TS34118

Voice Switched Speakerphone Circuit

9/23

Version: A08

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