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Wideband audio boosts VoIP quality

By Michael Ward, special to Network World
October 06, 2006 12:04 AM ET
This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

Network World - Companies have rapidly deployed VoIP technology to reduce operational costs and increase productivity by integrating telecom services with other enterprise workflows. However, the codecs most widely used today that let speech be transmitted over IP data networks do not reproduce speech faithfully for a variety of dialects.

Typical human speech, while consisting of a wide range of frequencies, is very well reproduced by sampling frequencies up to 10KHz. Unfortunately, most VoIP codecs capture well under a half of this frequency spectrum. The G.729 voice codec, one of the most common codecs used in VoIP systems, is a narrowband codec that samples across a frequency range from about 200Hz to just under 4KHz. As a result much of the frequency spectrum of typical human voice is not captured.

To minimize the amount of data required to encode the sampled speech to limit the requirements of network bandwidth as much as possible, algorithms such as G.729 use a variety of encoding techniques that make certain assumptions about speech.

In many cases these assumptions have been made based on a hypothetical Western male voice. These predictive encoding techniques coupled with the limited frequency sampling result in a voice codec that poorly recreates Asian speakers because of the higher frequency content of many Asian dialects. These narrowband codecs also do a poor job in the transmission and encoding of music.

In an attempt to solve these problems, other codecs were defined that double the audio-frequency spectrum encoded. Wideband codecs sample frequencies up to 8KHz. By doubling the frequencies that are encoded, a truer representation of the speed or audio can be encoded and recreated on the remote end. As most speech can be represented with only 10KHz of frequency, these wideband codecs are able to more faithfully reproduce the original speech than narrowband codecs. Music transmission also becomes possible using wideband codecs.

The G722.2, or Wideband Audio Modem Riser, wideband codec was defined initially by the European Telecommunications Standards Institute/Third Generation Partnership Project as Wideband AMR for use in cellular and mobile applications and then ratified by the International Telecommunication Standardization Sector as G.722.2 for use in VoIP and other applications. G.722.2 supports nine bit rates from 6.6K to 23.85Kbps. Bit rates as low as 12.65Kbps still can deliver acceptable voice quality.

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