PSB21384HV1.3 Infineon Technologies, PSB21384HV1.3 Datasheet - Page 148

• Manufacturer Part Number: PSB21384HV1.3
• Description: Telecom ICs AMuLaw/Speech CODEC w/ ST Transceiver
• Manufacturer: Infineon Technologies

Specifications of PSB21384HV1.3

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Data Sheet
A small fade constant (LP2N) enables fast settling down the 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 LP2L of this charging especially on the RX-path permits transmission of
continuous tones and “music on hold”.
The offset stage represents the exact level threshold in [dB] between the speech signal
and averaged noise.
4.4.3.2
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 is to bridge the very short speech pauses during a
monologue 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 rapidly to the average noise level.
Additionally the noise edges are to be smoothed. Therefore two time constants are
necessary and are separately programmable: PDS for speech and PDN for space
(background noise) signals.
The Peak Detector is very sensitive to spikes. The 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, where the upper half-byte features LIMX and the
lower half-byte LIMR. LIM is related to the maximum PCM level (+3.14 dBm0). 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.
Signal Processing
138
PSB 21381/2
PSB 21383/4
2001-03-12
Codec