tea1085t NXP Semiconductors, tea1085t Datasheet - Page 11

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

Philips Semiconductors
Larsen limiter detector (DTI and DCA) pins 15 and 16
The QLA output signal is AC coupled to the detector input
DTI. DTI is biased by potential divider R30 and R31. The
voltage applied to DTI of the Larsen level limiter is
converted into a current for further processing in this
circuit. Current adjustment is achieved using the network
connected between DCA and V
The equation for DC current is:
The equation for AC current is:
In the basic application:
R30 = 100 k , R31 = 220 k , R33 = 500 , R32 = 100 k
and C25 = 330 nF
This results in I
High-pass filter
A third order high-pass filter is created between the
microphone input voltage and the current flowing into
DCA. The cut-off frequencies (see Fig.9) of the three
sections are:
Where: R25 = R26 and C22 = C23
The filter reduces the sensitivity of the system to own
speech.
Normal speech is in the frequency range 300 Hz to
3400 Hz, however, the Larsen signal normally occurs at a
frequency
With the component values as used in the basic
application (see Fig.16); f1 = 500 Hz, f2 = 1 kHz and
f3 = 3 kHz
March 1992
I
i
f1
f2
f3
i
----------- -
V
DCA
DCA
DCA
DTI
Listening-in circuit for line-powered
telephone sets
=
=
=
=
=
---------------------------- - where R
2 R
------------------------------
2 R33C24
------------------------------
2 R26C23
=
V
----------- - for f >
---------------------------- -
R30
R33
2 (mA/V)
DTI
eg
1
1
1
R30
3 kHz.
C24
+
R31
DCA
=
= 11 A and the equation:
1
-- - R33 C25
2
1/(2 R25C22
V
BB
eq
---------------------------- -
R32
=
BB
R30 R31
---------------------------- -
R30
1
+
(see Fig.8).
R33
+
R31
11
Where:
Larsen limiter capacitor (LLC) pin 12
A 1 F capacitor (C26) is connected externally between
V
the Larsen level limiter in the listen-in and Larsen mode.
The timing is also dependent on the value of the resistor
connected between SIC and V
Larsen level limiter threshold (THL1 and THL2) pins
13 and 14
When the signal at DTI exceeds the first threshold level the
capacitor connected to LLC will start to discharge. The first
threshold level is determined by the value of the resistor,
R35, connected to THL1 and V
discharge of C26 depends on how much the level of the
signal at DTI exceeds the first threshold level (for normal
speech the discharge is small).
The Larsen effect is generally defined as a signal level of
The Larsen signal must be reduced to a low level within
200 ms. For Larsen signal levels (f
Fig.16, the system will switch from the listen-in mode to the
Larsen mode in a time period of 100 ms to 200 ms;
consequently, the initial Larsen effect will last only for a
short period of time.
handbook, halfpage
SS
100 mV(RMS), on line, for a period of more than 100 ms.
100 mV(RMS) at DTI and, with the component values of
20 log
and LLC to determine the attack and release timing of
(dB)
g
g o
g
0
=
Fig.9 Third-order high-pass filter.
i
---------- -
DCA
V
m
18 dB per octave
f1
TEA1085; TEA1085A
speech
12 dB per octave
f2
g
o
SS
SS
=
.
6 dB per octave
Preliminary specification
. The amount of
A
---------- -
R33
f3
pre
f3 in Fig.9) of
Larsen
20 log f
MGR038