U4084B Datasheet, PDF (17/26 Pages) TEMIC Semiconductors – Low-Voltage, Voice-Switched Circuit for Hands-Free Operation

U4084B
Design Hints
Switching Time, Figure 6
The switching time of the U4084B circuit is determined
by C_T (Pin 11, refer to figure 6), and the capacitors at the
level-detector outputs (RLO1, RLO2, TLO1, TLO2), see
figure 2.
The switching time from idle to receive or transmit mode
is determined by the capacitor at C_T, together with the
internal current sources. The switching time is:
+ DT
DV C_T
I
+ + 240 5
60
20.0 ms
where:
DV =
C_T =
I=
240 mV
5 mF
60 mA
If the circuit switches directly from receive to transmit
mode (or vice-versa), the total switching time is 40 ms.
The switching time depends on the mode selection. If the
circuit is switching to “fast idle”, the time constant is
determined by
resistor. With
the C_T
C_T =
capacitor,
5.0 mF,
and
the
the internal 2.0-kW
time constant is
approximately 10 ms, resulting in a switching time of
approximately 30 ms (for 95% change). Fast idle is mode
may occur if both persons are talking at the same time,
thus trying to get control of the circuit. The switching
time from idle back to either transmit or receive mode is
described above.
By switching to “slow idle”, the time constant is
determined
resistor (see
fbiygutrhee6)C. W^T icthapCa_T^ci=to5r.0am^nFd,
R_T,
and
the
R_T =
external
120 kW,
the time constant is approximately 600 ms, resulting a
switching time of approximately 1.8 seconds (for 95%
change). The switching to slow idle starts when both
speakers have stopped talking. The switching time back
to the original mode depends on how fast that person
starts talking again. The sooner the speaking starts during
the 1.8-second period, the faster the switching time since
a smaller voltage excursion is required. The switching
time is determined by the internal current source as
described above.
The above switching times occur after the level detectors
have detected the appropriate signal levels, since their
outputs operate the attenuator control block. The rise time
of the level detectors’ outputs to new speech is quick by
comparison (approximately 1.0 ms), determined by the
internal 350 W resistor and the external capacitor
(typically 2.0 mF). The output’s decay time is determined
by the external capacitor, and an internal 4.0-mA current
source giving a decay rate of 60 ms for a 120 mV
excursion at RLO or TLO. The total response time of the
circuit is not constant as it depends on the relative strength
of the signals at the different level detectors and the
timing of the signals with respect to each other. The
" ^{capacitors at the four outputs (RLO1, RLO2, TLO1,}
TLO2) must have equal values ( 10%) to prevent
problems in timing and level response.
The rise time of the level detector’s outputs is too short
to be of significant. The decay time, however provides a
significant part of the “hold time” necessary to hold the
circuit during the normal pauses in speech.
The components at the inputs of the level detectors (RLI1,
RLI2, TLI1, TLI2) do not affect the switching time but
rather affect the relative signal levels required to switch
the circuit and the frequency response of the detectors.
Design Equations
Following definitions are used @ 1.0 kHz with reference
to figures 2 and 21 whereas coupling capacitors are
omitted for the sake of simplicity:
–
G_MA is the gain of the microphone amplifier
measured from the microphone output to TI
(typically 35 V/V, or 31 dB);
–
G_T is the gain of the transmit attenuator,
measured from TI to TO;
–
G_EXT is the gain of an external transmit amplifier
(typically 10.2 V/V, or 20.1 dB)
–
G_ST is the side-tone gain;
–
G_EXR is the gain of an external receive amplifier;
–
G_R is the gain of the receive attenuator measured
from RI to RECO;
–
G_SA is the gain of the speaker amplifier, mea-
sured from RECO to the differential output of the
speaker amplifier (typically 22 V/V or 26.8 dB);
–
G_AC is the acoustic coupling, measured from the
speaker differential voltage to the microphone
output voltage.
TELEFUNKEN Semiconductors
Rev. A1, 31-Jan-97
Preliminary Information
17 (26)