3 - Electronic Speech Circuits

Abstract

This chapter discusses the circuits that provide two-way speech in the telephone set. The discussion moves from a description of the telephone system, as it presently exists, to a discussion of how it is changing and improving, as mechanical and conventional electrical devices are replaced by electronic devices. These devices have most of the required components on the chip, with connections provided for outboard components, such as resistors and capacitors that are used to “program” the chip; that is, to set the electrical operating parameters for specific applications. In this way, one design can be used for a variety of similar, yet different requirements. This approach lowers cost and improves performance. For most conventional telephone sets in the United States, the component that determines the minimum operating current in the local loop is the carbon transmitter. Under these conditions, the on-hook current drawn by the electronics inside the telephone set must be well below the minimum current required by the line relay; otherwise, the line relay would energize and incorrectly indicate an off-hook condition.