Partial response coding for a multi-level optical recording channel

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to data recording, and more specifically to a system and method for multi-level recording in a partial response channel.

2. Related Art

Conventional recording techniques use saturation recording to store information on the recording media. Saturation recording techniques typically store the information in a two-level (i.e., binary) form, using digital data encoding methods to mark the recording medium. The process used to encode the data is limited to codes requiring no more than two, or possibly three, symbol amplitudes. Thus, such techniques provide limits to data storage capacity and transfer rates of the medium.

The most common signal processing technique employed in saturation recording is run-length limited coding and peak detection. The application of more advanced methods of coding, such as partial response maximum likelihood (PRML) sequence detection has recently been considered for saturation recording applications. PRML techniques provide an increase in recording density and reliability over runlength limited codes and peak detection.

Significant advances in electron trapping materials have lead to the development of a storage medium that provides a linear response characteristic. Such a linear response characteristic provides an advantage over saturation-type media in that it adds an analog dimension to the storage capacity of the medium. Because the response is linear, the electron trapping material presents the ability to encode information in two dimensions--amplitude and phase. As a result, the storage medium is no longer confined to storing binary or tri-level data. Instead, the concept of M-ary, or non-binary, data coding and storage is provided. The increased symbol alphabet allowed by such encoding provides the opportunity to increase dramatically the data recording density and transfer rate of the storage device. For example, the potential storage capacity of a single 51/4 inch disk can be extended to several gigabytes if that disk were to be implemented using the electronic trapping materials.

Examples of materials that can be used as the storage media for M-ary storage are described in U.S. Pat. Nos. 4,915,982, 4,834,536, and 4,830,875. Other materials useful as the storage media are disclosed in U.S. Pat. Nos. 4,839,092 and 4,806,772, and 4,842,960. Examples of an optical disk and an optical disk drive incorporating electron trapping materials for data storage are disclosed in U.S. Pat. Nos. 5,142,493 and 5,007,037, respectively. The full disclosure of each of these Patents is incorporated by reference herein.

SUMMARY OF THE INVENTION

The present invention is directed toward a system and method for recording multilevel data to a multi-amplitude recording channel. According to the invention, binary data are encoded to form multi-level data. The multilevel data are recorded to the storage media for later recall.

One key difference between data recording as disclosed herein and conventional recording techniques is that of the storage media. Conventional storage media are saturation-type recording media, capable of storing only two-level (binary) data. To record a large number of information bits to the media requires a large number of channel symbols.

In contrast, the present invention utilizes linear, multi-amplitude recording media which allows dam to be stored as multi-level data--requiring fewer symbols to represent the same number of information bits. Preferably, this media is an optical storage media that is written to and read from using a write and a read laser, respectively.

To obtain greater data density in the storage media, a diffraction limited write laser is utilized, resulting in a smaller write-spot size. This smaller spot size yields a higher recorded data density. Because the read laser is of a longer wavelength, its diffraction limited spot size is larger. As a result, more than one mark is read at a given read time resulting in inter-symbol interference.

To take full advantage of this multi-amplitude storage media trellis coded modultation techniques are adopted. Such techniques convert the binary input data into M-ary data having M levels. Further coding is then performed to compensate for the effects of the inter-symbol interference. This is accomplished by precoding the data using a Tomlinson-Harashima precoder. The precoding results in multi-level data (of m levels, where m.ltoreq..infin.). The data are precoded prior to recording so that the decoding process can be implemented in a somewhat straightforward manner.

Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the-accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.

FIG. 1 is a diagram illustrating a portion of a track written with "marks" having different intensity and length.

FIG. 2 is a block diagram illustrating a systems-level channel model for the M-ary recording channel.

FIG. 3A through 3C illustrate the effects of intersymbol interference that occurs during the read process.

FIG. 4 is a block diagram illustrating the system model of FIG. 2, with encoder 204 divided into the two phases to implement the preferred embodiment.

