  We show the equivalence of two different notions of quantum channel capacity:
that which uses the entanglement fidelity as its criterion of success in
transmission, and that which uses the minimum fidelity of pure states in a
subspace of the input Hilbert space as its criterion. As a corollary, any
source with entropy rate less than the capacity may be transmitted with high
entanglement fidelity. We also show that a restricted class of encodings is
sufficient to transmit any quantum source which may be transmitted on a given
channel. This enables us to simplify a known upper bound for the channel
capacity. It also enables us to show that the availability of an auxiliary
classical channel from encoder to decoder does not increase the quantum
capacity.
