8/9 GMTR code is constructed and its bit error rate(BER)performance is evaluated. We assumed three types of read/write channels using a longitudinal medium, a single layered perpendicular medium and a double layered perpendicular medium. BER performance is obtained by computer simulation, and compared with those of 16/17(0.6/6)code and 16/17 MTR code. The results show that the proposed 8/9 GMTR coded E^3PR4ML system exhibits the excellent performance in all channels assumed here.
The influence of jitter-like noise on the performance of PRML systems in the perpendicular magnetic recording channel using a double layered medium is studied. Both the bit error rate(BER)performance and the required SNR to achieve a BER of 10^<-4> are obtained by computer simulation and are compared. The results show that PRML systems with the only positive coefficients exhibit the good performance even for the channel in which the power ratio of the jitter-like noise to the white noise at the reading point is fifty-fifty.
Head sensitivity function of MR heads for perpendicular double layer media was derived for the sake of the head response calculations by reciprocity theorem. Approximated Fan's function was given. Comparisons with the exact solution showed the validity of the function. Hyperboric tangent function and error function also agreed well with the Fan's solution. However, for the agreement with expermental results, assumption of an extremely narrow transition length of recorded magnetization, i.e. less than 10 nm or almost zero, was required. Further investigation would be needed for this point. Noise spectrum could also be explained by the obtained head sensitivity finctions.
Noise properties of perpendicular magnetic recording were investigated for some PR channels; PR4, EPR4 and E^2PR4 by computer simulation. A measured readback pulse and a real noise spectrum were used as well as an arctangent pulse and a white noise, which is mathematically easy to adapt. Results for the measured signal and noise were relatively different from that for the arctangent signal and the white noise. Noise correlation after PR equalization was also discussed. And considering this noise correlation, we thought a detection based on regression analysis was good, and analized it.
An application of the neural network equalization to the PRML channel with jitter-like media noise is studied. First, a model of the read/write channel with jitter-like media noise and a neural network equalizer are described. Then, the bit-error rate of the 16/17(0, 6/6)coded EPR4ML and E^2PR4ML systems using a neural network equalizer is evaluated by computer simulation and the performance is compared with that of the conventional systems using a transversal filter. The results show that the neural network equalization can reduce the influenece of jiter-like media noise on the performance.
A new iterative decoding scheme was developed for the cyclic-redundancy-check error-correction-code(CRCC), a concatenated error-correction coding system. This simple iterative decoder achieves efficient cooperation between an outer Reed-Solomon error-correction-code(RS-ECC)decoder and a PRML trellis-detector. By using "error-free" decoded bit-information feedback from the outer RS-ECC decoder, the iterative scheme employs "state pinning" in the ML trellis-detector. This enables long error-events in the ML-decoded data to be corrected efficiently without adding extra coding redundancy and specific decoding complexity. The iterative scheme improves the correction capability of the outer RS-ECC coding by making use of a CRCC coding for specific correcting short error-events. Simulation shows that the iterative scheme achieves significant improvement in RS-ECC error-rate performance for a rate 16/17 quasi-MTR-coded Modified E^2PRML(ME^2PRML)channel in conjunction with CRCC coding.
Assuming only jitter occurred at magnetization transition as media noise, noise spectra were analyzed by Fourier transformation. For a double-layered perpendicular medium, the noise spectrum appeared to triangularly increase at low frequency corresponding to experimental data. Also for a longitudinal medium the noise was reduced at low frequency while boosted at moderate frequency. The agreements suggest that the transition noise will mainly derive from the transition jitter. The channel equalization which has lower frequency suppression property such as a differentiation will be effective for double-layered medium jitter noise.
A reduction method of the medium noise which will be a serious problem for future ultra high density recording was studied. It was thought that a double layered perpendicular recording medium developed so far shows a rather large medium noise. It was found that a medium noise of a double layered medium could be reduced when the noise from a recording layer was reduced. To realize this, we proposed(1)to insert a very thin intermediate layer between a soft magnetic back layer and a recording layer and(2)to add a small amount of nitrogen during deposition of the back layer. Though the intermediate layer deteriorates the crystal growth of the recording layer, a fine grain size and a fine magnetic domain size of the recording layer could be realized on the soft magnetic back layer. Nitrogen addition to the back layer had an effect to grow it with multiple crystal orientations that led to realize a fine grain growth of the recording layer. Finally, the noise from the recording layer rather than that from the soft magnetic backlayer is dominant medium nooise for the double layered medium.