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U4082B-MFLG Datasheet(PDF) 4 Page - ATMEL Corporation |
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U4082B-MFLG Datasheet(HTML) 4 Page - ATMEL Corporation |
4 / 30 page ![]() 4 4743D–CORD–03/06 U4082B 3. Introduction 3.1 General The fundamental difference between the operation of a speakerphone and a handset is that of half duplex versus full duplex. The handset is full duplex since conversation can occur in both directions (transmit and receive) simultaneously. A speakerphone has higher gain levels in both paths, and attempting to converse in full-duplex mode results in oscillatory problems due to the loop that exists within the system. The loop is formed by the receive and transmit paths, the hybrid and the acoustic coupling (speaker to microphone). The only practical and economical solution used to date is to design the speakerphone to oper- ate in half-duplex mode. That is, only one person speaks at a time, while the other listens. To achieve this, a circuit is required which can detect who is talking, switch on the appropriate path (transmit or receive), and switch off (attenuate) the other path. In this way, the loop gain is main- tained less than unity. When the talkers exchange functions, the circuit must quickly detect this, and switch the circuit appropriately. By providing speech-level detectors, the circuit operates in a “hands-free” mode, eliminating the need for a “push-to-talk” switch. The handset has the same loop as the speakerphone. Oscillations do not occur because the gains are considerably lower and the coupling from the earpiece to the mouthpiece is almost nonexistent (the receiver is normally held against a person's ear). The U4082B provides the necessary level detectors, attenuators, and switching control for a properly operating speakerphone. The detection sensitivity and timing are externally controlla- ble. Additionally, the U4082B provides background noise monitors (which make the circuit insensitive to room and line noise), hybrid amplifiers for interfacing to Tip and Ring, the micro- phone amplifier, and other associated functions. 15 VB Output voltage ≈ V S/2. It is a system AC ground and biases the volume control. A filter cap is required 16 CPT An RC at this pin sets the time constant for the transmit background monitor 17 TLI2 Transmit level detector input on the microphone/speaker side 18 TLO2 Transmit level detector output on the microphone/speaker side, and input to the transmit background monitor 19 RLO2 Receive level detector output on the microphone/speaker side 20 RLI2 Receive level detector input on the microphone/speaker side 21 RI Input receive attenuator and dial tone detector. Maximum input level is 350 mVrms. Input impedance is approximately 10 k Ω 22 RECO Receive attenuator output. DC level is approximately VB 23 TLI1 Transmit level detector input on the line side 24 TLO1 Transmit level detector output on the line side 25 RLO1 Receive level detector output on the line side and input to the receive background monitor 26 RLI1 Receive level detector input on the line side 27 CPR An RC at this pin sets the time constant for the receive background monitor 28 GND Ground Table 2-1. Pin Description (Continued) Pin Symbol Function |
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