Dynamic information storage or retrieval – Binary pulse train information signal – Binary signal gain processing
Reexamination Certificate
1998-05-29
2001-01-30
Tran, Thang V. (Department: 2753)
Dynamic information storage or retrieval
Binary pulse train information signal
Binary signal gain processing
C369S047360
Reexamination Certificate
active
06181658
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical record carrier comprising a recording layer for recording information in a pattern of optically detectable marks in substantially parallel tracks, the tracks comprising alternating first and second servo tracks, the servo tracks having a track modulation different from the information pattern and indicative of position information.
The invention also relates to a method of scanning a record carrier having substantially parallel tracks, the tracks comprising alternating first and second servo tracks, the servo tracks having a track modulation different from the information pattern and indicative of position information.
The invention further relates to an apparatus for scanning said record carrier, comprising an optical system for scanning first and second servo tracks having a track modulation by a radiation beam, a detector for detecting the radiation beam coming from the record carrier and modulated by the track modulation, and a signal processor for deriving a position-information signal from an output signal of the detector.
The invention additionally relates to an apparatus for manufacturing said record carrier, comprising an optical system for scanning a radiation-sensitive layer of a record carrier by a radiation beam along a path corresponding to the servo tracks to be formed and a modulation unit for modulating the radiation beam in such a way that the pattern formed by the radiation beam in the radiation-sensitive layer corresponds to a control signal applied to the modulation unit.
BACKGROUND OF THE INVENTION
A track is a line on the record carrier to be followed by a scanning device and having a length of the order of a characteristic dimension of the record carrier. A track on a rectangular record carrier may have a length substantially equal to the length or width of the record carrier. A track on a disc-shaped record carrier is a 360° turn of a continuous spiral line or a circular line on the disc.
This type of record carrier including associated apparatuses is known from the Japanese patent application no. 06338066. The record carrier described therein comprises first and second alternating servo tracks in the form of adjacent grooves in a substrate. The first servo tracks are frequency modulated at a relatively low frequency, the second servo tracks are frequency modulated at a relatively high frequency. On scanning the record carrier, the scanning device switches between the low and high frequency when changing from one servo track to the next servo track.
The conventional record carrier has the disadvantage that the two frequencies must have a relatively large difference, or frequency guard space, to be able to separate them in the scanning device and thereby create a low cross-talk between the servo tracks. Hence, the total bandwidth of the low-frequency range, the guard space and the high-frequency range is relatively large. However, the total bandwidth should be small to avoid interference with the information recorded in the marks. Moreover, a frequency-modulated signal gives more triangle noise at increasing frequency, making it necessary to increase the bandwidth of the high-frequency range or increase its track modulation to reduce the noise.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a record carrier, scanning method and associated apparatuses that allow equal information densities in the servo tracks while keeping the crosstalk at a low level.
In accordance with an aspect of the invention, the record carrier as described in the opening paragraph is characterized in that the position information in the first servo tracks is encoded into a group of m first words of channel bits and the position information in the second servo track is encoded into a different group of n second words of channel bits, where m and n are integers, all first words are different and all second words are different.
In accordance with another aspect of the invention, the method as described in the opening paragraph is characterized in that, when scanning the first servo track, the position information is derived from the modulation of the first servo track in which the position information is encoded into a group of m different first words of channel bits, when scanning the second servo track, the position information is derived from the modulation of the second servo track in which the position information is encoded into a different group of n second words of channel bits, where m and n are integers.
In accordance with a further aspect of the invention, the scanning apparatus as described in the opening paragraph is characterized in that the signal processor comprises a demodulator for recovering a channel-bit pattern from the output signal of the detector, and a decoder for deriving the position-information signal by decoding the channel-bit pattern according to a group of first code words or a different group of second code words, depending on whether one of the first or second servo tracks is being scanned.
In accordance with a still further aspect of the invention, the apparatus for manufacturing the record carrier comprises an encoder for encoding position information to a channel-bit pattern according to a group of first words or a different group of second words, the first and second words being used for alternating servo tracks, and also comprises a modulator for deriving the control signal from the channel-bit pattern.
The use of two different groups of words, or two different code tables, for encoding position information for neighbouring servo tracks reduces the crosstalk between servo tracks. When scanning a first servo track, the code words in the channel bit stream must appear in the first code table. Since the information in neighbouring servo tracks is coded in words from a different code table, crosstalk in a detector signal from these servo tracks can easily be distinguished from the detector signal from the servo track being followed. The invention reduces the bandwidth of the track modulation as compared to the bandwidth used in the mentioned prior art by a factor of three and provides a significantly better signal-to-noise ratio. Since the number of channel bits in each code word can be chosen equal for both the first and second servo track, the information density in the servo tracks can be made substantially equal. A simplification of the coding is obtained by choosing the number of words m and n in the code tables to be equal.
The two groups of code words must be different. In the preferred embodiment the m words of one group should not appear among the n words of the other group or vice versa. However, it is possible to have one or more words appear in both groups. Preferably, all words in the groups are different, which reduces the crosstalk for all words in the servo tracks. For coding purposes an identification pattern and a synchronisation pattern are not regarded as code words and may be identical in neighbouring servo tracks. Such patterns are preferably aligned in neighbouring servo tracks.
The encoding of position information using different code tables for neighbouring tracks can advantageously be used in record carriers in which the tracks form a repeated succession of a first servo tracks, a non-servo track, a second servo tracks and a non-servo track. A non-servo track does not comprise its own position information, and a scanning device scanning over such a non-servo track should derive the position information from one or both of the neighbouring servo tracks. When scanning a first or second servo track, a scanning device preferably reads the information stored in that track and uses the appropriate code table to decode the channel bits into information. When scanning a non-servo track, the scanning device may either read the position information from one neighbouring servo track, from the other neighbouring servo track, or from both neighbouring servo tracks.
A scanning device may read the neighbouring servo track by means of a special detector which
Schouhamer Immink Kornelis A.
Van Den Enden Gijsbert J.
Belk Michael E.
Tran Thang V.
U.S. Philips Corporation
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