AV Technology - March 2008 - (Page 36)

The SOUND of HD CONFERENCING PHOTO COURTESY LIFESIZE COMMUNICATIONS Videoconferencing systems once supported only narrowband 3 kHz audio, which provided a hollow, almost “tinny” sound. Newer systems now support 7 kHz, 14 kHz, and even 22 kHz wideband audio. These expanded frequency ranges provide a richer sound (similar to how FM radio provides a fuller sound compared to AM radio) and a better meeting experience. More importantly, however, the new systems deliver important additional audio information that makes speech more intelligible, thereby increasing meeting effectiveness and reducing the “meeting fatigue” that occurs from participants having to strain to understand the talker. Most videoconferencing events involve people speaking into one or perhaps two microphones, so there is no real need for 5-channel surround sound, and there are unlikely to be extremely low bass tones or extremely high frequencies, as would be the case with music. In fact, the concept of HD videoconferencing has focused on supporting HD video resolution, with most vendors selecting some wideband audio algorithm (often proprietary) because HD users expect an improved audio experience. However, when HD systems from different vendors try to connect, the audio codec negotiated is typically ITU G.722 (7 kHz bandwidth) since this is a common standard. CODECS MAKE THE DIFFERENCE Understanding the challenges of sound quality in a videoconference start with understanding audio compression. Capturing voice or other audio signals for transmission in a digital format, as is the case for a videoconference, requires filtering, processing, and digitizing the signals, and then compression of the results. There are many ways to process audio signals, each representing a tradeoff between multiple performance and efficiency parameters. The compression-decompression algorithms used for videoconferencing and voice over IP networks have been designed to optimize around three basic parameters: • Frequency Response (or Input Bandwidth): High bandwidth systems reproduce more of the fundamental frequencies and overtones of the original signal, and also require input and output devices (microphones and loudspeakers) capable of handling these signals. As an example, the common PSTN telephone network, with its roots in analog transmission, is designed to handle input signals up to just over 3 kHz. This limited frequency range is marginally acceptable for voice, but not for music, where many critical frequencies above 3 kHz are present. 30 DOES VIDEO LOOK BETTER WITH SOUND? A research project, conducted in 1999 by Michael R. Frater, John F. Arnold, and Abedin Vahedian from the School of Electrical Engineering at the University of New South Wales (Australia), investigated whether the mere presence of audio had any effect on the perceived quality of a video image. In their study, viewers were asked to rate the quality of two video sequences known as “A” and “B” on a continuous scale ranging between “Excellent” and “Bad.” Either A or B (chosen at random) was an original sequence with no degradation, but the other sequence was visually degraded by the addition of white Gaussian noise to the picture. In addition, they were able to selectively degrade the video image of the “foreground” (for example, a person talking) separately from the “background” (for example, the wall behind the person talking). They found that the difference between sensitivity to foreground and background degradation is increased by the presence of audio corresponding to speech of the foreground person. Conclusion: the presence of high-quality audio has a significant impact on the subjective quality of the video. In other words, video looks better when there is sound with it. 36 dB signal-noise 32 dB signal-noise 28 dB signal-noise Degradation (subjective rating) • Compression Rate: Some audio codecs can compress an input signal by a factor of ten or more. Obviously, the higher the compression rate, the smaller the output bit stream, leaving more network bandwidth available for video (or other data). Generally, the higher the compression rate, the lower the quality of the resultant audio signal. • De lay: Delay (or latency) is a crucial determinant of videoconferencing quality and of user satisfaction. With a highlatency communications environment, such as experienced with a satellite-based phone or video call where the delay is caused by the network, two-way conversation becomes very awkward and unnatural. The latency causes people to trip over each other in conversation or to pause frequently to see if the other side is speaking. Latency can also be introduced 36 | AVTECHNOLOGY | march 2008 25 20 15 10 5 0 m Co bin ed B k ac gro un d F g ore rou nd m Co bin ed B k ac gro un d F g ore rou nd m Co bin ed B k ac gro un d F g ore rou nd With Audio Without Audio 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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