Lecture 25: Audio CD - MIT OpenCourseWare Signals and Systems From LPs to CDs – and how 6.003 helps get you there. December 8, 2011. 1":;' •

6.003: Signals and Systems

From LPs to CDs – and how 6.003 helps get you there

December 8, 2011 1

":;'

•

Edison’s Phonograph

2


3


4


5


6

Image by Infrogmation on Wikimedia Commons.

http://commons.wikimedia.org/wiki/File:BRStateMuseumJuly08Amberola.jpg


7

Photo of Pioneer record player removed due to copyright restrictions.


8


9

Photo of Grado phono cartridge removed due to copyright restrictions.


LPs: 100 years of optimization → good fidelity, but

• fragile: easily scratched

• lots of distortions: e.g., wow and flutter

• expensive

CDs: much higher fidelity

• nearly indestructible

• very low distortion

• inexpensive

→ many of these advantages made possible by concepts from Signals and Systems!

10


11

Image by Dante Alighieri on Wikimedia Commons.

http://commons.wikimedia.org/wiki/File:CDR-large.jpg

What’s on a CD?

12

0.83-3.56 Jlm

k"'" protective layer (plus label) reflective layer (typically aluminum) polycarbonate (injection molded)

~~------~ +--~~------~ +--

Image by Dante Alighierion Wikimedia Commons.



LPs: 100 years of optimization → good fidelity, but

• fragile: easily scratched

• lots of distortions: e.g., wow and flutter

• expensive

CDs: much higher fidelity √

• nearly indestructible

• very low distortion

• inexpensive

→ many of these advantages made possible by concepts from Signals and Systems!

13

What’s on a CD?

14

0.83-3.56 Jlm


~~------~ +--~~------~ +--



What’s on a CD?

Continuous signal (audio)

Discrete storage (pits and lands)

→ sampling!

15

• • ...

What’s on a CD?

0

20

40

60

80

100

120

10 100 1000 10000 100000

Thre

shol

d (d

B S

PL)

Frequency (Hz)

Dadson & King, 1952Yeowart, Brian, & Tempest, 1967Green, Kidd, & Stevens, 1987

16

is = 44.1kHz !

Xm Ct) Anti-aliasing x Ct) Sample and x Ct) Analog-to- x [n) digital

CT filter hold converter

What’s on a CD?

17

Xm(f) Ideal

anti-al iasi ng filter

-20 20 f (kHz)

What’s on a CD?

18

XU)

JMM IM M(o 44.1 f (kHz)

X(!)

J (\ 11 (\ 111(\ (JI(\ l1i(o 20 t 44.1 f (kHz)

24.1

Without anti-aliasing

filter

With ideal anti-aliasing

filter

What’s on a CD?

19

---- 0 co "0

-20 1 ---<l> "0 -40 :::J -c -60 0> co -80 ~ 0 ----0> <l>

"0 --- -500 <l> 0> C « -1000

1 0° 1 0 1 1 02

Frequency (kHz)

What’s on a CD?

20

jm

What’s on a CD?

21

---- 0 co "0

-20 1 ---<l> "0 -40 :::J -c -60 0> co -80 ~ 0 ----0> <l>

"0 --- -500 <l> 0> C « -1000

1 0° 1 0 1 1 02

Frequency (kHz)

What’s on a CD?

22

Is = 176.4kHz

What’s on a CD?

23

0 176.4 f (kHz) XU)

, ,

~ , ,

~ ~ ~ A 0 176.4 f (kHz)

What’s on a CD?

24

Q)

-g -40 -c 0)

~ -80 ~~~~~--~~~~~~~~~ ~ 0 - r=~~~~-:~~-:~~ 0) Q)

::s.-200 Q) 0)

~ -400 ~~~~~~~ ____ ~;::::::~~~ 10° 10

1 102 103

Frequency (kHz)

What’s on a CD?

25

Is = 176.4kHz

What’s on a CD?

26

o Q) -40

"0 ::::l .~ -80 0)

20. r 24.1 kHz 88.2 kHz.

~ -120 L-~~-!---+.L.....UJL...1...-~LJ.J..:..~'..LL..----1L~..L.l.l.':"'-O ~~~~~~~--~~~--~-!-40 N

0) -80 "'v..----------c : « -120 L-~~-'-'--~~~ __ ~~~ __ ~_

o O. 0.2 0.3 0.4 0.5 DT frequency, Q /21t

What’s on a CD?

27

t:: co 0.. >.

1

~o c en co E

-1

-1

24.1 = 0.1366 176.4 20

..cfIII/J-......... / / = 0.1134

o

, 176.4

1 Real part

2

o

3

What’s on a CD?

28

<J.) '"d

;::I

0.2

~ 0.1 p, S

<r;

n

What’s on a CD?

29

X(Q)

n m m OT m fl OT signal

after sampling _ filter at 176 kHz

0 27t Q

Y(Q)

~ ffi & ffi n After

filtering with OT filter

0 27t Q

Ydown (Q)

J (\ 11 (\ 111(\ 11(\ l1i[ down~~~Pling 0 27t Q

What’s on a CD?