FIG. 5A is a block diagram illustrating a finite state machine used to implement a simple rate one-half binary convolutional code.

FIG. 5B is a trellis illustrating the output bits that result for a stream of input bits for the specific coder illustrated in FIG. 5A.

FIG. 6A depicts the trellis of FIG. 5B when the four output sequences 00, 10, 01, and 11 are replaced with four signal sets C.sub.0 through C.sub.3.

FIG. 6B depicts an example of partitioning of signal sets C.sub.0 through C.sub.3.

FIG. 7 illustrates an example of a time varying trellis.

FIG. 8 is a block diagram illustrating one embodiment of a Tomlinson-Harashima precoder.

FIG. 9 depicts a time varying one dimensional 8 state trellis.

DETAILED DESCRIPTION OF THE EMBODIMENTS

1. Overview and Discussion of the Invention

The present invention is directed to a system and method for multi-level encoding of data for a multiple amplitude optical recording channel.

Traditional optical recording products for data storage utilize binary (digital) encoding methods to mark the optical media. The intrinsic nature of optical marking requires the use of such encoding methods. For example, write-once recording, commonly found in compact disks, is accomplished by marking the optical material with pits or other similar features to indicate the data recorded. For example, a 1 may correspond to the presence of a pit, and a 0 to the absence of a pit. Similarly, phase-change recording is accomplished by manipulating crystalline or amorphous features of the recording material to indicate the presence of a 1 or a 0. Finally, magneto-optic recording is accomplished by altering the polarities of the magnetic material. Due to the nature of the materials, these marking techniques have limited channel encoding to binary digital code sets. Conventional magnetic recording is likewise limited to two-state encoding at very high dam densities where the feature dimensions approach the minimum domain dimensions.

As described above, the implementation of storage media using electron trapping materials allows data to be stored at multiple levels. Thus, the use of a M-ary, or non-binary code set would enable the recording system to take full advantage of the properties of the electron trapping material. This results in significant increases in both data density and transfer rate. The full potential of optical recording media based on materials which exhibit the electron-trapping phenomenon and provide a very broad linear amplitude response can be realized by implementing M-ary data code sets.

2. The Multi-Amplitude Optical Channel

2.1 Multi-Level Recording Media

An example of a multi-amplitude optical storage media is now described. Electron trapping is an opto-electronic approach to optical recording. To prepare the electron-trapping media, a disk, or other storage substrate, is coded with a II-VI phosphor material that is doped with two rare earth metals. The fundamental process responsible for the storage of information in the electron-trapping material is the transfer of an electron charge from one dopant atom to a neighboring different dopant atom under the stimulus of incident light radiation. Thus, to facilitate the operation of writing to the material, the material is illuminated with light at a first wavelength so that electrons from one dopant atom are accelerated to a higher energy state in a second dopant atom, where they remain trapped at energy levels determined by the dopant materials.

To read the information, the material is illuminated with light of a second wavelength. The absorption of a photon at this second wavelength provides the trapped electron with enough energy to elevate it out of the trap, and return it to the ground state of the first dopant atom, thereby releasing the stored energy in the form of visible light.

Although this is a two-state process, the effective "domain" is delimited by the adjacent dopant pair within the crystal lattice. Because the effective domain is very small as compared to the marking resolution (defined by the spot size of the light radiation), a marking region contains many effective domains. The high ratio of mark-to-domain dimension provides a linear amplitude response in a similar manner to analog audio-video magnetic recording. That is, the number of emitted photons is proportional to excitation energy, and hence, is linear in this sense.

The above description provides one example of how an electron-trapping material can be implemented to provide multi-level recording media. This description is provided by way of example only, and it is not intended to limit the scope of the invention in any way. It will become apparent to a person skilled in the relevant art how to implement the invention using other types of multi-amplitude recording media.

