Methods and apparatus for rejection of interference in a digital

Abstract

A digital communications system of the type transmitting data on a plurality of modulated FDM carriers interspersed between harmonics of a system-wide signal and utilizing a main synchronized detector at a receiving end, is immunized to signal degradation due to reception of 'noise' from both the shifting of signal harmonics and the modulation sidebands of adjacent carriers into the passband of the desired communications signal during at least synchronization acquisition by methods of interference minimization including: emplacing each data carrier at a frequency mid-way between adjacent harmonics; locking the frequency of the data carrier to the system-wide signal frequency to maintain the communications frequency exactly mid-way between two adjacent harmonics of the latter signal; transmitting the data in digital form and modulating each data carrier at a baud essentially equal to an exact even submultiple of the system-wide potentially-interfering signal, and detecting the transmitted digitally-modulated carrier at the receiving end by use of long-time-constant bandpass filters. Similar integrating filters, having means for discharging the storage elements thereof after each baud time period as determined by a baud clock synchronized to the system-wide signal, are utilized for the main synchronized data detector.