Group Code Recording (Paperback)

High Quality Content by WIKIPEDIA articles In computer science, group code recording (GCR) refers to several distinct but related encoding methods for magnetic media. The first, used in 6250 cpi magnetic tape, is an error-correcting code combined with a run length limited encoding scheme. The others are different floppy disk encoding methods used in some microcomputers until the late 1980s.In order to reliably read and write to magnetic tape, several constraints on the signal to be written must be followed. The first is that two adjacent flux reversals must be separated by a certain distance on the media. The second is that there must be a flux reversal often enough to keep the reader's clock in phase with the written signal; that is, the signal must be self-clocking. Prior to 6250 cpi tapes, 1600 cpi tapes satisfied these constraints using a technique called phase encoding, which was only 50% efficient. For 6250 GCR tapes, a (0,2)RLL code is used. This code requires five bits to be written for every four bits of data.