Optical Recording Channels Research Articles

Turbo-coded optical recording channels with DVD minimum mark size

Recent work on turbo codes applied to partial response (PR) optical recording channels has focused on unconstrained channels. In this paper, we consider the application of turbo codes to a (1,7) constrained, PR-equalized optical recording channel with digital versatile disc (DVD) parameters. The addition of a (1,7) run-length-limited (RLL) constraint requires the use of a soft RLL decoder to communicate with the turbo code. Although soft RLL decoders were previously developed for use with iterative decoding, application to a practical optical channel has not been addressed until now. Here, results on both correlated noise and media noise optical recording channel models are given for two PR targets, 1+D and 1+D+D/sup 2/+D/sup 3/. We achieved coding gains of 4 to 6.3 dB over a baseline RLL-coded system. We also evaluated system performance at smaller mark sizes, and found that density gains of 17% and 22% are achievable for the two targets, respectively.

Decision-Directed Correction for Bloom in Optical Recording Channels

A threshold adaptation scheme is proposed in a non-Viterbi simple detector for d=2 minimum run-length-constraint coded optical recording schemes, to increase robustness to bloom. The detector has a simple structure comprising of a threshold detector, post processing error correction, and decision directed threshold adaptation. The equalization target is a free symmetric 7-tap target and the detection exploits the d=2 constraint in the input sequence. The detector is evaluated for digital versatile disk (DVD) specifications and compared with other threshold-based detectors as well as Viterbi detectors following targets of length up to 5. Performance improvement of over 1.5 dB is observed with the proposed feature for bloom larger than 15% of the channel bit period.

Coded modulation and precoding for electron-trapping optical memories

This paper develops coding and signal processing approaches for a novel optical recording channel that arises from electron-trapping phosphor materials. The recording medium allows multiple reads and writes, and one important feature is that the read process serves to erase the disk. This feature would enable vendors of prerecorded video to provide customers with one-time services. For applications where this feature is not desirable, the data can be immediately rewritten. From a communications viewpoint, the most important feature of this new channel is that, subject to a peak constraint, it supports a continuum of recording levels. The combination of read and write processes creates a partial-response channel, and the ability to write a continuum of levels makes it possible to employ precoding techniques, such as the one developed by Tomlinson (1971) and by Miyakawa and Harashima (1969). This is fundamentally different from magnetic data storage, where the read/write process creates a partial-response channel but where it is only possible to write two levels at the input to that channel. This paper shows that the use of precoding and coset codes can significantly improve upon the recording densities (and recording rates) that can be achieved by using M-ary run length constrained codes to eliminate intersymbol interference (ISI) at the output of the read/write process. The approach presented is applicable to any optical recording channel that supports a continuum of recording levels.

Imaging spectrometer for ultraviolet–near-infrared microspectroscopy

We describe the design and performance of a new high-speed imaging spectrometer system developed for the spectral analysis of micrometer-sized features in transmitted light. The instrument utilizes the wavelength range from ultraviolet (UV) to near infrared (NIR) (250–975 nm). The extension into the ultraviolet range is to our knowledge unique. The system comprises two white light sources (xenon and halogen lamps), a motorized sample stage, microscope optics with slit-determined field of view, and three (UV, visible and NIR) parallel optical recording channels. The dispersive optical recording system is based on holographic gratings and uses charge-coupled-device (CCD) cameras as imaging detectors. The instrument has been fully integrated with data acquisition and control systems. Subsequent data analysis is performed by dedicated software. The instrument provides spectral images from narrow wavelength bands (5 nm). In such an image the spectral information from a selected wavelength band is depicted with its corresponding geometric location, i.e., a single pixel on the CCD chip. Hence, objects can be visualized in different wavelength bands and a spectrum can easily be produced for each location of the object. The instrument can measure objects up to 900 μm×27 mm with a spectral resolution better than 5 nm and a spatial resolution better than 5 μm. The imaging spectrometer system provides a powerful combination of spectroscopy and image processing for use in many applications in research, control and quality assessment, where spectral information and its location are of interest. Although the system has been primarily developed for applications in the pulp and paper industry, it has a vast potential for spectroscopic analysis with a micrometer spatial resolution in general and for measurements of impurities and their locations in products in particular. As an example of a measurement, spectral image data from a wood fiber are presented.

Multi-channel equalization for PWM channels in magneto-optical recording

We show by modeling and simulations that multitrack readback and equalization can lead to significant increases in the track density in magneto-optical recording without performance degradation due to two dimensional interference. We assume an architecture in which an isolated set of closely spaced tracks is read simultaneously. In an earlier paper, we described a multi-channel zero-forcing equalizer and a two-dimensional decision-feedback equalizer for this architecture, and presented their performance for PPM (pulse position modulation) optical recording channels. Since PWM (pulse width modulation) recording is more popular in magneto-optic (MO) disks, we model the PWM channel in this paper and show simulation results for the performance improvement obtained by using multi-channel equalization for data recorded in PWM format.

High capacity optical recording channels

Several capacity enhancement techniques that are compatible with the systems requirements of erasable optical recording channels are reviewed. Recent simulation and experimental results on PRML detection, thermal analysis of mark formation, and crosstalk cancellation are presented.

Performance of Modulation Codes in Various Optical Recording Media

The performance of modulation codes has been measured in ablative and magneto-optic recording media as well as in mastered direct discs. The modulation codes studied include NRZ (i.e. the uncoded bit stream), runlength limited sequences, multi-level recording schemes, and differentially detectable codes. We also studied the performance of codes when the optical channel is equalised, particularly the case of partial response detection of NRZ. The general conclusions are that: 1) differentially detectable codes require the full penalty in data density caused by the loss in code rate, 2) multi-level recording does not offer significant advantages, and step-constrained multi-level codes perform worse than their unconstrained counterparts, 3) runlength limited sequences are useful codes to enhance the density while offering additional practical advantages, and 4) partial response detection of uncoded bit streams transmitted through an equalised optical recording channel achieves the highest density.

Channels and codes for magnetooptical recording

The author outlines the technology used in the optical recording channel concentrating on magnetooptical recording and describes some of the major formats, codes, and data detection methods which are being developed. A sophisticated model of the optical recording channel was used as a way to make fair comparisons of these candidates, without skewing the results because of different disk or drive quality, degrees of hardware development, test procedures, etc. The author describes strengths and weaknesses of some of the leading contenders for current and future optical recording channels.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

SNR merits of binary modulation codes in equalized digital recording systems

For a digital recording system with a binary modulation encoder and a linearly dispersive channel with additive noise, the author determines optimum mean-square performance of the linear, partial-response, and decision-feedback equalizers. The analysis revolves around the power spectral density A( Omega ) of the code and the folded signal-to-noise ratio X( Omega ) of the channel. The latter function is analyzed for stylized optical and magnetic recording channels. For all equalizers it is shown that the effect of coding is similar to increasing X( Omega ) by an additive portion 1/A( Omega ). This favors depressions of A( Omega ) at frequencies where X( Omega ) is poor. However, for A( Omega ) to have any depressions, the signaling rate must be increased with respect to uncoded storage, and this inevitably degrades X( Omega ). At high information densities the degradation is often too large for coding to be rewarding. Examples serve to illustrate these results. >