AV Technology - February/March 2009 - (Page 50) MAKE ROOM FOR VIDEO Touch comes to Listen • Ideal for boardrooms, universities, courtrooms, council meetings, and government conferences • Built-in high resolution configurable touchscreen • Software for control, voting, polling, video display, and web streaming • Stand-alone “plug & play” • Uses Cat-5e cabling • Intuitive and easy to use • Makes meetings efficient and productive Don’t miss a single sound. ming as delivered by providers such as Comcast, Verizon FIOS, AT&T U-verse, or Time Warner. Type 2: Videoconferencing (VC) is usually two-way point to point, although it is occasionally a multipoint mesh network. All parties can see each other. Historically, it has used telco circuits such as T-1 or T-3 circuits. More recently, VC is embracing IP and MPEG compression using the H.264 standard. Videoconferencing has traditionally used large specialized endpoints. More recently, VC can be accomplished from the desktop using a conventional PC. Type 3: Internet streamed video is usually sent from a conventional server and played by a player, often from within a browser. It is unique because it is most often transported using TCP rather than UDP. Examples of internet-streamed video are YouTube, forecast video downloads from the Weather Channel, and Internet TV. Type 1 and Type 2 video use UDP because of strict timing constraints. They allow time for the retransmission of packets that are provided by TCP. According to Cisco and other experts, these forms of video are slowly merging together. The future will see nearly all forms of video as MPEG compressed content transported in IP packets. Because conventional video and VC do not use TCP, they have rather predictable behavior on the network. For example, conventional video carried in IP packets often has a rather precise bandwidth requirement. On the other hand, internet streamed video will download much like a data file transfer and will take as much bandwidth as TCP can negotiate to obtain from the network. Its bandwidth requirement is less predictable. BANDWIDTH REQUIREMENTS Table 1 shows some typical bandwidth usage with the various methods of transporting video. Conventional video currently uses 2 to 4 Mbps for standard definition video. Most often it is sent at a fixed bit rate. First, the encoder creates the MPEG stream of bits. Then they are deposited in 188byte MPEG packets, seven of which are placed in each IP packet. The bit rate is set in the encoder, and that determines how frequently the IP packets are sent. But, since they are uniform in size, the frequency doesn’t vary much. Videoconferencing generally uses less bandwidth. There are several reasons for this. One is that the MPEG compression is much more efficient when there is little motion in the scene, as in a conference session. Also, the conference unit video encoder can be set to collect only 7.5 or 15 frames per second rather than the conventional 30 frames per second used in television. For some systems (non-HD), the resolution can be reduced from 720 x 480 (standard definition TV) to 360 x 240 (VHS). Combining the lower resolution with half the frame rate generates only about oneeighth the number of bits in a conventional video stream. In some VC systems, the MPEG stream is created and placed in TCP packets, which are sent based on an algorithm that combines the amount of motion the camera sees and the capacity of the TCP packet. Therefore, the traffic load increases ➤ TABLE 1: TYPICAL VIDEO BANDWIDTH UTILIZATION Conventional Conventional Conventional Video Video Video (SD, MPEG-2) (SD, MPEG-4) (HD, MPEG-4) Video Conference (non-IP) 200 kbps1 Video Conference (IP, MPEG-2, 4) 460 kbps2 Internet Streamed Video 600 kbps3 Touch and hear for yourself Free no obligation demonstration +1.800.330.0891 North America or +1.801.233.8992 sales@listentech.com • www.listentech.com 3.5 Mbps 2 Mbps 10 Mbps 1. Computed at Phil Hippensteel Associates Labs, 2008. 2. YouTube download. 3. Polycom whitepaper, “Converged Network Design for Telepresence Solutions” (2006), and discussions with Polycom employees. Figure listed is based on 384 kbps standard definition setting and is considered a minimum. HD may require 1 to 2 Mbps. Telepresence may require 10 to 20 Mbps. assistive listening • soundfield • tour group • language interpretation • co ening conferencing 50 | AV TECHNOLOGY | february | march 2009 http://www.listentech.com http://www.listentech.com
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