TEA1098ATV PHILIPS [NXP Semiconductors], TEA1098ATV Datasheet - Page 12

• Manufacturer Part Number: TEA1098ATV
• Description: Speech and handsfree IC
• Manufacturer: PHILIPS [NXP Semiconductors]

Philips Semiconductors
AGC (pin AGC)
The TEA1098A performs automatic line loss
compensation, which fits well with the true line attenuation.
The automatic gain control varies the gain of some
transmit and receive amplifiers in accordance with the DC
line current. The control range is 6.45 dB for G
G
approximately to a line length of 5.5 km for a 0.5 mm
twisted-pair copper cable.
To enable this gain control, pin AGC must be shorted to
pin LN. The start current for compensation corresponds to
a line current of typically 23 mA and the stop current to
57 mA. The start current can be increased by connecting
an external resistor between pins AGC and LN. It can be
increased up to 40 mA (using a resistor typically 80 k ).
The start and stop current will be maintained in a ratio
equal to 2.5. By leaving the AGC pin open-circuit, the gain
control is disabled and no line loss compensation is
performed.
Handsfree application
As can be seen from Fig.4, 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.
2000 Mar 21
handbook, full pagewidth
v(IR-RECO)
Speech and handsfree IC
and 6.8 dB for G
telephone
line
v(HFTX-LN)
sidetone
, which corresponds
Fig.7 Handsfree telephone set principles.
HYBRID
v(MIC-LN)
and
12
This is achieved by the duplex controller. The duplex
controller of the TEA1098A detects which channel has the
‘largest’ signal and then controls the gains of the
microphone and loudspeaker amplifiers so that the sum of
the gains remains constant.
As a result, in handsfree application, the circuit can be in
three stable modes:
1. Transmit mode (TX mode).
2. Receive mode (RX mode).
3. Idle mode.
The difference between the maximum gain and minimum
gain is called the switching range.
CONTROL
DUPLEX
The gain of the microphone amplifier is at its maximum
and the gain of the loudspeaker amplifier is at its
minimum.
The gain of the loudspeaker amplifier is at its
maximum and the gain of the microphone amplifier is
at its minimum.
The gain of the amplifiers is halfway between their
maximum and minimum value.
MGM299
acoustic
coupling
Preliminary specification
TEA1098A