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DENNIS FREEMAN: Hello.

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So welcome to the
last lecture in 003.

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So there are more
lectures scheduled.

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But I don't believe
in trying to cram

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new material in the last couple
of hours before the exam.

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So no new material after today,
although it will be helpful

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if you continue to try
to work on problems,

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00:00:46,200 --> 00:00:49,510
and try to internalize
what we did more recently.

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00:00:49,510 --> 00:00:50,830
That's the idea.

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00:00:50,830 --> 00:00:52,890
So I'm not trying to
pump new information in.

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00:00:52,890 --> 00:00:55,260
That doesn't mean that
there aren't things

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that you should be doing.

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But after today,
no new information.

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In fact, today there
is no new information.

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There will be a
session on Tuesday.

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00:01:05,760 --> 00:01:10,410
But the idea is primarily to
get feedback from you people.

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00:01:10,410 --> 00:01:12,960
As I mentioned last time,
we've changed a lot of things.

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We change a lot of
things every term.

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And our primary
source of information

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is the end of term surveys.

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Tell us what works.

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Tell us what didn't work.

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Tell us what you liked.

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00:01:21,750 --> 00:01:22,958
Tell us what you didn't like.

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00:01:22,958 --> 00:01:24,900
If you didn't like the
electronic submission

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00:01:24,900 --> 00:01:26,917
of homework, tell us.

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00:01:26,917 --> 00:01:28,500
If you didn't like
the tutor, tell us.

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00:01:28,500 --> 00:01:31,320
If you would rather have
individual office hours instead

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of block office hours,
it's a good chance for you

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to tell us what you think.

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And it's especially
important that you fill out

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the online subject evaluation.

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00:01:42,180 --> 00:01:45,390
So please, if you have a laptop
with a certificate on it,

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feel free to bring it
to lecture next time.

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That's one of the main ideas.

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But we'll also spend
some time just discussing

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what you would suggest for
improvements for next time.

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00:02:00,414 --> 00:02:01,330
Questions or comments?

46
00:02:04,690 --> 00:02:07,710
OK, so today is
mostly just for fun.

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Today is mostly just to
tell you about the way

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6.003 plays out in
technologies that you

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might have some interest in.

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00:02:17,050 --> 00:02:20,470
And I chose a technology that I
am particularly interested in.

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Because as I probably have
mentioned in the past,

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like many of you, I was
afflicted with an addiction

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with music at about your age.

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I recovered.

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You will too.

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But I was very interested in it.

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It was a very hot topic.

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And I want to talk about some
of the things that 003 has done

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to make that industry very
different today from what it

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was when I was an undergraduate.

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It may not surprise
you, but it surprises me

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that the technology that
we used was really not

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very different
from the technology

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that the inventor invented.

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So who invented the phonograph?

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You can look at the notes.

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You're supposed to know that.

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Just shout.

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Makes me feel better.

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AUDIENCE: [INAUDIBLE]

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DENNIS FREEMAN: Thank you--

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Edison.

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So Edison was, of course, a
genius and a prolific inventor.

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At the time he invented
the phonograph,

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he was interested in
two things that he

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was trying to draw
relations between,

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telegraphy and telephony.

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We talked about
telegraphy before.

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Telegraphy, telegraph, was
a way of getting messages

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point to point.

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So person A wanted to get
a message to person B.

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And it would be transmitted
via the telegraph system.

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Now I kind of
oversimplified the way

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the telegraph
system worked when I

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talked about it the last time.

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It really worked a
lot more like message

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passing than you
might have thought.

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What would happen is
that there wouldn't

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be a direct connection
between person A and person B.

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00:04:02,440 --> 00:04:05,930
It might have to go
through six relays.

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00:04:05,930 --> 00:04:08,270
So in order to
get to California,

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00:04:08,270 --> 00:04:10,850
it might be important
that a message that

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00:04:10,850 --> 00:04:13,880
originated in Boston
would go to New York,

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00:04:13,880 --> 00:04:16,730
and from New York
to Pennsylvania,

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00:04:16,730 --> 00:04:20,000
from Pennsylvania to
Chicago, et cetera.

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00:04:20,000 --> 00:04:24,080
So there may not be a direct
route over long distances.

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00:04:24,080 --> 00:04:26,930
So the way that was done,
you would take your message

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00:04:26,930 --> 00:04:32,990
to the telegraph office A.
The operator would key it in.

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00:04:32,990 --> 00:04:35,790
It would get written
down by the receiver,

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00:04:35,790 --> 00:04:39,750
say, in New York, who would
listen to the message,

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00:04:39,750 --> 00:04:44,140
write it out, basically
reproducing your message there,

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00:04:44,140 --> 00:04:47,990
then key it in to
the next relay.

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00:04:47,990 --> 00:04:50,900
So it might actually be
keyed in a dozen times

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before it hits the
final destination.

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00:04:52,730 --> 00:04:55,370
And that was tedious,
laborious, hard.

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00:04:55,370 --> 00:04:59,630
And Edison was interested
in fixing that.

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And to do so, he invented
a paper tape system,

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00:05:02,990 --> 00:05:06,400
so that the receiver
wouldn't need to listen it.

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00:05:06,400 --> 00:05:10,539
They would just punch out paper
tape as the clicks came in.

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00:05:10,539 --> 00:05:12,080
So as the dots and
dashes came in it,

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00:05:12,080 --> 00:05:13,700
it would make holes in paper.

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00:05:13,700 --> 00:05:15,350
The holes in paper,
then, could be

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00:05:15,350 --> 00:05:18,050
used to transmit to
the next station.

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00:05:18,050 --> 00:05:20,510
So Edison did that.

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00:05:20,510 --> 00:05:22,040
He was interested
in thinking about

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00:05:22,040 --> 00:05:23,600
whether you could do
the same sort of thing

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00:05:23,600 --> 00:05:25,880
with other kinds of signals
besides telegraph signals.

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00:05:25,880 --> 00:05:28,070
And that's the origin
of the phonograph.

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He was interested in, could
you send a voice message

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00:05:31,580 --> 00:05:35,570
to a relay, have it
automatically recorded,

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00:05:35,570 --> 00:05:39,704
so that it could then be
rebroadcast over the next link?

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00:05:39,704 --> 00:05:41,870
So he was kind of imagining
a telephone network that

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00:05:41,870 --> 00:05:46,610
was based on the same principle
that the telegraph network had

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00:05:46,610 --> 00:05:47,720
been.

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00:05:47,720 --> 00:05:50,960
So the idea was to try
to record an audio signal

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00:05:50,960 --> 00:05:55,820
of the type that would come
across a telephone wire.

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00:05:55,820 --> 00:05:59,690
So he got together with
his aide, Batchelor,

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00:05:59,690 --> 00:06:01,100
and made this drawing.

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00:06:01,100 --> 00:06:04,460
This was 1877.

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00:06:04,460 --> 00:06:06,440
And 30 hours later--

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00:06:06,440 --> 00:06:09,080
kind of rapid prototyping
for the time--

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00:06:09,080 --> 00:06:11,750
his machinist made this.

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00:06:11,750 --> 00:06:13,610
So that's the first phonograph.

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00:06:13,610 --> 00:06:21,230
The idea was you wrap that
disk with a piece of wax paper.

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00:06:21,230 --> 00:06:23,480
You turn the crank.

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00:06:23,480 --> 00:06:29,320
As you turn the crank, this
thing is connected to a screw,

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00:06:29,320 --> 00:06:32,450
so that it's moving this way at
the same time this big thing is

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00:06:32,450 --> 00:06:34,820
going around that way.

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00:06:34,820 --> 00:06:38,300
This is a diaphragm, so
that you shout in here.

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00:06:38,300 --> 00:06:40,710
And there's a needle
connected on the other end,

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00:06:40,710 --> 00:06:43,190
so that as you shout,
the needle gets

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00:06:43,190 --> 00:06:48,500
moved with audio frequencies
contained in your voice.

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00:06:48,500 --> 00:06:50,300
So how do you do this, then?

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00:06:50,300 --> 00:06:52,970
So you grab the handle.

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00:06:52,970 --> 00:06:55,700
You put a new sheet
of wax paper on it.

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00:06:55,700 --> 00:06:56,810
You grab the handle.

147
00:06:56,810 --> 00:07:01,200
You turn at a constant
speed, and shout.

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00:07:01,200 --> 00:07:05,300
OK, it took some coordination.

149
00:07:05,300 --> 00:07:09,140
The remarkable
thing is it worked.

150
00:07:09,140 --> 00:07:12,986
So Edison said, Mary
had a little lamb.

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00:07:12,986 --> 00:07:17,390
And on replaying it, the device
said, Mary had a little lamb.

152
00:07:17,390 --> 00:07:20,020
The very first try, it worked.

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00:07:20,020 --> 00:07:22,220
Now it didn't work
all that great.

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00:07:22,220 --> 00:07:25,535
What do you suppose happened?

