Digital data communications system.

摘要

A modulation method and apparatus is provided which can coexist with conventional Manchester (Ethernet) coding schemes and which can be transmitted and received over twisted pair cabling. The method according to the invention is a three-level D.C.-free code without bipolar violations wherein intersymbol interference is sufficiently controllable that the received signal can be detected as a two-level code. The code has a different power spectrum than Manchester coding for the same baud rate. The modulation method is a modified frequency modulation (Miller) code comprising three levels wherein each of the transitions is of a limited duration and the transition returns to zero d.c. after a predetermined pulse duration. The modulation method may be termed pulsed modified frequency modulation or pulsed Miller encoding. The signal so modulated is intended to be transmitted over twisted pair cabling of limited bandwidth such that the cabling serves as a low-pass filter. The pulsed modified frequency modulation encoding method according to the invention produces a fundamental at a frequency which is about half of the fundamental of a Manchester coding scheme at the same bit rate.