Imedia's CherryPicker.............................
Publication Date: November 18, 1996 (Page 43)
Copyright (C) ELECTRONIC ENGINEERING TIMES 1996
Section: Technology
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Algorithm crunches MPEG data
Technology
Publication Date: November 18, 1996 (Page 43)
Copyright (C) ELECTRONIC ENGINEERING TIMES 1996
Algorithm crunches MPEG data
By Chappell Brown
San Francisco - A new approach to video compression that can squeeze 24 digital video channels into one analog channel promises new capabilities for cable programming. The editing system is only one application of developer Imedia Inc.'s "statistical multiplexing" technology, which the company believes could be used in a variety of ways to make more efficient use of video broadcasting.
Imedia calls its editing system CherryPicker; it was built around an MPEG-2-based algorithm. It allows cable content to be merged from a variety of programming sources-digital-satellite broadcasts, optical fiber, local stations, video archives-and then piped down a single RF television channel. The resulting video broadcast offers viewers a large number of channels with a wider range of unique programming.
Interactive uses
"We've applied the technique to creating systems such as video-on-demand and foresee other interactive uses when it is applied to systems such as ADSL," said Adam Tom, an Imedia founder who helped develop the technology. By working directly with digital video streams that have been encoded using MPEG-2 hardware, the statistical multiplexing operation not only compresses the data stream but allows for a wide variety of other editing functions. "For example, with a video-on-demand system we can cache a small amount of the stream to allow viewers to perform VCR functions such as fast forward or frame-by-frame viewing all within a broadcast format," he explained.
Caching image data also means that a single copy of a movie can be archived and rebroadcast over the same channel in 24 parallel streams, each one starting every 2.5 minutes. The technique would allow the creation of highly efficient video-on-demand distribution systems. Interactivity can also be built into the system via set-top boxes.
"Cable companies are excited about the technology because it will allow them to produce highly tailored content. For example, you could stream advertising into homes based on the viewer's buying preferences," said Tom. In addition to offering what the company calls the highest compression rate in the industry, the statistical multiplexing method also enables a highly flexible method of structuring a video stream. The Cherry-Picker system, which runs on a Silicon Graphics workstation, will be demonstrated next month at the Western Cable show in Anaheim, Calif.
The compression technique keys off MPEG coding and decoding. Once an analog signal has gone through an MPEG coder, statistical multiplexing identifies the relatively rare segments of the digital-video stream that contain large amounts of data. "Fairly static scenes produce relatively little data. But once in a while, during a fast pan with high detail or a scene showing something exploding, you get a burst of data which actually uses the full bandwidth of the channel," Tom said.
The statistical multiplexing algorithm grabs the high-data-burst segments and forms a data packet that is reallocated to another channel that is not running at maximum. "We put a 1-Mbit ceiling on the digital stream and repackage anything that goes over that limit, putting it somewhere else where there is more temporary capacity," he said. The method allows a normal 6-Mbits/second channel to be squeezed down to 1 Mbit/s so that more channels can be broadcast within the same bandwidth.
"We benefit from some of the special features of MPEG coding. For example, at the receiver, the MPEG decoder automatically identifies and reassembles the packets into a continuous high-quality video image," he said. MPEG breaks up the digital stream into packets with an identifier linking it to a given channel. Thus, the compression scheme and CherryPicker's ability to merge diverse video content work smoothly with the packet-coding system. So functionality can be added to systems without requiring more hardware at the receiving end.
HDTV background
Tom's previous duties as head of the decoder work on General Instrument's MPEG-2/DigiCipher II system gained him an intimate knowledge of MPEG decoding. Imedia itself was hatched by Tom and fellow students Paul Shen and Ed Krause, who had worked on MIT's high-definition television project. That project eventually merged with General Instrument's HDTV initiative. "At General Instrument we began looking at the compression problem in terms of MPEG coding," Tom said.
After working out the details of statistical multiplexing, the three decided in 1994 to found a company to develop and market the concept. The result was Imedia, which has raised $12 million in private capital. The fledgling company's first contract was with Tele-Communications Inc., the giant cable service provider.
Copyright c 1996 CMP Media Inc. |