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00:07:25,535 --> 00:07:26,970
AUDIENCE: [INAUDIBLE]

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00:07:26,970 --> 00:07:27,886
DENNIS FREEMAN: Noise?

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00:07:27,886 --> 00:07:29,538
Of course, there was noise, yes.

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00:07:29,538 --> 00:07:30,900
AUDIENCE: [INAUDIBLE]

159
00:07:30,900 --> 00:07:32,525
DENNIS FREEMAN:
Frequency was terrible.

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00:07:34,669 --> 00:07:35,710
Anything more disastrous?

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00:07:35,710 --> 00:07:36,340
Yeah?

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00:07:36,340 --> 00:07:37,510
AUDIENCE: [INAUDIBLE]

163
00:07:37,510 --> 00:07:39,509
DENNIS FREEMAN: Cranking
is not at all constant.

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00:07:39,509 --> 00:07:42,520
So you can make it sound like
Alvin the chipmunk if you like.

165
00:07:45,980 --> 00:07:46,670
Something worse.

166
00:07:50,120 --> 00:07:53,734
How long do you think
the recording lasted?

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00:07:53,734 --> 00:07:55,210
AUDIENCE: [INAUDIBLE].

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00:07:55,210 --> 00:07:57,586
DENNIS FREEMAN:
Yeah, about once.

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00:07:57,586 --> 00:08:01,262
AUDIENCE: [INAUDIBLE].

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00:08:01,262 --> 00:08:02,720
DENNIS FREEMAN: So
the problem was,

171
00:08:02,720 --> 00:08:07,190
to get the energy back
out, you have to--

172
00:08:07,190 --> 00:08:12,500
so after recording-- so
clean sheet of wax paper,

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00:08:12,500 --> 00:08:15,760
crank, crank,
crank, yell, scream,

174
00:08:15,760 --> 00:08:18,340
and now reset the
needle to the beginning.

175
00:08:18,340 --> 00:08:21,010
And now crank without screaming.

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00:08:21,010 --> 00:08:22,990
In order to get the
message back out,

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00:08:22,990 --> 00:08:25,960
you had to push on the needle,
so that it would follow

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00:08:25,960 --> 00:08:29,100
the indentations on the wax.

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00:08:29,100 --> 00:08:31,860
But if you pushed hard
enough for it to follow it,

180
00:08:31,860 --> 00:08:34,559
what would happen?

181
00:08:34,559 --> 00:08:37,130
It would erase the message.

182
00:08:37,130 --> 00:08:40,429
So it was kind of a
destructive read out.

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00:08:40,429 --> 00:08:42,799
It didn't last very long.

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00:08:42,799 --> 00:08:47,850
Because the act of reading it
back out tended to erase it.

185
00:08:47,850 --> 00:08:51,960
So he redid the
thing with tinfoil.

186
00:08:51,960 --> 00:08:54,250
That worked much better.

187
00:08:54,250 --> 00:08:57,780
And in fact, it caught
on like overnight.

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00:08:57,780 --> 00:09:00,060
So within two years,
he was invited

189
00:09:00,060 --> 00:09:01,770
to the National
Academies of Sciences

190
00:09:01,770 --> 00:09:03,180
to give a talk on this.

191
00:09:03,180 --> 00:09:06,000
That's him posing at the
National Academy of Sciences

192
00:09:06,000 --> 00:09:09,210
with his then phonograph.

193
00:09:09,210 --> 00:09:12,460
They loved it.

194
00:09:12,460 --> 00:09:15,260
Now despite the
fact they loved it,

195
00:09:15,260 --> 00:09:17,750
nobody knew what to do with it.

196
00:09:17,750 --> 00:09:20,570
Everybody thought it was just
the neatest thing in the world.

197
00:09:20,570 --> 00:09:22,550
But they didn't have a
clue what to do with it.

198
00:09:22,550 --> 00:09:25,590
And of course, Edison, one,
was a bit self-promoting.

199
00:09:25,590 --> 00:09:28,940
So he wrote articles about
it, and tried to explain.

200
00:09:28,940 --> 00:09:31,160
In the patent, he listed
about seven or eight things

201
00:09:31,160 --> 00:09:32,990
you could do with this thing.

202
00:09:32,990 --> 00:09:37,070
Today they sound a
little far-fetched.

203
00:09:37,070 --> 00:09:42,080
Recording important
historical addresses,

204
00:09:42,080 --> 00:09:44,870
medical transcription--

205
00:09:44,870 --> 00:09:46,780
the one that seems most--

206
00:09:46,780 --> 00:09:49,610
I don't know-- sort
of gruesome to me,

207
00:09:49,610 --> 00:09:52,610
recording the last will
and testament of a person

208
00:09:52,610 --> 00:09:55,070
as they're dying.

209
00:09:55,070 --> 00:09:59,230
Now that sounds gruesome to me.

210
00:09:59,230 --> 00:10:01,280
Then imagine what it
was like back then.

211
00:10:01,280 --> 00:10:05,260
Back then, nobody had ever
heard somebody's voice

212
00:10:05,260 --> 00:10:06,040
who was not alive.

213
00:10:10,800 --> 00:10:13,140
The technology didn't exist.

214
00:10:13,140 --> 00:10:15,090
So all of the sudden,
he's proposing

215
00:10:15,090 --> 00:10:17,630
to record the dying
breath of, say--

216
00:10:17,630 --> 00:10:19,970
it kind of mortified everybody.

217
00:10:19,970 --> 00:10:21,960
What Edison didn't
like was the idea

218
00:10:21,960 --> 00:10:24,420
that it would be used for music.

219
00:10:24,420 --> 00:10:25,830
That was trivial.

220
00:10:25,830 --> 00:10:28,410
He was doing important science.

221
00:10:28,410 --> 00:10:31,350
So he didn't like the idea that
this would be used for music.

222
00:10:31,350 --> 00:10:33,900
And so he downplayed
that his entire life.

223
00:10:33,900 --> 00:10:39,300
Nevertheless, the commercial
success was music.

224
00:10:39,300 --> 00:10:45,510
He was trying to promote this as
a system for recording speech,

225
00:10:45,510 --> 00:10:49,350
medical transcription, buying
breaths, historical talks.

226
00:10:49,350 --> 00:10:50,520
That didn't really catch on.

227
00:10:50,520 --> 00:10:53,520
This was a playback device.

228
00:10:53,520 --> 00:10:56,160
This really did catch on.

229
00:10:56,160 --> 00:10:58,050
This was about 60 years later.

230
00:10:58,050 --> 00:11:00,886
It's virtually identical.

231
00:11:00,886 --> 00:11:02,760
It became a piece of
furniture that everybody

232
00:11:02,760 --> 00:11:05,460
had in their house.

233
00:11:05,460 --> 00:11:08,100
The cylinders had
gone back to wax.

234
00:11:08,100 --> 00:11:10,410
Alexander Graham Bell had
worked on the problem.

235
00:11:10,410 --> 00:11:14,010
He figured out how to
make long-lasting wax.

236
00:11:14,010 --> 00:11:17,920
This crank cranked a spring.

237
00:11:17,920 --> 00:11:20,205
So you could crank it up,
but over time it would play.

238
00:11:20,205 --> 00:11:22,330
So you didn't need to go
at exactly the right speed

239
00:11:22,330 --> 00:11:24,090
anymore--

240
00:11:24,090 --> 00:11:25,920
a number of big improvements.

241
00:11:25,920 --> 00:11:30,660
The cylinders held two
minutes of recording.

242
00:11:30,660 --> 00:11:33,780
So they weren't
exactly long-playing.

243
00:11:33,780 --> 00:11:42,620
OK, this was the same model of
phonograph I had in college.

244
00:11:42,620 --> 00:11:46,680
And it's actually
remarkably similar.

245
00:11:46,680 --> 00:11:50,340
So there's some differences.

246
00:11:50,340 --> 00:11:52,560
There was no crank.

247
00:11:52,560 --> 00:11:55,140
We had electricity.

248
00:11:55,140 --> 00:11:56,265
It had a motor.

249
00:11:56,265 --> 00:11:57,660
So the motor propelled it.

250
00:11:57,660 --> 00:11:59,100
It was no longer a cylinder.

251
00:11:59,100 --> 00:12:01,020
Now it's a desk.

252
00:12:01,020 --> 00:12:01,815
There was no screw.

253
00:12:04,700 --> 00:12:08,690
So how does the needle track
the grooves if there's no screw?

254
00:12:11,325 --> 00:12:12,710
AUDIENCE: [INAUDIBLE]

255
00:12:12,710 --> 00:12:14,085
DENNIS FREEMAN:
There's a spiral.

256
00:12:14,085 --> 00:12:15,409
So the records had a spiral.

257
00:12:15,409 --> 00:12:15,950
That's right.

258
00:12:19,760 --> 00:12:22,400
This was the thing
that read the grooves.

