Speech compensation in dual-mode telephone

Abstract

A dual-mode telephone has transmit and receive channels suitable for cellular telephony, and is operative in analog and digital modes of communication. Each of the channels has parallel branches of circuitry wherein the circuitry of one branch accommodates voice signals in accordance with an analog protocol and the second branch accommodates voice signals processed by digital compression and expansion circuitry. The digital compression circuitry, which is employed for outgoing voice signals, and the digital expansion circuitry, which is employed for incoming voice signals, constitute a vocoder operative under control of a microcontroller unit. Operation of the vocoder during a digital transmission mode introduces a digitally processed quality to received voice signals, this characteristic being distinguishable from the voice quality of pulse-code modulation employed in the analog communication mode. In order to introduce uniformity of voice quality during both analog and digital modes of communication, the telephone includes circuitry for inserting the vocoder circuitry into the analog branch of the receive channel as a speech compensator with the compression and expansion circuitry operated in a back-to-back arrangement. The input and output signal formats of the speech compensator are identical, but the resulting output signal of the compensator has a digitally processed voice quality. Thereby, the digitally processed voice quality is present in both analog and digital modes of communication so as to eliminate a disturbance in change of voice quality during a hand-off between the two communication modes.