Patent US 3,965,309

A test system for a T carrier type telephone PCM communications system converts typical trio type pulse sets to quadruple sets in order to eliminate the dc offset. Thus, the test signal can be passed through several successive spans or through marginal repeaters in a manner similar to an ordinary data signal. The trio to quadruple conversion is accomplished by a stuffing procedure which compensates for any ambiguity in the audio crossover when the initial test signal is being produced. A reconverter may be remotely actuated to test any particular span or to loopback the test signal whereby testing may be carried on at a single location for both incoming and outgoing lines.