The MC34118 Voice Switched Speakerphone Circuit incorporates the necessary amplifiers, attenuators, level detectors, and control algorithm to
form the heart of a high quality hands-free speakerphone system. Included are a microphone amplifier with adjustable gain and MUTE control. Transmit and Receive attenuators which operate in a complementary manner, level detectors at both input and ouput of both attenuators, and background noise monitors for both the transmit and receive channels. A Dial Tone Detector prevents the dial tone from being attenuated by the Receive background noise monitor circuit. Also included are two line driver amplifiers which can be used to form a hybrid network in conjunction with an external coupling transformer. A high-pass filter can be used to filter out 60Hz noise in the receive channel, or for other filtering functions. A Chip Disable pin permits powering down the entire circuit to conserve power on long loops where loop current is at a minimim.
The MC34118 may be operated from a power supply, or it can be powered from the telelphone line, requiring typically 5.0 mA. The MC34118 can be interfaced directly to Tip and Ring (through a coupling transformer) for stand-alone operation, or it can be used in conjunction with a hand-set speech network and/or other features of a featurephone.
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