1. CDs and DVDs store data digitally as a series of pits and lands representing ones and zeros that are read by a laser beam. 2. The laser beam reflects off the pits and lands at varying intensities which are converted into electrical signals representing the binary data. 3. CDs can store about 650-750 MB of data in this way while DVDs use a shorter wavelength laser and smaller track sizes to store about 30 times more data than a CD.

1. CD’s and DVD’s.
1. http://electronics.howstuffworks.com/cd.ht
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2. Exploring Sound: Digital Sound
Laser discs such as CDs and DVDs carry digital
information, which is represented by the binary
code -- combinations of 1s and 0s.
Any number can be represented in binary code.
Learn how this information is encoded in the clip in
the above link.

2. CD’s and DVD’s
1. Data is stored digitally
A series of ones and zeros read by laser light reflected
from the disk
2. Strong reflections correspond to
constructive interference
These reflections are chosen to represent zeros
3. Weak reflections correspond to
destructive interference
These reflections are chosen to represent ones

3. CD’s and Thin Film Interference
1. A CD has multiple tracks
The tracks consist of a sequence of pits of
varying length formed in a reflecting
information layer
2. The laser beam shines on a metallic
layer through a clear plastic coating

4. A CD’s pits and bumps
www.physics.byu.edu/faculty/rees/106/PPT/Class26.ppt

5. 1
Reading a CD
1. As the disk rotates, the
laser reflects off the
sequence of bumps and
lower areas into a
photodetector
2. The photodetector
converts the fluctuating
reflected light intensity
into an electrical string
of zeros and ones
3. The pit depth is made equal to one-quarter of the
wavelength of the light

6. 2
Reading a CD
4. When the laser beam hits a rising or falling
bump edge, part of the beam reflects from
the top of the bump and part from the lower
adjacent area
5. Light reflecting from the top and bottom of
the pit is a half-wavelength out of phase, so
the intensity drops.

7. 3
Reading a CD
6. The bump
edges are
read as 1’s
7. The flat bump
tops and
intervening
flat plains are
read as 0’s

8. DVD’s
1. DVD’s use shorter wavelength lasers
2. The track separation, pit depth and
minimum pit length are all smaller
3. Therefore, the DVD can store about 30
times more information than a CD

9. Quality?
1. In the case of CD sound, fidelity (the
similarity between the original wave and the
DAC's output ) is an important goal, so the
sampling rate is 44,100 samples per second
and the number of gradations is 65,536.
2. At this level, the output of the DAC so closely
matches the original waveform that the
sound is essentially "perfect" to most human
ears .

10. Why is a CD’s capacity approximately 750 MB?
1. One thing about the CD's sampling rate and precision is
that it produces a lot of data.
2. On a CD, the digital numbers produced by the ADC are
stored as bytes, and it takes 2 bytes to represent 65,536
gradations.
3. There are two sound streams being recorded (one for
each of the speakers on a stereo system).
4. A CD can store up to 74 minutes of music, so the total
amount of digital data that must be stored on a CD is:
44,100 samples/(channel*second) * 2 bytes/sample * 2 channels *
74 minutes * 60 seconds/minute
= 783,216,000 bytes