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– 5 –

Y

The signals recorded on an MD are rewritten using a new process called “magnetic field modulation overwriting”*.

Y

In this process, a laser beam spot of about 5 mW is focused on the location on the disc to be rewritten, heating that
location to the Curie temperature (180

p

C) and thus canceling the magnetization.

Y

At the same time, current flows to the optical pickup and to the magnetic head opposite it, between the two of which
the disc is held, thus generating a magnetic field.

Y

When the disc revolves so the laser spot moves from the location to be rewritten, the temperature drops below the
Curie temperature and the magnetic field generated by the magnetic head re-magnetizes that location.

Y

At this time, if the direction of the current flowing to the magnetic head is reversed in accordance with whether the
data being recorded is “1” or “0”, the direction of the magnetic field also changes between north and south, and
accordingly, the direction of the magnetization of the recording film changes between upward-facing and downward-
facing.  Thus, it is possible to directly magnetize the recording film on the disc in accordance with the “0” and “1”
digital signals.

Y

Thus, the new recording data is overwritten regardless of the direction of the previously recorded magnetization,
eliminating the need for an erasing head.

Y

This process is called “magnetic field modulation overwriting”.

Y

Because this “magnetic field modulation overwriting” makes it possible to directly overwrite the new signals on top
of the old signals in a single process, re-recording on a MD is just as easy as with a magnetic tape, making the MD
ideally suited for use in personal audio equipment.

D

 Rewriting action of a magnet optic disc

U

MD rewriting process

U

No need for a erasing head

*Overwrite means to write new data while erasing the old data.

– 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

*Spindle servo control means control of the disc’s rotation speed.

– 7 –

Y

“ATRAC” (adaptive transform acoustic coding) is the name given to the signal compression technology that is one
of the most important technologies used in the MD system.  The principles of this technology are extremely complex,
so here we will only explain the basic concept.

Y

Simply speaking, utilizing the characteristics of the human auditory sense, the sounds that cannot be heard by the
human ear are eliminated so that only the sounds that can be heard remain, thus reducing the amount of data that
must be recorded.  This principle is similar to the PASC signal compression that is used for digital compact discs.

Y

The amount of data that can be recorded per second (the bit rate) is 16 bits x 44,100 x 2 channels = 1,411,200 bits
per second, or approximately 1.4 megabits per second (1 megabit = 1,000 kilobits = 1 million bits).

Y

However, because the diameter of an MD is only about one-half that of a CD, it only has a capacity of about 160
megabytes.  In order to record the same 74 minutes of data as a CD, the bit rate of 1.4 megabits per second must
be compressed to one-fifth.  “ATRAC” is the signal compression technology that was developed in order to do that.
(Signal compression is rather like the technology for orange juice concentrate, in which the best part of the fresh
orange is concentrated for easy transportation and then reconstituted with water before drinking.)

Y

The basic concept of ATRAC utilizes the characteristics of the human auditory sense.  The frequency spectrum
that cannot be heard by the human ear (Fig. B) and the frequency spectrum that cannot be heard because it is
masked by high-level sounds (Fig. C) are cut, and the bit rate is compressed by appropriately arranging the bits,
thus making it possible to fit all of the necessary data within the capacity of the disc.

Y

Technically speaking, the analog signals are converted to 44.1-kHz 16-bit digital signals, and these signals are then

processed by the ATRAC encoder.  Using a maximum time of 11.6 ms as a single unit, the encoder converts the
digital waveforms within each single unit into about 500 different frequency spectra and then analyzes the strength
level of each frequency.  Then, as shown in the figures below, utilizing the two principles of the “threshold of
audibility” and the “masking effect”, only the signals actually heard by the human ear are selected and compressed
to one-fifth the original volume.

Y

This complex process is performed by an LSI chip called the ATRAC.

D

 ATRAC signal compression technology

U

What does the word ATRAC
mean?

U

Why is signal compression
necessary?

U

Basic concept of ATRAC

Fig.A

Fig.C

Fig.B

– 8 –

Y

The figure below shows the composition of an MD system.  An MD system uses six LSIs that were specifically
developed for MD.  The following is a description of the functions of those six LSIs.

Y

This LSI controls the laser and performs the detection processing of the servo signals and of the audio signals from
the disc needed to correct the position of the laser spot.

Y

This LSI receives the laser spot position correction signals from the RF amplifier and controls the focus servo that
ensures that the laser spot is correctly focused on the disc surface and the tracking servo that controls the disc
rotation direction to ensure that the pits on the disc surface are traced correctly.

Y

This LSI demodulates the address signals that are recorded in the pre-group on a playback/record magnetooptic
disc MD.  In addition to reading the absolute addresses, the address decoder also functions to control the rotation
speed so that the servo signal is obtained at a constant linear speed at the location being read.

Y

This is the main signal processing LSI.  EFM is a circuit which converts 8-bit digital signals into the 14-bit format
recorded on the disc.  It also performs signal processing for error correction, changing the interleaving format in
order to use the ideal algorithm for MD.  In addition, this LSI also performs other tasks such as encoding during
recording and decoding during playback.

Y

This memory controller provides a "shock-proof" memory.

Y

This LSI performs the signal processing for the ATRAC compression technology that was specifically developed
for MD.

Y

New genaration LSIs developed for higher performance of MD system.
There are reduced to three LSIs.

1. RF amplifire
2. 4 ch driver (servo control)
3. Signal processor

U

Development of 6 LSIs

A

RF amplifier

B

Servo control circuit

C

Address decoder

D

EFM and ACIRC encoder/
decoder

E

A n t i - v i b r a t i o n   m e m o r y
controller

F

Audio signal compression
encoder/decoder

U

New generation LSIs

D