Dropout-tolerant read channels
Dropouts are intermittent losses of signal commonly seen in magnetic tape recording readout. The main reason for such losses is the increased spacing between the head and the medium due to media defects or debris particles. The resulting signal is not only degraded by the apparent amplitude loss, but the characteristics of the pulses due to transitions are also changed. Moreover, in many cases, the locations of the pulses are altered, causing excessive amounts of peakshift. In this paper, a model, linking the liftoff to these effects, is presented. Experimental verification of the model using actual signals from a test tape drive is also given. Artificial dropout waveforms generated using this model are used to test two different read channel strategies. The first approach is an equalization-based correction scheme that attempts to undo the dropout. Results indicate that the dropout effects can be almost completely eliminated if an appropriate equalization procedure is applied. As an alternative approach, it is shown that the use of turbo coding in the presence of dropouts appears to be promising.