Abstract: Incoming data in binary form is retimed, and under control of a framing clock, a first counter clocks in four bits in the first stage of a preview store; and a second counter clocks in eight bits, four bits in each of the second and third stage of the preview store. The first bit in each instance is the framing bit for the next previous frame location and the second next previous frame location so long as an in-frame condition exists, and the frame control is in synchronism with the framing bits contained in the incoming data.The current data bit is compared with the bits appearing at the output of the first and second stages of the preview store to determine if the three bits comprise a valid framing sequence for a non-winking framing sequence in which any combination of the following frame bit indications may occur-F1 F0 F0 F1-where the number following the F indicates the state of the framing bit.

Abstract: One or a burst of error signals from an error detector are applied to one input of a latch. The first error received during a "window" forces the latch to a set position, and a subsequent enabling pulse reads the error occurrence into the count up input of an up-down counter. Following the enable pulse, the latch is reset. If one or more errors occur during the subsequent window period, another error occurrence is read into the count up input. Otherwise, the occurrence of the enable pulse reads the absence of an error into the down count input of said up-down counter. On a full count, an output latch circuit is set by an output signal from the counter. The latch is not reset until a zero count is obtained in the up-down counter. Overflow and underflow are prohibited by circuits external to the up-down counter.