CD’s and DVD’s.

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

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.

CD’s and DVD’s1. Data is stored digitally

A series of ones and zeros read by laser light reflected from the disk

2. Strong reflections correspond to constructive interferenceThese reflections are chosen to represent zeros

3. Weak reflections correspond to destructive interferenceThese reflections are chosen to represent ones

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

A CD’s pits and bumps

www.physics.byu.edu/faculty/rees/106/PPT/Class26.ppt

Reading a CD1. 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

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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.

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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

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DVD’s

1. DVD’s use shorter wavelength lasers2. The track separation, pit depth and

minimum pit length are all smaller3. Therefore, the DVD can store about 30

times more information than a CD

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 .

Quality?

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