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5-22] How fast is 1x? What are CAV, CLV, PCAV, and ZCLV?




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This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

5-22] How fast is 1x? What are CAV, CLV, PCAV, and ZCLV?

(2004/03/03)

A player spinning a CD at 1x reads 75 sectors per second. On a CD-ROM,
where a sector has 2048 bytes, this is exactly 150KB/sec. On an audio CD,
with 2352 bytes per sector, this works out to about 172.27KB/sec. (Note for
the nit-pickers: the actual bit rate is considerably higher, because of EFM,
CIRC, L2 ECC, and other magic acronyms. The channel bit rate is 4.3218MHz.
See Ken Pohlmann's _Principles of Digital Audio_, 4th edition, page 249.)

In terms of revolutions per minute, the answer varies depending on which
part of the CD is being read. At 1x, the speed at which bits flow under
the read head (the "linear velocity") needs to be fairly constant. You can
get more bits in a circle at the outside of the disc than you can in a
circle at the inside of the disc, because the circumference is greater.
This means that the disc needs to spin more slowly (reduced "angular
velocity") at the outside than it does at the inside.

To play an audio CD, you always want to be reading at 1x. This means you
need a constant linear velocity that gives you 172.27KB/sec. The angular
velocity changes as you move toward the outside of the disc.

To read files from a CD-ROM, you want to be reading as fast as you can.
This means you'd like to maintain a constant angular velocity, spinning
as fast as the spindle can go, with a linear velocity that increases as
you move out to the outside of the disc. This is why a drive like the
Plextor 12/20 reads at 12x at the start of the disc and 20x near the end.

In practice, there is a maximum angular velocity because of physical
constraints, and a maximum linear velocity because of hardware and software
constraints. This results in drives that use constant angular velocity
for the first part of the disc, but limit themselves to a maximum linear
velocity. As the read head moves further out on the disc, the drive
switches to constant linear velocity mode.

Devices that always spin at the same rate are called CAV (Constant Angular
Velocity) drives. Devices that maintain a fixed linear velocity are called
CLV (Constant Linear Velocity) drives. Devices that switch from CAV to CLV
when the maximum speed is reached are called PCAV (Partial Constant Angular
Velocity) drives. Most of the recent high-speed CD-ROM drives are PCAV.
Devices that are CLV, but use different speeds on different parts ("zones")
of the disc, are called ZCLV. Most CD recorders use CLV while writing,
but some (e.g. 20x and higher) use PCAV or ZCLV.

See http://www.plextor.be/english/technical/zoneclv.html for a graph
illustrating ZCLV. http://www.cdspeed2000.com/go.php3?link=faq_general.html
has some nice charts showing CDSpeed output on different drive types.

You can compute how long it will take to record a disc with a CLV drive by
taking the amount of data and dividing it by the record speed of the drive.
A 74-minute disc will take about 19 minutes to record at 4x and a little
under 10 minutes at 8x. With a PCAV drive, this calculation is no longer
valid, because the velocity changes as the write head moves outward.

In terms of actual rotational speeds, a disc being read at 1x spins at
about 530rpm when reading near the center of the disc, slowing to about
200rpm at the outer edge. The linear velocity is constant, ranging from
1.2 m/s to 1.4 m/s depending on the disc. Discs with longer playing times
(e.g. 74 minute discs vs 60 minute discs) use the slower velocity.

It has been stated that, at a rotational speed equivalent to about 50x
at the inside of a disc, the polycarbonate starts to deform and the disc
becomes unreadable. Experiments (e.g. an episode of the "Mythbusters"
TV show from 2003) have demonstrated that discs will warp when they get up
around 25,000 to 30,000 RPMs. However, recent 52x drives only read data
that quickly from the outside of the disc, actually reading at about 21x
near the inside. This requires a speed of 10,000 to 12,000 RPM, which is
safe for discs in good condition. Reading at 52x from the very inside of
the disc would require a speed of about 27,500 RPM, and read data at 137x
near the outside.

Discs with minor defects can and will shatter at these speeds, so some
care must be taken with drives rated at 40x and above. See section (7-25)
for more information.

An unbalanced disc can cause noisy vibrations in high speed drives.
Some devices will actually reduce the spindle speed if the vibrations
become too severe.

Incidentally, "1x" on a DVD-ROM drive is 1353KB/sec, which is roughly 9x
the speed of a "1x" CD-ROM drive. A 16x DVD-ROM drive reads at a speed
equivalent to a 144x CD-ROM drive! The DVD doesn't actually spin 9x as
fast, though, because the DVD "bit density" is higher. The drive can read
roughly three times more data in a single revolution from a DVD than it
can from a CD. (Incidentally, the 1353KB/sec figure comes from the DVD
maximum user data rate of 11.08Mbps, where the 'M' is 1000*1000.) For
more details, see http://www.dvddemystified.com/dvdfaq.html#4.2.


 

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