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Pseudo-error detectors are devices which show great potential for the measurement of the bit error rate of an on-line digital communications link. They are implemented in the form of a second detector (in addition and in parallel to the traffic data bit detector) which is very perturbation-sensitive. They do not compromise the traffic handling capacity of the system. Four methods of generating the pseudo-error characteristic are described: i) shifted detection threshold; ii) intersymbol interference enhancement; iii) noise addition; iv) sampling phase offset. Practical considerations generally govern the choice of method. Experimental results of pseudo-error detector behavior in the presence of Gaussian noise show that stable characteristics can be achieved to estimate a wide range of bit error rate (BER's) in very modest time intervals. In addition, experimental results in a real complex environment consisting of a 1.544 Mbits/s T1 capacity digital link using QPSK modulation techniques, including both Gaussian and non-Gaussian perturbations, show pseudo-error detection to be reliable for the measurement of BER and for controlling channel switching.