30

LPs: 100 years of optimization, good fidelity, but • fragile: easily scratched • lots of distortions: e.g., wow and flutter • expensive

CDs: much higher fidelity / • nearly indestructible v / • very low distortion v • very cheap

~ many of these advantages made possible by concepts from Signals and Systems!

What’s on a CD?

31

What’s on a CD?

Audio --7 bits: • sampling • filtering • DT processing of CT signals • downsampling

Next issue: how to you build a player?

32

YdownCQ)

What’s on a CD?

33

J {\ 11 {\ 111{\ 11 {\ 11 ( Spectrum of samples

on CD

Upsampled 4 X O

After filtering with

DT filter

o 211: Q DT filter .. ... . · m··· · ....

n ffiffiffimffiffiffi ffiffiffiffiffiffiffifl o 211: Q

~ ill ffi ffi ~ o 211: Q

What’s on a CD?

34

What’s on a CD?

35

0.83-3.56 Jlm


~~------~ +--~~------~ +--



What’s on a CD?

36

What’s on a CD?

37

Interferometric sensing: 6.003 explanation

mirror

a ~ f\ f\ f\ f\ f\ [t VVVVVV II: 11

b

What’s on a CD?

38


mirror

a ~ f\ f\ f\ f\ f\ [t VVVVVV • 12

b

What’s on a CD?

39


mirror

a ~ f\ f\ f\ f\ f\ [t VVVVVV II: 11

b

What’s on a CD?

40


mirror

a ~ f\ f\ f\ f\ f\ [t VVVVVV • 12

b

What’s on a CD?

41


mirror

bl f\!\!\!\[ VVVV b

What’s on a CD?

42

Focusing with feedback control

front view of quadrant

detector

What’s on a CD?

43


quadrant detector


detector

What’s on a CD?

44


quadrant detector


detector

What’s on a CD?

45



detector

What’s on a CD?

46


quadrant detector


detector

What’s on a CD?

47


quadrant detector


detector

What’s on a CD?

48


electromagnetic positioner


detector

A+C- 8- D

What’s on a CD?

49

What’s on a CD?

50

Image removed due to copyright restrictions. Please see Brain, Marshall."How CDs Work: CD Player Components." HowStuffWorks, April 2000.

http://electronics.howstuffworks.com/cd4.htm

Focusing with feedback

+ controller ~ focusing • magnet

7'

1'-

A+C-8-0 quadrature sensor

What’s on a CD?

51

Translating pits to bits

What’s on a CD?

52


What’s on a CD?

53


What’s on a CD?

54


What’s on a CD?

55


What’s on a CD?

56


What’s on a CD?

57


What’s on a CD?

58


What’s on a CD?

59


What’s on a CD?

60


What’s on a CD?

61


What’s on a CD?

62

Tracking with feedback control

What’s on a CD?

63

What’s on a CD?


64

What’s on a CD?


65

What’s on a CD?


66

What’s on a CD?


67


What’s on a CD?

68


What’s on a CD?

69


What’s on a CD?

70

What’s on a CD?


71

E-F-t?-positioner

What’s on a CD?


72

What’s on a CD?

73

Image removed due to copyright restrictions. Please see Brain, Marshall."How CDs Work: CD Player Components." HowStuffWorks, April 2000.

http://electronics.howstuffworks.com/cd4.htm

Tracking the tracks with feedback

desired C71 track --+.

location + ~-

controller ~ track drive motor

E-F differential sensor

• ,

What’s on a CD?

74

What’s on a CD?

LPs: 100 years of optimization, good fidelity, but • fragile: easily scratched • lots of distortions: e.g., wow and flutter • expensive

CDs: much higher fidelity / • nearly indestructible v / • very low distortion v/ • very cheap v

~ many of these advantages made possible by concepts from Signals and Systems!

75

From LPs to CDs -and how 6.003 helps get you there

f , = 176.4kHz

Sampling

i~ 1 S I i200~ I «.4rX.:O"~~--:';:'O ~--'3"?---;;1 0'

Frequency (kHz)

Filtering

'""l_ii~_ =--.~.~ Feedback

., , Real part

DT processing of CT signals

What’s on a CD?

76

What’s on a CD?

References:

“The history of the Edison Cylinder Phonograph”

http://memory.loc.gov/ammem/edhtml/edcyldr.html

“Audio Compact Disk – An Introduction” by Kelin J. Kuhnhttp://audio18.stormloader.com/Audio/compact_disc/audioCD.htm

“How CDs Work” by Marshall Brain http://electronics.howstuffworks.com/cd.htm

aborah

Line

http://memory.loc.gov/ammem/edhtml/edcyldr.html

http://audio18.stormloader.com/Audio/compact_disc/audioCD.htm

http://electronics.howstuffworks.com/cd.htm

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Documents

Lecture 25: Audio CD - MIT OpenCourseWare Signals and Systems From LPs to CDs – and how 6.003 helps get you there. December 8, 2011. 1":;' •