Method and apparatus for retrofitting a standard coin...

Details Method and apparatus for retrofitting a standard coin... Method and apparatus for retrofitting a standard coin...

H - Electricity

04

M

344/1.9

H04M 17/02 (2006.01) H04M 1/60 (2006.01)

Patent

CA 2040431

A method and apparatus to enable a low power telephone controller (50) to operate with either a carbon microphone (51) or a dynamic microphone (51) is described. A jumper (JP1) in a preamplifier and filter circuit (33) is preset to accommodate the audio transfer characteristics of one or the 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 (51) to the appropriate level to be amplified so that the input to an audio circuit in the controller (50) is 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.

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