Source Encoding (Language of Coins: 4/9)

0:04  0:06We begin with a problem.

0:06  0:08[WIND BLOWING]

0:15  0:16Alice and Bob live in tree forts,

0:16  0:18which are far apart,

0:18  0:21with no line of sight between them.

0:21  0:23And they need to communicate.

0:23  0:25So they decide to run a wire

0:25  0:27between the two houses.

0:40  0:42They pull the wire tight,

0:42  0:45and attach a tin can to each end –

0:52  0:54allowing them to send their voices

0:54  0:56faintly along the wire.

0:59  1:02[BOB  MUFFLED] "Hello?"

1:02  1:06[ALICE  MUFFLED] Hello? I can't hear you.

1:06  1:09[BOB  MUFFLED] I can hear you, but just barely.

1:09  1:15[ALICE  MUFFLED] 1. 2. 3. 4. 5.

1:15  1:18However, there is a problem:

1:18  1:21'noise.'

1:21  1:22Whenever there is a high wind,

1:22  1:24it becomes impossible to hear

1:24  1:27the signal over the noise.

1:29  1:30So they need a way to increase

1:30  1:32the energy level of the signal,

1:32  1:35to separate it from the noise.

1:35  1:37This gives Bob an idea.

1:40  1:43They can simply pluck the wire,

1:43  1:47which is much easier to detect over the noise.

1:47  1:49But this leads to a new problem.

1:49  1:53How do they encode their messages as plucks?

1:57  1:58Well, since they want to play

1:58  2:00board games across a distance,

2:00  2:03they tackle the most common messages first –

2:03  2:06the outcome of two dice rolls.

2:06  2:09In this case, the messages they are sending

2:09  2:11can be thought of as a selection

2:11  2:14from a finite number of 'symbols' –

2:14  2:17in this case, the eleven possible numbers,

2:17  2:20which we call a 'discrete source.'

2:24  2:27At first, they decide to use the simplest method.

2:27  2:31They send the result as the number of plucks.

2:31  2:34So, to send a '3,' they send three plucks.

2:34  2:36'9' is nine plucks.

2:36  2:38And '12' is twelve plucks.

2:38  2:41However, they soon realize that this takes

2:41  2:43much longer than it needs to.

2:44  2:48From practice, they find that their maximum pluck speed

2:48  2:51is two plucks per second.

2:51  2:54Any faster, and they will get confused.

2:54  2:57So two plucks per second can be thought of as the 'rate' –

2:57  3:01or 'capacity' – for sending information in this way.

3:01  3:06[SOUND OF PLUCKING]

3:06  3:07And it turns out that

3:07  3:10the most common roll is a 7 –

3:10  3:14so it takes 3.5 seconds to send the number 7.

3:14  3:20[THE SOUND OF 7 PLUCKS]

3:22  3:24Alice then realizes they can do much better

3:24  3:27if they change their coding strategy.

3:27  3:30She realizes that the odds of each number being sent

3:30  3:32[follow] a simple pattern.

3:32  3:34There is one way to roll a 2.

3:34  3:36[There are] two ways to roll a 3.

3:36  3:38Three ways to roll a 4.

3:38  3:40Four ways to roll a 5.

3:40  3:43Five ways to roll a 6.

3:43  3:45And six ways to roll a 7 –

3:45  3:46the most common [result].

3:46  3:49And five ways to roll an 8.

3:49  3:50Four ways for a 9 –

3:50  3:54and so on, back to one way for a 12.

3:54  3:55This is a graph showing

3:55  3:58the number of ways each result can occur.

3:58  4:00And the pattern is obvious.

4:00  4:02So now, let's change the graph to

4:02  4:05'number of plucks versus each symbol.'

4:05  4:07She proceeds by mapping

4:07  4:08the most common number –

4:08  4:127 – to the shortest signal – one pluck.

4:12  4:14[SOUND OF ONE PLUCK]

4:14  4:17She then proceeds to the next most probable number.

4:17  4:20And if there is a tie, she picks one at random.

4:20  4:23In this case, she selects 6 to be two plucks,

4:23  4:25and then 8 to be three plucks,

4:25  4:28and then back to 5 to be four plucks,

4:28  4:30and 9 is five plucks,

4:30  4:34and back and forth, until we reach 12,

4:34  4:36which is assigned to 11 plucks.

4:36  4:39Now, the most common number, 7,

4:39  4:42can be sent in less than a second –

4:42  4:44a huge improvement.

4:44  4:46This simple change allows them to send

4:46  4:52more information in the same amount of time, on average.

4:52  4:54In fact, this coding strategy is optimal

4:54  4:56for this simple example –

4:56  4:58in that it's impossible for you

4:58  5:00to come up with a shorter method

5:00  5:05of sending two dice rolls – using identical plucks.

5:05  5:09However, after playing with the wire for some time,

5:09  5:11Bob hits on a new idea.

5:11  5:13[PLUCKING SOUNDS BEING PLAYED BACKWARDS]

5:27  5:32[PLUCKS SHOWN IN SLOW MOTION – NO SOUND]
 Τίτλος:
 Source Encoding (Language of Coins: 4/9)
 Περιγραφή:

Introduction to coding theory (variable length source coding) with a lossless compression problem. This simplified problem only deals with sending unary symbols (plucks) to send single symbols. Source encoding attempts to compress the data from a source in order to transmit it more efficiently.
 Video Language:
 Japanese
 Duration:
 05:57
Mike Ridgway edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9)  
Mike Ridgway edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9)  
Mike Ridgway edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9)  
Mike Ridgway edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9)  
Mike Ridgway edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9)  
Mike Ridgway edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9)  
Mike Ridgway edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9)  
edojur2 edited Αγγλικά subtitles for Source Encoding (Language of Coins: 4/9) 