A linear correspondence is developed between the states of a rate-

convolutional encoder

and the states of a corresponding syndrome former

, where

is an encoder of the code dual to the code generated by

. This correspondence is used to find an expression for the number of all-zero paths of length

in the code trellis; the answer depends only on the constraint lengths of the dual code. A partial answer to the resynchronization problem also falls out of this development.