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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

The TEA1099H 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 TXIN contains both the speech and the

input signal from the loudspeaker (acoustic coupling).

When receiving, the contribution from the loudspeaker

overrules the speech. 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 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

TEA1099H 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.

In the same way, a transmit detector is integrated which, in

standard applications, does not consider input signals

between TXIN and GNDTX as noise when they have a

level greater than 0.75 mV (RMS). This level is

proportional to RTSEN.

The output of the decision logic is a current source

(see Fig.11). The logic table gives the relationship

between the inputs and the value of the current source.

It can charge or discharge the capacitor CSWT with a

current of 10 µA (switch-over). If the current is zero, the

voltage on SWT becomes equal to the voltage on IDT via

the high-ohmic resistor RIDT (idling). The resulting voltage

difference between SWT and IDT determines the mode of

the TEA1099H and can vary between −400 and +400 mV

(see Table 1).

The switch-over timing can be set with CSWT, the Idle

mode timing with CSWT and RIDT. In the basic application

given in Fig.16, CSWT is 220 nF and RIDT is 2.2 MΩ. This

enables a switch-over time from transmit to receive mode

or vice-versa of approximately 13 ms (580 mV swing on

SWT).

The switch-over time from Idle mode to transmit mode or

receive mode is approximately 4 ms (180 mV swing on

SWT).

The switch-over time, from receive mode or transmit mode

to Idle mode is equal to 4 × RIDTCSWT and is approximately

2 seconds (Idle mode time).

The input DLC overrules the decision logic. When the

voltage on pin DLC goes lower than 0.2 V, the capacitor

CSWT is discharged with 10 µA thus resulting in the

transmit mode.

Table 1 Modes of TEA1099H

VSWT − VIDT (mV)

<−180

0

>180

MODE

transmit mode

Idle mode

receive mode

Voice-switch: pins STAB and SWR

A diagram of the voice-switch is illustrated in Fig.12. With

the voltage on SWT, the TEA1099H voice-switch

regulates the gains of the transmit and the receive

channels so that the sum of both is kept constant.

In the transmit mode, the gain of the microphone amplifier

is at its maximum and the gain of the loudspeaker amplifier

is at its minimum. In the receive mode, the opposite

applies. In the Idle mode, both microphone and

loudspeaker amplifier gains are halfway. The difference

between maximum and minimum is the so called switching

range. This range is determined by the ratio of

RSWR and RSTAB and is adjustable between 0 and 52 dB.

RSTAB should be 3.65 kΩ and sets an internally used

reference current. In the basic application diagram given in

Fig.16, RSWR is 365 kΩ which results in a switching range

of 40 dB. The switch-over behaviour is illustrated in Fig.13.

In the receive mode, the gain of the loudspeaker amplifier

can be reduced using the volume control. Since the voice

switch keeps the sum of the gains constant, the gain of the

microphone amplifier is increased at the same time (see

dashed curves in Fig.13). In the transmit mode, however,

the volume control has no influence on the gain of the

microphone amplifier or the gain of the loudspeaker

amplifier. Consequently, the switching range is reduced

when the volume is reduced. At maximum reduction of

volume, the switching range becomes 0 dB.

1999 Apr 08

17

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