ts34118 Taiwan Semiconductor Company, Ltd. (TSC), ts34118 Datasheet - Page 9

• Manufacturer Part Number: ts34118
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: Taiwan Semiconductor Company, Ltd. (TSC)

Background Noise Monitors
The purpose of the background noise monitors is to distinguish speech (which consists of bursts) from background noise
(a relatively constant signal level). There are two background noise monitors-one for the receive path and one for the
transmit path. Referring to Figure 4, the receive background noise monitor is operated on by the RLI1-PLO1 level
detector, while the transmit background noise monitor is operated on by the TLI2-TLO2 level detector. They monitor the
background noise by storing a dc voltage representative of the respective noise levels in capacitors at CPR and CPT.
The voltages at these pins have slow rise times (determined by the external RC), but fast decay times. If the signal at
RLI1 (or TLI2) changes slowly, the voltage at CPR (or CPT) will remain more positive than the voltage at the non-inverting
input of the monitor’s output comparator. When speech is present, the voltage on the non-inverting input of the
comparator will rise quicker than the voltage at the inverting input (due to the burst characteristic of speech), causing its
output to change. This output is sensed by the Attenuator Control Block.
The 36mV offset at the comparator’s input keeps the comparator from changing state unless the speech level exceeds
the background noise by§4.0dB. The time constant of the external RC (§4.7 seconds) determines the response time to
background noise variations.
Volume Control
The volume control input at VLC (pin 13) is sensed as a voltage with respect to V
The volume control affects the
B.
attenuators only in the receive mode. It has no effect in the idle or transmit modes.
When in the receive mode, the gain of the receive attenuator will be +6.0dB, and the gain of the transmit attenuator will
be –46dB only when VLC is equal to V
. As VLC is reduced below V
, the gain of the receive attenuator is reduced (see
B
B
Figure 14), and the gain of the transmit attenuator is increased such that their sum remains constant. Changing the
voltage at VLC changes the voltage at C
(see the Attenuator Control Block section), which in turn controls the
T
attenuators.
The volume control setting does not affect the maximum attenuator input signal at which noticeable distortion occurs. The
bias current at VLC is typically 60nA out of the pin, and does not vary significantly with the VLC voltage or with Vcc.
Dial Tone Detectors
The dial tone detector is a comparator with one side connected to the receive input (RXI) and the other input connected
to V
with a 15mV offset (see Figure 5). If the circuit is in the receive mode, and the incoming signal is greater than 15mV
B
(10mVrms), the comparator’s output will change, disabling the receive idle mode. The receive attenuator will then be at a
setting determined solely by the volume control.
The purpose of this circuit is to prevent the dial tone (which would be considered as continuous noise) from fading away
as the circuit would have the tendency to switch to the idle mode. By disabling the receive idle mode, the dial tone
remains at the normally expected full level.
Figure 5. Dial Tone Detectors
TS34118
9-20
2004/09 rev. B