U4082B Datasheet, PDF (6/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

output of the speaker amplifier. On the transmit side, one level detector (TLI2) is at the output of
the microphone amplifier, while the other (TLI1) is at the hybrid output. The outputs RLO1 and
TLO1 feed a comparator, whose output is fed to the attenuator control block. Likewise, outputs
RLO2 and TLO2 feed a second comparator which also goes to the attenuator control block. The
truth table for the effects of the level detectors is given in the section “Attenuator Control Block”
on page 8.
3.4 Background Noise Monitors
This circuit distinguishes speech (which consists of bursts) from background noise (a relatively
constant signal level). There are two background noise monitors, one for the receive path and
the other for the transmit path. The receive background noise monitor is operated by the
RLI1-RLO1 level detector, while the transmit background noise monitor is operated by the
TLI2-TLO2 level detector (Figure 3-2).
They monitor the background noise by storing a DC voltage representative of the respective
noise levels in capacitors at CPR and CPT. The voltages at these pins have slow rise times
(determined by the external RC), but fast decay times. If the signal at RLI1 (or TLI2) changes
slowly, the voltage at CPR (or CPT) will remain more positive than the voltage at the non-invert-
ing input of the monitor's output comparator. When speech is present, the voltage on the
non-inverting input of the comparator will rise more quickly than the voltage at the inverting input
(due to the burst characteristic of speech), causing its output to change. This output is sensed by
the attenuator control block.
Figure 3-2. Level Detectors
(TLI2) RLI1
(17) 26
5.1 kΩ
-
+
V_B
0.1 µF
Signal input
Level detector
Background noise monitor
4 µA
350Ω
-
+
+
-
-
+
(TLO2) RLO1 ⁽¹⁸⁾²⁵
2 µF
56 kΩ
33 kΩ
15 V_B
36 mV
(CPT) ^VS
CPR
(16) _{100 kΩ}
27
47 µF
(RLI2) TLI1
(20) 23
5.1 kΩ
0.1 µF
V_B
+
-
Level detector
4 µA
350Ω
TLO1
24
(RLO2) ⁽¹⁹⁾
2 µF
-
C4 (C3)
+
C2 (C1)
Comparator
To attenuator
control block
Signal input
The 36 mV offset at the comparator's input keeps the comparator from changing state unless the
speech level exceeds the background noise by approximately 4.0 dB. The time constant of the
external RC (approximately 4.7s) determines the response time to background noise variations.
6 U4082B
4743D–CORD–03/06