259
00:12:22,400 --> 00:12:27,020
What held that in the groove?

260
00:12:27,020 --> 00:12:27,850
It was gravity.

261
00:12:30,500 --> 00:12:33,080
So it was the same kind of idea.

262
00:12:33,080 --> 00:12:41,150
You had to push on the needle to
hold the needle in the groove,

263
00:12:41,150 --> 00:12:45,020
so that the needle could
report the vibrations that

264
00:12:45,020 --> 00:12:48,110
were recorded there.

265
00:12:48,110 --> 00:12:54,515
But this was much classier than
the version in Edison's time.

266
00:12:54,515 --> 00:13:01,670
And in fact, you could tell
how serious the audio file was

267
00:13:01,670 --> 00:13:04,604
by the different kinds of
performance enhancements

268
00:13:04,604 --> 00:13:05,520
of this sort of thing.

269
00:13:05,520 --> 00:13:09,660
So for example,
this little knob,

270
00:13:09,660 --> 00:13:12,995
this controls the
weight, the force

271
00:13:12,995 --> 00:13:14,711
that's placed on the needle.

272
00:13:14,711 --> 00:13:16,460
If you had a good set
up, you could set it

273
00:13:16,460 --> 00:13:18,890
for about 1.2 grams.

274
00:13:18,890 --> 00:13:21,050
If you had a bad set up,
it'd go up to 2 grams.

275
00:13:21,050 --> 00:13:29,879
So you could tell how serious
a person was by how much force

276
00:13:29,879 --> 00:13:30,795
was required to track.

277
00:13:34,010 --> 00:13:36,790
So for example,
the primary thing

278
00:13:36,790 --> 00:13:38,540
that determined how
well the system worked

279
00:13:38,540 --> 00:13:41,092
was the cartridge.

280
00:13:41,092 --> 00:13:42,550
So here's a blow
up of a cartridge.

281
00:13:42,550 --> 00:13:46,320
The cartridge--
anybody who is serious

282
00:13:46,320 --> 00:13:47,970
had a diamond-tipped needle.

283
00:13:47,970 --> 00:13:49,980
If they didn't have a
diamond-tipped needle,

284
00:13:49,980 --> 00:13:51,990
you just didn't even
think about them.

285
00:13:51,990 --> 00:13:54,120
So everybody I knew had
a diamond-tipped needle.

286
00:13:54,120 --> 00:13:56,970
That was just the way it was.

287
00:13:56,970 --> 00:13:59,160
The least expensive
kind of cartridges

288
00:13:59,160 --> 00:14:03,020
was a little piece
of piezoceramic.

289
00:14:03,020 --> 00:14:07,470
The piezoceramic
was stiff, so that,

290
00:14:07,470 --> 00:14:09,900
as you played the records,
they tended to wear out.

291
00:14:09,900 --> 00:14:12,450
Because you had to put
a lot of force on it.

292
00:14:12,450 --> 00:14:17,400
Mine, I had the V15 type 4.

293
00:14:17,400 --> 00:14:20,280
So this was a magnetic device.

294
00:14:20,280 --> 00:14:22,890
So there were a tiny
little electromagnets,

295
00:14:22,890 --> 00:14:24,450
so that the up and
down motions would

296
00:14:24,450 --> 00:14:26,130
be converted into
an electrical signal

297
00:14:26,130 --> 00:14:28,700
by magnetism, not by
piezoelectric effect.

298
00:14:28,700 --> 00:14:32,550
And you could track that
with a much smaller force.

299
00:14:32,550 --> 00:14:35,460
And I should just mention
just so you know that we

300
00:14:35,460 --> 00:14:36,810
were serious back then--

301
00:14:36,810 --> 00:14:40,890
so this cost 160 then dollars.

302
00:14:40,890 --> 00:14:45,780
There's been a factor of six or
so inflation, this cost $1,000.

303
00:14:45,780 --> 00:14:49,160
And if you were
serious, you had these.

304
00:14:49,160 --> 00:14:54,650
So that's kind of the
technology as it existed

305
00:14:54,650 --> 00:14:56,690
when I was an undergraduate.

306
00:14:56,690 --> 00:14:59,150
And it was good.

307
00:14:59,150 --> 00:15:02,480
It had existed for
over 100 years.

308
00:15:02,480 --> 00:15:07,100
But it was still basically the
same thing Edison had done.

309
00:15:07,100 --> 00:15:09,590
The media-- oh, I should
show it just in case

310
00:15:09,590 --> 00:15:12,020
you've never seen a media.

311
00:15:12,020 --> 00:15:13,730
So this is a record.

312
00:15:13,730 --> 00:15:14,670
This was my record.

313
00:15:20,080 --> 00:15:23,892
And the problem is, of
course, they scratch.

314
00:15:23,892 --> 00:15:25,600
So over time, these
things would scratch.

315
00:15:25,600 --> 00:15:27,599
So the technologies
for things like,

316
00:15:27,599 --> 00:15:29,140
how do you keep them
from scratching,

317
00:15:29,140 --> 00:15:30,020
how do you wear them out?

318
00:15:30,020 --> 00:15:32,680
And you can also appreciate sort
of where the music industry is

319
00:15:32,680 --> 00:15:33,490
coming from.

320
00:15:33,490 --> 00:15:35,680
This costs about $15--

321
00:15:35,680 --> 00:15:38,770
15 then dollars.

322
00:15:38,770 --> 00:15:41,660
So with factor six,
it's about $100.

323
00:15:41,660 --> 00:15:44,110
So from the point of
view of buying music,

324
00:15:44,110 --> 00:15:46,792
it was much more
expensive back then.

325
00:15:46,792 --> 00:15:49,000
And from the point of view
of the recording industry,

326
00:15:49,000 --> 00:15:49,720
think of it.

327
00:15:49,720 --> 00:15:51,400
You had all these
addicted people

328
00:15:51,400 --> 00:15:54,010
wanting to buy this stuff.

329
00:15:54,010 --> 00:15:55,120
And it wears out.

330
00:15:57,760 --> 00:16:00,190
So you play it for
two or three years,

331
00:16:00,190 --> 00:16:02,090
you have to buy a new one.

332
00:16:02,090 --> 00:16:04,560
OK, so you can sort of
appreciate where the music

333
00:16:04,560 --> 00:16:05,730
industry is coming from.

334
00:16:05,730 --> 00:16:08,360
Because things have changed.

335
00:16:08,360 --> 00:16:14,070
OK, so distortions, expensive,
fragile, blah, blah, blah.

336
00:16:14,070 --> 00:16:17,280
Then in the 1980s,
everything changed.

337
00:16:17,280 --> 00:16:20,400
In the 1980s, there was
the invention of the CD.

338
00:16:20,400 --> 00:16:23,250
The CD was invented
by Philips Corp

339
00:16:23,250 --> 00:16:25,830
and Sony Corp working together.

340
00:16:25,830 --> 00:16:29,930
And just everything
was revolutionized.

341
00:16:29,930 --> 00:16:35,960
So we'll talk a little bit
about how the CD works.

342
00:16:35,960 --> 00:16:38,300
The important thing
for the discussion

343
00:16:38,300 --> 00:16:43,410
is that, unlike records, CDs
are virtually indestructible.

344
00:16:43,410 --> 00:16:44,660
They have very low distortion.

345
00:16:44,660 --> 00:16:47,060
They are next to free.

346
00:16:47,060 --> 00:16:50,390
And all of that is
technology that's

347
00:16:50,390 --> 00:16:51,800
in large part enabled by 6.003.

348
00:16:51,800 --> 00:16:54,920
Of And that's what I want
to actually talk about.

349
00:16:54,920 --> 00:16:57,585
So what is a CD?

350
00:16:57,585 --> 00:17:00,620
A CD is not very
different from a record.

351
00:17:00,620 --> 00:17:02,979
So a CD has tracks.

352
00:17:02,979 --> 00:17:04,520
The information is
written in tracks.

353
00:17:04,520 --> 00:17:06,369
The tracks are
written as a spiral.

354
00:17:06,369 --> 00:17:10,359
The spiral starts at the
inside and spirals out.

355
00:17:10,359 --> 00:17:14,319
And so the idea is that you
read a spiral of messages.

356
00:17:14,319 --> 00:17:16,780
And just like a record--

357
00:17:16,780 --> 00:17:19,450
the way you make a record
is called embossing.

358
00:17:19,450 --> 00:17:21,319
You make a master.

359
00:17:21,319 --> 00:17:28,460
And then you take molten
plastic, and press it,

360
00:17:28,460 --> 00:17:32,780
and copy the patterns
to make that record.

361
00:17:32,780 --> 00:17:35,570
In fact, there is a big
technology on the formula

362
00:17:35,570 --> 00:17:38,030
that you used for the vinyl.

