(IBM Mainframe Discussion List) wrote:
In a message dated 6/20/2006 10:01:30 A.M. Central Daylight Time,
[EMAIL PROTECTED] writes:
the old Apple "Twiggy" drive (which appeared briefly
on the unlamented Lisa) varied its motor speed in order to pack more
data on those outer tracks while keeping the data transfer rate
constant. It was slow, since the rotational speed had to stabilize
after each seek.
How bizarre. Didn't know about that. There's always more than one way to
solve a problem. [There's another rule with no exceptions allowed. One or
more exceptions soon to follow.]
Every CD drive was designed that way. Bad way.
Precisely CD tracks have constant number of of bit per inch (cm in EU).
To have constant I/O rate of 150kB/s (remember first application: MUSIC!
bitrate MUST BE constant) there was Constant Linear Velocity (CLV),
which mean the spindle rotation must vary, depending on the head position.
It was good for music, but slow for computers. Veeeery slow.
The revolution came with ZCAV (Zone Constant Angular Velocity). CAV is
typical fo HDD, ZCAV + some intelligence in chipset allowed to achieve
speeds of 10x and more. Hint: '10x' means 10 x 150kB/s. Currently it is
approx. 52x and is stable for few years (there no more faster devices).
One of the limitations is durability of CD polycarbonate - centrifugal
force can destroy the CD.
Due to ZCAV issues CD is quite fast when reading large file, but
horribly slow when seeking and reading small pieces of data.
--
Radoslaw Skorupka
Lodz, Poland
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