bandwidth? - Sixty Symbols)
Uploaded: 2021-01-14T10:30:49.000Z
Duration: 11 min 39 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

情態副詞

I'm gonna talk about bandwidth on, actually, just to be really old fashioned about this.

It's 60 symbols video, and it's got a symbol, which would be Delta F.

基礎被動語態

Because it's a range of frequencies, it is related to the amount of information you can get down the fiber.

So you know the band width of your broadband is telling you how much information you actually getting there is largely limited by how much you're prepared to pay your broadband company.

But there's a sort of fundamental physics limit as to how much information you can actually get down safe.

You can apply this concept to anything to sounds to radio waves to light down optical fiber whatever you like.

But the idea is the same, which is that essentially light sound.

Whatever comes in waves on the classic sound wave or light wave is a sine wave, a little app here, which will produce a total.

That's a sign wave, and there's no information content in there at all, right?

You can't use that to actually transmit information if you actually want to convey information in particular you want to confide digital information you need ones and zeros on that essentially means you need pulses.

You can't do it with just a sine wave like that.

Then nothing that beep that nothing you need.

You need to break it up into pulses, and you can't do that with a single frequency.

I could turn it on and off, but actually, then he wouldn't be a single frequency anymore.

Okay, let's do something a little bit more sophisticated, which is So there was my tone and I've got Brady's.

I found my phone is June to produce a tone off kilohertz 1000 Hertz and I've set Brady's up to produce a tone off 1001 hurts.

Okay, so you know we can play Brady's That's what 1000 1 who it sounds like.

And that's and that's what 1000 sounds look really very similar.

The interesting thing the hams is if we play them both at once.

And if you listen carefully, you can hear that this is strange beating effect that it sometimes louder, sometimes quiet.

One way might might make it clear is, if I change the frequency of it, you'll hear that the bee change that.

If I take Brady's phone from 1000 1 hurts 1002 you hear that the beats get quicker, so let's turn off the annoying noises for a moment.

So physically, what's happening is you got these two ways of slightly different frequencies, and that means they're just kind of overtime drifting in and out of phase with each other.

And when they're in phase, the waves, all that up when you get allowed to sound them, when they're out of phase two, the waves cancel each other out, and then it goes quiet again.

So you get these beats of loud and quiet, and what I really want is I just want one of those beets.

I can't quite do that with only two frequencies where I've got only got two frequencies here.

Then, actually, I can produce a single beep, a single blip.

That's not good, because then basically everything a group you know, adds up in coherently at one point.

So you end up with lots of sound there, but then it destructively interferes everywhere else so I can produce a single little pulse of beep on.

If I got to convey information digitally, I want to send a whole string of these beeps.

But in order to do that, I needed to combine different frequencies together, right?

It wasn't just one frequency, it was a whole range of frequencies.

And this is very simple relationship between the range of frequencies I add up and how short the beepers.

And essentially, it's only write this one down.

So essentially, if I've got a range of frequencies Delta F on, I'll produce pulses duration delta t.

So the bit the bleeps end up producing the product of the two always comes out to be approximately one.

So you see, if I want Thio produce very short pulses if I want to produce it, you know, if I want to pack a lot of information in a finite period of time.

I can put produce, you know, a whole lot of code in a very short period of time.

You can see from this former if I want Delta T to be small on the product of Delta F in Delta, T has to be one.

As this term gets smaller, the only way I could do that is by making this term bigger.

In other words, to make shorter and shorter pulses of sound or light or whatever it is, I need a wider and wider range of frequencies to do it on.

The fundamental limits to putting information down a fiber optic cable is what range of frequencies can I put down Because low fiber optic cable, you know, I'll probably let Blue Light go through a red light go through it won't let X ray through radio waves through.

It's just no optical fibers don't work that way.

And so there's a finite range of frequencies, a bandwidth of lights that I can put down the fiberoptic tape cable on that fundamentally limits.

How short a beep I can send down how short a blip of light I can send down on.

That's what limits the amount of information you can put down the fiber optic cable.

So, professor, when I'm using the Internet and I get a one, is that what's happening to create the one?

They're interfering all these different colors of light except a certain one.

So if someone that if some of the path between you and whatever server is that you got that one form is down a fiber optic cable, that's exactly what's happening.

A little blip of light has been sent down that fiber optic cable to represent the one, and to produce a little blip of light, you need a whole range of frequencies of light to do it.

It's not just a single color that can't just switch something on and off, but okay, so that's the weird thing, right?

Even if you've got supposing so, everyone thinks like a laser produces light of a single color, single wavelength single frequency.

Actually, that's only true if you leave it on forever.

So you're not switching, then you do indeed have a sine wave that goes on forever on that is indeed strictly a single frequency.

As soon as you switch it on and off over some finite period, it's no longer a single frequency, so even a laser.

So again, maybe a picture will help with this, right?

If I've got my light, my light wave that goes on and on forever, that really is.

If I were a competent of drawing a single frequency, a single wavelength of light.

If, however, I just switch it on for a little while, So it's off.

Then I got a pulse of light, and then it's off again.

This no longer has a single wavelength associated with.

It looks like it, cause you can see what it's going up.

It's going down, but actually to produce something like this.

This is clearly different from this, and to go from having a really single wave to having this, I've got to add a whole want your waves together of slightly different frequencies to produce something like this.

And so this is actually the superposition of a whole bunch of slightly different frequencies, even a laser, which you think overs while it's a single wavelength of light.

If you switch it on and switch it off again.

You've actually produced a pulse of light that has by its nature to contain more than one frequency.

If I have a laser, I could just put you on and off.

I'm not saying I'm producing a little bit of this red light and then this little bit that's a bit bluer.

And this little bit it's a bit redder in order to make a pulse.

What nature does all that for you, that actually, by switching on and off the laser instead of producing this single for wavelength of light, he's actually producing the right superposition of wavelengths of light to make a pulse.

So they're on a bunch of engineers and experts that had to come up with a way to make all these things interfering.

Don't you know that nature that takes care of it, that just by switching the thing on and off of the right wavelength, the frequency you want to switch things on?

The fact that the producing pulses that by its nature produces light which is no longer a single waving.

But it turns out that actually this Ah quantum mechanical application of all this as well.

One of the things that people know about quantum mechanics is this wonderful thing called the uncertainty principle that says there are various things that you can't measure at the same time, so you can't measure the position of a particle and its momentum at the same time.

And one of the other forms of the uncertainty principle is that you can't measure the time when something arrives and its energy at exactly the same time.

So if you measure the energy of something and the time of arrival, then that's an answer you can.

You can trade them off against each other.

You can measure times very accurately, and then you get rubbish measurement of energy and vice versa.

And turns out this is a simple example of exactly that.

And the reason is we go back to this equation again for a second, that the frequency here, instead of thinking in terms of these pulses of light and the classical picture of waves, think about photons and when a photon arrives, if we've got a pulse of light like that, we know that the light has to arrive sometime within this time, So the photo on we detect is going to arrive within some finite time or other.

So because of this relationship up here, because it's some finite time of arrival, that means there's also a finite range of frequencies that that photo on can have, what frequency that we actually measure for the photo.

字幕列表影片播放

最大帶寬是多少？- 60個符號 (What is the maximum Bandwidth? - Sixty Symbols)

essentially

ultimate

slightly

period