2.2 A Multi-Level Recording Channel

A model of an M-ary channel according to the invention is now described. FIG. 1 is a diagram illustrating a portion of a track written with "marks" having different intensity and length. The channel is written and read optically by separate lasers focused to `spots` on the media. A write spot 102 of diameter L.sub.W and a read spot 104 of diameter L.sub.R result from the focusing of a write laser and a read laser onto the media. Because of the gaussian properties of the coherent laser beams, the spots are generally circular. However, because the disk is spinning during read and write operations, the remnant spots 108 take the oblong shape that is illustrated in FIG. 1.

For writing multiple levels of data, the write laser is positioned above a track on a rotating disk and its intensity is modulated. The strength of the remnant mark on the disk is proportional to the write laser intensity. Specifically, the trapping level, or number of electrons trapped, is proportional to the intensity of light impinging on the media. Thus, multiple levels of encoding are provided by modulating the intensity of the write laser.

For reading, the read laser is positioned above the track to be read and a constant intensity illumination is provided to the written-to media. As a result of the excitation provided by the read laser, trapped electrons are released from the trapped state resulting in the emission of photons. The intensity of the emission is proportional to the number of electrons that were trapped during the write process. This allows the multiple levels to be detected at the read stage.

According to a preferred embodiment of the invention, the wavelength of the read laser, .lambda..sub.R, is different from the wavelength, .lambda..sub.W, of the write laser. Specifically, in one embodiment, the data are written with a blue light laser having a .lambda..sub.W of 488 nanometers (nm) and they are read with a red laser having a .lambda..sub.R of 647 nm.

2.3 A Multi-Level Channel Model

FIG. 2 is a block diagram illustrating a systems-level channel model for the M-ary recording channel. This model includes an encoder 204, an optical channel 208 and a multiplier 212. In this model, b.sub.k denotes a user bit sequence (i.e., the data to be recorded). Typically, the user data to be recorded as provided by the user are in digital (binary) form. To take full advantage of the multi-level properties of the storage medium, these digital data are encoded by encoder 204 to provide multi-level data. To this end, encoder 204 receives the user bit sequence b.sub.k and codes this sequence to produce a coded channel symbol sequence a.sub.k. The coded channel symbol sequence is the actual multi-level coded data that are to be recorded onto the multi-level recording channel.

To record the sequence a.sub.k, the sequence is used to modulate the write laser as represented by multiplier 212. Variations in the amplitude of a.sub.k result in a variation in the intensity of the modulated write-laser signal w(t). Because sequence a.sub.k is a sequence having m levels (theoretically m.ltoreq..infin.), the write laser is modulated to m levels. Therefore, m levels of data are written to the medium.

In the model illustrated in FIG.2, w(t) represents the modulated laser light, or, in reality, the mark pattern written on the disk where ##EQU1## In equation (1), b(t-kT.sub.5) is a "box" function representing one pulse of the unmodulated write laser and having a nominal height of one (1), a duration time of T.sub.S and n<.infin.. Although there is flexibility in choosing a value for T.sub.S, in one embodiment, T.sub.S is chosen to be substantially close to or equal to T.sub.W =L.sub.W /v, where L.sub.w is the spotsize of the write laser and v is the velocity of the medium. In an alternative embodiment, at least one class of modulation codes, however, uses T.sub.S <T.sub.W (M-ary runlength limited codes are considered later).

During the read process, information is read from the media using a read laser with spotsize L.sub.R. The read laser illuminates the recorded marks and the media releases a number of photons. The number released is proportional to the mark intensity as originally determined by the intensity of the write laser (i.e., as modulated by a.sub.k). The photons are counted using a photodetector whose output is an analog voltage r(t). An approximation to r(t) is a sliding, ideal integrator, namely ##EQU2## where the duration of the impulse is T.sub.R =L.sub.R /v. K is a proportionality constant and n(t) is a white gaussian noise. Without loss of generality it can be assumed that K=1. A decoder (not illustrated in FIG. 2) uses r(t) to estimate the code sequence a.sub.k.

The purpose of encoder 204 is to convert a sequence of user data bits to a channel waveform. Specifically, encoder 204 converts the user binary data to multi-level data. The performance of the code is measured by its storage density D (in bits/unit area) and error probability P.sub.e (bit error rate, or BER).

