TS34118CS28

Taiwan Semiconductor

8

Version: E1608

APPLICATION INFORMATION (CONTINUE)

Level Detectors

There are four level detectors-two on the receive side and two on the transmit side. Refer to Figure 4. The terms in

parentheses from one system, and the other terms from one system. Each level detector is a high gain amplifier

with back-to-bank diodes in the feedback path, resulting in non-linear gain, which permits operation over a wide

dynamic range of speech levels. The sensitivity of each level detector is determined by the external resistor and

capacitor at each input (TLI1, TLI2, RLI1 and RLI2). Each output charge an external capacitor through a diode and

limiting resistor, thus providing a do representation of the input as signal level. The outputs have a quick rise time

(determined by the capacitor and an internal 350Ω resistor), and a slow decay time set by an internal current

source and the capacitor. The capacitors on the four outputs should have the same value (±10%) to prevent timing

problems. Referring to Figure 2, on the receives side, one level detector (RLI1) is at the receive input receiving thus

same signal as at Tip and Ring, and the other (PLI2) is at the output of the speaker amplifier. On the transmit side,

one level detector (TLI2) is at the output of the microphone amplifier, while the other (TLI1) is at the hybrid output.

Outputs RLO1 and TLO1 feed a comparator, the output of which goes to the Attenuator Control Block. Likewise,

outputs RLO2 and TLO2 feed a second comparator which also goes to the Attenuator Control Block. The touch

table for the effects of the level detectors on the Control Block is given in the section describing the Control Block.

Figure 4. Level Detector

Volume Control

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

reduced (see Figure 14), and the gain of the transmit attenuator is increased such that their sum remains constant.

Changing the voltage at VCI 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 VCI is typically 60nA out of the pin, and does not vary significantly with the VCI voltage