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BACKGROUND OF THE INVENTION
This invention relates generally to method and apparatus for optically
recording and reproducing information in and from an optical disc, and
more particularly, the present invention relates to method and apparatus
with which it is possible to rewrite or renew prerecorded information or
data.
In known optical disc recording and reproducing systems, a light beam, such
as laser light, having a diameter of 1 micrometer or less is incident on a
rotating disc having a photosensitive recording material layer thereon. As
a result, information is recorded in the form of pits, i.e. concavo-convex
portions, holes or light and shade portions. Namely, the energy of the
laser light beam is converted into heat so that the optical
characteristics, such as the reflection coefficent or the transmittance,
are changed to record the information at a high density.
In order to keep accurately the positional relationship between an optical
head and the optical disc irrespective of the variations in the optical
head feeding mechanism or in the disc drive mechanism, one or more guide
tracks which are optically detectable are provided so that tracking
control can be effected. As one example of such a tracking control,
coaxial or spiral guide tracks are made on the entire area of the
information signal recording region on the optical disc where the guide
tracks are made of grooves each having a width of approximately 0.5
micrometers and a depth which is approximately one eighth the wavelength
of the light used for recording and reproducing information. As another
example of tracking control, a servo track carrying a predetermined servo
signal is made so that recording and reproducing will be performed along
the servo track.
When using such an optical disc having one or more guide tracks, track
addresses are assigned respectively to the guide tracks so that recording
(writing) and reproducing (reading) of digital information or data can be
performed at any desired places on the optical disc. The guide tracks are
divided into a plurality of regions which are called sectors so that a
unit of information is recorded or reproduced by designating a region or
area defined by the track address and the sector address.
The track and sector addresses are recorded in a directory region also made
on the optical disc so that the positional relationship between the
optical head and a given portion of the disc can be controlled to write or
read information on or from the given portion. When it is intended to make
some changes in the prerecorded information, however, the prerecorded
information cannot be simply erased as in a magnetic recording medium
because optically changed portions cannot be restored to the original
state. For this reason, when it is desired to rewrite or edit prewritten
information on an optical disc, the information has to be read out first
from where it has been recorded to be editted, and then the editted or
corrected information has to be recorded on a portion where no information
has been recorded yet. In other words, the same portion or area on the
optical disc cannot be used for repetitive recording. When rewriting or
editting, which will be referred to as renewing hereafter, it is necessary
to employ an external nonvolatile storage, such as a magnetic disc or the
like, to store directories of various pieces of information or data
prewritten in an optical disc. Each directory includes a track address and
a sector address of a portion or region on an optical disc so that a
particular region can be found to reproduce or read data therefrom.
Each of the directories has an identification name or code to constitute an
index so that a given unit of information or data can be read out by
designating a predetermined identification. Identifications with which
directories of information pieces are designated should not be changed
even if the contents of information are renewed so that the renewed
information can be read out with its original identification. Therefore,
in conventional optical recording systems, when one or more pieces of
information prerecorded in an optical disc are to be renewed by writing
the editted information on an unused portion on the disc, the
corresponding directory or directories have to be changed and stored in
the external storage again. Namely, in the conventional technique of
renewing data on an optical disc, a nonvolatile external storage is needed
for each disc to keep the directories of all pieces of information
recorded in an optical disc. Accordingly, optical discs have to be used
with associated nonvolatile external storages in pairs. For this reason,
it has been difficult to handle optical discs. Furthermore, since it has
been impossible to erase prewritten information from an optical disc,
optical discs have to be kept with care so that secret information once
written is not read out by others.
SUMMARY OF THE INVENTION
The present invention has been developed in order to remove the
above-described drawbacks caused by the conventional optical recording and
reproducing systems.
It is, therefore, an object of the present invention to provide apparatus
for optically recording and reproducing information on and from an optical
disc, with which apparatus prewritten information or data can be renewed
without employing a nonvolatile external storage for storing directories
of pieces of information prewritten in an optical disc.
According to a feature of the present invention, a laser light beam
modulated by a predetermined d.c. or a.c. signal is applied to a given
region on an optical disc so that superposing writing is effected to
substantially erase prewritten information.
According to another feature of the present invention an output signal from
an optical head responsive to light reflected at an optical disc is
detected so that erased portions or regions can be distinguished from
unerased regions to skip erased regions on reading.
According to another feature of the present invention a given piece or
pieces of information can be ruined so that they cannot be read out even
if the optical head is correctly accessed by some means, thereby
protecting secret information.
