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Old 23rd February, 2007, 11:04 AM
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noob noob is offline
Join Date: June 2005
Location: Auckland NZ
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The speed limit for DVD's and CD's has to do with physical media tensile strengths.

CD x1 ~ 500 RPM
DVD x1 ~1500 RPM

CD's have a max sustained rotational speed of ~23,000 RPM (46x)
DVD's have a max sustained rotational speed of ~32,000 RPM (21x)

Because of the new nature of Blu ray / HDDVD, using a blue-violet laser as opposed to a red one, the pits are far closer together. This allows a much faster transfer speed without the initial requirement to speed up the disc.

Also because of certain design aspects (real blue-violet lasers haven't been around long), it has been harder to get similar power outputs as the more established red lasers, even though the shorter wavelengths have a higher intensity. Especially with the miniaturisation required.

As for speed -

Blu ray 1x = ~2000 RPM

assuming same parameters as DVD's 32,000 RPM max sustained = 16x
If they could be made stronger and reach say 40,000 RPM that would be 20x

Data speeds are also different;

CD 1x = 150kB/s
@ 12x (6000 RPM) = 1.8MB/s
@ max 46x = 6.9MB/s

DVD 1x = 1.32MB/s
@ 4x (6000 RPM) = 5.28MB/s
@ max 21x = 27.72MB/s

Blu ray 1x = 4.5MB/s
@ 3x (6000 RPM) = 13.5 MB/s
@ max 16x = 72MB/s
@ max 20x = 90MB/s

Max sustained speeds allow for a margin of safety. Mainly for unbalance to begin with, but now also includes sub standard widely avaliable media.
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Last edited by noob; 23rd February, 2007 at 11:05 AM.
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