| A significant challenge in magnetic tape recording comes from signal dropouts which are partial or total loss of readback signal amplitude for a temporary time interval. Dropouts are considered to be a major limiting factor in the performance of the read channel as they cause long bursts of bit errors. Naturally, the task of combating them is generally considered to be in the realm of error correction codes (ECC). In this study, we take a different approach. Our goal is to attack this problem at the signal level where appropriate equalization and detection strategies can be employed. Despite the complexity and the variety of the underlying dropout mechanisms, a simple dropout model was developed and shown to be consistent with the experimental data. At the heart of the model is the assumption that the dropout event is formed through a series of events causing the spacing between the head and the media to be increased. Tape liftoff during both write and read changes the amplitude and the shape of the pulses through an exponential function of this spacing profile. Tape liftoff during the write process results in the transitions to be written further down the track. It was found that a linear relation is sufficient to link the spacing profile to the resulting extra peakshift. Based on this model, a dropout correction scheme has been designed and tested on synthetic and real dropout waveforms. |