In accordance with the present invention there is provided apparatus for
optically recording and reproducing information comprising: first means
for driving an optical disc; second means for emitting laser light beam so
that the beam is incident on said disc; third means for modulating said
laser light beam in accordance with an information signal; fourth means
for driving said second means; fifth means for controlling said first to
fourth means so that information is written on a file region on said
optical disc, while directory of said region is also written in a
directory region; sixth means for generating a superposing writing signal;
seventh means for modulating said laser light beam by said superposing
writing signal; eighth means responsive to light reflected at said disc;
and ninth means for detecting a region on which said laser light beam
modulated by said superposing writing signal has been irradiated from the
output signal from said eighth means; said fifth means being responsive to
an output signal from said eighth means so that region or regions, on
which said laser light beam modulated by said superposing writing signal
has been irradiated, are skipped when reproducing information from said
optical disc.
In accordance with the present invention there is also provided a method of
renewing information prewritten on an optical disc having a file region
and a directory region, comprising the steps of: searching directory of a
designated data to be renewed; reading out said data to renew the same;
storing said directory in a memory; erasing said data prewritten on said
disc by superposing writing which is effected by irradiating a laser light
beam on a corresponding region on said file region, where said laser light
beam is modulated by a predetermined d.c. or a.c. signal; searching the
last position of data prewritten on said disc; writing the renewed data in
an unused region which follows said last position; renewing the directory
stored in said memory so that a new directory designates the position
where said renewed data has been written; searching the last position of
directories which are prewritten in said directory region; and writing
said new directory at an unused region in said directory region which
follows said last position of directories.
In accordance with the present invention there is also provided a method of
renewing information prewritten on an optical disc having a file region
and a directory region, comprising the steps of: searching directory of a
designated data to be renewed; reading out said data to renew the same;
storing said directory in a memory; erasing said directory prewritten on
said disc by superposing writing which is effected by irradiating a laser
light beam on a corresponding region in said directory region, where said
laser light beam is modulated by a predetermined d.c. or a.c. signal;
searching the last position of data prewritten on said disc; writing the
renewed data in an unused region which follows said last position;
renewing the directory stored in said memory so that a new directory
designates the position where said renewed data has been written;
searching the last position of directories which are prewritten in said
directory region; and writing said new directory at an unused region in
said directory region which follows said last position of directories.
In accordance with the present invention there is further provided a method
of reading out information prewritten on an optical disc having a file
region and a directory region, comprising the steps of: detecting a
reproduced signal to check the envelope thereof so that it can be
ascertained whether information in each region can be read out or not; and
reading out information from each region by skipping one or more regions
when it has been detected that information therein cannot be read out.
BRIEF DESCRIPTION OF THE DRAWINGS
The object and features of the present invention will become more readily
apparent from the following detailed description of the preferred
embodiments taken in conjunction with the accompanying drawings in which:
FIGS. 1A and 1B are a top plan view and a side view of an example of an
optical disc which is used in the recording and reproducing apparatus
according to the present invention;
FIGS. 2A to 2C are explanatory diagrams showing the detailed structure of
the optically changed portion on or along a single track corresponding to
a portion defined by the line II--II in FIG. 1A;
FIG. 3 is a schematic block diagram of an embodiment of the apparatus for
optically recording and reproducing information according to the present
invention;
FIGS. 4A, 4B, 4C, and 4D show a recording state in a sector whose
information is to be erased;
FIGS. 5A to 5D are waveform charts useful for understanding various signals
with which a laser light beam is modulated to substantially erase
prerecorded information;
FIG. 6 is a schematic top plan view of an optical disc having a directory
region and a file region;
FIGS. 7A, 7B and 7C are explanatory diagrams showing a recorded state of a
plurality of files which are prewritten or registerd in the optical disc
of FIG. 6;
FIGS. 8A, 8B and 8C are explanatory diagrams useful for understanding an
embodiment of the way of renewing one of the files in the optical disc of
FIG. 6;
FIG. 9 is a flowchart showing the steps for renewing a file in accordance
with the embodiment of FIGS. 8A to 8C;
FIG. 10 is a flowchart showing the steps for reproducing or reading a
renewed file in accordance with the embodiment of FIGS. 8A to 8C;
FIG. 11 is a schematic block diagram of the envelope detecting circuit
shown in FIG. 3;
FIGS. 12A to 12E are waveform charts useful for understanding the operation
of the envelope detecting circuit of FIG. 11;
FIGS. 13A to 13C are waveform charts useful for understanding how a renewed
information is read out;
FIG. 14 is a flowchart showing the steps for the reading operation;
FIGS. 15A, 15B and 15C are explanatory diagrams useful for understanding
another embodiment of the way of renewing one of the files in the optical
disc of FIG. 6; and
FIG. 16 is a flowchart showing the steps for renewing a file in accordance
with the embodiment of FIGS. 15A to 15C.
