Stochastic dc control

摘要

When recording data on a record carrier control of the DC content of the data as recorded is important in order to allow accurate reproduction of the data. The Digital Sum Value represents the DC content, The Digital Sum Value can be controlled by replacing code words at the output of an encoder by code words that can never occur during encoding. The replacement code word has different parity than the code word it replaces. The resulting stream of code words is subsequently encoded using an NRZI coder, so that the change in parity resulting from the replacement code word results in a change of polarity of the NRZI output. The replacement code word can thus be used to change the polarity of the NRZI output if required to keep the Digital Sum Value low.