Specifications

Net-2000™ VCU Users Manual
Version 4.0, December, 2013
VO
IP Mode
4.8.1 Rate “Bit Steal 1 Bit”
The two “Bit Steal 1 Bit” rates allow for customized data signaling. When running in one of these rates the value of
the least significant bit of the data bits has no effect on the vocoder. The “Bit Steal one bit - 2450 bps speech source
with 3550 bps FEC” rate has 72 bits of data, so the value of the LSB (bit 71 of the channel data bits) does not
influence the vocoder. The “Bit Steal one bit - 2450 bps speech with 0 bps FEC” rate has 49 bits of data, so the
value of LSB (bit 48 of the channel data bits) has no effect on the vocoder. Since the vocoder does not care about
the value of this data bit, users can steal this bit for customized use.
4.9 Vocoder Options Configuration
To fine tune the capability of either vocoder mode the NET-2000™-VCU provides VAD, Echo cancellation Tone
Detector and Automatic Gain Control options. The availability of these extended features depends on the vocoder
selection. To turn ON an option use the home page navigation menu to get to the VOIP Page and click on the check
box for the desired vocoder option. Then click on the Start button to enable the new setting selection.
Figure 21 Vocoder Options
4.9.1 Voice Activation Detection (VAD)
The Voice Activation Detection (VAD) algorithm along with the Comfort Noise Insertion (CNI) feature of the NET-
2000™-VCU is a useful function for systems trying to save in system bandwidth or to reduce transmit power during
times of conversational silence.
With the VAD functions enabled, periods of speech silence will be recognized by the encoder. Instead of
compressing the silence as speech the encoder outputs a silence frame (in-band). This silence frame contains
information regarding the level of background noise, which allows the corresponding decoder to synthesize a
“Comfort Noise” signal at the other end. The comfort noise is intended to give the listener the feeling that the call is
still connected, as opposed to producing absolute silence, which can give the impression that, the call has been
“dropped”.
The silence threshold value is -25 dBm0 in the VAD algorithm. Each frame that exceeds this level will be classified
as voice. If the frame level is less than -25 dBm0 the voice/silence decision will be determined based upon various
adaptive thresholds.
(Subject to Change) Page 22
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