An Improved TA Suppression Method for Coded PR Channels Piya Kovintavewat 1,a and Santi Koonkarnkhai 2,b 1,2 Data Storage Technology Research Center, Nakhon Pathom Rajabhat University, Thailand a piya@npru.ac.th, b santi@npru.ac.th Keywords: Perpendicular magnetic recording, thermal asperity, turbo equalization Abstract. Thermal asperity (TA) resulting from the collision between the slider and the asperity on a magnetic medium during read process can deteriorate the performance of hard disk drives (HDDs). Without TA detection and correction algorithms, the system performance can be unacceptable, depending on how severe the TA is. This paper presents an improved TA suppression method for coded partial response (PR) channels, which consists of two channels running in parallel. Specifically, one channel is matched to the target H(D), while the other is matched to the target H(D)G(D), where G(D) = 1 – D 2 is a bandpass filter and D is a delay operator. The soft-output Viterbi algorithm (SOVA) detector in the H(D) channel yields the high-quality soft information in absence of the TA, while that in the G(D)H(D) channel produces the high-quality soft information in presence of the TA. Then, the overall soft information chosen from these two detectors, depending on if a TA is detected or not, is sent to the decoder according to the turbo equalization principle. Experimental results show that the proposed method performs better than the conventional and the previously proposed ones, when operating at high signal-to-noise ratio (SNR) region where a practical HDD works. Introduction In practice, a magneto-resistive (MR) read head is used in current HDDs to sense a magnetic flux directly from the transitions of the magnetization pattern, resulting in an induced voltage pulse called a transition pulse. While reading the data bits on a magnetic medium, if the slider contacts with an asperity (or a surface roughness), both the slider surface and the asperity tip are heated. This causes an additive voltage transient known as thermal asperity (TA) [1] in the readback signal. Typically, a TA signal has a short rise time (50 – 160 ns) with a long decay time (1 – 5 μs), and its peak TA amplitude is 2 to 3 times the peak of the readback signal [2, 3]. In general, the TA can cause severe transient noise burst, loss of timing synchronization, or off-track perturbation, which leads to an error burst. This error burst could easily exceed the correction capability of the error- correction code, thus leading to a sector read failure. As the recording density keeps increasing and the flying height keeps decreasing, the TA effect becomes even more severe in HDDs. Hence, an efficient method to combat the TA effect is crucial. Many works have been proposed to alleviate the TA effect. Because the TA causes a shift in the baseline of the readback signal, the average value of the normal readback signal is zero, whereas that of the TA-affected readback signal is not. Therefore, Klaassen and van Peppen [2] introduced the TA detection by looking at the baseline of the averaged readback signal, while the TA correction was performed by a high-pass filter. Dorfman and Wolf [3] presented a TA suppression method by feeding the TA-affected readback signal through a filter (1 – D), which performed well in longitudinal magnetic recording channel. However, for a perpendicular magnetic recording (PMR) channel, which has a d.c. component, Erden and Kurtas [4] proposed a TA detection and correction method by use of different low-pass and high-pass filters, whereas Mathew and Tjhia [5] presented a simple threshold-based approach to detect and mitigate the TA effect. Additionally, Kovintavewat and Koonkarnkhai [6] proposed a TA suppression method based on a least-squares fitting technique, and they also presented a joint TA suppression and turbo equalization method on the partial-response (PR) channels with ECCs [7]. Although the method in [7] performs Advanced Materials Research Vol. 979 (2014) pp 46-49 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.979.46 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 202.29.10.6-17/06/14,12:47:15)