363
00:17:38,030 --> 00:17:40,460
One of the secrets of
RCA at the time when

364
00:17:40,460 --> 00:17:44,540
I was in graduate school,
they had a secret recipe

365
00:17:44,540 --> 00:17:48,680
that included dropping four
slices of American cheese

366
00:17:48,680 --> 00:17:50,300
in their vinyl producing.

367
00:17:50,300 --> 00:17:51,890
Because somebody had
accidentally once

368
00:17:51,890 --> 00:17:53,450
dropped a sandwich.

369
00:17:53,450 --> 00:17:56,970
And those records came out good.

370
00:17:56,970 --> 00:17:59,030
So there were magical
things like that that

371
00:17:59,030 --> 00:18:01,670
were trade secrets at the time.

372
00:18:01,670 --> 00:18:05,600
OK, so that technology
was basically embossing.

373
00:18:05,600 --> 00:18:09,410
And that's the same thing
you do when you make a CD.

374
00:18:09,410 --> 00:18:14,720
CD is poly carbonate,
which is injection molded.

375
00:18:14,720 --> 00:18:18,500
So you make a master.

376
00:18:18,500 --> 00:18:21,150
The master is usually
made out of aluminum.

377
00:18:21,150 --> 00:18:23,150
The master is made
very similar to the way

378
00:18:23,150 --> 00:18:24,530
that Edison would make a master.

379
00:18:24,530 --> 00:18:26,870
There's a big device
with an electromagnet

380
00:18:26,870 --> 00:18:30,710
with a small stylus
and it punches them.

381
00:18:30,710 --> 00:18:35,520
But then you copy the
CDs by injection molding.

382
00:18:35,520 --> 00:18:38,510
So you take that original.

383
00:18:38,510 --> 00:18:40,320
And there's actually
a complicated process.

384
00:18:40,320 --> 00:18:44,930
You use the original to
make a half a dozen fathers.

385
00:18:44,930 --> 00:18:47,030
The fathers are used
to make mothers.

386
00:18:47,030 --> 00:18:48,530
Each father makes 20 mothers.

387
00:18:48,530 --> 00:18:51,050
Then each of the
mothers make children.

388
00:18:51,050 --> 00:18:54,050
And so you end up being
able to get a lot of copies

389
00:18:54,050 --> 00:18:55,175
out of the original master.

390
00:18:58,460 --> 00:19:01,520
There's a reflective
layer of aluminum

391
00:19:01,520 --> 00:19:03,920
that's evaporated on top
of the polycarbonate.

392
00:19:03,920 --> 00:19:09,320
And then there's a thin layer
of plastic put over the top.

393
00:19:09,320 --> 00:19:14,450
That results in something
that's extremely robust.

394
00:19:14,450 --> 00:19:18,050
And we'll see in a minute
how they make it robust.

395
00:19:18,050 --> 00:19:22,640
But this dimension is
like over a millimeter.

396
00:19:22,640 --> 00:19:29,450
So this is the CD version
of the same album.

397
00:19:29,450 --> 00:19:34,460
So this surface is the
information reading side.

398
00:19:34,460 --> 00:19:36,930
There's a millimeter
of stuff here.

399
00:19:36,930 --> 00:19:39,830
There's a millimeter of
polycarbonate protecting

400
00:19:39,830 --> 00:19:42,320
this surface that
has the information

401
00:19:42,320 --> 00:19:44,030
from the environment.

402
00:19:44,030 --> 00:19:47,549
So in fact, if you want
to destroy somebody's CD,

403
00:19:47,549 --> 00:19:49,340
you might think the
way to do that would be

404
00:19:49,340 --> 00:19:51,080
to write in felt tip on here.

405
00:19:51,080 --> 00:19:52,310
No, don't do it that way.

406
00:19:52,310 --> 00:19:56,150
Use a ballpoint
pen on this side.

407
00:19:56,150 --> 00:19:59,060
Because that layer of
plastic is very thin.

408
00:19:59,060 --> 00:20:00,760
So use a ballpoint pen here.

409
00:20:00,760 --> 00:20:02,500
It will completely screw it up.

410
00:20:02,500 --> 00:20:08,335
So it's very robust when it gets
imperfections on the read side.

411
00:20:10,930 --> 00:20:15,350
OK, so that's kind
of the structure.

412
00:20:15,350 --> 00:20:19,510
It's virtually
indestructible is the point.

413
00:20:19,510 --> 00:20:22,090
So then, how do you get
the information on it?

414
00:20:22,090 --> 00:20:25,540
Well, the information is
coded in these patterns.

415
00:20:25,540 --> 00:20:27,350
The patterns are small.

416
00:20:27,350 --> 00:20:29,785
The pattern width
is half micron.

417
00:20:29,785 --> 00:20:34,940
A human hair is 50
to 100 microns thick.

418
00:20:34,940 --> 00:20:37,020
And I have white here.

419
00:20:37,020 --> 00:20:38,790
Well, I used to
have blonde hair.

420
00:20:38,790 --> 00:20:40,160
I have gray here now.

421
00:20:40,160 --> 00:20:42,470
Ignoring that for the moment,
I used have blond hair.

422
00:20:42,470 --> 00:20:45,860
Blonde hair is about
50 microns diameter.

423
00:20:45,860 --> 00:20:48,830
Black hair is about
100 microns diameter.

424
00:20:48,830 --> 00:20:50,630
So depending on the
color of your hair,

425
00:20:50,630 --> 00:20:55,100
it would obscure 30
to 60 tracks if you

426
00:20:55,100 --> 00:20:56,720
were to lay a hair across it.

427
00:20:56,720 --> 00:20:59,960
So these are very,
very small patterns.

428
00:20:59,960 --> 00:21:08,960
And so the trick for how you
code them is a big problem.

429
00:21:08,960 --> 00:21:12,657
Before we get there, though,
how do you code the audio?

430
00:21:12,657 --> 00:21:14,240
You won't be too
surprised to find out

431
00:21:14,240 --> 00:21:16,190
that it's a sample data system.

432
00:21:16,190 --> 00:21:19,060
So we do sampling just the way
we've talked about it before.

433
00:21:19,060 --> 00:21:22,550
Audio, you can hear
sounds from about 20 hertz

434
00:21:22,550 --> 00:21:24,170
to about 20 kilohertz.

435
00:21:24,170 --> 00:21:26,330
This is an ideogram.

436
00:21:26,330 --> 00:21:28,880
Audiogram's plot is a
function of frequency.

437
00:21:28,880 --> 00:21:30,950
What's the minimum
amplitude required

438
00:21:30,950 --> 00:21:33,129
to be able to hear
that frequency?

439
00:21:33,129 --> 00:21:35,045
You're most sensitive
in the kilohertz region.

440
00:21:37,600 --> 00:21:40,072
The scale over here
is DB in something

441
00:21:40,072 --> 00:21:41,280
we call sound pressure level.

442
00:21:41,280 --> 00:21:43,880
Sound pressure level is
the centigrade equivalent

443
00:21:43,880 --> 00:21:45,590
for hearing.

444
00:21:45,590 --> 00:21:47,090
So 0 is just audible.

445
00:21:47,090 --> 00:21:49,580
100 starts to hurt.

446
00:21:49,580 --> 00:21:53,910
120 is permanent
damage kind of thing.

447
00:21:53,910 --> 00:21:59,990
So you can hear about
100 decibels over a range

448
00:21:59,990 --> 00:22:02,890
from 20 to 20 kilohertz.

449
00:22:02,890 --> 00:22:07,260
And so the way they
coded it for the CD

450
00:22:07,260 --> 00:22:10,200
was sampled at 44.1 kilohertz.

451
00:22:10,200 --> 00:22:13,380
They chose that
funny number to be

452
00:22:13,380 --> 00:22:16,560
a little bit bigger
than twice the highest

453
00:22:16,560 --> 00:22:19,590
frequency that you can hear.

454
00:22:19,590 --> 00:22:21,450
But they made it
the funny number

455
00:22:21,450 --> 00:22:26,950
because they wanted it to
be an integer multiple of 60

456
00:22:26,950 --> 00:22:29,580
so it was easy to synchronize
with television signals.

457
00:22:29,580 --> 00:22:31,300
That's where the funny
number came from.

458
00:22:31,300 --> 00:22:37,470
So 44.1 kilohertz divided
by 60 is some integer.

459
00:22:37,470 --> 00:22:40,500
The problem is, of course,
that they wanted to be greedy.

460
00:22:40,500 --> 00:22:43,590
They wanted all the
bits to code audio.

461
00:22:43,590 --> 00:22:47,880
So they only sampled
barely over the frequency

462
00:22:47,880 --> 00:22:49,860
that was required.

463
00:22:49,860 --> 00:22:52,715
So according to the sampling
theorum, you need 40.

464
00:22:58,640 --> 00:23:02,700
So if you had a sample
data system running at 44,

465
00:23:02,700 --> 00:23:09,430
any amount of signal above 22
would alias and sound terrible.