The same or corresponding elements and parts are designated at like
reference numerals throughout the drawings.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIGS. 1A and 1B an example of an optical disc 1, which is
used in the recording and reproducing apparatus according to the present
invention, is shown. The disc 1 is made of the same material as those of
conventional optical discs. Spiral or coaxial tracks 2 are formed on the
optical disc 1 and it is shown that the tracks 2 are divided into a
plularity of information regions or sectors S1 to S10 and a track address
region TA. Each of the information regions S1 to S10 is equiangularly
arranged at an angle .beta. so that information will be recorded and
reproduced to and from each sector. The sectors are distinguished from
each other by using sector marks SM1 to SM9 and an index mark IM both
coaxilly provided at the inner most portion of the disc 1. The index mark
IM is radially alinged with the track address region TA both having an
angle .alpha. when viewed from the center of the disc 1. The index mark IM
will be used to designate the track address region TA and to indicate the
starting point for counting the sector number. In FIG. 1, the reference W
indicates an effective recording region within the coaxial tracks 2.
FIGS. 2A to 2C are explanatory diagrams showing the detailed structure of
the optically changed, i.e. recorded portion on or along a single track 2
corresponding to a portion defined by the line II--II in FIG. 1A. FIG. 2A
is a top plan view; FIG. 2B is a cross-sectional view; and FIG. 2C is a
waveform chart corresponding to the track portion illustrated in FIGS. 2A
and 2B. In FIGS. 2A and 2B, the reference 4 indicates a track address
portion of the track address region TA, while the reference 6 indicates a
guide track of the sector S1. The guide track 6 is formed by a flat groove
which extends coaxially, and the width and depth of the guide track 6 are
respectively expressed in terms of w and .delta.. This depth .delta. is
selected to be substantially equal to one eighth the wavelength of the
laser light beam so that a tracking signal for controlling a tiny light
spot will be readily obtained. The track address portion 4 is formed when
manufacturing the disc 1. Namely, a given pit pattern is cut in the
original or master disc so that the same pit pattern is copied by well
known stamping techniques.
As best seen in the cross-sectional view of FIG. 2B, a metallic film or the
like is vapor deposited on both the address track portion 4 and the guide
track 6 to provide a recording or writing layer 6a. Information recording
is actualized by either making holes in the recording layer 6a or changing
the optical characteristics thereof in the same manner as in conventional
optical disc recording techniques. Such recording techniques are known by
the name of heat mode recording. Prerecorded signals will be derived or
picked up from the recording layer 6a with a light beam whose power is low
such that it is below the threshold for sensitization of the recording
layer 6a. Thus, the intensity of reflected light will be detected so that
a reproduced signal will be obtained as shown in FIG. 2C.
In the track address portion 4, since track address information has been
prerecorded in the form of phase structure, the track address information
will be reproduced by using the variation in the intensity of the
reflected light which is obtained when a light spot of 1 micrometer is
diffracted at the concavo-convex portion of the track address portion 4.
Because of the difference in recording forms the amplitude of the
reproduced signal corresponding to the track address portion 4 is
different from that of the reproduced signal corresponding to the guide
track 6.
FIG. 3 illustrates a schematic block diagram of an embodiment of the
apparatus for optically recording and reproducing information according to
the present invention. The apparatus of FIG. 3 is arranged, as will be
described in detail hereafter, to write and read information or data on or
from a designated sector, and to renew or rewrite a designated sector.
The apparatus comprises a disc motor 7 so that the optical disc 1 is
rotated stably. A sector position detector 8 comprises a light-emitting
element and a photosensitive element to form a reflected light detector,
and is arranged to detect light reflected at the sector marks SM1 to SM10
and at the index mark IM which are coaxially arranged as shown in FIG. 1A.
An output signal from the sector position detector 8 is fed to a
separation circuit 9 in which an index signal "a" and a sector signal "b"
are separated from each other. The index signal Sa is then fed to a disc
motor driving circuit 10 so that synchronization of the disc motor 7 will
be obtained.
