psb21391 Infineon Technologies Corporation, psb21391 Datasheet - Page 135

• Manufacturer Part Number: psb21391
• Description: Siemens Codec With Upn Transceiver
• Manufacturer: Infineon Technologies Corporation

Figure 68
Speech Detector Signal Flow Graph
4.4.3.1
The tasks of the noise monitor are to differentiate voice signals from background noise,
even if it exceeds the voice level, and to recognize voice signals without any delay.
Therefore the background noise monitor consists of the low-pass filter 2 (LP2) and the
offset in two separate branches. Basically it works on the burst-characteristic of the
speech: voice signals consist of short peaks with high power (bursts). In contrast,
background noise can be regarded approximately stationary from its average power.
Low-pass filter 2 provides different time constants for noise (non-detected speech) and
speech. It determines the average of the noise reference level. In case of background
noise the level at the output of LP2 is approximately the level of the input. Due to the
offset OFF the comparator remains in the initial state. In case of speech at the
comparator input the difference between the signal levels of the offset branch and of the
LP2-branch increases and the comparator changes state. At speech bursts the digital
signals arriving at the comparator via the offset branch change faster than those via the
LP2-branch so that the comparator changes its polarity. Hence two logical levels are
generated: one for speech and one for noise.
Data Sheet
Background Noise Monitor
125
PSB 21391
PSB 21393
2001-03-07
Codec