An integrated error control and dc-free code based on multimode coding

Optical disk storage systems use both error control (EC) codes and dc-free constrained sequence (CS) codes in order to increase the reliability of the data read from the disk. We present a method of integrating these coding techniques in order to overcome drawbacks with the conventional manner in which these coding procedures are concatenated. Our technique is based on the principle of multimode coding. Each source word (SW) is represented by a set of complementary EC codewords (CWs), and from this set the encoder selects the EC CW that best meets the CS goals of the system. The decoding structure avoids the problem of error propagation during CS decoding by performing error correction before removing the effects of the CS code. Power spectra of encoded sequences generated by a hardware implementation have a null at 0 Hz, confirming that these coded sequences are dc-free. BER simulations demonstrate the superior performance of this combined EC and CS code on a dc-constrained noisy channel.