Method and apparatus having cascaded decoding for multiple runlength-limited channel codes

Abstract

Two different (d,k)-RLL codes can be used in systems such as erasable and writable dense optical disks. This is because it is typically possible to make readers capable of reading information written at a higher resolution (by a short wavelength authoring system) than the resolution that can be written by a consumer writer (using a laser of the same or longer wavelength as the reader). For one embodiment, two different decoders are used to decode the data. Alternately, we present an encoding scheme which operates with a single decoder. In one case, q,&equals;q,(and as a result p,<p,), so that the decoders D,and D,actually process blocks of the same length. The decoding of the sequences encoded by E,is done using the decoder D,plus an additional function &psgr; which maps the restored p,-blocks into p,-blocks in a to very simple manner. This way, we do not need a separate circuit for the (d,,k,)-RLL decoder, because D,is obtained as a cascading of D,and another very simple circuit &psgr;. For one embodiment, the decoder D,is a block decoder, and, consequently D,is also a block decoder. The conversion performed by the function &psgr; is simply a removal of bits in fixed and pre-specified locations, thus getting the shorter p,-block out of the p,-block. The use of two coding schemes provides a method of making a lower performance recording system that produces recordings that are compatible with standard full performance readers. For example, this technique will allow a half capacity DVD to be written with a lower resolution system.