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30. E. Soljanin, “On–track and off–track distance properties of Class 4 partial response channels,” in<br />

Proc. 1995 SPIE Int. Symp. on Voice, Video, and Data Communications, Philadelphia, PA, vol. 2605,<br />

pp. 92–102, Oct. 1995.<br />

31. E. Soljanin, “Simple soft-output detection for magnetic recording channels,” in 1998 IEEE Int. Symp.<br />

Inform. Theory (ISIT’00), Sorrento, Italy, June 2000.<br />

32. A. J. van Wijngaarden and K. A. Schouhamer Immink “Combinatorial construction of high rate<br />

runlength-limited codes,” Proc. 1996 IEEE Global Telecommun. Conf. (GLOBECOM ’96), London,<br />

U.K., pp. 343–347, Nov. 1996.<br />

33. A. J. van Wijngaarden and K. A. Schouhamer Immink, “Maximum run-length limited codes with<br />

error control properties,” IEEE J. Select. Areas Commun., vol. 19, April 2001.<br />

34. A. J. van Wijngaarden and E. Soljanin, “A combinatorial technique for constructing high rate<br />

MTR–RLL codes,” IEEE J. Select. Areas Commun., vol. 19, April 2001.<br />

34.6 Data Detection<br />

Miroslav Despotović and Vojin ∨<br />

Senk<br />

Introduction<br />

Digital magnetic recording systems transport information from one time to another. In communication<br />

society jargon, it is said that recording and reading information back from a (magnetic) medium is<br />

equivalent to sending it through a time channel. There are differences between such channels. Namely,<br />

in communication systems, the goal is a user error rate of 10 −5 or 10 −6 . Storage systems, however, often<br />

require error rates of 10 −12 or better. On the other hand, the common goal is to send the greatest possible<br />

amount of information through the channel used. For storage systems, this is tantamount to increasing<br />

recording density, keeping the amount redundancy as low as possible, i.e., keeping the bit rate per recorded<br />

pulse as high as possible. The perpetual push for higher bit rates and higher storage densities spurs a<br />

steady increment of the amplitude distortion of many types of transmission and storage channels.<br />

When recording density is low, each transition written on the magnetic medium results in a relatively<br />

isolated peak of voltage, and peak detection method is used to recover written information; however,<br />

when PW50 (pulse width at half maximum response) becomes comparable with the channel bit period,<br />

the peak detection channel cannot provide reliable data detection, due to intersymbol interference (ISI).<br />

This interference arises because the effects of one readback pulse are not allowed to die away completely<br />

before the transmission of the next. This is an example of a so-called baseband transmission system, i.e.,<br />

no carrier modulation is used to send data. Impulse dispersion and different types of induced noise at<br />

the receiver end of the system introduce combination of several techniques (equalization, detection, and<br />

timing recovery) to restore data. This chapter section gives a survey of most important detection techniques<br />

in use today assuming ideal synchronization.<br />

Increasing recording density in new magnetic recording products necessarily demands enhanced<br />

detection techniques. First detectors operated at densities at which pulses were clearly separated, so that<br />

very simple, symbol-by-symbol detection technique was applied, the so-called peak detector [30]. With<br />

increased density, the overlap of neighboring dispersed pulses becomes so severe (i.e., large intersymbol<br />

interference—ISI) that peak detector could not combat with such heavy pulse shape degradation. To<br />

accomplish this task, it was necessary to master signal processing technology to be able to implement<br />

more powerful sequence detection techniques. This chapter section will both focus on this type of detection<br />

already applied in commercial products and give advanced procedures for searching the detection trellis<br />

to serve as a tutorial material for research on next generation products.<br />

Partial-Response Equalization<br />

In the classical peak detection scheme, an equalizer is inserted whose task is just to remove all the ISI so<br />

that an isolated pulse is acquired, but the equalization will also enhance and colorize the noise (from<br />

readback process) due to spectral mismatch. The noise enhancement obtained in this manner will increase<br />

© 2002 by CRC Press LLC

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