U4091BM-N Datasheet, PDF(18/38 Page) ATMEL Corporation – PROGRAMMABLE TELEPHONE AUDIO PROCESSOR

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U4091BM-N Datasheet, PDF (18/38 Pages) ATMEL Corporation – PROGRAMMABLE TELEPHONE AUDIO PROCESSOR

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Figure 10. Power-on Reset (Ringing)

VRING

IVDD

VMP

OSCOUT

Reset

ton

trt

Figure 11. Power-on Reset (Low Voltage Reset)

Line

LID

VMP

LVI

LVR

LVI

Reset

OSCOUT

Dial-Tone Detector

Background Noise

Monitors

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 on page 21). If the circuit is

in idle mode, and the incoming signal is greater than 35 mV (25 mVrms), the compara-

tor's output will 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 cir-

cuit 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.

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 moni-

tor is operated on by the transmit level detector (see Figure 13 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 C), 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 out-

put 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.

18 U4091BM-N

4666BâCORDâ08/04

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