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For magnetic disk drives, we have developed a scheme for synchronous servo with maximum-likelihood (SSML) detection that is synchronous around the entire circumference of the track and utilizes ML estimation to detect head position, clock mark phase, and track addresses. Compared to the conventional asynchronous sector servo scheme, the SSML scheme has many, much shorter, servo areas-typically several hundred-distributed on the disk. This results in higher servo information rate, which will be needed for future high-track-density disk drives with a wide servo bandwidth. We analyze the performance of the SSML scheme, including the format efficiency. We discuss a limitation of the synchronous servo scheme with respect to the number of servo areas, and we show an appropriate region for its application. Feasibility of the SSML scheme has been demonstrated in prototype 2.5-in hard disk drives.

The effect of the nonlinear readback process of an MR head on a PRML digital magnetic recording channel was studied. Although the combination of a highly sensitive MR read head and a PRML channel is promising for high areal-density recording, nonlinearity in the MR readback process is a critical issue in the case of a channel in which linearity is assumed. We theoretically analyzed the characteristics of SHD (second harmonic distortion) as an index of the nonlinearity. Then, the effect of nonlinearity on the error rate was studied by computer simulation and in experiments. The results show that an SHD of less than ?25 dB is required in order for the channel to be free of nonlinear distortion.

The partial-response maximum likelihood (PRML) system currently is investigated for practical applications as a modulation/demodulation scheme for digital magnetic recording equipment. This paper proposes a construction of the bit synchronizer by complete digital signal processing. The operational principle is verified by experiment. The proposed bit synchronizer is composed of the decision-feedback digital PLL and the interpolator. The decision-feedback digital PLL receives the data-asynchronous sampled signal sequence as the input, which is the result of sampling by the fixed clock of two samples/bit, and regenerates the clock. The interpolator calculates the signal value at the data point indicated by the phase of the decision-feedback digital PLL output. The result is given to the Viterbi decoder. By applying the proposed scheme, the PRML demodulator from the equalizer to the Viterbi decoder can be realized as a digital signal processing circuit, which operates synchronously with the single fixed master clock. A PRML demodulator is constructed based on the proposed bit synchronizer, and the operational principle is verified by an experiment using a magnetic disk device. The bit-error rate less than 10−6 is obtained for the channel bit rate of 10 Mbps.

For hard disk drives (HDDs) having multiple disk rotational speeds, a new data channel and head positioning servo system was developed. Head positioning performance at the low spin-speed was preserved by using a synchronous servo scheme with more servo areas than the conventional scheme. Using a common physical disk format, sufficient servo performance and the same data recording density of 1.3 Gbit/inch/sup 2/ were obtained on two 2.5 inch prototype HDDs with a low spin-speed of 37.5 Hz and a high spin-speed of 75 Hz. Disk interchangeability was also confirmed.

A track density simulation model for a discrete track medium and an MR/Inductive head was developed based on an iterative geometric signal-to-noise ratio calculation. Using this model, we optimized design parameters of discrete track media related to track density. Our model suggests that discrete track media can achieve higher track density than conventional continuous track media.