466
00:23:09,430 --> 00:23:11,180
So you need to put an
anti-aliasing filter

467
00:23:11,180 --> 00:23:12,060
to prevent that.

468
00:23:14,820 --> 00:23:16,820
We've kind of cheated the
whole way through 003.

469
00:23:16,820 --> 00:23:18,960
We always said, when you
need a low-pass filter,

470
00:23:18,960 --> 00:23:19,890
use an ideal one.

471
00:23:19,890 --> 00:23:22,640
Well, they don't exist.

472
00:23:22,640 --> 00:23:26,030
we use ideal filters because
they're conceptually simple.

473
00:23:26,030 --> 00:23:27,770
There is no such thing.

474
00:23:27,770 --> 00:23:31,820
If you actually try to make a
filter using designs like Russ

475
00:23:31,820 --> 00:23:33,110
talked about in recitation.

476
00:23:33,110 --> 00:23:35,780
Butterworth filters,
things like that, they

477
00:23:35,780 --> 00:23:38,000
have transitioned bandwidths.

478
00:23:38,000 --> 00:23:40,310
So there is a range
of frequencies

479
00:23:40,310 --> 00:23:42,880
for which the filter is
neither fully on nor fully off.

480
00:23:42,880 --> 00:23:45,490
And it's in transition.

481
00:23:45,490 --> 00:23:48,440
You would like that range of
frequencies to be very small,

482
00:23:48,440 --> 00:23:49,880
so you don't waste bandwidth.

483
00:23:52,940 --> 00:23:56,180
So they would like to
reconstruct up to 40.

484
00:23:56,180 --> 00:24:02,210
And they allowed 4
kilohertz in the guard band.

485
00:24:02,210 --> 00:24:04,580
That was intentional,
because they

486
00:24:04,580 --> 00:24:06,770
didn't want to have a
lot of bits that were not

487
00:24:06,770 --> 00:24:09,037
useful for recording audio.

488
00:24:09,037 --> 00:24:11,120
That's the reason they
only went up to 44, instead

489
00:24:11,120 --> 00:24:14,690
of, say, 80, which would have
made the anti-aliasing problem

490
00:24:14,690 --> 00:24:16,700
simple.

491
00:24:16,700 --> 00:24:20,900
But it complicated the making
of the anti-alias filter.

492
00:24:20,900 --> 00:24:24,980
Because there is sounds
that come into microphones

493
00:24:24,980 --> 00:24:26,150
about 20 kilohertz.

494
00:24:26,150 --> 00:24:29,390
It's just that you
can't hear them.

495
00:24:29,390 --> 00:24:33,060
But if you were to record those
sounds, and then sample them

496
00:24:33,060 --> 00:24:36,879
at 40 kilohertz, you
would hear the alias.

497
00:24:36,879 --> 00:24:38,420
And it would be
highly objectionable.

498
00:24:38,420 --> 00:24:40,790
So you have to anti-alias.

499
00:24:40,790 --> 00:24:42,650
And that's actually hard.

500
00:24:42,650 --> 00:24:47,270
Because if you'd like
to hear up to 20,

501
00:24:47,270 --> 00:24:52,650
and if you're
sampling at 44.1, 20

502
00:24:52,650 --> 00:24:56,340
reflected down to
24.1, which means

503
00:24:56,340 --> 00:25:00,930
that you want the transition
band to be 34 kilohertz.

504
00:25:04,030 --> 00:25:06,540
And if you think
about the design

505
00:25:06,540 --> 00:25:09,960
of a filter like a
Butterworth filter,

506
00:25:09,960 --> 00:25:15,512
in order to get a transition
that attenuates by ADDB--

507
00:25:15,512 --> 00:25:17,220
think about the dynamic
range of hearing.

508
00:25:17,220 --> 00:25:22,170
You have to make the
loudest sounds small enough,

509
00:25:22,170 --> 00:25:23,730
so that you can no
longer hear them.

510
00:25:23,730 --> 00:25:28,050
So in general, you have to
attenuate by about ADDB.

511
00:25:28,050 --> 00:25:29,820
You need about 50 poles.

512
00:25:29,820 --> 00:25:37,030
To get an ADD attenuation
using a Butterworth design

513
00:25:37,030 --> 00:25:39,610
in 4 kilohertz at 20 kilohertz.

514
00:25:39,610 --> 00:25:43,470
Everybody sort of know
what I'm talking about?

515
00:25:43,470 --> 00:25:45,920
That's impossible.

516
00:25:45,920 --> 00:25:50,060
Lining up filters-- so the way
the Butterworth filters work,

517
00:25:50,060 --> 00:25:50,690
you remember?

518
00:25:50,690 --> 00:25:59,240
So you have a bunch
of poles like so.

519
00:25:59,240 --> 00:26:01,550
The way you think
about that is that you

520
00:26:01,550 --> 00:26:07,160
start with a design that looks
like a not-so-good low-pass

521
00:26:07,160 --> 00:26:09,530
filter.

522
00:26:09,530 --> 00:26:14,010
Then you add these
poles, which give you

523
00:26:14,010 --> 00:26:19,180
a peaky response like that, so
that when this is going down,

524
00:26:19,180 --> 00:26:23,800
the peakiness here
helps to cancel that.

525
00:26:23,800 --> 00:26:26,740
Then you do that again.

526
00:26:26,740 --> 00:26:30,820
So you can think about
these successive pole pairs

527
00:26:30,820 --> 00:26:35,060
as sharpening the
transition by pushing up

528
00:26:35,060 --> 00:26:38,400
on the center frequency
and rolling off

529
00:26:38,400 --> 00:26:39,790
on the edge frequency.

530
00:26:39,790 --> 00:26:42,850
But in order for that to work,
they have to all be lined up.

531
00:26:42,850 --> 00:26:44,320
So if you've got
50 of them, you've

532
00:26:44,320 --> 00:26:47,380
got to be able to line these
up to about a percent or so.

533
00:26:47,380 --> 00:26:50,830
Otherwise, they don't perform
the way they're supposed to,

534
00:26:50,830 --> 00:26:53,840
and they can actually
make things worse.

535
00:26:53,840 --> 00:27:00,130
So making an analog
filter of that type

536
00:27:00,130 --> 00:27:02,980
was impossible then
and is difficult now.

537
00:27:02,980 --> 00:27:06,190
Here is sort of the
state of the art.

538
00:27:06,190 --> 00:27:08,400
So this actually does
have ADDB of roll

539
00:27:08,400 --> 00:27:10,060
off, because it's
a fancier design.

540
00:27:10,060 --> 00:27:12,070
As an elliptic design.

541
00:27:12,070 --> 00:27:14,620
The trick here, though,
is that all the parts

542
00:27:14,620 --> 00:27:16,510
are laser trimmed.

543
00:27:16,510 --> 00:27:18,250
So they're all done
on one substrate.

544
00:27:18,250 --> 00:27:20,110
All the inductors,
all the capacitors

545
00:27:20,110 --> 00:27:22,652
are done on a single substrate.

546
00:27:22,652 --> 00:27:24,610
Then they're laser trimmed,
so that they follow

547
00:27:24,610 --> 00:27:27,550
the exactly the right places.

548
00:27:27,550 --> 00:27:30,850
Then it's encased
in aluminum to help

549
00:27:30,850 --> 00:27:34,030
to hold the temperature
constant and to keep

550
00:27:34,030 --> 00:27:37,420
the electromagnetic
interference out.

551
00:27:37,420 --> 00:27:42,290
And even so, this has
just 11 pole pairs.

552
00:27:42,290 --> 00:27:47,600
So this is not nearly adequate
for that kind of a design.

553
00:27:47,600 --> 00:27:56,000
So what we do instead is we use
something called oversampling.

554
00:27:56,000 --> 00:27:58,820
So instead of sampling
at 44.1, which

555
00:27:58,820 --> 00:28:01,190
is what you would think
would happen in a CD,

556
00:28:01,190 --> 00:28:04,680
you sample it four times that.

557
00:28:04,680 --> 00:28:07,590
By sampling at a
very high frequency,

558
00:28:07,590 --> 00:28:11,970
you make it easy to make
an anti-aliasing filter.

559
00:28:11,970 --> 00:28:14,130
Because now, you
only need to worry

560
00:28:14,130 --> 00:28:18,690
about saving the frequencies
up to 20 kilohertz.

561
00:28:18,690 --> 00:28:26,100
But you've got 170 kilohertz
to do the transition.

562
00:28:26,100 --> 00:28:27,990
So the filter design
becomes very easy.

563
00:28:27,990 --> 00:28:32,820
In fact, a typical design only
has five poles, and so just

564
00:28:32,820 --> 00:28:33,880
a handful of capacitors.

565
00:28:33,880 --> 00:28:38,020
Just two capacitors and
two op amps will do that.

566
00:28:40,960 --> 00:28:47,212
So then you've got a system that
is sampled at 176 kilohertz.

567
00:28:47,212 --> 00:28:48,420
What's the problem with that?

