1997 Nov 25
9
Philips Semiconductors
Product specification
Voice switched speakerphone IC
TEA1095
The capacitors connected in series with the two resistors
block any DC component and form a first order high-pass
filter. In the basic application (see Fig.12), it is assumed
that V
TXIN
= 1 mV (RMS) and V
RXIN
= 100 mV (RMS)
nominal and both R
TSEN
and R
RSEN
have a value of 10 k
.
With the value of C
TSEN
and C
RSEN
at 100 nF, the cut-off
frequency is at 160 Hz.
The buffer amplifiers leading the compressed signals to
TENV and RENV have a maximum source current of
120
μ
A and a maximum sink current of 1
μ
A. Together with
the capacitors C
TENV
and C
RENV
, the timing of the signal
envelope monitors can be set. In the basic application, the
value of both capacitors is 470 nF. Because of the
logarithmic compression, each 6 dB signal increase
means 18 mV increase of the voltage on the envelopes
TENV or RENV at room temperature. Thus, timings can be
expressed in dB/ms. At room temperature, the 120
μ
A
sourced current corresponds to a maximum rise-slope of
the signal envelope of 85 dB/ms. This is enough to track
normal speech signals. The 1
μ
A current sunk by TENV or
RENV corresponds to a maximum fall-slope of 0.7 dB/ms.
This is enough for a smooth envelope and also eliminates
the effect of echoes on switching behaviour.
To determine the noise level, the signal on TENV and
RENV are buffered to TNOI and RNOI. These buffers have
a maximum source current of 1
μ
A and a maximum sink
current of 120
μ
A. Together with the capacitors C
TNOI
and
C
RNOI
, the timing can be set. In the basic application of
Fig.12, the value of both capacitors is 4.7
μ
F. At room
temperature, the 1
μ
A sourced current corresponds to a
maximum rise-slope of the noise envelope of
approximately 0.07 dB/ms. This is small enough to track
background noise and not to be influenced by speech
bursts. The 120
μ
A current that is sunk corresponds to a
maximum fall-slope of approximately 8.5 dB/ms. However,
during the decrease of the signal envelope, the noise
envelope tracks the signal envelope so it will never fall
faster than approximately 0.7 dB/ms. The behaviour of the
signal envelope and noise envelope monitors is illustrated
in Fig.7.
Fig.6 Signal and noise envelope detectors.
handbook, full pagewidth
MBG355
LOG
LOG
from
transmit
amplifier
from
receiver
amplifier
TSEN
RTSEN
CTSEN
CTENV
CTNOI
RRSEN
CRSEN
CRENV
CRNOI
TENV
TNOI
RSEN
RENV
RNOI
to logic
to logic