Digital front-end functions include asymmetry cancellation [2006-1], adaptive equalization [2006-2], timing recovery and gain control as shown in the figure below. As indicated there, decisions of the detected bits that are provided by the detector are preferably used to drive the data-aided digital front-end functions. A hard-decision detector can for instance be a slicer, a partial-response Maximal likelihood (PRML) sequence detector, or a noise-predictive maximum-likelihood (NPML) sequence detector. Data-aided digital front-end functions and detection are part of the inner channel.
The overall system performance after error correction coding is very sensitive to the performance of the inner channel. Careful design of digital front-end functions is therefore critical to ensure optimal signal quality. Hence, the study and selection of algorithms for adaptive equalization, asymmetry cancellation, timing recovery [2009-11] and gain control are very important as they enable significant improvement in overall system performance.
Reliable timing recovery is of vital importance in tape-based data storage, since disturbances such as dropout events, instantaneous speed variations, or nonlinear signal distortions lead to degraded tape channel conditions.
One approach for robust timing recovery is called global timing control (GTC), which exploits the multi-channel nature of tape storage systems. The idea of GTC is to combine the timing information provided by a plurality of channels and generate “global” control signals that are used by all channels for their timing recovery functions. Channels subject to disturbance are aided with the timing information from channels operating under more reliable conditions [2008-4, 2008-5].
Figure 1. Building blocks of the read channel (inner channel).
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