Identification of Nonlinear Reader Distortion in Magnetic Recording

The “extracted dipulse” technique is widely used in magnetic recording to characterize both the linear and the various nonlinear components in the readback signal. However, it is not always straightforward to identify the exact source and nature of the distortions since they may arise during either the writing process or the readback process. This article analyzes the distortion caused by the read sensor. Reader asymmetry and saturation are described by quadratic and cubic components, respectively. We compute the exact positions and the shapes and magnitudes of the resulting “echo” responses that appear in the extracted dipulse. It is important that the positions are identified exactly, as this can help distinguish reader distortion from distortion during writing. The shapes of the echo responses are also important. The widths of echoes caused by reader distortion are significantly narrower than those that arise from the writing process, and they become progressively narrower as the order of the reader distortion increases. It is also possible to reconstruct the nonlinear transfer function of the reader and derive the nonlinear coefficients. A set of five heads were measured. Good agreement is shown between the reconstructed transfer function and the details of the echoes appearing in the extracted dipulse.