1997 Nov 25
8
Philips Semiconductors
Product specification
Voice switched speakerphone IC
TEA1095
Receive channel
Fig.5 Receive channel.
handbook, full pagewidth
MBG356
V
I
I V
RGARX
CGARX
GARX
RXOUT
MUTERX
to loudspeaker
amplifier
to/from
voice switch
to
envelope
detector
RXIN
from transmission
circuit
VOL
VOLUME
CONTROL
RVOL
R
ECEIVER AMPLIFIER
:
PINS
RXIN, GARX, RXOUT
AND
MUTERX
The TEA1095 has an asymmetrical input (RXIN) for the
receiver amplifier with an input resistance of 20 k
. The
gain of the input stage varies according to the mode of the
TEA1095. In the receive mode, the gain is at its maximum;
in the transmit 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.
In the receive mode, the overall gain of the receive
amplifier can be adjusted from
14 dB to +26 dB to suit
application specific requirements. The gain from RXIN to
RXOUT is proportional to the value of R
GARX
and equals
6.5 dB with R
GARX
= 16.5 k
. A capacitor connected in
parallel with R
GARX
can be used to provide a first-order
low-pass filter.
By applying a HIGH level on pin MUTERX, the receiver
amplifier is muted and the TEA1095 is automatically
forced into the transmit mode.
V
OLUME CONTROL
:
PIN
VOL
The receiver amplifier gain can be adjusted with the
potentiometer R
VOL
. A linear potentiometer can be used to
obtain logarithmic control of the gain of the receiver
amplifier. Each 950
increase of R
VOL
results in a gain
loss of 3 dB. The maximum gain reduction with the volume
control is internally limited to the switching range.
Duplex controller
S
IGNAL AND NOISE ENVELOPE DETECTORS
:
PINS
TSEN,
TENV, TNOI, RSEN, RENV
AND
RNOI
The signal envelopes are used to monitor the signal level
strength in both channels. The noise envelopes are used
to monitor background noise in both channels. The signal
and noise envelopes provide inputs for the decision logic.
The signal and noise envelopes detectors are shown in
Fig.6.
For the transmit channel, the input signal at TXIN is 40 dB
amplified to TSEN. For the receive channel, the input
signal at RXIN is 0 dB amplified to RSEN. The signals from
TSEN and RSEN are logarithmically compressed and
buffered to TENV and RENV respectively. The sensitivity
of the envelope detectors is set with R
TSEN
and R
RSEN
.