Error density detector

Claim 1

In a digital transmission system in which the presence or absence of errors in a periodic framing pattern is used to determine if the system is properly synchronized, an error density detector comprising: an error detector which provides an error output pulse at an output whenever an error is detected in the framing pattern; a timer which provides at a first output a sample pulse at the beginning of each sample interval, said sample interval including a plurality of frames and said sample pulse occurring intermediate of said framing pattern; and at a second output a sample clock pulse which occurs within the time slot of the sample pulse; and logic means comprising: a programmable register having n programmable input terminals l to n, having a mode and a first clock input connected, respectively to the first and second outputs of said timer, said programmable input terminals being enabled during the occurrence of the sample pulse so that programmed information may be read into said input terminals l to n in parallel, having a second clock input connected to receive the error output pulses so as to cause the register to shift one bit for each error pulse, having n output terminals Ql to Qn, and having a serial input terminal connected to the first output terminal Ql, whereby the state programmed into the first input terminal will be propagated by said error pulse and; gating means for providing the program information and for providing an alarm indication, having a first input connected to Ql, having second and third inputs connected to appropriate output terminals of said register so that an alarm indication will occur only when at least a first predetermined number of errors have occurred during a sample interval and so that the alarm lead state will subsequently change if less than a second predetermined number of errors have occurred during a subsequent sample interval.