DITECH Packet Voice Processor, Руководство по управлению

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16 250-0800-06, Version A
D I T E C H N E T W O R K S
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Introduction
Packet Voice Processor Commands Manual
1.4.2 VQA Technology
The Packet Voice Processor supports a variety of voice quality enhancements,
including the following:
Hybrid Echo Cancellation - Section 1.4.2.1 on page 16
Acoustic Echo Control - Section 1.4.2.2 on page 17
Adaptive Noise Cancellation - Section 1.4.2.3 on page 17
Automatic Level Control - Section 1.4.2.4 on page 18
Enhanced Voice Intelligibility - Section 1.4.2.5 on page 18
Internally to the Packet Voice Processor, voice quality enhancements are applied to
streams encoded in G.711 format. The voice assurance capabilities take advantage
of the encoding and decoding facilities of the Packet Voice Processor.
1.4.2.1 Hybrid Echo Cancellation
Traditional echo in a telecommunication circuit is caused by the conversion point,
also known as the hybrid, between the 4-wire and 2-wire transmission facilities. The
hybrid connects the 4-wire carrier facility to the 2-wire access line within a switched
network. The energy transfer across the 4-wire-to-2-wire connection is not perfectly
transparent and causes a reflection of energy, referred to as echo.
Signal amplitude and transmission path characteristics affect perception of the echo.
Hybrid echo cancellation detects and compensates for signal amplitude and
transmission path characteristics. The set of transmission path characteristics is
known as round-trip delay.
1.4.2.1.1 Round-Trip Delay
As the round-trip delay increases, the echo arrives later at the originating (near) end,
making it more intolerable and increasing the need for hybrid echo cancellation. The
Packet Voice Processor eliminates the echo.
Round-trip delay is increased by characteristics that differ from one facility to another.
Table 1-4 on page 17 describes the three delay characteristics (types) that impact
echo generation.