The sector signal Sb is fed to a sector address generating circuit 11 in
which counting is effected so as to designate a sector address of an
optical head 12.
Writing or recording in a sector of the optical disc 1 will be performed as
follows. Data to be written is fed from an input/output device 14 to a
buffer memory 15 under the control of a computer or central processing
unit (CPU) 13 as much as the capacity of a single sector. The CPU 13 sets
a writing track address in a track address register 16, and a writing
sector address in a sector address register 17. After this the CPU 13
produces a write instruction. On the other hand, a read out signal from
the optical head 12 is waveform equalized and pulse shaped in an RF
reproducing circuit 18, and a present track address is demodulated in a
demodulator 19, and is then stored in a present track address register 20.
The present track address will be compared with a writing track address
from the track address register 16 in a track address comparator 21. When
these two data are not equal to each other, a linear motor 22, which feeds
the optical head 12 at a high speed, is driven as much as the difference
therebetween. When the optical head 12 accesses a writing track,
comparison between a present sector address and the writing sector address
is effected in a sector address comparator 23. When these two sector
addresses are equal to each other, the output of the buffer memory 15 is
modulated by a modulator 24 to be fed to a semiconductor laser driving
circuit 25. The semiconductor laser driving circuit 25 modulates the
intensity of the laser light beam emitted from a semiconductor laser built
in the optical head 12 so that writing on the optical disc 1 is performed.
With this modulation holes or light and shade patterns are made on the
recording layer of the disc 1 resulting in variation in reflection
coefficient. When reading out prerecorded data, the output power level of
the semiconductor laser is set to be lower than that on writing so that
the level is below the threshold for sensitization of the recording layer
6a. A read/write control circuit 26 controls the timing of writing the
data of the buffer memory 15 into a sector and reading out, and also
controls the switching of the level of the output power of the
semiconcutor laser.
When reading out from a sector, a given sector is designated by setting a
reading track address and a reading sector address in the same manner as
in writing. When a reading sector is detected, a read out signal, which is
pulse shaped in the RF reproducing circuit 18, is then demodulated into
data in a demodulator 19. After demodulation, the read out data is written
in the buffer memory 15. Nextly, the read out data written in the buffer
memory 15 is fed to the data input/output device 14 by the CPU 13.
The reference 27 designates a directory buffer in which index information
of the data written in the optical disc 1 is temporarily stored, and this
directory buffer 27 is arranged to write the index information in a
directory region of the optical disc 1 when the amount of the same equals
the maximum amount of data which can be written in a single sector.
Prewritten information will be renewed as will be described hereinbelow. As
described in the above, since information is recorded as the change in
optical characteristics in the optical disc 1, erasure of the prerecorded
information cannot be effected in the same manner as in magnetic recording
media. In the present invention, therefore, one or more sectors in which
information to be renewed has been prerecorded, are made impossible to be
read out, and then renewed information is written in one or more unused
sectors. In other words, only renewed information can be read out while
old information has been ruined so that the apparatus according to the
present invention functions as if old information, which is no longer
needed after renewal, were erased from the disc 1. This is achieved by
writing a signal on a sector in which information to be renewed has been
written. This way of writing doubly is referred to as superposing writing
hereafter. In the following description, the word "erasing" is used to
mean that prewritten information is made to be unreadable because of
superposing writing.
When it is intended to erase information in a given sector, a superposing
signal "d" from a superposing signal generating circuit 28 is fed via a
superposing writing combiner 29 to the semiconductor laser driving circuit
25. FIGS. 4A and 4B show a recording state in a sector whose information
is to be erased. When a superposing signal of a d.c. like state, of FIG.
4D is recorded on a prewritten sector, the state of pits on the track is
made uniform as shown in FIGS. 4A and 4B along the track. As a result,
when the erased sector is scanned by the optical head 12 to reproduce
information therefrom, the reproduced signal takes a waveform as shown in
FIG. 4C having no information. Therefore, originally written information
can be regarded as if it has been erased. A signal to be superposed is not
limited to the above-mentioned d.c. like signal.
Other examples of superposing signals will be described with reference to
FIGS. 5A to 5D. FIG. 5A shows a waveform of a reproduced signal from
consecutive sectors whose numbers are #n, #n+1 and #n+2. FIG. 5B shows a
d.c. signal as a superposing signal in the same manner as in FIG. 4D. FIG.