568
00:28:53,560 --> 00:28:56,623
What's the problem with
sampling at 176 kilohertz?

569
00:29:00,700 --> 00:29:02,630
Too much information.

570
00:29:02,630 --> 00:29:05,277
So what you've done by sampling
at the higher frequency

571
00:29:05,277 --> 00:29:07,610
is you've increased the amount
of information that needs

572
00:29:07,610 --> 00:29:10,210
to be stored by a factor of 4.

573
00:29:10,210 --> 00:29:14,980
So instead of being able to
record 74 minutes of music,

574
00:29:14,980 --> 00:29:18,310
you can record 74 divided
by 4, 18 minutes of music.

575
00:29:18,310 --> 00:29:20,800
Nobody would like that, right?

576
00:29:20,800 --> 00:29:23,960
So how do you fix that?

577
00:29:23,960 --> 00:29:27,300
AUDIENCE: [INAUDIBLE]

578
00:29:27,300 --> 00:29:29,820
DENNIS FREEMAN:
Downsample, exactly.

579
00:29:29,820 --> 00:29:36,690
So what you do is you run
that discrete time signal that

580
00:29:36,690 --> 00:29:42,330
was sampled at 176 through a
discrete low-pass filter, which

581
00:29:42,330 --> 00:29:46,050
you implemented digitally with
arbitrary precision as much

582
00:29:46,050 --> 00:29:48,520
as you want.

583
00:29:48,520 --> 00:29:57,930
So here's a design based on
a filter whose length is 200.

584
00:29:57,930 --> 00:30:01,350
By making a length 200
discrete time filter,

585
00:30:01,350 --> 00:30:04,222
you can capture the
basic sine T over T shape

586
00:30:04,222 --> 00:30:06,180
that you're trying to
get with an ideal filter.

587
00:30:09,300 --> 00:30:14,680
So here with 200 samples,
it's easy to reproduce

588
00:30:14,680 --> 00:30:17,500
a large number of ripples
similar to the large number

589
00:30:17,500 --> 00:30:19,850
of ripples that I
wanted over here,

590
00:30:19,850 --> 00:30:22,540
but now with digital
multiplies, not

591
00:30:22,540 --> 00:30:26,490
with resistors and capacitors.

592
00:30:26,490 --> 00:30:30,420
And this shows where the poles--

593
00:30:30,420 --> 00:30:32,940
this is an all 0 filter.

594
00:30:32,940 --> 00:30:34,350
It's a finite length filter.

595
00:30:34,350 --> 00:30:36,090
We didn't talk about that.

596
00:30:36,090 --> 00:30:39,270
It's an easy way to
implement filter design.

597
00:30:39,270 --> 00:30:41,620
It's an easy way
to design filters.

598
00:30:41,620 --> 00:30:43,530
It's a very popular
way of doing it.

599
00:30:43,530 --> 00:30:44,810
So it's an all 0 filter.

600
00:30:44,810 --> 00:30:46,560
And you can sort of
figure out by thinking

601
00:30:46,560 --> 00:30:50,100
about the spacing of the
zeros relative to the unit

602
00:30:50,100 --> 00:30:53,940
circle, how that implements
a low-pass filter.

603
00:30:53,940 --> 00:30:56,850
That was done with an
automated optimizing

604
00:30:56,850 --> 00:30:59,540
algorithm for finding
the best low-pass filter.

605
00:30:59,540 --> 00:31:03,780
The method is called the
Parks-McClellan algorithm.

606
00:31:03,780 --> 00:31:05,580
And it makes a very nice filter.

607
00:31:05,580 --> 00:31:10,680
So this shows the
transition of ADDB

608
00:31:10,680 --> 00:31:17,280
passing 20 kilohertz signals
and attenuating 24 and above.

609
00:31:17,280 --> 00:31:22,130
So you get a very nice filter
by doing that in discrete time.

610
00:31:22,130 --> 00:31:26,174
So then having done
the digital filter,

611
00:31:26,174 --> 00:31:27,590
you can throw away
the frequencies

612
00:31:27,590 --> 00:31:30,700
that would alias
and then downsample.

613
00:31:30,700 --> 00:31:31,314
Yeah?

614
00:31:31,314 --> 00:31:34,237
AUDIENCE: Is the filter
in those zero locations

615
00:31:34,237 --> 00:31:35,470
standardized by anybody?

616
00:31:35,470 --> 00:31:37,060
Or does any manufacturer--

617
00:31:37,060 --> 00:31:39,470
DENNIS FREEMAN: Any
manufacturer-- yes.

618
00:31:39,470 --> 00:31:41,132
Every manufacturer is different.

619
00:31:41,132 --> 00:31:42,590
What's happening
over time, though,

620
00:31:42,590 --> 00:31:45,290
is that the chips are
becoming standardized.

621
00:31:45,290 --> 00:31:48,487
So if you buy a particular chip,
it has a particular filter set.

622
00:31:48,487 --> 00:31:50,070
But it's not
standardized in the code.

623
00:31:53,750 --> 00:32:01,340
OK, so that's how you generate
a signal that is robust.

624
00:32:01,340 --> 00:32:03,530
And it's basically just 003.

625
00:32:03,530 --> 00:32:06,980
Now we have to think about,
how would you make the player?

626
00:32:06,980 --> 00:32:09,200
Because what we'd
like to do is enable

627
00:32:09,200 --> 00:32:16,140
you to use a CD in a
more portable fashion

628
00:32:16,140 --> 00:32:19,140
than you could use
this technology.

629
00:32:19,140 --> 00:32:21,060
So the question is
now, how would you

630
00:32:21,060 --> 00:32:27,120
make a player that can pick up
this information off the CD,

631
00:32:27,120 --> 00:32:30,540
so that you can
reproduce it in a Walkman

632
00:32:30,540 --> 00:32:35,440
or in a more
portable environment?

633
00:32:35,440 --> 00:32:38,620
So I had this brilliant
idea about two years ago

634
00:32:38,620 --> 00:32:45,070
that I would take this CD and
I would use my microscope.

635
00:32:45,070 --> 00:32:47,770
I may have mentioned
I light microscopy.

636
00:32:47,770 --> 00:32:48,880
I went into my microscope.

637
00:32:48,880 --> 00:32:50,630
I used my research-quality
microscope,

638
00:32:50,630 --> 00:32:52,810
which costs $70,000.

639
00:32:52,810 --> 00:32:57,730
And I'll take a beautiful
picture of the bits on here.

640
00:32:57,730 --> 00:33:03,100
And I'll write a problem set
for 003 where you have to decode

641
00:33:03,100 --> 00:33:04,780
the bitstream.

642
00:33:04,780 --> 00:33:06,940
I thought it was brilliant.

643
00:33:06,940 --> 00:33:09,520
And it was like a week
before the homework

644
00:33:09,520 --> 00:33:11,520
had to be written, you
know, the standard thing.

645
00:33:11,520 --> 00:33:14,020
And of course, I couldn't
get it to work in time.

646
00:33:14,020 --> 00:33:18,940
The problem was that even my
$70,000 research microscope

647
00:33:18,940 --> 00:33:22,620
has trouble seeing the bits.

648
00:33:22,620 --> 00:33:25,520
The bits are really small.

649
00:33:25,520 --> 00:33:31,020
So when I first tried it,
I couldn't see anything.

650
00:33:31,020 --> 00:33:32,760
So I fiddled around
with the microscope.

651
00:33:32,760 --> 00:33:35,280
I tried all different
kinds of magnifications.

652
00:33:35,280 --> 00:33:37,260
I tried all kinds of
different optical tricks.

653
00:33:37,260 --> 00:33:39,190
But I know I couldn't
figure out anything.

654
00:33:39,190 --> 00:33:39,780
I couldn't get it to work.

655
00:33:39,780 --> 00:33:41,029
So I got my graduate students.

656
00:33:41,029 --> 00:33:43,200
They're always much better
at this sort of thing.

657
00:33:43,200 --> 00:33:44,880
And we finally got it to work.

658
00:33:44,880 --> 00:33:47,520
We cheated like crazy.

659
00:33:47,520 --> 00:33:50,610
So we had to use something
called water immersion.

660
00:33:50,610 --> 00:33:53,490
So you put a drop of
water on the surface,

661
00:33:53,490 --> 00:33:55,170
so that you can use the--

662
00:33:55,170 --> 00:34:00,620
you lose resolution when
you go through a glass

663
00:34:00,620 --> 00:34:03,380
to air interface.

664
00:34:03,380 --> 00:34:05,960
That's a bunch of microscopy
that you don't need to know.

665
00:34:05,960 --> 00:34:07,010
But you do.

666
00:34:07,010 --> 00:34:08,810
So there are tricks
in microscopy

667
00:34:08,810 --> 00:34:13,250
where you can avoid having the
air interface by using water.

668
00:34:13,250 --> 00:34:16,880
The water is more closely
optically matched to glass.

