U4091BM TEMIC Semiconductors, U4091BM Datasheet - Page 13

• Manufacturer Part Number: U4091BM
• Description: Programmable Telephone Audio Processor
• Manufacturer: TEMIC Semiconductors

Dial-Tone Detector
The dial-tone detector is a comparator with one side
connected to the speaker amplifier input and the other to
V
idle mode, and the incoming signal is greater than 35 mV
(25 mVrms), the comparator’s output will change
disabling the receive idle mode. This circuit prevents the
dial tone (which would be considered as continuous
noise) from fading away as the circuit would have the
tendency to switch to idle mode. By disabling the receive
idle mode, the dial tone remains at the normally expected
full level.
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
*
path. The receive background-noise monitor is operated
on by the receive level detector, while the transmit
background noise monitor is operated on by the transmit
level detector (see figure 12). They monitor the
background noise by storing a DC voltage representative
of the respective noise levels in capacitors at CBNMR and
CBNMT. The voltages at these pins have slow rise times
(determined by the internal current source and an exter-
nal C), but fast decay times. If the signal at TLDR (or
TLDT) changes slowly, the voltage at BNMR (or BNMT)
will remain more positive than the voltage at the non-
inverting input of the monitor’s output comparator. When
speech is present, the voltage at the non-inverting input
of the comparator will rise quicker 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
mode-control block.
4-Point Sensing
In 4-point sensing mode, the receive- and the transmit-
sensing path include additional CLOGs (Calculated
Logarithmical amplifier). The block MODECON
compares the detector output signals and decides whether
receive-, transmit- or idle mode has to be activated.
Depending on the mode decision, MODECON generates
a differential voltage to control AFSCON.
The MODECON block has seven inputs:
D The output of the transmit log (LOGT)
D The output of the receive clog (CLOGR)
D The output of the transmit clog (CLOGT)
Rev. A1, 02-Jun-98
M
one for the receive path and the other for the transmit
the comparison of LOGT, CLOGR
– designated I1
the comparison of CLOGT, LOGR
with a 35-mV offset (see figure 11). If the circuit is in
Target Specification
D The output of the receive log (LOGR)
D The output of the transmit background-noise monitor
D The output of the receive background-noise monitor
D The output of the dial-tone detector
The differential output (AFST, AFSR) of the block
MODECON controls AFSCON. The effect of I1-I4 is as
follows:
X = don’t care; Y = I3 and I4 are not both noise.
LOGT > CLOGR
LOGT < CLOGR
LOGR < CLOGT
LOGR > CLOGT
BNMT detects speech
BNMT detects noise
BNMR detects speech
BNMR detects noise
Term Definitions
1. ‘Transmit’ means the transmit attenuator is fully on,
2. ‘Receive’ means the receive attenuator is fully on, and
3. In ‘Idle’ mode, the transmit- and receive attenuator
4. Switching to the full transmit or receive modes from
– designated I2
(BNMT) – designated I3
(BNMR) – designated I4
I1
R
R
R
R
T
T
T
T
and the receive attenuator is at maximum attenuation.
the transmit attenuator is at maximum attenuation.
are at the half of their maximum attenuation.
a) ‘Change mode’ means both transmit and receive
b) ‘Idle’ means speech has ceased in both transmit
idle mode is at the fast rate (30 ms).
speech are present in approximately equal levels.
The attenuators are quickly switched (30 ms) to
the opposite mode until one speech level
dominates the other.
and receive paths. The attenuators are then
slowly switched (1.5 seconds) to idle mode.
I2
T
R
T
R
T
R
T
R
Inputs
I3
Y
Y
X
N
N
N
X
S
I1=T
I1=R
I2=T
I2=R
I3=S
I3=N
I4=S
I4=N
U4091BM
I4
X
Y
Y
X
N
N
N
S
Change mode
Change mode
Transmit
Receive
Output
Mode
Idle
Idle
Idle
Idle
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