PSB4860 Просмотр технического описания (PDF) - Siemens AG
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PSB4860 Datasheet PDF : 250 Pages
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PSB 4860
Functional Description
comparator signals speech. Therefore the output of the background noise monitor is a
digital signal indicating speech (1) or noise (0).
A small fade constant (LP2N) enables fast settling of LP2 to the average noise level after
the end of speech recognition. However, a too small time constant for LP2N can cause
rapid charging to such a high level that after recognizing speech the danger of an
unwanted switching back to noise exists. It is recommended to choose a large rising
constant (LP2S) so that speech itself charges the LP2 very slowly. Generally, it is not
recommended to choose an infinite LP2S because then approaching the noise level is
disabled. During continuous speech or tones the LP2 will be charged until the limitation
LP2L is reached. Then the value of LP2 is frozen until a break discharges the LP2. This
limitation permits transmission of continuous tones and “music on hold”.
The offset stage represents the estimated difference between the speech signal and
averaged noise.
Signal Preprocessing
As described in the preceding chapter, the background noise monitor is able to
discriminate between speech and noise. In very short speech pauses e.g. between two
words, however, it changes immediately to non-speech, which is equal to noise.
Therefore a peak detection is required in front of the Noise Monitor.
The main task of the Peak Detector (PD) is to bridge the very short speech pauses during
a monolog so that this time constant has to be long. Furthermore, the speech bursts are
stored so that a sure speech detection is guaranteed. But if no speech is recognized the
noise low-pass LP2 must be charged faster to the average noise level. In addition, the
noise edges are to be smoothed. Therefore two time constants are necessary. As the
peak detector is very sensitive to spikes, the low-pass LP1 filters the incoming signal
containing noise in a way that main spikes are eliminated. Due to the programmable time
constant it is possible to refuse high-energy sibilants and noise edges.
To compress the speech signals in their amplitudes and to ease the detection of speech,
the signals have to be companded logarithmically. Hereby, the speech detector should
not be influenced by the system noise which is always present but should discriminate
between speech and background noise. The limitation of the logarithmic amplifier can be
programmed via the parameter LIM. LIM is related to the maximum PCM level. A signal
exceeding the limitation defined by LIM is getting amplified logarithmically, while very
smooth system noise below is neglected. It should be the level of the minimum system
noise which is always existing; in the transmit path the noise generated by the telephone
circuitry itself and in receive direction the level of the first bit which is stable without any
speech signal at the receive path. Table 6 shows the parameters for the speech detector.
Semiconductor Group
33
10.97
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