Abstract: CMI code has many features, but the interference on a transmission line resulted from radiant noise is an unavoidable problem. In the invention, WALSH 1 code is employed to solve the problem. Clock pulses having a frequency twice that of code on a transmission line in the WALSH 1 code are extracted. The extracted clock pulses consists of zero phase clock pulses and pi phase clock pulses, wherein the zero phase clock pulses are accurately extracted. An embodiment comprises an clock extraction circuit for extracting extracted clock pulses of 2f.sub.0 from a receive pulse train of frequency f.sub.

Abstract: A start bit detecting circuit is disclosed, which employs a waveform reshaper, a detection and start circuit and a decision circuit. The waveform reshaper has threshold values T+ and T-. The detection and start circuit starts the start bit detecting circuit upon detection of a predetermined pulse width. The decision circuit judges, for a plurality of decision points, coincidence between the input signal and a specific pattern having a start-signal element, which comprises a plurality of bits.

Abstract: A control signal transmission system for a key telephone system, in which control signal information is transmitted in a parallel signal configuration between a plurality of key telephone sets and a key service unit by the use of transmission lines one-to-one corresponding to the individual key telephone sets, and in which the information between the key telephone sets is transmitted using a serial signal format of a frame configuration so that time slots of respective frames are synchronized with respect to all of the transmission lines.