Abstract: A communication system for connecting a telephone switching office to a data processing center is described. A telephone trunk line from the switching office carries data which is digitally encoded and may be in a compressed format, for example mu-law PCM, and which is derived from remote data terminals. A novel type of modem is connected between the trunk and the data processing center. This is a direct synthesis device which converts the modulated digital data directly to a non-modulated digital format readable by the data processing center and, for digital data originating at the data processing center, produces modulated digital data which is transmitted along the trunk to the switching office. The direct synthesis modem is a single, purely digital device which replaces many separate digital/analog modems which are required in conventional systems.