Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUN~ OF THE INVENTION
Field of the Invention:
This invention relates to a method and an apparatus for
recording information signals on an optical disc, ln which
the digital multi-value recording is performed.
Conventionally, when digi-tal signals are recorded on
a record medium using a laser beam, digital signals of two
values representative of a lower level (0) and a higher
level (1~ are recorded.
The data transfer rate of 30 - 100 MBS is generally
necessary to record and reproduce the high quality digital
video signals in and from an optical disc. A conventional
optical disc on which analog video signals are recorded is
rotated at a speed of 1800 rpm/ so that it is difficult to
realize the above-mentioned data transfer rate in this case.
In addition, the recording surface density which is
2 - 10 times that of a present compact disc is required to
realize a digital video audio disc in diameter of, for
example, 20 cm, wherein video and audio signals are digitally
processed for record and reproduction.
The recording surface density a is expressed by the
following expression.
-n 2NA
~ = p
This expression may be obtained as follows.
Assuming that the most outer diameter of the program
area on an optieal dise is rO and the most inner diameter
is r1, the recording area S will be
S = 7r(rO ~ r,2)
Assuming that the truck pitch is P, the whole lPngth Q of
the reeording truek will be
Q = P 1 2~r dr = p ~r 2 _ rl2) = S
rl
When the recording is performed in the eonstant linear
velocity mode, and assuming that the recording linear
veloeity at that time is V, the reeording time T will be
Q S
V PV
On the other hand, assuming that the use wave
length o~ a light of the playbaek optieal system is ~
and the numerieal aperture of the lens is NA, the cut-off
frequency fc of the reproduction signal will be
fc = ~ x V~
Thus, we have the following expression.
T = S = S , 2NA
PV P ~ fc
It will be appreciated from this expression that the time
T when the record and reproduction are possible on a dise
is determined by the term of the dise parameter eonsisting
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of the recording area and truck pitch, the term of the
parameter of the playback optica:L system, and the cut-off
frequency.
When the data rate of information is assumed to be a
(bits/sec), the frequency band necessary to transmit the
data is determined in accordance with a Nyquist theor~m.
The ratio of this data rate to the necessary frequency
band is called a transmission efficiency and expressed by
a character n. That is,
n = f [BP Hz)
In case of 100% roll-off in the NRZ sys-tem, (n = 1),
and in case of 50% roll-off, (n = 1. 5). If the ideal
Nyquist filter was realized, (n = 2); however, in this case
the partial response method must be used.
As described above, it will be understood that the
cut-off fre~uency of the playback optical system is
determined by the data rate a (bits/sec) of the information
to be transmitted and the transmission efficiency n
Therefore, the recording time of a disc will be
T = S 2NA . 1 = S . 2NA . n
fc P ~ a
Now, assuming that the total amount of the data to be
recorded on a disc is A (bits), we have
A = a-T = p ~ n
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Thus, the recording surface density 5 will be
~ = A = n 2NA
It will be appreciated from the above expression that
in order to increase the recording density, it is necessary
firstly to increase NA of the playback optical system;
secondly to shorten the wave length of the playback optical
system; thirdly to make the truck pitch small; and fourthly
to elevate the transmission efficiency. However, with
respect to the first method, the numerical aperture may be
at most 0.45 - 0050 because an increase in NA causes the
skew angle tolerance of a disc to be reduced. With respect
to the second method, the short wave length of the semi-
conductor laser is limited presently to the order of at
most 0.7~ ~m. With respect to the third method, it is
difficult to realize a smaller truck pitch in consideration
of the playability. Therefore, it is difficult to remarkab-
ly improve the recording density by the above-mentioned three
methods~ Furthermore, as *he fourth method of elevating the ~;
transmission efficiency, it is considered the method in
that the information signals are transmitted in the Nyquist
frequency on the playback side in accordance with the partial
response, thereby obtaining (n = 2). However, it is impossi-
ble to realize (~ = 2) even if the partial response is used.
As described above, it is difficult to remarkably
improve the recording den ity of a disc even if any
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combinations of these conventional methods are used.
