DVD has the following specifications, compared with standard CD:

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                          CD                    DVD
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Disc Diameter             120 mm                120 mm
Disc Thickness            1.2 mm                1.2 mm
Disc Structure            Single Substrate      Two bonded 0.6 mm substrates
Laser Wavelength          780 nm                650 and 635 nm
Numerical Aperture        0.45                  0.60
Track Pitch               1.6 um                0.74 um
Shortest pit/land length  0.83 um               0.4 um
Reference Speed           1.2 m/sec, CLV        4.0 m/sec, CLV
Data Layers               1                     1 or 2
Data Capacity             680 megabytes         Single layer: 4.7 gigabytes
                                                Double layer: 8.5 gigabytes
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