U4091BM Datasheet, PDF (14/29 Pages) TEMIC Semiconductors – Programmable Telephone Audio Processor

U4091BM
Summary of the Truth Table
1. The circuit will switch to transmit mode if
a) Both transmit level detectors sense higher signal
levels than the respective receive level detectors
and
b) The transmit background-noise monitor indi-
cates the presence of speech.
2. The circuit will switch to receive mode if
a) Both receive level detectors sense higher signal
levels than the respective transmit level
detectors, and
b) The receive background-noise monitor indicates
the presence of speech.
3. The circuit will switch to the reverse mode if the level
detectors disagree on the relative strengths of the
signal levels, and at least one of the background-
noise monitors indicates speech.
4. The circuit will switch to idle mode when
a) Both talkers are quiet (no speech present), or
b) When one talker’s speech level is continuously
overridden by noise at the other speaker’s
location.
The time required to switch the circuit between transmit,
receive and idle is determined by internal current sources
and the capacitor at Pin CT. A diagram of the C_T circuitry
is shown in figure 13. It operates as follows:
D CCT is typically 4.7 mF.
D To switch to transmit mode, ITX is turned on (IRX is
off), charging the external capacitor to –240 mV
below VM. (An internal clamp prevents further
charging of the capacitor.)
D To switch to receive mode, IRX is turned on (ITX is
off), increasing the voltage on the capacitor to
+240 mV with respect to VM.
D To switch to reverse mode, the current sources ITX,
IRX are turned off, and the current source IFI is
switched on, discharging the capacitor to VM.
D To switch to idle mode, the current sources ITX, IRX,
IFI are turned off, and the current source ISI is charg-
ing the capacitor to VM.
IN
+
OUT
–
35 mV
to mode
I4 ^control
VM _DTD
14588
Figure 11. Dial tone detector
VB
TLDR
(TLDT)
–
+
+
–
–
+
56 k Ω
33 k Ω
36 mV
VM
BNMR
(BNMT)
1 µF
I4
(I3)
14589
Figure 12. Background noise monitor
14 (29)
AFS
to
control
attenuators
CT
CCT ^IRX
10µA ITX
10µA ^IFI
ISI
VM VM
Control
circuit
4
I1–4
Dial tone det.
14590
Figure 13. Generation of control voltage (CT) for mode switching
Target Specification
Rev. A1, 02-Jun-98