Therefore, another method is considered in that
multi-trucks are formed to substan-tially reduce the trans-
mission rate. However, this method results in the short
recording/playback time.
OBJ~CTS AND SUM~ARY OF THE INVENI'ION
It is an object of this invention to provide a method
and an apparatus for optically recording information signals
on a disc which can increase the recording information
amount and which is suitable for use in recording digital
video signals.
This invention provides advantages such that it is
possible to record multi-value digital signals of three
or more values on an optical disc, and therefore, it is
possible to increase the recording information amount more
than conventional method and apparatus in which two-value
digital signals are recorded.
~ ccording to the pres~nt invention, this and other
objects and advantages are accomplished by the provision
of a method and an apparatus for optically recording
digital signals on a disc, wherein the apparatus comprises:
a laser beam generating device; a modulator for modulating
the laser beam in accordance with multivalue recording
digital signals; and a device for forming pits in different
depths in a rotating record medium in correspondence to the
exposure amounts of the laser beam.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. lA - lD show waveform drawings of digital signals
of two to five values, respectively;
FIG. 2 is a schematic diagram showing the construction
of the optical system of a recording apparatus to which this
invention can be applied;
FIGo 3 is a schematic diagram showing an example of
the characteristic of a recording layer to which this
invention can be applied;
FIGS. 4A - 4D show schematic diagrams which are used
for describing this invention, wherein a recording signal
of the triangular wave is supplied to form the pits
representative of two values; and
FIGS. 5A and 5B show schematic diagrams which are used
for describing this invention, wherein recording signal of
three values is supplied to form the pits representative of
three vaiues.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT~
This invention provides a method and an apparatus which
are capable of recording a three-value digital signal of
l 1 and 2 (FIG. lB), a four-value digital signal (FIG. lC)
and a five-value digital signal (FIG. lD) as compared with
a conventional method and an apparatus wherein a two-value
digital signal at both a lower level (O) and a higher level
(1) as shown in FIG. lA iS recorded. In this way, by
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recording n values of three or more values, it is possible
to transmit larger information amount which is (log2 n) times
the information amount in the case of recording two values
(FIG. lA). Namely, it is possible -to transmit the infor-
mation amount which is incxeased by 1.58 times in the case
of three values; twice in the case of four values; and 2.3
times in the case of five values.
As shown in the waveform of FIG. lB, no transition
of the level is included between the minimum value (O) and
the maximum value (2) in case of three-value recording
signals, but the signals are defined so that the level of
the intermediate value (1~ always exists between them.
This is very effective to reproduce the waveforms of the
reproduction signals without any distortion.
FIG. 2 shows an example of a recording apparatus to
perform this invention. In FIG. 2, a reference numeral 1
represents a laser generator consisting of, for example,
a gas laser, from which a laser beam is supplied through
a mirror 2 to an optical modulator 3 using an acoustic
optical effect. A recording signal is supplied from a
terminal 4 to this optical modulator 3. The light path
of the laser beam to be output from the optical modulator
3 is changed by a mirror 5. The laser beam is then
converged by a lens 6 and its light path is again changed
by a mirror 7, then the laser beam is radiated through an
ob~ective 8 onto a recording layer 10 of a disc 9.
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The disc 9 is rotated at a predetermined angular
velocity by means of a spindle motor 12. The recording
layer 10 is coated on the surface of a glass base 11. A
number of pits in the depths correspondi.ng to the exposure
amounts are formed in the recording layer 10 in such a manner
that, for example, the development processing is performed
after exposure by utilizing a photoresist of the positive
type as the recording layer 10.
It can be mentioned, as an example of the photoresist
of the positlve -~ype, AZ-1350 of Shipley Corporation, where-
in sensitive material is added to a novolak resin.
FIG. 3 shows a relationship between an exposure level
L of this recording layer (photoresist) 10 and a depth D of
the pit to be formed therein, wherein a reference character
t indicates a thickness of the coating of the recording
layer 10. The exposure level L represents an energy to be
applied to the recording layer 10 at a linear velocity of
1 m/sec of the disc 9.
