A system is provided for selectively transmitting horizontal television lines including video and digital audio components or equivalent digital data lines containing a plurality of audio program signals. A conventional television signal transmission places a digital audio signal in the horizontal blanking interval, followed by analog video information. In accordance with the present invention, the "window" containing the analog video information is replaced with a plurality of digital audio signals that are time division multiplexed within the window. An additional audio channel is placed in the horizontal blanking interval, at the same location the audio is placed when video information is transmitted. Selector switches are provided in the encoder and decoder for processing a composite waveform as either a video signal with an associated audio channel, or as a multiple channel digital audio signal. Independent encryption and decryption of each of the multiple audio channels is provided.