|TEA1094AT||Hands free IC|
|TEA1094AT Datasheet PDF : 28 Pages |
Hands free IC
DECISION LOGIC: PINS IDT AND SWT
handbook, full pagewidth
X X 1 1 − 10 µA
X 1 0 X + 10 µA
1 X 0 X + 10 µA
The pin numbers given in parenthesis refer to the TEA1094A.
(1) When MUTET = HIGH, +10 µA is forced.
When DLC/MUTER < 0.2 V, −10 µA is forced.
Fig.9 Decision logic.
The TEA1094 and TEA1094A select their modes of
operation (transmit, receive or idle mode) by comparing
the signal and the noise envelopes of both channels. This
is executed by the decision logic. The resulting voltage on
pin SWT is the input for the voice-switch.
To facilitate the distinction between signal and noise, the
signal is considered as speech when its envelope is more
than 4.3 dB above the noise envelope. At room
temperature, this is equal to a voltage difference
VENV − VNOI = 13 mV. This so called speech/noise
threshold is implemented in both channels.
The signal on MIC contains both speech and the signal
coming from the loudspeaker (acoustic coupling). When
receiving, the contribution from the loudspeaker overrules
As a result, the signal envelope on TENV is formed mainly
by the loudspeaker signal. To correct this, an attenuator is
connected between TENV and the TENV/RENV
comparator. Its attenuation equals that applied to the
When a dial tone is present on the line, without monitoring,
the tone would be recognized as noise because it is a
signal with a constant amplitude. This would cause the
TEA1094 (TEA1094A) to go into the idle mode and the
user of the set would hear the dial tone fade away. To
prevent this, a dial tone detector is incorporated which, in
standard applications, does not consider input signals
between RIN1 and RIN2 as noise when they have a level
greater than 127 mV (RMS). This level is proportional to
1996 Jul 15
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