PSK-System und -Modem

A modem system is disclosed which has an improved reliability of detection of PSK data using a simple signal delay detection scheme. A transmission signal is produced which has a phase reference portion provided at at least one of the front and rear portions of a time slot. In a first half of the remaining portion of the time slot the phase of the transmission signal changes stepwise in a first predetermined direction for a "mark" state while in a second half of the remaining portion it changes stepwise back to the reference value. For a "space" state the phase changes stepwise in a second predetermined direction, opposite the first , in the first half of the remaining portion and then in the second half it changes stepwise back to the reference value. In a second aspect of the invention only one of the two digital states causes a corresponding phase change while for the other state the transmission signal retains the phase reference value throughout the time slot.