[KLUG Members] re: Revolutionary CD Drives & Rotations Per Minute (RPM)

[Mike Williams]

>
>
>From: magoo <magoo@voyager.net>
>Subject: [KLUG Members] Revolutionary CD Drives & Rotations Per Minute (RPM)
>  
>
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>   A CD drive does NOT use constant angular velocity (CAV)! 
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>
>CD drives run at a variable speed depending on the track number!
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>Constant Angular Velocity (CAV) and Constant Linear Velocity (CLV) 
>Magnetic and optical storage drives can rotate with constant angular
>velocity (CAV), or constant linear velocity (CLV). CAV, used by magnetic
>drives (and record players), is measured in RPM, and means that the read
>head sweeps the same angle, for the same amount of time, at all radii.
>CLV, used in CD-ROM, allows the head to read the same length of track at
>all times and radii, at a rate of 1.3 m/sec--also called 'reference
>speed, or scan rate.' CLV requires that the disc spin slower as the head
>moves to the outer edge of the disc; a CD-ROM, for example, spins from
>539 RPM at the inner edge, to 210 RPM at the outer edge. 
>
That's how the standard was originally defined, but drives haven't 
actually functioned this way for a long time.  Under Constant Linear 
Velocity (CLV) you can store more data on the outer tracks than the 
inner because they're longer, and you can calibrate things so that the 
media is always moving by the read head at the highest speed it is 
capable of.  The drawback to this is that when the head moves from 
inside to outside you have to change the rotational speed of the motor.  
Original CD-ROM's were CLV. 

Hard drives have always been Constant Angular Velocity.  CAV runs the 
motor at a constant speed, but the media will be going over the head 
faster on the outside than the inside.  This makes less efficient use of 
the storage capability since you will have small sectors on the inner 
tracks and larger ones on the outside.  (Think pie pieces).  The 
advantage is that the motor always runs at the same speed, so you don't 
have to change it when moving the head.

All current CD-ROM drives are either CAV or a bit of a hybrid.  The 
fastest CLV drive I've ever heard of was about 12X, maybe 16.  The first 
CAV's called themselves 24X Max because they're maximum ratio was 24.  
Manufacturers started quoting the ratio of their drive's rotational 
speed vs the slowest rotational speed of a 1x drive, reached when the 
head was at the outside edge and the drive's motor was running it's 
slowest.  Current electronics are smart enough to deal with the fact 
that the 1's and 0's are going by the head at different speeds at 
different areas of the disk.

(cross-posting to hardware for obvious reasons)