Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Patent
1993-11-22
1995-04-11
Dzierzynski, Paul M.
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
369110, G11B 700
Patent
active
054065402
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the improvement of optical disk recording performance.
During the recording process of writable optical disks, data is written as the disk rotates. To check that the data was written correctly, it is read back from the disk (usually known as verifying the data) on the following revolution. It therefore takes two revolutions of the disk to successfully write and verify data.
We have found that the time to write and verify data can be halved by reading the data from the disk while writing it (Direct Read During Write, DRDW).
Write Once optical disks (WORM disks) made by Plasmon Data Systems use a `moth eye` texture as the recording medium (see, for example, U.S. Pat. Nos. 4,616,237; 4,786,585; 4,724,444; and 4,758,307). Marks written by the laser form very quickly (typically 5-20 ns). Mark formation can be detected by monitoring the read photodiode during writing (see FIG. 1).
Accordingly, one aspect of the present invention provides a method of improving optical disk recording performance which comprises writing data to an optical disk with a write laser so as to form marks in the disk, reading said data immediately by monitoring the formation of said marks with a read photodiode during writing, characterised in that
a) said read photodiode is connected to a pulse detector arranged so as to measure the amplitude of a detected signal that was generated by the laser beam and has reflected from the marks being formed.
b) the power of said write laser is controlled as a function of the amplitude of a detected signal that was generated by the laser beam and has reflected from the marks being formed.
If every bit of data is written successfully there will be no errors when reading the data. Indeed, the read channel error correction can cope with several bits in error before reporting a verify failure.
The pulse detector is capable of detecting marks forming on the disk surface during write. If the normal mark formation is not seen, it is a bad mark, or bit in error, By simply counting the number of bad marks during writing, data which will pass the verify read can be positively identified by observing a low count (of the order of a few bits) on the bad pulse counter.
In cases of few bits in error being detected, no verification is required.
If more than a few bad marks are detected, the normal verify sequence is triggered. This will test whether the error correction can cope with the number of bits in error.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 4-6 illustrate the characteristics of a write laser with respect to a detected signal.
FIG. 2 shows the distribution of number bits in errors in sectors.
FIG. 3 shows the apparatus of the present invention.
FIG. 2 shows the distribution of numbers of bits in error in sectors on a typical Plasmon disk. Most sectors have no bits in error, and so a high hit rate of zero counts can be expected (approximately 99.3%). If two bits in error are taken as acceptable, the hit rate would be 99.8%. The time to perform most writes should therefore be halved.
Recording marks on optical disks requires accurate control of laser power. A small percentage change in the write power will cause a dramatic difference in the written mark size.
Accordingly, in the present invention, there is provided laser power control using feedback photodiodes within the laser module to provide the desired laser power. Outside of the laser module, optical elements direct the light to and from the disk surface.
In normal operation the optical elements get dirty with airborne dust, smoke particles etc. Each dirty surface reduces the laser power by a (small) percentage. The optical disk surface can also get dirty causing further signal loss.
There is a greater latitude in laser power during reading due to careful circuit design, while accurate write laser power is demanded by the physical properties of the disk. Writing data will become unreliable long before reading is impaired.
In practice, Plasmon have found the problem may cause poor disk drive performance (many write-retr
REFERENCES:
patent: 4606016 (1986-08-01), Verboom et al.
patent: 4648085 (1987-03-01), Shimonou
patent: 4680594 (1987-07-01), Bracht
patent: 5101396 (1992-03-01), Takeuchi et al.
Broadbent Mark J.
Helfet Peter R.
Lister Quentin J.
Longman Robert J.
Dzierzynski Paul M.
Plasmon Data Systems, Inc.
Wong Don
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