Group Coded Recording

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Group Coded Recording is an encoding system for storing data on magnetic tape. It was created by IBM in 1973, for use in recording data at a density of 6250 bpi.

It uses a run-length limited coding system; five bits are written for every four bits of data. This allows it to be self-clocking, which requires that ensuring that long runs of '0' or '1' data do not pass without any magnetic field flux reversals (which could cause the clock to lose synchronization). IBM also included a longitudinal error-correcting code to deal with the inevitable small storage errors.

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