Dynamic information storage or retrieval – Control of storage or retrieval operation by a control... – Mechanism control by the control signal
Reexamination Certificate
1999-02-18
2001-08-14
Huber, Paul W. (Department: 2753)
Dynamic information storage or retrieval
Control of storage or retrieval operation by a control...
Mechanism control by the control signal
C369S124120
Reexamination Certificate
active
06275458
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and apparatus for reading multilevel signals from an optical disc and writing multilevel signals to an optical disc. The invention relates to methods and apparatuses for processing signals that are eventually written to and read from an optical disc. These signals produce marks on the optical disc that may vary in both reflectivity and length. The system disclosed provides a method of encoding and decoding the data, correcting for errors, synchronizing the data, controlling the DC content, establishing and recovering a clock signal, establishing and recovering the envelope of the signal, and compensating for signal distortion.
2. Relationship to the Art
In order to increase the capacity and speed of optical data storage systems, multilevel optical recording systems have been developed. It should be noted that in this specification, the term multilevel is used to indicate greater than 2 levels. In a traditional optical recording system, reflectivity of the recording media is modulated between two states. The density of data recorded on an optical recording medium may be increased by modulating the reflectivity of the optical recording medium into more than two states.
One type of optical recording medium that appears to be particularly suitable for multilevel signal modulation is phase change optical material. When a phase change material is heated by a writing laser, the reflectivity of the phase change material may be changed. The change in reflectivity may be controlled by adjusting the amount of heating of the material and the rate at which the material cools. This process is described further in “Laser-induced crystallization phenomena in GeTe-based alloys. I. Characterization on nucleation and growth” (J. Appl. Phys. 78 (8), Oct. 15, 1995. p. 4906) by J. H. Coombs, et. al. (hereinafter “Coombs”).
After a phase change optical disc has been written, the intensity of a beam of light reflected from the disc is measured so that the multilevel data written to the disc may be recovered. U.S. Pat. No. 5,144,615 entitled APPARATUS AND METHOD FOR RECORDING AND REPRODUCING MULTILEVEL INFORMATION issued to Kobayashi (hereinafter “Kobayashi”) discloses a system for recovering multilevel data from such an optical disc.
FIG. 1
is a block diagram illustrating the system disclosed in Kobayashi for recovering such data. Analog data read from a detector is input from a mark length detecting circuit
101
and a reflectivity detecting circuit
102
. The outputs of these circuits are sent to an analog-to-digital (A/D) converter
103
. The A/D converter
103
includes an n-value circuit which determines the value that the signal corresponds to by comparing the signal to predetermined reference voltages. Subsequently, the n-value signal is converted into a binary signal by binary circuit
405
.
While this system discloses the concept of reading a multilevel signal and converting it into a digital signal in a basic sense, no method is disclosed of handling various imperfections in optically read multilevel signals that in fact tend to occur. For example, it is not clear how a clock is recovered for the purpose of precisely detecting mark lengths and no method is disclosed for handling problems that tend to occur in real systems such as amplitude modulation and DC offset of the optically detected signal and noise.
In a conventional two level optical data storage system, information is stored in the lengths of the marks and the spaces between them. So long as the edge of a mark can be detected with enough precision to distinguish between marks that differ in length by a minimum allowed amount, the system can operate reliably. This edge transition between one reflectivity state and another can be detected by setting a threshold value and determining the time when the signal crosses the threshold. Slow amplitude variations that might interfere with this edge detection are removed by AC coupling the photodetector signal before the threshold detection circuit. Mark and space lengths are measured by counting how many clock periods are between the edge transitions. The reader clock periods are synchronized to the mark/space edges, thus ensuring that there are an integral number of clock periods in each mark/space.
In contrast, in a multilevel recording system, it is the amplitude of the signal that carries information. The reader must interpret the data signal to determine the amplitude of the signal at certain times. Therefore, the reader clock must be synchronized to the data stream to ensure that the reader is interpreting the signal at the proper time. Because of the blurring effect of the optics in a reader, the transitions between the different levels do not create sharp edges. It is therefore difficult to synchronize the reader clock to the data stream. A method of precisely aligning a read data stream is needed. Further, a multilevel system is more sensitive to fluctuations in the overall envelope of the data signal. AC coupling alone is not adequate to enable a sufficiently precise determination of the different amplitude signals. Another problem encountered in a multilevel optical disc system is DC compensation.
In order for a multilevel optical read system to reliably record and recover data, a method of handling these sources of error in reading an optical signal is needed.
SUMMARY OF THE INVENTION
Accordingly, a system for writing and reading multilevel marks on an optical disc is disclosed. The system includes an error correction encoding and decoding system, modulation and demodulation system, DC control system, amplitude correction circuit, a clock recovery circuit, a write strategy system, a system to focus and track the laser spot on the surface of the disc, a system to rotate the disc, and an interface to a computer system. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium that includes certain types of marks that enable reliable data storage and recovery. Several inventive embodiments of the present invention are described below.
In one embodiment, method is disclosed for reading a multilevel signal from an optical disc. The method includes reading a raw analog data signal from a disc using an optical detector and adjusting the amplitude of the raw analog data signal. A timing signal is recovered from the amplitude adjusted analog data signal and correction is made for amplitude modulation of the raw analog data signal by processing the raw analog data signal and the timing signal.
In another embodiment, a method of reading a multilevel signal from an optical disc includes reading a raw analog data signal from a disc using an optical detector and recovering a timing signal from the raw analog data signal. The analog signal is converted to a digital data signal using an A/D converter. Amplitude modulation of the raw analog data signal is corrected by processing the digital data signal to obtain an amplitude adjusted digital data signal.
In another embodiment, a method of reading a multilevel signal from an optical disc includes reading a raw analog data signal from a disc using an optical detector and correcting for amplitude modulation of the raw analog data signal by processing the raw analog data signal to obtain an amplitude adjusted analog data signal. A timing signal is recovered from the amplitude adjusted analog data signal and the timing signal is used to further correct for amplitude modulation in the amplitude adjusted analog data signal.
In another embodiment, A method of reading a multilevel signal from an optical disc includes reading a raw analog data signal from a disc using an optical detector and correcting for amplitude modulation of the raw analog data signal by processing the raw analog data signal to obtain an amplitude adjusted analog data signal. A timing signal is recovered from the amplitude adjusted analog data signal
Lee David C.
Martin Richard L.
Spielman Steven R.
Wong Terrence L.
Huber Paul W.
Thyfault Paul
Van Pelt & Yi LLP
LandOfFree
Method and apparatus for reading and writing a multi-level... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for reading and writing a multi-level..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for reading and writing a multi-level... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2470967