tea1085t NXP Semiconductors, tea1085t Datasheet - Page 12

• Manufacturer Part Number: tea1085t
• Description: Listening-in Circuit For Line-powered Telephone Sets
• Manufacturer: NXP Semiconductors

Philips Semiconductors
This reaction time is the 'attack delay time' and ensures
minimum sensitivity of the system for own speech.
The first threshold level at DTI is determined by the
equation:
Where: I
With the component values given in Fig.16, I
thus V
Listen-in mode
During normal speech the discharge of the capacitor
connected to LLC is not sufficient to reach the threshold
level whereby the system switches to the Larsen mode.
This is because normal speech is not continuous, the
discharge of C26 is slow (attack delay) and the charge is
fast.
The slope of V
With C26 = 1 F and R36 = 120 k this results in
S
Discharge of the capacitor at LLC occurs when the signal
at DTI exceeds V
the attack delay time t
equation:
Where k = t
The duty cycle is determined by the time in which the first
threshold level (V
DTI (see Fig.11) thus for large signals; k
With the component values given in Fig.16; k
signals
Consequently 120 ms
V
March 1992
V
S
t
ad
1i
DTI
DTI1
1i
Listening-in circuit for line-powered
telephone sets
= 10 V/s.
=
=
DTI1
---------------- -
------------------------------------- -
2
100 mV(RMS)
=
C26
V
DCA
100 mV(RMS).
LLC
= 18.8 mV.
1.25
---------- -
R25
3 k 1
1
= the DC current into DCA
/ T
=
LLC
R36
–
---------------------------- - V s
C26 R36
–
DTI1
DTI1
I
----------- -
DCA
during charge is given in the equation:
2
1.25
, thus for a continuous signal at DTI
) is exceeded by the signal level at
ad
(see Fig.10) is determined by the
t
ad
2 R33 if f > f3 in Fig.9
160 ms, for
0.5.
DCA
0.457 for
= 11 A
12
Larsen mode
After the 'attack delay time' the circuit switches from the
listen-in mode to the Larsen mode. The gain of the
loudspeaker amplifier is reduced quickly to a value
(t
residual Larsen signal is determined by a second
threshold level. This level can be set by resistor R34
connected between THL2 and V
level must always be selected at a lower level than the first
threshold level thus R34
The time taken to effect gain reduction is very short. In the
Larsen mode the circuit acts as a dynamic limiter with peak
detector and regulates the gain so that the signal level at
DTI is determined by the second threshold level V
The second threshold level at DTI is determined by the
equation:
Where: I
With the component values given in Fig.16,
V
The charge current in the Larsen mode is reduced to half
the charge current in the listen-in mode.
The slope of V
equation:
Where: C26 = 1 F and R36 = 100 k , S
When the Larsen effect stops (total open-loop gain 1) the
gain of the loudspeaker amplifier will return to its normal
value in a time period known as the 'Larsen release time'
(t
connected to LLC and resistor R36 connected to SIC.
Where: C26 = 1 F and R36 = 120 k , t
In practice the choice of the threshold levels (determined
by R35 and R34) depends on the sensitivity of the
microphone and loudspeaker, the send and receive gains,
sidetone suppression and the acoustical properties which
are determined by the cabinet of the telephone set.
V
S
LAa
LAr
DTI2
DTI2
la
). This time period is determined by capacitor C26
=
= Larsen attack time, see Fig.10) whereby the
= 6.9 mV.
---------------- -
=
V
DCA
LLC
1.25
---------- -
R34
= the DC current into DCA
LLC
=
–
-------------------------------------- V s
2 C26 R34
during charge (see Fig.10) is given in the
I
----------- -
DCA
2
1.25
TEA1085; TEA1085A
2 R33 if f > f3 in Fig.9
R35.
SS
Preliminary specification
. The second threshold
LAr
la
= 5 V/s
= 250 ms
DTI2
.