5C shows an a.c. signal having a constant frequency. FIG. 5D shows an a.c.
signal having a random period. When an a.c. signal is to be superposed, it
is preferable to select the frequency thereof to be below one half the
demodulating frequency so that perfect erasing by superposing writing is
insured.
In the above described embodiment, although erasing or superposing writing
is effected with respect to one or more sectors, such erasing may be
effected with respect to one or more files when various data is written in
the optical disc 1 in each file made therein. The following embodiment is
useful for renewing data of a file which is controlled under a directory
provided on the disc 1.
The optical disc 1 shown in FIG. 6 has a directory region 30 at the
outermost portion and a file region 31 in an inner portion. The addresses
of the tracks on the disc 1 are written in a track address region TA,
while sectors constituting the tracks are not shown for simplicity. The
directory region 30 is used for providing an index for each of the
recorded files.
FIGS. 7A, 7B and 7C are explanatory diagrams for the recorded state of
files F1, F2 and F3 which are prewritten or registered in the optical disc
1 of FIG. 6. In detail, FIG. 7A shows the directory region 30 of FIG. 6;
FIG. 7B shows the contents of the directory buffer 27 of FIG. 3; and FIG.
7C shows the file region 31. In the directory region 30, directories of
the files F1 to F3 are written. Here the directory means information
indicative of the position of the corresponding file, such as F1, F2 or
F3, each written in the file region 31 as shown in FIG. 7C. Each of the
directories includes a head track address, a head sector address, the last
track address, a last sector address and various indicating information,
such as labels, dates and comments. The directory region 30 will be used,
therefore to search a desired file prewritten in the optical disc 1.
FIGS. 7A to 7C show an example where the capacity of a unit sector is large
so that a plurality of directories can be registered. In order to register
a plurality of directories in the directory region 30, the directory
buffer 27 of FIG. 3 is arranged to store therein the plurality of
directories. All the directories temporarily stored in the directory
buffer 27 will be written simultaneously when the directory buffer 27 is
filled so that a plurality of directories can be registered at once.
However, if desired, each directory may be written one by one.
It will be described how a file is renewed with reference to FIGS. 8A to 8C
showing the state of recording in the same manner as FIGS. 7A to 7C. Let
us assume that it is intended to renew or change the file F2 to F2'. FIGS.
9 and 10 are flowcharts for renewing a file and reading out a renewed file
F2'. When an instruction of renewing the file F2 is present, the directory
of the file F2 is searched so that the position of the file F2 in the file
region 31 is detected. The data in the file F2 is read out and is stored
in an external storage, such as a magnetic disc or the like. Then the
aforementioned erasing is effected in connection with the file F2. Namely,
an erasing signal or superposing signal is written on the prewritten
information of the file F2 so that the file F2 will not be able to be read
out.
The contents of the file F2 stored in the external storage will be renewed
or exited so that the contents of the data are ready to be written in the
optical disc 1 again. The renewed file is designated at a reference F2'.
After erasing, the last sector of the last file registered in the file
region 31 is detected so that the renewed file F2' will be written in an
unused sector which follows the last sector. Subsequently, the directory
of the renewed file F2' is stored in the buffer memory 27. When only a
single file is renewed, namely, only the file F2 is changed to F2' as
described in the above, the directory of the renewed file F2' stored in
the directory buffer 27 is written in the directory region 30. On the
other hand, when other files are also renewed or new files such as F4 and
F5 are to be added, the directories of these files are also stored in the
directory buffer 27 together with the directory of the file F2'. Suppose
new files F4 and F5 are to be added, the directories of these files F4 and
F5 are stored in the directory buffer 27 together with the directory of
the renewed file F2' as shown in FIG. 8B. In this case, each of the
directories is not written in the directory region 30 one by one but all
the directories are simultaneously written when the directory buffer 27 is
filled as described in the above. The above described steps for renewing a
file are shown in the flowchart of FIG. 9, and are executed by the control
of the CPU 13 of FIG. 3.
Renewed file or files will be read out from the optical disc 1 in the
following manner. When an instruction of reading out the file F2 is
present, the directory of the file F2' is searched so that the position of
the file F2, which has been erased as described in the above, is detected.
Thus the optical head 12 is directed to a position where the file F2 has
been written. However, the file data of the old file F2 cannot be read out
because of the above-mentioned superposing writing. The impossibility of
reading out will be detected as will be described in detail hereinafter,
and thus it will be ascertained that the file F2 has been renewed.
