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TEA1099

Speech and handsfree IC with auxiliary inputs/outputs and analog multiplexer

Philips Electronics 

Philips Semiconductors

Speech and handsfree IC with auxiliary

inputs/outputs and analog multiplexer

Product specification

TEA1099H

Handsfree application

As can be seen from Fig.3, a loop is formed via the

sidetone network in the line interface part and the acoustic

coupling between loudspeaker and microphone of the

handsfree part. When this loop gain is greater than 1,

howling occurs. In a full duplex application, this would be

the case. The loop-gain has to be much lower than 1 and

therefore has to be decreased to avoid howling. This is

achieved by the duplex controller. The duplex controller of

the TEA1099H detects which channel has the ‘largest’

signal and then controls the gain of the microphone

amplifier and the loudspeaker amplifier so that the sum of

the gains remains constant.

As a result, the circuit in this handsfree application can be

in three stable modes:

1. Transmit mode (TX mode).

The gain of the microphone amplifier is at its maximum

and the gain of the loudspeaker amplifier is at its

minimum.

2. Receive mode (RX mode).

The gain of the loudspeaker amplifier is at its

maximum and the gain of the microphone amplifier is

at its minimum.

3. Idle mode.

The gain of the amplifiers is halfway between their

maximum and minimum value.

The difference between the maximum gain and minimum

gain is called the switching range.

HANDSFREE MICROPHONE CHANNEL: PINS TXIN, GATX,

TXOUT AND GNDTX (see Fig.7)

The TEA1099H has an asymmetrical handsfree

microphone input TXIN with an input resistance of 20 kΩ.

The DC biasing of the input is 0 V. The gain of the input

stage varies according to the mode of the TEA1099H.

In the transmit mode, the gain is at its maximum; in the

receive mode, it is at its minimum and in the Idle mode it is

halfway between maximum and minimum.

Switch-over from one mode to the other is smooth and

click-free. The output TXOUT is biased at two diodes

voltage and has a current capability equal to 20 µA (RMS).

In the transmit mode, the overall gain of the microphone

amplifier (from pins TXIN to TXOUT) can be adjusted from

0 up to 31 dB to suit specific application requirements.

The gain is proportional to the value of RGATX and equals

15.2 dB with RGATX = 30.1 kΩ. Without limitation from the

output, the microphone input stage can accommodate

signals up to 18 mV (RMS) at room temperature for 2% of

THD.

handbook, full pagewidth

telephone

line

sidetone

HYBRID

DUPLEX

CONTROL

Fig.6 Handsfree telephone set principles.

acoustic

coupling

MGM299

1999 Apr 08

12

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