U4082B Datasheet, PDF (4/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

Table 2-1.
Pin
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Pin Description (Continued)
Symbol Function
VB
CPT
Output voltage ≈ V_S/2. It is a system AC ground and biases the volume control. A filter cap is required
An RC at this pin sets the time constant for the transmit background monitor
TLI2
Transmit level detector input on the microphone/speaker side
TLO2
Transmit level detector output on the microphone/speaker side, and input to the transmit background
monitor
RLO2 Receive level detector output on the microphone/speaker side
RLI2
Receive level detector input on the microphone/speaker side
RI
Input receive attenuator and dial tone detector. Maximum input level is 350 mVrms. Input impedance is
approximately 10 kΩ
RECO
TLI1
Receive attenuator output. DC level is approximately V_B
Transmit level detector input on the line side
TLO1 Transmit level detector output on the line side
RLO1 Receive level detector output on the line side and input to the receive background monitor
RLI1
Receive level detector input on the line side
CPR
An RC at this pin sets the time constant for the receive background monitor
GND
Ground
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.
4 U4082B
4743D–CORD–03/06