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A unique experimental digital data modem has been developed that achieves a degree of co-channel interference and multipath discrimination rejection, which are important features when using the HF medium. It has been found feasible to incorporate these provisions in a compact unit which is compatible (with minor interface changes), with existing standard military and civilian transmitting and receiving equipment. The technique used employs pseudorandom data encoding and correlation detection. To minimize the synchronization requirement between transmitting and receiving terminals a "programmable matched filter" is used to perform the cross correlation process. Thus, timing may drift and propagation time vary over a time range equal to a teletype band interval without loss of synchronization. Multipath rejection is achieved by use of time discrimination of the various correlation pulses corresponding to the discrete propagation paths.