The low-voltage voice-switched speakerphone circuit, U4082B, incorporates a wide range of functions. The versatility of the device is further enhanced by giving access to internal circuit points. The block diagram (see Figure 1-1) shows amplifiers, level detectors, transmit and receive attenuators operating in complementary functions, background noise monitors, chip disable, dial tone detector and mute function. Due to low-voltage operation, the device can be operated either by a low supply or via a telephone line requiring 5.5 mA typically. Stand-alone operation is enabled by a coupling transformer (Tip and Ring) or in conjunction with a handset speech network, as shown in Figure 1-2 on page 2.