The bit density D in user-bits/area is computed as D=b/vTW, where b is the number of bits stored during period T on a disk rotating at speed v with a track width W. Assuming a normalized product vW=1, and for convenience let T=T.sub.R, the linear density D (often called density ratio) becomes

(in bits/sec). D=b/R.sub.R, (3)

Given this model, the ideal coded modulation scheme is one that densely stores user data with high reliability.

2.4 Induced Intersymbol Interference

According to one embodiment of the invention, the read and write lasers are diffraction limited. As a result, the read and write spot diameters, L.sub.R and L.sub.W as focused on the medium are made as small as possible. This has the effect of maximizing storage density on the disk. The diffraction limits L.sub.W,diff and L.sub.R,diff are lower bounds on the spot sizes L.sub.W and L.sub.R, respectively. Because the diffraction limit is proportional to the laser wavelength, .lambda., and because the lasers are diffraction limited, the read spot size L.sub.R is larger than the write spot size L.sub.W. In fact, in the specific embodiment described above using a blue write laser and a red read laser, L.sub.R is approximately 11/3 times the size of L.sub.W. The manner in which this spot size differential is used to induce a controlled intersymbol interference (i.e. partial response) channel is now described.

As described above, both the read and the write laser are diffraction limited. The minimum mark T.sub.min is constrained by the spot size of the write laser L.sub.W and speed of the disk, namely T.sub.min =T.sub.W. The maximum mark T.sub.max is constrained by system timing requirements. Because many recording systems derive timing from transitions in the recorded signal level, periodic transitions in recorded amplitude are required, thus constraining the maximum width T.sub.max of a mark.

Due to the diffraction limiting, in the above-described embodiment having a blue write and a red read laser, read spot 104 is actually approximately 11/3 times the size of write spot 102. One reason for diffraction limiting the write laser is to increase the density at which marks, and thus dam, are written to the medium. Because spot size L.sub.W is constrained to the minimum beam waist size, the data density is maximized for a given write laser wavelength, .lambda..sub.W.

This advantage of a minimum L.sub.W, however, is somewhat offset by a resultant intersymbol interference that occurs during the read process. To elaborate, because L.sub.R is larger than L.sub.W, when data are read, more than one symbol are read at a time. For example, assume that L.sub.R is twice the diameter of L.sub.W. In this example, the read laser actually stimulates emission from the spatial equivalent of two marks. This is illustrated in FIGS. 3A and 3B. In FIG. 3A, read laser actually reads the levels stored in marks 302A and 302B. In FIG. 3B, read laser reads the levels stored in marks 302A, 302B, and 302C. Because the detector cannot distinguish which of the received photons are from which mark, the detector provides a signal level that is the total of all photons detected. Thus, this intersymbol interference can result in an erroneous reading.

As another example, consider the embodiment described above where L.sub.R is 11/3 times the size of L.sub.W. In this example, illustrated in FIG. 3C, the read laser results in the detection of the current mark 304 (the mark we want to detect) plus one third of the previous mark 303. This is illustrated in FIG. 3C.

To minimize the effects of the resultant intersymbol interference, two solutions can be implemented. A first solution attempts to decode the results of the read operation to remove the effects of the intersymbol interference. Such a solution requires a decoder that estimates the effect of the extra 1/3 of the mark that is read and subtracts this amount from the detected value to produce a decoded signal.

A second solution is to provide an additional encoding phase in encoder 204 to subtract the effect of the additional 1/3 of the mark before the data are written to the medium. It is this second solution that is the preferred embodiment of the invention.

Note that for the example embodiment described herein, there is intersymbol interference because the read laser spot size is approximately 11/3 that of the write laser. The present invention is described in terms of this example embodiment. Description in these terms is provided for convenience only. It is not intended that the invention be limited to application in this example embodiment, where L.sub.R =11/3L.sub.W. In fact, after reading the following