Therefore, the directory of the renewed file F2' is then searched to
obtain information of the position of the renewed file F2'. With this
directory information the optical head 12 is directed to the right
position of the renewed file F2' to read out the file data therefrom. At
this time it is detected whether the reading operation is normal or not.
If it is abnormal, the step of directory searching is again executed. This
normal/abnormal detection may be performed by detecting the address mark
which indicates the head of the data portion of the corresponding sector.
However, this system of normal/abnormal detection has a chance of
erroneously detecting the address mark due to defects of the optical disc
1. Therefore, it is necessary to ascertain that no error occurs
immediately after data has been written on the optical disc 1.
FIG. 11 illustrates a block diagram of an embodiment of the envelope
detecting circuit 32 of FIG. 3, which envelope detecting circuit 32 is
used to detect the impossibility of reading out the prewritten information
or data. Namely, the envelope detecting circuit 32 is responsive to the
frequency of the read out signal so that it is ascertained whether the
read out signal includes information or not. FIGS. 12A to 12E show various
waveforms useful for understanding the operation of the envelope detecting
circuit 32 of FIG. 11, and the operation of the envelope detecting circuit
32 will be described with FIGS. 11 and 12A to 12E.
FIG. 12A shows the state of variation in the amount of light reflected at
prerecorded sectors Si-1, Si and Si+1 where information in one sector Si
has been erased by superposing writing. In FIGS. 11 and 12B, the output
signal from the optical head 12 is designated at the reference "c", and it
is to be noted that the output signal "c" takes a waveform as shown in
FIG. 12B because of an a.c. amplifier built in the optical head 12. In
other words, a d.c. component in the reflected light variation is lost
when the output signal from a photosensitive element is passed through the
a.c. amplifier. A high pass filter 33 is responsive to the output signal
"c" from the optical head 12 so that a low frequency component included in
the read out signal "c" is suppressed. The output signal from the high
pass filter 33 is fed to a clamping circuit 34 in which d.c. restortion is
effected, so that the level of an envelope detecting signal will be large.
The output signal from the clamping circuit 34 is fed to an envelope
detector 35 which produces an envelope signal "k". This envelope signal
"k" has a waveform as shown in FIG. 12C, and is fed to a comparator 36
which also receives a reference voltage V.sub.R from a reference voltage
generator 37. As a result of the comparison, the envelope signal "k" is
converted into a binary signal "f" as shown in FIG. 12D. The output signal
"f" from the comparator 36 is fed to an ENABLE terminal of a counter 38
which is arranged to count the number of clock pulses "h" fed from a clock
pulse generator 39. The output signal "f" is also fed to a RESET terminal
R of a S-R latch 40. The counter 38 is arranged to count the number of the
clock pulses "h" when the output signal "f" of the counter assumes logic
"1", and when the count reaches a predetermined value, the output signal
"i" of the counter 38 is set to logic "1". In the presence of logic "1"
from the counter 38 the latch 40 is set to logic "1". The latch 40 is
arranged to be cleared when the output signal "f" from the comparator 36
becomes logic "0". FIG. 12E shows a waveform of the output signal "g" from
the latch 40, which output signal is the above-mentioned envelope
detection signal "g". As seen in FIG. 12E, no envelope detection signal
"g" is emitted when scanning the sector Si whose information has been
erased by superposing writing, while envelope detection signals "g" of
logic "1" are emitted when scanning other sectors Si-1 and Si+1 from which
written information can be read out. From the above it will be understood
that erased or doubly written sectors can be distinguished from unerased
sectors by the envelope detection signal "g" from the envelope detecting
circuit 32.
Now it will be described how the renewed data, which has been written into
an unused sector with old data being erased, is read out from the disc 1.
FIGS. 13A to 13C show the state in which a file prewritten into sectors S1
to S4 is renewed by rewriting the data of sectors S3 and S4 into unused
sectors S5 and S6 so that a new file is made in a region comprising the
sectors S1, S2, S5 and S6. Old data written in the sectors S3 and S4 have
been erased by superposing writing so that the data therein cannot be read
out. FIG. 13A is a waveform chart showing the amount of light reflected at
the disc 1, and it will be understood that the intensity of reflected
light from the sectors S3 and S4 is constant due to superposing writing.
Reading operation is effected in accordance with a flowchart of FIG. 14.
Namely, an envelope check is effected at the beginning | | |
