18

4872A–CORD–08/05

U4091BM-R

12.1

Dial-tone Detector

The dial-tone detector is a comparator with one side connected to the speaker amplifier input

and the other to VM with a 35-mV offset (see Figure 12-5 on page 21). If the circuit is in idle

mode, and the incoming signal is greater than 35 mV (25 mV

change thus disabling the receive idle mode. This circuit prevents the dial tone (which would be

considered as continuous noise) from fading away as the circuit would have the tendency to

switch to idle mode. By disabling the receive idle mode, the dial tone remains at the normally

expected full level.

12.2

Background Noise Monitors

This circuit distinguishes 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

the other for the transmit path. The receive background-noise monitor is operated on by the

receive level detector, while the transmit background noise monitor is operated on by the trans-

mit level detector (see Figure 12-6 on page 21). They monitor the background noise by storing a

DC voltage representative of the respective noise levels in capacitors at CBNMR and CBNMT.

The voltages at these pins have slow rise times (determined by the internal current source and

an external capacitor), but fast decay times. If the signal at TLDR (or TLDT) changes slowly, the

voltage at BNMR (or BNMT) will remain more positive than the voltage at the non-inverting input

of the monitor's output comparator. When speech is present, the voltage at the non-inverting

input of the comparator will rise more quickly 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 mode-

control block.

12.3

4-point Sensing

In 4-point-sensing mode, the receive- and the transmit-sensing paths include additional CLOGs

(calculated logarithmic amplifiers). The block MODECON compares the detector output signals

and decides whether receive, transmit or idle mode has to be activated. Depending on the mode

decision, MODECON generates a differential voltage to control AFSCON.

The MODECON block has seven inputs:

• The output of the transmit log (LOGT) – the comparison of LOGT, CLOGR

• The output of the receive clog (CLOGR) – designated I1

• The output of the transmit clog (CLOGT) – the comparison of CLOGT, LOGR

• The output of the receive log (LOGR) – designated I2

• The output of the transmit background-noise monitor (BNMT) – designated I3

• The output of the receive background-noise monitor (BNMR) – designated I4

• The output of the dial-tone detector

The differential output (AFST, AFSR) of the block MODECON controls AFSCON. The effect of

I1-I4 in Table 12-1 on page 19.