Abstract: A synchronous time division digital multiplexer apparatus utilizing frequency shift keying to transmit data from a plurality of parallel input channels that are sequentially sampled and converted to serial data having non-return to zero (NRZ) characteristics. Frequency shift keying is used to modulate the obtained serial data with a sinusoidal audio carrier. Simultaneously, the system clock is automatically modulated by means of frequency shift keying with another sinusoidal audio carrier, or manually by a repetitive frame synchronization pulse. The data carrier and clock carrier are linearly combined, bandpass filtered, and level adjusted for over a voice grade transmission media.