A digital signal transmitting system produces digital signals of up to four different modes for transmission over an unused television channel in an existing cable television transmission line and which requires no more bandwidth than that normally required by a single existing television channel, the various modes of digital signals all include synchronizing signals and service bit signals which are used to address any of the numerous terminals at the receiving side to control access to the various modes of digital signals transmitted over the single channel. By controlling the transmission of the modes to have constant time intervals, which can be selected based upon different sampling times, high quality audio signals may be transmitted or/and data channels or monaural audio signals, all of which may be transmitted over the single cable television transmission line.