convolutional codes with unit memory always achieve the largest free distance among all codes of the same rate
and same number
of encoder states, where
is the encoder memory. A unit-memory code with maximal free distance is given at each place where this free distance exceeds that of the best code with
and
relatively prime, for all
and for
. It is shown that the unit-memory codes are byte oriented in such a way as to be attractive for use in concatenated coding systems.