Apparatus and method for detecting errors in a 7-level correlative signal

Abstract

The correlation properties of a 7-level correlative waveform permit detection of errors due to irregularities and impairments in the transmission facility, where these errors violate the correlation patterns of the waveform. The received 7-level correlative waveform is first decoded to obtain a pair of binary outputs which are representative of the original binary signal at the transmitter, except for errors that might have occurred during the transmission period. The pair of binary signals are operated on to obtain coded binary signals and their delayed counterparts. These coded binary signals are used to establish the effect of presence or absence of a top or bottom level that would result from the formation of a nonbinary correlative signal. If an error has occurred during transmission, the extreme levels of the locally generated signal will not normally coincide with those of the received signal. To determine if an error has occurred it is necessary to compare the extreme levels of the incoming waveform with those locally generated. Thus, the decoder provides the bottom and top level outputs which are compared with the extreme levels of the equivalent signal from the binary coded signals, and when occurrences of extreme levels of the two signals differ, it is an indication that an error has occurred. The error indication results in an output signal from the error detector circuitry.