Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-11-23
2001-08-07
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S124050, C369S124150
Reexamination Certificate
active
06272102
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an amplitude detector for signals having a periodical character, recorded on a recording medium. More particularly, the invention relates to an amplitude detector for use in an optical tape recorder.
BACKGROUND OF THE INVENTION
In optical tape recorders of the type described in the article “Optical Tape System; evaluation of recorder and media” by G. W. R. Leibbrandt, J. A. H. Kahlman, G. E. van Rosmalen and J. J. Vrehen in SPIE Proceedings Series, vol. 3109, use is made of a rotating polygon mirror, for example a polygon having 10 facets, which images a laser beam directed onto the polygon via an objective on an optical tape. The plane of the polygon mirror is substantially perpendicular to the travel direction of the tape, and the information is recorded in narrow, parallel tracks on the tape. In the longitudinal direction of a tape having a width of, for example 12.7 mm, for example, 12 tracks of 1 mm may be present. Each track consists of parallel sub-tracks located transversely to the longitudinal direction of the track, and each sub-track is recorded during the rotation of one facet of the polygon mirror by the laser beam, and the next, juxtaposed sub-track is recorded during the rotation of the next mirror facet by the laser beam.
For example, 1500 bits can be recorded on each sub-track by modifying the surface of the tape by means of the laser beam, for example by forming pits on this tape in a way which is comparable with the method used for a compact disc. When reading the pits by means of the same laser beam, these pits have different reflection properties than the interposed areas, and the signal recorded on the tape can be derived from this information.
The above citations are hereby incorporated in whole by reference.
SUMMARY OF THE INVENTION
Large amplitude variations occur in the signal of a sub-track read by means of the laser beam. Moreover, the signal drops out completely for a short time during the transition to the next sub-track, every time after a limited number of, for example 1500 bits.
FIG. 1
shows an example of a read signal which corresponds to a signal recorded on 1.5 sub-tracks of a track. For such a read signal with a periodical character, it is difficult to detect the zero crossings in the signal by means of known amplitude detectors.
In a known amplitude detector consisting of a high-pass filter and a zero detector (slicer), the time constant of the high-pass filter is determined by the frequency of the amplitude variations of the read signal, which variations may be only several dozens of bits for optical tape recording because otherwise the amplitude variations cannot be followed sufficiently. To make this possible, the coding used for the recorded signal should be DC-free for a large part of the maximum bit frequency. This is possible but it is at the expense of the recording density of the signal, which is undesirable.
Also an amplitude detector in which the detection threshold is fixed halfway between the maximum and minimum signal level in known manner, is not very well usable in this specific case. In fact, for such an amplitude detector it is necessary to make use of peak detectors having a sufficiently small time constant to follow the signal variations, but this time constant should not be too small in order that there is not too much signal decay between consecutive signal pits in the tape. However, such peak detectors are very sensitive to disturbances such as noise, drop-outs, etc., while existing RF peak detectors are neither sufficiently accurate for use with signals of the type described.
It is an object of the invention to provide an amplitude detector which does not have the afore-mentioned drawbacks and, more particularly, is sufficiently accurate, fairly insensitive to disturbances and usable for read signals of different code formats.
To this end, the invention provides an amplitude detector for signals having a periodical character, recorded on a recording medium, and is characterized by a first delay line and a second delay line, wherein an average maximum value of the detected signal is stored in the first delay line for each one of a predetermined number of detection instants within a detection period, and an average minimum value of the detected signal is stored in the second delay line for each one of said detection instants, means for determining the average level between the average maximum and minimum values for each detection instant on the basis of the output signals of the two delay lines, and means for comparing the current signal to be detected for a given detection instant with the average level for said detection instant so as to provide a detection signal.
The invention is based on the recognition that in, for example, an optical tape recorder of the type described, the amplitude of the read signal changes only slowly with respect to time at corresponding positions in sub-tracks. There is a strong correlation between the signal amplitude and the rotation frequency of the polygon mirror. This is caused by deviations in the optical system, such as lens errors and reflection variations on the polygon and by the mutual inaccuracy of the polygon facets.
In accordance with a first aspect of the invention, the average maximum and average minimum values of the read signal are determined by means of two peak detector/memory loop combinations for each one of a number of positions of the polygon mirror, for example 256 positions for a mirror with 10 facets. The decision level for each position is subsequently chosen to be halfway between these two levels. The length of the memory loop corresponds to the rotation frequency of the polygon mirror so that two peak detectors are provided for each one of the, for example 256 positions of the mirror, and means are provided for determining the decision level for each position from the output signals of these detectors.
In accordance with a second aspect of the invention, means are provided which can detect also rapid variations of the amplitude of the read signal, hence rapid amplitude variations of the data which have been read and can adapt the decision level accordingly. Such rapid variations may occur, for example in the case of tracking errors and when switching on the recorder.
The invention also relates to an optical tape recorder comprising an amplitude detector as claimed in claims
1
to
8
.
Those skilled in the art will understand the invention and additional objects and advantages of the invention by studying the description of preferred embodiments below with reference to the following drawings which illustrate the features of the appended claims.
REFERENCES:
patent: 5448309 (1995-09-01), Won
patent: 5581534 (1996-12-01), Van Rosmalen et al.
patent: 5629914 (1997-05-01), Clark et al.
patent: 5659535 (1997-08-01), Kimura et al.
Optical Tape System; Evaluation of Recorder and Media, G.W.R. Leibrandt, J.A.H. Kahlman, G.E. Van Rosmalen and JJ. Vrehen, SPIE Proceedings Series, vol. 3109, pp. 1-9.
Belk Michael E.
Chu Kim-Kwok
Tran Thang V.
U.S. Philips Corporation
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