TS34118CS28C8 TSC [Taiwan Semiconductor Company, Ltd], TS34118CS28C8 Datasheet - Page 9

• Manufacturer Part Number: TS34118CS28C8
• Description: Voice Switched Speakerphone Circuit
• Manufacturer: TSC [Taiwan Semiconductor Company, Ltd]

TS34118
Voice Switched Speakerphone Circuit
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 VB. The volume control affects the
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 VB. As VLC is reduced below VB, the gain of the receive attenuator is reduced
(see 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 CT (see the Attenuator Control Block section), which in turn controls the
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 VB with a 15mV offset (see Figure 5). If the circuit is in the receive mode, and the incoming signal is
greater than 15mV (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 Detector
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Version: A08