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TEA1098ATV

Speech and handsfree IC

Philips Electronics 

Philips Semiconductors

Speech and handsfree IC

Decision logic (pins IDT and SWT)

Product specification

TEA1098A

handbook, full pagewidth

TENV 8

TNOI 7

13 mV

ATTENUATOR

RENV 12

RNOI 10

13 mV

Vdt

from logic

from dynamic

limiter

DUPLEX CONTROLLER

LOGIC(1)

28 IDT

Vref

RIDT

X X 1 1 −10 µA

X 1 0 X +10 µA

1 X 0 X +10 µA

XX1 0

0

0 00X

0

27 SWT

CSWT

FCA153

(1) When DLC < 0.2 V, −10 µA is forced.

Fig.11 Decision logic.

The TEA1098A selects its mode 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 pin TXI contains both speech and the signal

from the loudspeaker (acoustic coupling). When receiving,

the contribution from the loudspeaker overrules the

speech.

As a result, the signal envelope on pin TENV is formed

mainly by the loudspeaker signal. To correct this, an

attenuator is connected between pin TENV and the

TENV/RENV comparator. Its attenuation equals that

applied to the microphone amplifier.

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

TEA1098A 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

HFRX and GND as noise when they have a level greater

than 25 mV (RMS). This level is proportional to RRSEN.

2000 Aug 18

15

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