Tone Detector; Noise Suppressor; Echo Canceller (Echo); 2-Wire Wait On Ring - Digital Voice Systems Net-2000 User Manual

Net-2000™ Voice Codec Unit
User's Manual
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".
Enabling VAD also enables Discontinuous Transmission (DTX). DTX packets are simulated in the
Net-2000™ VCU protocol by setting all the channel data bytes in the channel packet to 1. This
represent packets that would not need to be transmitted to the decoder in a real system.
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.

Tone Detector

The Tone detector is always enabled when AMBE/AMBE+ is selected as the vocoder type. It can be
turned on or off when AMBE+2 is selected as the vocoder type. When either a-Law or U-law is
selected the tone detector is inactive. When the Tone detector in enabled the Net-2000™ VCU is
able to send and receive North American DTMF tones.

Noise Suppressor

The Noise suppressor feature of the Net-2000™ VCU provides clear voice in environments with
background noise. This feature work only when AMBE+2 is selected as the vocoder type. To enable
the feature select the check box in the Configuration table on the VOIP page. The noise suppressor
can be switched off by unchecking the check-box. When the noise suppressor is started it may take
up to a few seconds to converge allowing for it do begin fully working.

Echo Canceller (Echo)

The AMBE™ software on the NET-2000™ VCU employs an adaptive echo cancellation algorithm to
cancel echoes of the decoder output present at the encoder input. This provides a 16-millisecond
echo canceller that is suitable for canceling the linear portion of the local echo caused by a 2-to-4
wire hybrid and can achieve echo cancellation of approximately 30dB or more. The Echo canceller is
only really useful when connected via the 2-wire interface.

2-Wire Wait on Ring

The 2-Wire wait on ring feature allows the Net-2000™ VCU to be ready to answer an incoming call
from a standard POTS line. When selected the Net-2000™ VCU will not automatically open the
POTS line when the 2-Wire voice source is selected. This provides the benefit of freeing up the
phone line when the Net-2000™ VCU is not active.

DTMF Selection Page

The NET-2000™ VCU is capable of detecting, transmitting, and synthesizing DTMF tones. DTMF
features are only enabled in VOIP Mode with vocoders that support DTMF Tones. Selecting the
DTMF from the main navigation menu brings the operator to the Status page and opens the DTMF
pop-up window (see Figure 14 DTMF Key Pad).
The DTMF pop-up window allows the operator to send DTMF tones across the channel. The
operator can enter the tones by selecting the digits in the pop-up window or by selecting the DTMF
digit window and entering the numbers using the keyboard. The operator can select how the tone
should be generated and then can select send to send the information the Net-2000™ VCU.
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Section 3 – Net-2000™ VCU Control
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