AV Technology - March 2008 - (Page 38)

The SOUND of HD CONFERENCING Reverberation is the total soundfield in a room that remains after a sound source is silenced. Reverberation is affected by the physical characteristics of the room (reflective walls, etc.), room size, microphone type and pick-up pattern, and the orientation between the talker and the microphone. If the microphone is not pointed at the talker or is more distant, a greater proportion of the sound picked up by the microphone will be reverberation instead of direct speech, and the end result will be a decrease in intelligibility. 2 3 4 Amplitude refers to how loud the talker sounds to the listener. A quiet talker is more difficult to understand than a loud one, all things being equal. Interaction is the ability of two or more participants to interact naturally with each other in a telephone conference. It is essential that one talker be able to interrupt another without disturbing the flow of conversation, or the dialogue will feel stilted and unnatural. Interaction, or interactivity, is enabled by two parameters: low delay systems (algorithms, hardware, networks) and full duplex (simultaneous send and receive) capabilities. Noise refers to the proportion of ambient noise that is picked up along with speech. Room noise, such as air conditioning and projector fan noise, can be easily heard by microphones, and such noise can play a significant role in decreasing the intelligibility of speech. 5 STEREO AND SPATIAL AUDIO ENHANCEMENTS Recently, two audio enhancements have been introduced by videoconferencing vendors. One of these is stereo sound, a concept with which most people are very familiar. Since the early days of the industry, videoconferencing systems have mixed all microphone inputs (after echo canceling) with any line inputs (not echo canceled) into a single audio signal prior to processing. Hence, most echo cancellers have been designed with this single channel in mind, and vendors have placed their emphasis on trying to maintain full duplex operations Several new videoconferencing systems now include support for AAC compression (made extremely popular by the Apple iPod). Besides supporting wideband response (up to 22 kHz), AAC includes native support for stereo sound. Some of the video systems on the market today support stereo only on their line inputs (typically used for VCR or DVD sources), not on their microphone inputs. With stereo microphone support, the videoconferencing system can provide excellent separation when multiple people are speaking at the same time — voices are separated by left and right channel so they are clearly heard in one ear or the other and not combined in one monaural wave of noise. Related to stereo, but not exactly the same, is another audio enhancement dubbed “spatial audio.” With spatial audio (not yet standardized), multiple microphones are used with spatial processing software to capture position-relevant audio. This information is AUDIO CODEC SPECIFICATIONS More than 20 years ago, the ITU-T (the International Telecommunications Union’s standardization sector) created standards for codecs used in telecommunications. The ITU’s recommendations are important because the use of such standards is what enables interoperability different vendors in different countries. Interestingly, there is no standard referring to “HD Audio.” CODEC BIT RATE (KB/S) BANDWIDTH (FREQUENCY RESPONSE) SAMPLE RATE ALGORITHMIC DELAY (MS) 0.125 0.125 40 40 25 37.5 0.625 APPLICATIONS G.711 G.722 G.722.1 G.722.1Annex C G.722.2 G.723.1 G.728 64 48/56/64 24/32 24/32/48 6.6-23.85, typically 16 5.3/6.3 16 3 kHz 7 kHz 7 kHz 14 kHz 7 kHz 3.1 kHz 3.1 kHz 8 kHz 16 kHz 16 kHz 32 kHz 16 kHz 8 kHz 8 kHz base line telephony, videoconferencing videoconferencing videoconferencing videoconferencing mobile communications VOIP, videoconferencing videoconferencing, also frequently used in normal base line telephony (except last link to end-user) VOIP, videoconferencing MP3 music, MPEG video, also used between studios for broadcast, minimum bit rates are for mono only, AAC can support stereo sound, which requires 2x videoconferencing, also used between studios for broadcast; minimum bit rates are for mono only, stereo requires 2x VOIP, videoconferencing G.729 AAC 6.4/8/11.8, usually 8 typically ≥ 64 3.1 kHz 8 kHz 15 Typically 40 ms @ 48kHz sampling, 60 ms @ 32 kHz sampling 10 ms @ 48kHz sampling, 15 ms @ 32 kHz sampling 15 20 kHz @ 48/44.1 kHz sampling, 14 kHz @ 32 kHz sampling AAC-LD typically ≥ 24 20 kHz @ 48/44.1 kHz sampling, 14 kHz @ 32 kHz sampling G.729A 8 3.1 kHz 8 kHz 38 | AVTECHNOLOGY | march 2008 www.avtechnologyonline.com http://www.avtechnologyonline.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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