AV Technology - March 2008 - (Page 37)

by the codec hardware and software architecture. High-latency codecs can produce excellent quality sound with high compression rates, but are more suitable for archiving and retrieval applications where the delay is not noticeable. Some codecs such as the AAC algorithm are available in LD or “low delay” variations. ELEMENTS OF SPEECH INTELLIGIBILITY Speech intelligibility (how easily speech is correctly understood), or rather the lack of it, is a primary contributor to “meeting fatigue.” If, during a conference call, you have to work hard to understand the remote participants, have trouble determining who is talking at any given time, or have to speak in an unnatural cadence to accommodate system delays, your brain will tire quickly. A 2003 white paper published by Polycom’s Jeff Rodman describes the five key elements that lead to a user’s perception of speech quality: Bandwidth is the frequency range of audio signals that is carried to the listener. Telephones, which are limited (by filters installed on the network) to a frequency range between 300 Hz to 3.3 kHz, carry only 20 percent of the frequencies present in typical human speech. By comparison, AM radio extends to about 5 kHz, FM radio spans 30 Hz to 15 kHz, and common CD audio encodes signals from 20 Hz to 20 kHz. Modern audio systems designed with digital communications in mind, including IP telephony, speakerphones, and videoconferencing, now support up to 22 kHz frequency response. While the human voice has most of its power below 7 or 8 kHz, ENHANCED AUDIO SUPPORT SUMMARY Different equipment manufacturers support different audio standards. VENDOR G.722.1 ANNEX C? Yes Yes Yes No No AAC SUPPORT? STEREO/ SPATIAL EFFECTS? No Yes Yes Yes VCR/ DVD/ PC inputs Aethra LifeSize Polycom Sony Tandberg AAC-LD, 14 kHz, mono MPEG-4 AAC-LC, 16 kHz No MPEG-4 AAC, 22 kHz AAC-LD, 20 kHz 1 the human ear can hear all the way up to 15-20 kHz, varying from person to person, by age and other factors. Consonants in speech are a key factor in speech articulation and recognition as they separate words like “mold” from “bold” or “sailing” from “failing.” While the most of the energy in English speech in vowels lies below 3 kHz, the sound energy in consonant sounds is predominantly in frequencies above 3.3 kHz. For example, the sound that distinguishes the “s” in “sailing” from the “f” in “failing” occurs between 4 kHz and 14 kHz (depending upon the person speaking those words). When these frequencies are removed, whether by loudspeakers, microphones, or the codec engine, no cue remains (other than context and background knowledge) as to which word has been said. http://www.crwww.com http://www.crwww.com

Table of Contents Feed for the Digital Edition of AV Technology - March 2008

AV Technology - March 2008 Contents Precedent Corporate: Whose Job Is It Anyway? Education: There’s No Business Like Show Business Government: Technology and the New World Order Buying a Videoconference System They Will Actually Use AV After Hours The Sound of HD Conferencing Assembling the Design Team Concert Hall Acoustics on a High School Budget Audio Architecture Keeping a Watchful Eye Product Spotlight Tech Horizons Product Review New Products AV MO

AV Technology - March 2008

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