A laboratory bidirectional four-channel broadband coherent-type fibre-optic subscriber line (3.5 km) is reported. The carriers are centre-frequency-stabilised by a ‘heterodyne spectroscope’ and demultiplexed by a tunable heterodyne receiver. In a very special state of the system, BER increases for nonlinear interaction between two counterpropagating narrowband lightwaves because of Brillouin scattering.