A linear block code over a field can be derived from a unit scheme. Looking at codes as structures within a unit scheme greatly extends the availability of linear block and convolutional codes and allows the construction of the codes to required length, rate, distance and type. Properties of a code emanate from properties of the unit from which it was derived. %% can thus be constructed and analysed by designating the units whose properties would give the required codes. Orthogonal units, units in group rings, Fourier/Vandermonde units and related units are used to construct and analyse linear block and convolutional codes and to construct these to predefined length, rate, distance and type. Self-dual, dual containing, quantum error-correcting and complementary dual linear block and convolutional codes are constructed. Low density parity check linear block and convolutional codes are constructed using group rings and are constructed with no short cycles in the control matrix. From a single unit, multiple codes of a required type are derivable.

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