B.S.T.J. Briefs: HILO - An Improved Transmission Scheme for Semiconductor Switching Networks

Foremost among the problems in semiconductor switching network implementations is the difficulty of fabricating a semiconductor crosspoint which duplicates the characteristics of metallic crosspoints of nearly zero " o n " impedance. A low crosspoint impedance is necessary for low signal loss through the switching network. A transmission technique is described in this paper which relieves the requirement for low crosspoint " o n " impedance and which allows excellent voice-band transmission characteristics with unbalanced semiconductor switching networks. The technique called H I L O converts voltage inputs to the network into current changes using a high input impedance current source and transmits the current changes through the network crosspoints by modulating the network bias current. A low output impedance is provided at the networks receiving terminal, where the modulated currents are decoded to recover normal voltage-current levels.