TEA1098 Philips Semiconductors, TEA1098 Datasheet - Page 15

• Manufacturer Part Number: TEA1098
• Description: Speech and handsfree IC
• Manufacturer: Philips Semiconductors

Philips Semiconductors
To determine the noise level, the signals between
pin TENV and pin TNOI, and between pin RENV and
pin RNOI are buffered. The buffers have a maximum
source current of 1 A and a maximum sink current of
120 A.
Capacitors C
envelope detectors. In the basic application, see Fig.19,
the value of both capacitors is 4.7 F.
At room temperature, the 1 A sourced current
corresponds to a maximum noise envelope rise-slope of
approximately 0.07 dB/ms which is small enough to track
background noise without being affected by speech
bursts.
Decision logic: pins IDT and SWT
The TEA1098 selects its mode of operation (Tx, Rx or Idle)
by comparing the signal and noise envelopes of both
channels. This is executed by the decision logic.
The resulting voltage on pin SWT is the input to 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 of
1999 Oct 14
handbook, full pagewidth
Speech and handsfree IC
SIGNAL ENVELOPE
NOISE ENVELOPE
INPUT SIGNAL
TNOI
A: 85 dB/ms
B: 0.7 dB/ms
B: 0.7 dB/ms
C: 0.07 dB/ms
and C
RNOI
set the timing of both noise
C
A
Fig.11 Signal and noise envelope waveforms.
4 mV (RMS)
36 mV
36 mV
15
The 120 A sink current corresponds to a maximum
fall-slope of approximately 8.5 dB/ms. However, because
the noise envelope tracks the fall of the signal envelope, it
will never fall faster than approximately 0.7 dB/ms.
The behaviour of the signal envelope and noise envelope
detectors is illustrated in Fig.11.
V
threshold is implemented in both channels.
The signal on pin TXIN contains both speech and the
acoustically coupled signal from the loudspeaker.
In Rx mode, the loudspeaker signal overrides the speech.
Therefore, the signal envelope on pin TENV consists
mainly of the loudspeaker signal. To correct this, an
attenuator is connected between pin TENV and the
TENV/RENV comparator. Its attenuation is equal to that
applied to the microphone amplifier.
B
B
ENV
1 mV (RMS)
V
NOI
= 13 mV. This so called speech/noise
C
A
MBG354
time
B
B
Product specification
TEA1098