U4084B Datasheet, PDF (20/26 Pages) TEMIC Semiconductors – Low-Voltage, Voice-Switched Circuit for Hands-Free Operation

U4084B
Some comments on the graph (figure 22):
D Acousting coupling and side-tone coupling were not
included in equations 6 and 11. Those couplings will
affect the actual performance of the final speaker-
phone due to their interaction with speech at the
microphone, and the receive signal coming in at Tip/
Ring. The effects of those couplings are difficult to
predict due to their associated phase shifts and fre-
quency response. In some cases, the coupling signal
will add, and other times substract from the incoming
signal. The physical design of the speakerphone en-
closure, as well as the specific phone line to which it
is connected, will affect the acoustic and side-tone
couplings, respectively.
D The M_R line helps define the maximum acoustic
coupling permissible in a system, which can be found
from the following equation:
+ G_AC(MAX(
2
R₁
R₃
G_MA ^{. . . . .}
12
Equation 12 is independent of the volume control setting.
Conversely, the acoustic coupling of a designed system
helps determine the minimum slope of that line. Using the
component values of figure 2 in equation 12 yields a
G_AC(MAX) of –37 dB. Experience has shown, however,
that an acoustic coupling loss of >40 dB is desirable.
D The M_T line helps define the maximum sidetone cou-
pling (G_ST) permissible in the system, which can be
found from the following equation:
+ G_ST
2
R₁
R₂
.
.
.
.
.
13
Using the component values of figure 2 in equation 13
yields a maximum side-tone of 0 dB. Experience has
shown, however, that a minimum of 6.0-dB loss is prefer-
able.
The above equations can be used to determine the resistor
values for the level detector inputs. Equation 5 can be
used to determine the R₁, 3 ratio and equation 9 can be
used to determine the R₁–R₂ ratio. In figure 21, R₁–R₄
each represent the combined impedance of the resistor
and coupling capacitor at each level detector input. The
magnitude of each RC’s impedance should be kept within
W the range of 2.0 to 15 k in the voiceband (due to the typi-
cal signal levels present) to obtain the best performance
from the level detectors. The specific R and C at each
location will determine the frequency response of that
level detector.
Application Information
Dial-Tone Detector
The threshold for the dial-tone detector is internally set at
15 mV (10 mV_rms) below V_B (see figure 5). That
threshold can be reduced by connecting a resistor from RI
+ ƪ ƫ ^{to ground. The resistor value is calculated from:}
R
10 k
V_B
DV
–
1
where V_B is the voltage at Pin 12, and DV the amount of
threshold reduction. By connecting a resistor from V_S to
RI, the threshold can be increased. The resistor value is
+ ƪ ƫ ^{calculated from:}
R
10 k
V_S–V_B
DV
–
1
where DV is the amount of the threshold increase.
Background-Noise Monitors
For testing or circuit analysis purposes, the transmit or
receive attenuators can be set to the “on” position by
disabling the background noise monitors, and applying a
signal so as activate the level detectors. Grounding the
CPR pin will disable the receive background-noise
monitor, thereby indicating the “presence of speech” to
the attenuator control block. Grounding CPT does the
same for the transmit part.
Additionally, the receive background-noise monitor is
automatically disabled by the dial-tone detector when-
ever the receive signal exceeds the detector’s threshold.
Transmit/Receive Detection Priority
Although the U4084B was designed to have an idle mode
such that the attenuators are halfway between their full-on
and full-off positions, the idle mode can be biased towards
the transmit or the receive side. By doing so, gaining
control of the circuit from idle will be easier for that side
towards which it is biased since that path will have less
attenuation at idle.
By connecting a resistor from C_T (Pin 11) to ground, the
circuit will be biased towards the transmit side. The resis-
+ ƪ ƫ ^{tor value is calculated from:}
R
R_T
V_B
DV
–
1
where:
D^RTV
=
=
120 kW (typ.)
V_B – V11 (see
connected
figure 8).
between
Pin
11
and
12.
By connecting a resistor from C_T (Pin 11) to V_S, the cir-
cuit will be biased towards the receive side. The resistor
+ ƪ ƫ ^{value is calculated from:}
R
R_T
V_S – V_B
DV
–
1
The switching time will be somewhat affected in each
case due to the different voltage excursions required to get
to transmit and receive from idle. For practical consider-
ations, the DV shift should not exceed 100 mV.
20 (26)
TELEFUNKEN Semiconductors
Preliminary Information
Rev. A1, 31-Jan-97