Abstract: When a digital muldem employs protective switching, a requirement exists to perform revertive switching. The present disclosure illustrates a way to test the failed module after a switch to determine whether the module is actually failed or the decision to switch was based on momentary external conditions. The test is accomplished by looping back the "failed" module, passing data from one of the operative channels through the "failed" module and doing a bit by bit compare of input and output data for identity. If all of the muldems convert signals to a common frequency, only a single compare circuit need be used on a multiplexed basis.

Abstract: Method and apparatus for displaying measures of distortion present at the outputs of a quadrature demodulator. From each of the in-phase and quadrature channels of the demodulator there are acquired n parallel bits, m of these bits being the demodulator data output of the associated channel and (n-m) bits being error bits for the channel. The n bits for each channel are converted to a signal acceptable to a plot display device and applied to orthogonal axes of the device. In addition, an absolute value of the (n-m) error bits for each channel is derived. This absolute value is filtered and displayed, as by a meter, for each channel.

Abstract: Actual customer data is routed through a composite muldem which comprises a high speed section and a plurality of low speed sections with the signals being switched or transferred between the high speed and low speed sections all at a common frequency. This same customer data is then tested by a bit by bit comparison circuit which tests the data after it is split, rerouted in various datapath configurations and compared in a logical fashion to first detect datapath failure and then isolate the failure to a specific part of the composite muldem by a set of logical deductions.