669
00:34:16,880 --> 00:34:20,120
So what we did was we
did immersion microscopy,

670
00:34:20,120 --> 00:34:22,969
put a drop of water on here,
put the lens in the water.

671
00:34:22,969 --> 00:34:25,780
And that's the picture I got.

672
00:34:25,780 --> 00:34:32,580
So the point is that I had
to work to get a $70,000

673
00:34:32,580 --> 00:34:37,290
microscope to read
the bits off here.

674
00:34:37,290 --> 00:34:39,120
And I sort of gave
up with the idea

675
00:34:39,120 --> 00:34:41,340
that I would record a track.

676
00:34:41,340 --> 00:34:45,210
Because it was just too painful
to just get this one image.

677
00:34:45,210 --> 00:34:51,510
Now the trick is they make
players that cost $10.

678
00:34:51,510 --> 00:34:55,650
What do they know
that I don't know?

679
00:34:55,650 --> 00:34:58,230
So here I am with a
$70,000 research microscope

680
00:34:58,230 --> 00:35:00,860
having a difficult time
getting the bits off.

681
00:35:00,860 --> 00:35:07,070
How do they make a
CD player for $10?

682
00:35:07,070 --> 00:35:09,499
Just for fun, I did the
same thing with a DVD.

683
00:35:09,499 --> 00:35:11,790
After I figured out how to
do the immersion microscopy,

684
00:35:11,790 --> 00:35:14,840
I figured I'd at least--
so this is a DVD done

685
00:35:14,840 --> 00:35:17,800
with the same sort of thing.

686
00:35:17,800 --> 00:35:19,810
So how do they do it?

687
00:35:19,810 --> 00:35:21,250
They do very clever things.

688
00:35:21,250 --> 00:35:23,265
The first trick they
use is interference.

689
00:35:23,265 --> 00:35:25,560
So I talked about
interference last time

690
00:35:25,560 --> 00:35:30,510
with our standing wave
illumination microscope.

691
00:35:30,510 --> 00:35:32,400
They do interference.

692
00:35:32,400 --> 00:35:36,221
So I told you that this is
polycarbonate with an aluminum

693
00:35:36,221 --> 00:35:36,720
coding.

694
00:35:36,720 --> 00:35:39,930
The aluminum is actually
working like a mirror.

695
00:35:39,930 --> 00:35:41,380
And that's the intent.

696
00:35:41,380 --> 00:35:46,780
So they don't actually try
to code block and white.

697
00:35:46,780 --> 00:35:49,090
What they try to
code is distance.

698
00:35:49,090 --> 00:35:51,250
So the idea is that
you take a laser beam,

699
00:35:51,250 --> 00:35:55,240
and you out reflect
it off of the CD much

700
00:35:55,240 --> 00:35:58,580
like it reflects off a mirror.

701
00:35:58,580 --> 00:36:03,260
And then the features
that were embossed

702
00:36:03,260 --> 00:36:05,420
are at different depths.

703
00:36:05,420 --> 00:36:08,000
And the depth's chosen
very specifically

704
00:36:08,000 --> 00:36:11,250
to be lambda over 4, the
wavelength of the light.

705
00:36:11,250 --> 00:36:15,290
They use a 700 nanometer laser.

706
00:36:15,290 --> 00:36:24,290
And the features are
offset by lambda over 4.

707
00:36:24,290 --> 00:36:27,410
The idea is that--

708
00:36:27,410 --> 00:36:32,730
so if you have a
feature, and you have--

709
00:36:32,730 --> 00:36:40,120
so you compare, what would be
the timing for this laser beam

710
00:36:40,120 --> 00:36:42,106
if it struck the mirror?

711
00:36:42,106 --> 00:36:43,480
You compare that
to what it would

712
00:36:43,480 --> 00:36:45,250
be if it struck a feature.

713
00:36:45,250 --> 00:36:48,790
And you rig it so
that those two times

714
00:36:48,790 --> 00:36:52,610
are different by lambda
over 2, lambda over 4 in,

715
00:36:52,610 --> 00:36:56,140
and lambda over four out.

716
00:36:56,140 --> 00:37:02,830
So then if you rig the beam
to be half of the light

717
00:37:02,830 --> 00:37:08,190
falls on both, what's the
sum of these waveforms

718
00:37:08,190 --> 00:37:13,240
going to be if you
have a component that's

719
00:37:13,240 --> 00:37:16,630
at the mirror and a
component at the indented?

720
00:37:16,630 --> 00:37:19,810
If these are different in
timing by lambda over 2

721
00:37:19,810 --> 00:37:22,240
when you add them,
the answer is?

722
00:37:25,138 --> 00:37:26,110
AUDIENCE: [INAUDIBLE]

723
00:37:26,110 --> 00:37:27,550
DENNIS FREEMAN: Yeah, you
get complete destructive

724
00:37:27,550 --> 00:37:28,091
interference.

725
00:37:28,091 --> 00:37:29,950
You get an out answer of 0.

726
00:37:29,950 --> 00:37:31,780
So the idea in the
reading of them

727
00:37:31,780 --> 00:37:35,530
is to use interference
and rig it

728
00:37:35,530 --> 00:37:40,380
so that 0s are
represented by no light

729
00:37:40,380 --> 00:37:43,780
and 1s are represented
by lots of light.

730
00:37:43,780 --> 00:37:48,910
So that's the first
clever trick they do.

731
00:37:48,910 --> 00:37:52,540
Then the question is,
how do you focus it?

732
00:37:52,540 --> 00:37:56,470
When I was doing my research
microscope, the research

733
00:37:56,470 --> 00:38:00,730
microscope-- and first off, it
sits on a half ton of granite

734
00:38:00,730 --> 00:38:02,050
to make it all very stable.

735
00:38:02,050 --> 00:38:05,530
Because when you're
doing the focusing,

736
00:38:05,530 --> 00:38:07,360
you don't want--
so the vibrations

737
00:38:07,360 --> 00:38:13,230
of the floor in my lab
are about 100 micrometers.

738
00:38:13,230 --> 00:38:14,920
So it's about 100 micrometers.

739
00:38:14,920 --> 00:38:19,160
And I have to have the stability
of the system sub-micron.

740
00:38:19,160 --> 00:38:23,060
So I use a table made out
of a half ton of granite.

741
00:38:23,060 --> 00:38:24,890
And the microscope
sits on top of that.

742
00:38:24,890 --> 00:38:27,370
And that makes everything
nice and stable.

743
00:38:27,370 --> 00:38:29,150
They can't do that.

744
00:38:29,150 --> 00:38:32,470
So how do they control the
focus without using a half--

745
00:38:32,470 --> 00:38:35,470
I mean, it wouldn't work
to make a Walkman that

746
00:38:35,470 --> 00:38:38,620
is a half ton granite, right?

747
00:38:38,620 --> 00:38:41,560
So what do they
do to enable them

748
00:38:41,560 --> 00:38:45,430
to do the same kind of
focusing that I was doing?

749
00:38:45,430 --> 00:38:47,050
They use feedback.

750
00:38:47,050 --> 00:38:52,480
So the idea is to substitute
feedback for precision.

751
00:38:52,480 --> 00:38:55,210
So rather than trying to
make the parts precise,

752
00:38:55,210 --> 00:38:59,900
they put the imprecise
parts in a feedback loop--

753
00:38:59,900 --> 00:39:03,210
very powerful method.

754
00:39:03,210 --> 00:39:04,610
So here's what they do.

755
00:39:04,610 --> 00:39:05,990
They take the laser.

756
00:39:05,990 --> 00:39:09,030
It's trying to focus on to the
CD that has patterns in it.

757
00:39:12,040 --> 00:39:15,850
And so there's a beam splitter,
so that part of the light

758
00:39:15,850 --> 00:39:18,150
goes over to the detector.

759
00:39:18,150 --> 00:39:18,650
Excuse me.

760
00:39:18,650 --> 00:39:19,660
I didn't do that right.

761
00:39:19,660 --> 00:39:21,520
The laser, it
squirts up to here,

762
00:39:21,520 --> 00:39:23,380
goes straight through
the beam splitter,

763
00:39:23,380 --> 00:39:25,660
comes back through
a focusing lens,

764
00:39:25,660 --> 00:39:27,910
bounces off the beam splitter
through another lens,

765
00:39:27,910 --> 00:39:29,930
and hits a detector.

766
00:39:29,930 --> 00:39:32,170
So the detector is this way.

767
00:39:32,170 --> 00:39:33,462
And I'm showing this view then.

768
00:39:33,462 --> 00:39:34,795
So the detector should be there.

769
00:39:34,795 --> 00:39:36,280
I'm sharing a view
of it over here.

770
00:39:36,280 --> 00:39:39,760
And it's rigged, so
that these two lenses--

771
00:39:39,760 --> 00:39:43,720
one of the lenses is spherical
and one is cylindrical.

