The L3 Coaxial System

Abstract

Television terminals are required at circuit ends of the L3 coaxial system; at the transmitting end to condition video signals for carrier transmission and at the receiving end to detect the transmitted signals. Special signal characteristics, e.g., a degree of modulation which exceeds the value commonly referred to as 100 per cent modulation, require departures from standard modulating and detecting processes. The high degree of modulation requires both careful control of transmitted wave form and at the receiver product demodulation with phase synchronous carrier (homodyne detection). Carrier regeneration requirements result in the choice of one step frequency translation from the video frequency spectrum to the allocated vestigial sideband carrier spectrum. The one step process using a single modulator results in unusual balance requirements for the modulator itself and an unusual circuit configuration. Transmission quality objectives for the terminals are such as to permit six pairs of television terminals to operate in tandem in a transcontinental circuit. This permits a degree of interconnecting flexibility in operation with other systems, e.g., L1 coaxial or microwave systems. These objectives place severe requirements upon the transmission stability of various fillers and other circuits within the terminals. New network techniques both in design and fabrication are brought to bear in the effort to achieve required performance. The transmitting and receiving terminals are described, illustrating the functional operation and mechanical and electrical arrangements of the equipment.