Abstract: A data modem that operates to permit the concurrent high speed transmission/reception of digital data over two independent data links or systems, is disclosed. Data bits concurrently applied to a transmitting modem from two independent data sources are interlaced to form a stream of data bits that are effectively grouped as data words to be differentially phase modulated. Modulated data may be transmitted over ordinary voice grade telephone lines to a distant receiver modem at effectively twice the bit rate of the individual data sources. Successive groups of six interlaced data bits are divided into two groups of three bits which are uniquely identified by having a selected phase angle added to or subtracted from the differential phase angle for alternate groups of three bits.