772
00:39:43,720 --> 00:39:45,330
What that means is--

773
00:39:45,330 --> 00:39:48,260
you remember I've talked
about microscopy in the past.

774
00:39:48,260 --> 00:39:51,530
Out of focus means blurred.

775
00:39:51,530 --> 00:39:55,080
Blurred for a spot means bigger.

776
00:39:55,080 --> 00:40:01,100
As a microscope goes out of
focus, the spot gets bigger.

777
00:40:01,100 --> 00:40:03,230
What you do with
these two lenses--

778
00:40:03,230 --> 00:40:05,870
because one is
circularly symmetric,

779
00:40:05,870 --> 00:40:08,210
and the other is
slenderly symmetrical,

780
00:40:08,210 --> 00:40:13,010
what you do is you make
the beam in this direction

781
00:40:13,010 --> 00:40:17,538
go out of focus as it's going
into focus in this direction.

782
00:40:22,380 --> 00:40:28,410
So because you get two lenses,
one is symmetric in xy.

783
00:40:28,410 --> 00:40:33,690
And one has no magnification in
y, but big magnification in x.

784
00:40:33,690 --> 00:40:37,620
You can rig it so that
this dot will go out

785
00:40:37,620 --> 00:40:45,230
of focus horizontally while it's
coming into focus vertically.

786
00:40:45,230 --> 00:40:53,070
The result is that, if you move
the CD closer, it becomes fat.

787
00:40:53,070 --> 00:40:57,640
And if you move the CD
further, it becomes skinny.

788
00:40:57,640 --> 00:41:03,070
So all they need to do,
then, is focus up and down

789
00:41:03,070 --> 00:41:05,080
until the dot is
as close to being

790
00:41:05,080 --> 00:41:07,120
circular as they can get it.

791
00:41:07,120 --> 00:41:10,370
That's why they use
a quadrant detector.

792
00:41:10,370 --> 00:41:14,100
So the quadrant detector
lets them calculate

793
00:41:14,100 --> 00:41:18,140
a plus c minus b plus d.

794
00:41:18,140 --> 00:41:21,140
And you would like
that answer to be 0.

795
00:41:21,140 --> 00:41:23,280
So now you use
that as your error.

796
00:41:23,280 --> 00:41:27,620
They use the light that
hits this quadrant detector

797
00:41:27,620 --> 00:41:30,530
to compute this number,
put that in a feedback loop

798
00:41:30,530 --> 00:41:34,160
that drives the stage
up and down to focus.

799
00:41:34,160 --> 00:41:36,410
So here's an ancient CD.

800
00:41:36,410 --> 00:41:38,350
The CD would be here.

801
00:41:38,350 --> 00:41:42,140
Here is the reading lens.

802
00:41:42,140 --> 00:41:46,490
Here is a platform that has
an electromagnet under it that

803
00:41:46,490 --> 00:41:48,335
will move it up and
down 100 microns.

804
00:41:54,410 --> 00:42:00,240
So this electromagnet
is under the stage.

805
00:42:00,240 --> 00:42:03,760
So this whole thing
goes up and down

806
00:42:03,760 --> 00:42:06,570
in a feedback loop that
tries to make this error

807
00:42:06,570 --> 00:42:12,090
signal be 0, tries to make the
pattern circularly symmetric.

808
00:42:12,090 --> 00:42:15,210
So it's extremely clever.

809
00:42:15,210 --> 00:42:21,430
Then you have this pattern.

810
00:42:21,430 --> 00:42:23,470
So the light is twice as big.

811
00:42:23,470 --> 00:42:26,650
Half the photons are
supposed to fit in the pit.

812
00:42:26,650 --> 00:42:28,210
And half of the
photons are supposed

813
00:42:28,210 --> 00:42:32,290
to hit in the land
that surrounds the pit,

814
00:42:32,290 --> 00:42:44,190
so that as the CD is spinning,
if the spot illuminates

815
00:42:44,190 --> 00:42:46,260
a place that's
entirely in the pit,

816
00:42:46,260 --> 00:42:50,640
the dot is small
because of interference.

817
00:42:50,640 --> 00:42:53,190
And if it hits a place
that doesn't have a pit,

818
00:42:53,190 --> 00:42:56,930
the dot is large, because
there's no interference.

819
00:42:56,930 --> 00:42:58,340
But then you have
the problem of,

820
00:42:58,340 --> 00:43:04,860
how do you keep the dot reading
the center of the groove?

821
00:43:04,860 --> 00:43:07,080
So in this technology,
we kept the dot

822
00:43:07,080 --> 00:43:11,040
in the center of the
groove by using gravity.

823
00:43:11,040 --> 00:43:13,560
We obviously don't want
to touch that surface.

824
00:43:13,560 --> 00:43:15,300
So that's not the
technology we use.

825
00:43:15,300 --> 00:43:18,210
We don't use a screw.

826
00:43:18,210 --> 00:43:19,065
We use feedback.

827
00:43:22,410 --> 00:43:27,420
So here what we do, instead
of projecting a single dot,

828
00:43:27,420 --> 00:43:29,700
we use a beam
splitter that turns

829
00:43:29,700 --> 00:43:32,880
the one dot into three dots.

830
00:43:32,880 --> 00:43:35,100
And the three dots
are positioned

831
00:43:35,100 --> 00:43:38,970
so that they hit two
more photo sensors, one

832
00:43:38,970 --> 00:43:42,520
on each side of the
main photo center.

833
00:43:42,520 --> 00:43:46,560
And the idea is that, if the
light is perfectly centered

834
00:43:46,560 --> 00:43:50,810
on a track, these will
have equal brightness.

835
00:43:50,810 --> 00:43:56,490
But now if the beam is off
center, one of the dots

836
00:43:56,490 --> 00:44:00,600
will be bright, because
it hit all land, compared

837
00:44:00,600 --> 00:44:05,070
to the other one, which some of
the light is hitting the pit.

838
00:44:05,070 --> 00:44:09,340
So this is just like the
head tracking thing in 601.

839
00:44:09,340 --> 00:44:13,720
So whichever of the two photo
sensors gets more light,

840
00:44:13,720 --> 00:44:17,120
you move the track to
compensate for that.

841
00:44:17,120 --> 00:44:19,910
So it all works in
a feedback loop.

842
00:44:19,910 --> 00:44:27,380
So the idea here is
the same sort of deal.

843
00:44:27,380 --> 00:44:28,910
You have two sensors here.

844
00:44:28,910 --> 00:44:33,940
And you use the error, which is
the difference between the two,

845
00:44:33,940 --> 00:44:36,260
to move the head to the track.

846
00:44:36,260 --> 00:44:39,120
And the head motion is
this thing over here.

847
00:44:39,120 --> 00:44:42,210
So here's a rack and pinion
connected up to a motor.

848
00:44:42,210 --> 00:44:45,680
And so you have that
motor servo-ed back

849
00:44:45,680 --> 00:44:50,630
to some photo sensors over here
that keep the beam centered.

850
00:44:54,470 --> 00:44:59,210
So the idea, then, is
that what you get for $10

851
00:44:59,210 --> 00:45:02,780
to read this thing is
completely remarkable.

852
00:45:02,780 --> 00:45:07,010
The idea is that it's got very
complicated servo mechanisms

853
00:45:07,010 --> 00:45:11,480
that can read very small
things that even a $70,000

854
00:45:11,480 --> 00:45:14,960
microscope has trouble reading.

855
00:45:14,960 --> 00:45:16,130
And it's stable.

856
00:45:16,130 --> 00:45:18,980
And you can run them, or
you can jostle them around,

857
00:45:18,980 --> 00:45:20,660
and jog with them.

858
00:45:20,660 --> 00:45:24,650
And that's all because they use
feedback, rather than trying

859
00:45:24,650 --> 00:45:27,170
to make precision machining.

860
00:45:27,170 --> 00:45:30,770
And then they make the
signals robust by using

861
00:45:30,770 --> 00:45:34,065
the kind of DT signal
processing that we talked about.

862
00:45:34,065 --> 00:45:36,290
So anyway, the point
of today was just

863
00:45:36,290 --> 00:45:40,010
to give it some motivation
of the way 6.003 factors

864
00:45:40,010 --> 00:45:43,380
into technologies that you
might be interested in.

865
00:45:43,380 --> 00:45:44,680
And this is just one example.

866
00:45:44,680 --> 00:45:47,150
I could have chosen any
number of different devices

867
00:45:47,150 --> 00:45:49,020
and they would
have been similar.

868
00:45:49,020 --> 00:45:50,960
So I hope you
enjoyed the course.

869
00:45:50,960 --> 00:45:54,060
I hope that you have
fun with the final.

870
00:45:54,060 --> 00:45:58,100
Please come on
Tuesday, and tell us

871
00:45:58,100 --> 00:46:00,630
what we should do
differently next time.

872
00:46:00,630 --> 00:46:02,180
Thanks.