AV Technology - June 2008 - (Page 71)

codecs from the internet. Most hardware video encoders in use today are using an MPEG-2 codec. You need to select the codec based in part on what the receiver can decode in the decoder or set-top box (STB). Next we will discuss what is specified in the standards. In MPEG-1, the video compression supports only progressive output devices. These devices display 30 full frames per second. Older TVs displayed 60 half-frames per second, showing the odd lines first and then the even lines. This was called interlaced format. Newer TVs that are interlaced have an “i” designation as in 720i or 1080i. So, MPEG-1 codecs cannot have the output displayed on older standard CRT TVs. On the other hand, computer monitors are progressive and will display MPEG-1 streams. Also, MPEG-1 first described a technique that involved predicting the position of an object in the scene based on where that object first appeared to remove spatial redundancy. Suppose the camera presents 12 frames of data to the codec. In the first frame an arrow is being released from a bow. In the fourth, the arrow is 30 feet in front of the bow. The codec records nearly all of the information about the first frame and the new position of the arrow in the fourth frame. The receiver calculates the position of the arrow in frames two and three. That information is not included in the MPEG stream. This method allowed three kinds of frames to be created. I-frames are essentially JPEG images of the entire scene, as in frame one. Frames with predicted information, such as frame four, are called P-frames. Finally, frames that are reconstructed based on previous and subsequent frames such as frames two and three are called B-frames. Using techniques like this allows for a significant reduction in the amount of information that must be sent to the receiver. While a typical I-frame might be several tens of thousands of bytes, a P-frame might be half that much information and a B-frame might be as small as several thousand bytes. The main variable in the size of the frames is the amount of variability in brightness and color and the amount of motion between frames. The MPEG-2 standard described a codec that supported interlaced presentation and improved the compression algorithm. It also increased the levels of resolution supported and introduced a framework for the formats used in high definijune 2008 | AV TECHNOLOGY | 71 tion TV (HD). This is currently the most widely deployed codec. MPEG-4 and its variation H.264 improved the compression algorithm even more and expanded the range of supported resolutions and frame rates. With H.264, there are formats to support mobile devices like cell phones that require only 56kb/sec. On the other hand, there are formats for production studios that offer such high quality that they are streamed at 27 Mb/sec. Finally, a word about encoders. There is little difference between an encoder and a video codec. In common usage, a codec is a device that does the quantization and compression. An encoder includes this function but usually does even more preparation of the stream such as the creation of the IP packets that are sent. More often than not, the terms are interchangeable. Phil Hippensteel is an industry consultant and researcher. He routinely presents at major conferences and is an assistant professor at Penn State University. He can be reached at pjh15@psu.edu. Portable To Start is a self-contained portable PA system with outstanding sonic performance and easy portability. It is well-suited to a wide range of commercial applications including outdoor events, houses of worship, conference facilities, museums, theme parts, government facilities, rental inventories, auditoriums, athletic facilities, shopping malls and trade show/exhibit booths, to name a few. It is also available in a wireless powered support speaker. The Califone ® PowerPro™ PA 916-SD Front panel LEDs indicate status of power, RF and audio reception for each of its two wireless microphone receivers. Programmable CD player. MP3 player/recorder with 512 MB of onboard memory. Plays to/from SD memory card port. Mini-USB port offers easy connectivity to other media devices or computers. Two wired microphone controls with full line-in and line-out capabilities. 16-channel UHF transmission with 300’ range to an unlimited number of UHF Califone loudspeakers. DC power supplied by an onboard rechargeable battery (up to 8 hours) or universal AC power. Voice Priority feature automatically lowers music volume for announcements. Speaker mute switch mutes main speakers and connects speakers (wired & wireless). Califone, Inc. http://califonepro.com http://califonepro.com

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

AV Technology - June 2008 Contents Precedent Corporate: Global Standardization Government: Strategic Technology Planning Education: It’s All Just a Click Pre-Empting Projector Breakdown Dealing With Technophobes Keeping Ahead of Technology Obsolescence Understanding Video Codecs Rules of Engagement Network, See & Learn Sound From the Heart Sportsbook Makes an Impact Upgrading an AV Image Product Review: Polycom’s RealPresence Experience HD Tech Horizons: A True AV Networking Standard? Product Spotlight: Digital Media Servers; Flat Panel New Products Ad Index AV MO: Calculating Fiber Optic Transmission Distance

AV Technology - June 2008

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