High Speed RW Technology
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CD-R versus CD-RW technology
Both rely on the same logical standards. The same number of bits
is placed onto the surface of a recordable or a ReWritable disk. At exactly
the same place and format that the reading head of CD-ROM would expect them
to be. Consequently, the same on-drive electronics can be used for reading both
types of disks. This seems to have been the biggest advantage enabling quick
acceptance of CD-RW technology among drive manufacturers. Both types of disks
are recorded in one of two similar ways. Either on a track-by-track basis or
in small amounts of "chunks" called packets.
Furthermore, one or more non-packet written tracks can be recorded
under a single continuous head movement constituting a single session. If the
laser is turned on once during the recording phase of all adjacent tracks of
this section the recording is called Session-at-Once (SAO). Otherwise, it is
called Track-at-Once (TAO). Disk-at-Once is a special case of SAO mode when
no further recording on a disk is possible. Both recorded-once and ReWritable
ones behave from a user's perspective identically in all these recording modes.
Due to limitations of the encoding / error correcting features
of the original CD-audio format specifications at least 2 seconds of "silence"
have to be embedded when the laser is turned on/off. Each second of added silence
is equivalent to 75 frames, so a total of 150 frames of audio or data get lost
each time a new track is added on a disk. In terms of data bytes this is 150x2048
bytes, roughly 300KB!
Packet writing overcomes this limitation and (for drives particularly
designed for this, as all the latest ones) allows recording to be continued
by wasting just 7 frames (aka, linking blocks). 15KB versus 300KB of the earlier
case, an improvement of 2000%! The vision of a data CD becoming a large floppy
has been got very close. (Not by so much though, as we will see below
In the case of non-packet written tracks, the data files contained
on a single track follow the strict rules defined by the ISO-9660 standard and
some of its proprietary extensions (such as Joliet). A lot of (disk) space is
wasted for joining adjacent tracks, resulting to inefficient disk-space use,
as noted above.
When packet writing is used, the recording head with the help
of the underlying drive-logic turns the recording of the "actual"
data on and off at a much smaller time frame. By adjusting suitably the way
the "Table of Contents" (TOC) is stored on the disk, a much better
disk-space use is being made. The drawback here is that reading might be slowed-down
and a (predefined) longer-than-necessary TOC might incur additional disk storage
waste on a disk.
There are, however, a number of important, and in some cases even
difficult to "cure", disadvantages that (current) RW technology imposes...