TS34118

8-20

2004/09 rev. B

Attenuators

the most direct method of monitoring the circuit’s mode.

The inputs to the Control Block are seven, 2 from the comparators operated by the level detectors, 2 from the

background noise monitors, the volume control, the dial-tone detector, and the AGC circuit. These seven inputs are

described below.

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 Detectors