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CD-ROM padder

Put CD ROM data at the outside of the disk, where it can be accessed most quickly
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In the early days of CD-ROM drives, data that was located near the center could be accessed more quickly than data near the edges, and could be read just as fast. On a 1x drive, for example, the disk would rotate at about 3.5 revolutions/second when accessing data near the middle and about 9.6 revolutions/second when accessing data near the edge. Thus, waiting for the drive to rotate to the correct spot would take an average of about 1/7 second for data near the center and 1/19 second for data near the edge.

On newer drives, however, the rotational speed doesn't very much on different parts of the disk, but the data rate does. Many so-called 48x drives are able to read data nearly twice as fast near the edge as near the center, with minimal difference in rotational latency.

I would therefore suggest that it might be advantageous to create a dummy session at the start of a disk that would fill up all of the unused area, so that all of the useful information was around the edges. For data which was only going to be written a couple times after it was written, this wouldn't be worth the extra writing time. For data that would be used a lot, however, the improved access speed could be quite useful.

Incidentally, writing data in this fashion would probably reduce slightly the power consumption of portable CD-MP3 players. Not sure the savings would amount to much, but it could be an added benefit.

supercat, Feb 18 2005


       I've always been under the impression that a 1x drive read at the same speed from center to edge, but that it spun much faster when reading from the center.   

       This would speed transfer rates for high speed drives, but wouldn't help access times very much. Since most high-speed drives spin up to a set speed (constant angular velocity) regardless of where it's reading, you'll still have to wait for the disk to spin up.   

       Do portable CD-MP3 players play at constant linear velocity? If they do, this might help quite a bit on power saving.
Freefall, Feb 18 2005

       Putting data at the edge might slightly impede certain access times, but for many applications with high speed drives that would be more than outweighed by the increase in transfer speeds.   

       Portable MP3 players typically use constant linear velocity, so the motor current would be reduced as a result of moving data out to the edges. On the other hand, such players often have sufficiently good bearings that unless a disk is singificantly off-balance (and in need of the CD Balancer) the current required to spin the disk is actually very small. I once popped the batteries out of a CD-MP3 player as it was running and the disk kept spinning for an amazingly long time--over 30 seconds IIRC).
supercat, Feb 18 2005

       Good idea, but it has a few flaws. One: the outside is most prone to damage (putting the disk in the tray can sometimes knick the edge) 2: Music is listened to at 1 to 1 speed on cd players, and probably the same with mp3 players. Except for finding the song there's not much more to deal with...and honestly, is a 1/7th second delay on getting to the spot on the disk, instead of 1/19th anything that a human can reliably notice? approx 0.09022 of a second is the difference...
Sabriand, Feb 18 2005

       On slower drives, the access speed is faster near the middle than near the edge, while the transfer speed is the same. It's on faster drives (32x+) that the transfer rates change with position, and on those they get faster near the edge. Many 48x drives transfer data almost twice as fast near the edge as near the hub.
supercat, Feb 18 2005

       As a power saving method in a portable device, maybe....but putting data that will be used often in the most damage prone parts of a disk? isn't that asking for the misfortune fairy to notice you?
Sabriand, Feb 18 2005

       This is baked for pirate/homebrew Dreamcast games. The system has a constant angular velocity, and reads the outer edge much faster. Without this, some games can't load textures fast enough for a seamless game. It would make more sense to write all discs from the outside-in, but that would involve hardware changes.
Aq_Bi, Feb 18 2005


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