Method and apparatus for retrofitting a standard coin operated telephone employing a carbon microphone with a line powered electronic controller

摘要

A method and apparatus to enable a low power telephone controller to operate with either a carbon microphone or a dynamic microphone is described. A jumper in a preamplifier and filter circuit is preset to accommodate the audio transfer characteristics of one or other type of microphones. For either microphone, a small DC current of less than 500uA is used to sense the presence of either type microphone and serves the dual role of biasing the carbon microphone to the appropriate level to be amplified so that the input to an audio circuit in the controller us very nearly the same for either microphone. The audio circuit then processes and dynamically balances the signal to the telephone line, where it is output.