Referring now to FIGS. 4A - 4D, when a recording signal
of the triangular wave, for example such as shown in FIG. 4A,
is supplied to the terminal 4 of the optical modulator 3,
a laser beam is generated wherein the exposure energy
changes gradually as shown in FIG. 4B due to the non-linear
modulation property of the optical modulator 3. In FIG. 4B,
a character Ll shows a threshold value of the exposure
energy necessary to form a hole in the recording layer 10;
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L2 indicates the exposure energy in correspondence to the
depth at which a pit to be formed in the recording layer 10
reaches the surface of the glass base 11.
Thus, as shown in FIG. 4C, a slant portion is formed
in the recording layer 10 of the disc 9 wherein the depth
of the hole gradually becomes large in the range of Ll - L2
of the exposure energy and the width of the pit gradually
becomes large and finally almost equals to the diameter of
the recording beam spot. In the range where the exposure
energy is Lz or more, the hole in the depth which reaches
the surface of the glass base 11 is formed. FIG. 4C
illustrates a plan view and a cross sectional view of the
pit consisting of this slant portion and hole.
The thic~ness t of the coating of the recording layer
10 is selected to be a value which is slightly larger than
~/4 when the wave length of the laser beam for reproduction
is ~. In`the case where replica discs with an aluminum
reflection film surface are stamped using this disc 9 as a
"mother disc", the depth of each pit in this disc is also
set into a slightly larger value than ~/4. As described
above, since the depth of a pit is preset, when the pit is
read out, there is a difference in phase of ~/2 between the
laser beam reflected by the pit and the laser beam reflected
by the land around the pit, so that both phases will be set
off and no reproduction output will be generated. In the
case where the laser beam spot for reproduction is radiated
g
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substantially on the land, the reflected light consists of
the same phase component, so that the reproduction output
will be generated. Therefore, when the disc of which the
pits as shown in FIG. 4C have been formed in the reflection
surface is played back, a reproduction signal of such a
waveform as shown in FIG~ 4D will be obtained.
As described above, when a signal of the triangular
wave is recorded and reproduced, it is possible to obtain
a reproduction signal having the same inclination as that
of the recording signal. This inclination is caused on the
basis of the facts that slant portions exist between the
land and the pit and that the width of the pit changes.
That is to say, it is possible to perform the multi-value
recording using this principle.
For example in case of performing the three-value
recording, the bias center of the exposure energy is set
into the center Lb of the region where the exposure energy
is almost proportional to the depth D of the pit to be formed
in the characteristic shown in FIG. 3, and the exposure
energy by which a deep pit and a land can be formed around
this bias center is generated.
FIG. 5A shows an example of such a three-value
recording signal. The laser beam of the above-mentioned
exposure energy Lb is generated in response to the central
value Vl of this recording signal. The laser beam
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generation is stopped or the laser bearn of the exposure
energy which doesn't reach a threshold value Ll (refer to
FIG. 4B) is generated in correspondence to a low level VO.
The laser beam of the exposure energy of L2 or more (refer
to FIG. 4B) is generated in accordance with a high level V2.
Thus, as shown in FIG. 5B, the lands are formed in the
recording layer 10 of the disc 9 in the interval of the
recording signal of VO; the intermediate pits are formed
in the interval of V~; and the deep pit is formed in the
interval of V2. Although it is not shown, photoresist has
an excellent S/N characteristic; there~ore, it is also
possible to perform the recording of a five-value digital
signal by precisely controlling the exposure energy and its
recording and playback process.
This invention may be applied to the case where two
signal tracks are simultaneously formed using two laser
beams for recording. In this case, the transmission rate
of digital signals can be substantially increased, and
moreover, since there is no need to raise the recording
density, it is possible to easily recover the bit clock
when playing back a disc.
With this detailed description of the specific appa-
ratws used to illustrate the preferred embodiments of the
present invention, it will be obvious to those skilled in
the art that various modifications can be made in the
present method and apparatus described herein without
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71~
departing from the spirit and int:ended scope of the
invention which is limited only by the appended claims.
