U4082B-DEV-BOARD Atmel, U4082B-DEV-BOARD Datasheet - Page 6

• Manufacturer Part Number: U4082B-DEV-BOARD
• Description: WiFi / 802.11 Modules & Development Tools Voice Switched Circuit
• Manufacturer: Atmel

3.4
Figure 3-2.
6
Background Noise Monitors
U4082B
Level Detectors
(RLI2) TLI1
(TLI2) RLI1
Signal input
5.1 k
(17) 26
Signal input
5.1 k
(20) 23
0.1 µF
0.1 µF
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
on page
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
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.
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.
V
V
B
B
-
+
-
+
8.
Level detector
(TLO2) RLO1
Level detector
(RLO2)
TLO1
350
350
4 µA
(18)25
(19)
(Figure
2 µF
2 µF
4 µA
24
15
3-2).
V
33 k
56 k
B
+
-
Background noise monitor
-
+
Comparator
-
+
C2 (C1)
36 mV
To attenuator
control block
C4 (C3)
-
+
(CPT)
(16) 100 k
CPR
27
“Attenuator Control Block”
47 µF
V
S
4743D–CORD–03/06