SJ-MD100
– 6 –
Y
With a disc, high-speed random access is possible, something which is not possible with a tape.
Y
The optical pickup moves quickly in the radial direction, thus directly accessing the start of each track.
Y
With an optical disc, which is capable of playback only, the addresses in the TOC (table of contents) in the lead-in
area are all read beforehand in order to access the start of each track.
Y
With a recordable MD, which is capable of both recording and playback, blank guide grooves, called “pre-grooves”,
are formed around the entire surface of the disc at the production stage.
Y
As shown in the illustration below, the pre-grooves wobble very slightly in a regular pattern, and that curve is fine-
modulated (at intervals of 13.3 ms) to record beforehand the addresses of the continuous time data.
Y
Technically speaking, the pre-grooves in the disc is not perfect spiral but is wobble with :
– a typical amplitude of 30 nm.
– a spatial period of 54 to 64
¨
m.
When this wobble is locked to a frequency of 22.05 kHz, the velocity of the disc should be in the range of 1.2 to 1.4 m/s.
Y
In this way, even if the MD is blank, because the addresses are already recorded around the entire circumference
of the disc, when the MD is recorded, those addresses are read in order to control the movement of the optical
pickup and perform CLV control.
Y
To record immediately, quick access is first performed to quickly search for an blank area on the disc, and then
recording will automatically begin. When recording is completed, that address is automatically recorded in the U-
TOC (user table of contents) area, so it can be read during playback for quick and easy access. This also makes
it easy to edit the track number.
Y
Thus, with an MD, when the recording is completed, the track data is automatically written in the U-TOC area at the
innermost part of the magnetooptic disc. The data is then read during playback for quick and easy access.
Y
The addresses for the start locations of each track (track mark), and also the addresses for the end locations, are
all recorded in the U-TOC, so when editing a track all that has to be rewritten is the area address.
Y
This is similar to the directory that a computer writes on a floppy disk.
D
Random access on a recordable MD
U
R a n d o m a c c e s s o n a
playback-only MD
U
R a n d o m a c c e s s o n a
recordable MD
U
User TOC area
Pre-grooves on a magnet optic disc
*Spindle servo control means control of the disc’s rotation speed.
