A combining filter for vision and sound transmission

A short review of possible methods of combining vision and sound transmissions into a common aerial is given. The use of the Maxwell bridge (M.b.) principle leads to a compact design with good performance. The fundamental characteristics of a filter of this type are derived. It is shown that tapped coaxial stubs are unsuitable as frequency discriminative elements, and instead, T-connected coaxial stubs are used with the connecting arms forming part of the resonators; the mathematical analysis of these is presented. Consideration is given to problems associated with high-power transmission, such as the dissipation of heat and the presence of high voltages. An expression for the minimum insertion loss for the sound transmission is derived and generalized for any frequency-discriminative combining filter. A description is given of an M.b. combining filter, of which the performance was in close agreement with the calculations. The measured results are shown for insertion loss between vision and sound inputs, insertion loss of each transmission, and phase delay, envelope delay and input admittance for the vision transmission. Other possible uses are suggested for Maxwell-bridge-type filters.