MC34018

The MC34018 has the following features including All necessary level detection and attenuation controls for a hands-free telephone in a single integrated circuit;Background noise level monitoring with long time constant;Wide operating dynamic range through signal compression;On-chip supply and reference voltage regulation;Typical 100 mW output power (into 25 Ohms) with peak limiting to minimize distortion;Chip Select pin for active/standby operation;Linear Volume Control Function;Standard 28-pin plastic DIP package 10.600 inch wide) and SOIC package.

The MC34018 Speakerphone integrated circuit incorporates the necessary amplifiers, attenuators, and control functions to produce a high quality hands-free speakerphone system. Included are a microphone amplifier, a power audio amplifier for the speaker, transmit and receive attenuators, a monitoring system for background sound level, and an attenuation control system which responds to the relative transmit and receive levels as well as the background level. Also included are all necessary regulated voltages for both internal and external circuitry, allowing line-powered operation (no additional power supplies required. A Chip Select pin allows the chip to be powered down when not in use. A volume control function may be implemented with an external potentiometer. MC34018 applications include speaker-phones for household and business use, intercom systems, automotive telephones, and others.

The Tx-Rx comparator responds to the voltages at TLO and RLO, which in turn are functions of the currents sourced out of TLI and RLI, respectively. If an offset at the comparator input is desired, e.g., to prevent noise from switching the system, or to give preference to either the transmit or receive channel, this may be achieved by biasing the appropriate input (TLI or RLI).A resistor to ground will cause a DC current to flow out of that input, thus forcing the output of that amplifier to be biased slightly higher than normal. This amplifier then becomes the preferred one in the system operation. Resistor values from 500 k to 10 M ohms are recommended for this purpose.