Dynamic information storage or retrieval – Binary pulse train information signal – Binary signal level detecting using a reference signal
Reexamination Certificate
1999-09-02
2002-07-16
Hindi, Nabil (Department: 2651)
Dynamic information storage or retrieval
Binary pulse train information signal
Binary signal level detecting using a reference signal
C369S059220
Reexamination Certificate
active
06421309
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data retrieval systems, and more particularly to an apparatus and method for detecting a maximum mark length recorded on an optical disk.
2. Description of Related Art
In an optical disk such as a compact disc (CD) or a digital versatile disc (DVD), the frequency of signals read from the disk vary due to disk rotation speed variations. Also, the greater the radial distance of a track to the center of the disk the longer its length. Accordingly, in a constant angular velocity (CAV) system, the linear velocity is greater toward the outer circumference of the disk. As a result, even when the disk is rotated at a constant speed, a signal read from the outer circumference has a higher frequency. In general, the frequency may vary over an operating range of approximately ±30% to ±50%.
As a general countermeasure against such variation in the frequency, a reference or basic frequency is calculated based on signals recorded on the disk, and then the calculated reference frequency is set as the center frequency of a phase-locked loop (PLL). The reference frequency is generally calculated by using a maximum mark length obtained in a frame synchronous area (sync area) which are placed at regular intervals in the frames. A sync area as used herein means an area where the starting position of a data area is defined by a code not existing in the data area. In such a sync area, a frame number (frame ID) and a frame sync code used for frame synchronization are written. The frame sync code includes a longest mark for the synchronization (frame synchronization) that is longer than any other mark in the data areas. This longest mark is usually detected for computing the reference frequency.
In an optical disk such as a DVD, data are spirally or concentrically recorded on the surface of the disk. The data are recorded by using marks each having a length along the circumferential (tracking) direction. A mark is detected by using a difference in the quantity of reflected light depending upon the presence or absence of a mark (that is, a pit or a land) under irradiation with a laser beam or the like. For example, a mark can be detected on the basis of an interval between crossing points of a preset slice level and the quantity of reflected light (output waveform). In general, the length of a mark (hereinafter referred to as the “mark length”) is represented by using a time interval nT between the crossing points, where T indicates time per bit (namely, a fundamental period), and n is an integer. The mark is recorded with a length of an integral multiple of the fundamental period T. The reference frequency can be obtained as 1/T based on the fundamental period T. Hereinafter, a mark with a mark length of nT is expressed as a mark nT.
A conventional maximum mark length detector has, for example, a structure as shown in FIG.
6
. The maximum mark length detector
90
comprises a measuring device
92
for measuring mark lengths Lk, a maximum value register
94
for storing a maximum value Lmax of measured mark lengths Lk, and a comparator
96
for comparing the maximum value Lmax stored in the maximum value register
94
with a measured mark length Lk. When the measured mark length Lk is greater than the current maximum value Lmax in the register
94
, a control unit
98
substitutes the measured mark length Lk for the maximum value Lmax in the maximum value register
94
.
The maximum mark length can be detected, for example, by using a procedure as shown in
FIG. 7
in which the detection is conducted twice. It is assumed that a first maximum mark length Lmax
1
is first detected (step S
180
), and subsequently, a second maximum mark length Lmax
2
is detected (step S
182
). Then, the comparator
96
compares these maximum mark lengths Lmax
1
and Lmax
2
(step S
184
). When they are equal to each other, the detection is completed because the maximum mark length has been detected. In contrast, when they are different from each other, the detection is started over again. The measurement is carried out twice in order to confirm the value of the maximum mark length Lmax
1
. On the basis of the maximum mark length thus detected, the reference frequency of signals to be read from the disk can be obtained. However, usually the maximum mark lengths Lmax
1
and Lmax
2
do not exactly accord with each other due to the influence of measurement error, noise and the like. Therefore, typically they are considered to be equivalent, when:
L
max
1
−
L
max
2
<&Dgr;L
where &Dgr;L indicates a tolerance which takes into account measurement error, noise and the like.
In the detection of the maximum mark lengths (step S
180
and step S
182
), a mark detection time Tw is set, which has a duration of an integral multiple of a system clock and is selected to necessarily include a maximum length mark so that a maximum mark length can be detected within the detection time Tw. The detection (step S
180
and step S
182
) can be carried out by using a procedure as shown in
FIG. 8
, for example. First, when measuring device
92
provides a new measured mark length (measured value Lk) (step S
192
), the measured value Lk is compared with a current maximum value Lmax in the register
94
(step S
194
). When the measured value Lk is greater than the current maximum value Lmax, the measured value Lk is substituted for the maximum value Lmax in the register
94
as a new current maximum value (step S
196
). The maximum value Lmax in the register
94
is initially set to zero (step S
190
). Subsequently, the comparator
96
compares the elapsed time count measured by control unit
98
with the mark detection time Tw (step S
198
). When the measured elapsed time reaches the detection time Tw, the measurement of mark lengths is completed. In contrast, when the measured elapsed time does not reach the detection time Tw, a subsequent mark length is measured (step S
192
).
As described above, in the prior art, a fixed value related to the system clock has been used as the mark detection time Tw. However, as is shown in
FIG. 9
, when a disk
60
is rotated at a constant speed, the time interval between two sync areas
62
(hereinafter referred to as the “sync area interval”) varies due to the difference in the linear velocity between the inner circumference and the outer circumference. Accordingly, a signal read from the outer circumference has a higher frequency.
FIG. 9
, a reference numeral
64
denotes a data area, Tso indicates a sync area interval in the outermost circumference and Tsi indicates a sync area interval in the innermost circumference. Furthermore, the mark detection time Tw is required to include at least one sync area
62
(where a longest mark is recorded). Therefore, the mark detection time Tw is set on the basis of the sync area interval in the innermost circumference where the lowest frequency is obtained.
When such a fixed mark detection time Tw is used, as shown in
FIG. 9
, for example, the mark detection time Tw includes merely one sync area
62
in the innermost circumference but includes plural (seven in
FIG. 9
) sync areas
62
in the outermost circumference. However, one sync area
62
is sufficient for the detection. Therefore, in the outermost circumference, the maximum mark length detection is carried out even in the superfluous six sync areas
62
. Accordingly, six sevenths of the detection time is wasted in the outermost circumference. This increases the wait time before the start of a data read operation.
Moreover, a mark length is detected on the basis of the interval between the crossing points of the output waveform obtained from the disk and the slice level. Therefore, referring to an output waveform
70
shown in
FIG. 10
, when the slice level lowers (as is shown with a reference numeral
74
), two consecutive mark lengths L′ (k-1) and L′ (k) to be measured are:
L
′(
k
−1)=
L
(
k−
1
)−
2&Dgr;T
L
′(
k
)=
L
(
k
)+2&Dgr;
T
where L(k
Kanai Toshio
Ushio Teruhiko
Bluestone Randall J.
Hindi Nabil
International Business Machines - Corporation
Kunzler Brian C.
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