Technology Primer
on Digital Video Communications
by Charter Cable
440 Science Drive, Suite 302
Madison, WI 53711
Technology is changing very rapidly, and this is an area where most of us are not experts by any stretch of the imagination. We have had television in our homes in this country for well over half of this century, even color TV since the late 1950$#146;s. While nearly all of that video is in an analog format, computers (CD-ROMS) and even new VCRs work with digital video. Digital video because of its large bandwidth, needs to be compressed (i.e.. reduced in size) to be stored or transported. The JEDI and MPTC network use analog video, by far the most common and popular video format today. The ubiquity of analog video makes it very easy to interconnect with any type of digital network, and will continue to make analog video popular well into the next century.
Compressed Digital Video. So why go to digital video? Digital video allows higher quality video transmissions over very long distances, and with high powered computers available at lower costs video can be digitized more cost effectively than before. But why compress the digital video? The reason to compress video (shrink it down) is that uncompressed digital video requires over 160 million bits per second of bandwidth – and that’s huge! Uncompressed digital video is a bandwidth “hog” which makes video expensive to transmit and store. Compared to your 28.8 kilobit (28,800 bits per second) modem, 160 Mbps is over 5,300 times more bandwidth. To transport this digital information economically, the digital video needs to be compressed.
How is video compressed? Video can be compressed because there is a significant amount of redundancy in video pictures. Also, picture information that doesn’t change from one "scene" to another does not need to be transmitted.
Let’s define these terms: MPEG-2 (pronounced EM peg two) and ATM. MPEG-2 stands for "Moving Pictures Experts Group – 2", standard number two. MPEG-2 is an international standard that all vendors can build equipment to for high quality compression of video. ATM or "Asynchronous Transfer Mode" describes a specific set of rules to transport high speed voice, video, or data across any network in the world. ATM advancements are ratified by the international standards body the International Telecommunications Union.
But why are standards important? Standards tend to insure that what people buy will interoperate with other vendors equipment. Standards also tend to bring down prices and to provide additional choices for consumers, as well as protect consumers investments. Some products produced before standards would only work when the same vendors equipment was on both ends of the network. Early fax machines, modems, and local area networks (LANS), current AM stereo, and of course Beta type VCRs are examples of where technology was ahead of the standards bodies. Not being able to use some of this equipment for long has delayed technology deployment for years and cost customers and the industry millions of dollars and untold frustration – because similar equipment doesn’t work together. The MPEG-2 group set out to define the standards before multiple vendors had developed their competing products and formats. This international standard document is administered by the public – not one firm or a closed group of companies. So any company can choose to build products that comply with the standard.
The History of MPEG Standards. The idea behind a moving pictures experts group began to form in the late 80’s, driven initially by scientists in Europe and Japan that wanted to avoid multiple, proprietary standards for video and audio compression schemes. These people had the vision to se how important compressed video could become, leading to some very beneficial applications. The first task for the MPEG group was computer (CD-ROM) based digital video. A number of different vendors had different formatted products which only worked in certain CD players, with certain software. MPEG-1, approved in 1993, set an international standard for 1.5 Megabit per second video (for CD players) that all vendors could build to. Standards, drive down costs for consumers, and ensure interoperability.
MPEG-2 provides for much higher quality video at variable bit rates that can go much higher than 1.5 Million bits per second. MPEG-2 even provides for the transmission of High Definition Television, or Advanced Television (ATV) as it is now called.
The important thing to remember is that MPEG-2 is an international standard that ensures that the vendors equipment you buy will work with other vendor’s equipment in the future. MPEG-2 also provide significantly higher video quality at lower bit rates than Motion JPEG video. Standards ensure that consumers can shop around and buy from a number of companies as long as their product meets the standards.
What is JPEG? This is another term that you will hear. It stands for "Joint Photographic Experts Group". The JPEG group preceded the MPEG work. The JPEG group worked to develop a process to compress a still picture, or photo, digitally. JPEG is now also an international standard.
Before the MPEG-2 standards were finished, some companies created their own derivative of JPEG, called Motion-JPEG to transporting video. Motion-JPEG is not, nor will this ever be, a standard. So one vendor’s Motion-JPEG equipment will not work with another vendors Motion-JPEG equipment. The reason that Motion-JPEG will not become a standard is that MPEG-2 has been embraced as the new video format. In other words, it would be a waste of time for the standards body to attempt to standardize on one of the various Motion JPEG schemes that are being used today since MPEG-2 is now defined.
ATM. The dream of the Bell laboratory researchers was a high speed, yet scalable network that could provide all services with high reliability. ATM delivers on that dream. Many people have at least heard of SONET. SONET stands for “Synchronous Optical Network”. SONET allows many slower speed services to be stacked together to be transported efficiently over the telephone network.
Asynchronous Transfer Mode (ATM) is a more efficient relative of SONET. Here’s the difference; SONET dedicated a full-time bandwidth to a customer, whether you use it, or not. While ATM can reserve the capacity for you, it only sends data back and forth when you need to.
The History of ATM. The concept of Asynchronous Transfer Mode got its start in the 1970’s with Bell Lab’s researchers envisioning future data services and a concept of one powerful service that could deliver everything to anybody. The service had to be fast, and also had to accommodate voice, data, and video traffic, and be scalable; that is, where people would only pay for what they needed. The international support began moving faster during the 1980’s and lead to the creation of the ATM Users Forum in 1991. The ATM Forum began with 4 members in 1991, and today its membership is in excess of 750 companies, governmental and research organizations. The ATM Forum continues to play a vital role in the advancement of ATM standards and deployment.
The advantages of ATM are that it is extremely flexible, very efficient, it is scalable ("bandwidth on demand"), and it is granular, meaning you can have as small a slice of the bandwidth as you require.
ATM works by changing the information, whether it be voice, data, or video into so-called "cells" or blocks of data. Cells are a fixed sized group of data that is routed through the ATM network very rapidly and efficiently. The network knows’ when the cell has to be delivered to the receiver. So each data "cell", just like a letter in the US postal service, has the address that it came from, its destination address, the time that is has to be delivered by, and its order in the transmission --- if it is part of a group of data. This ensures that video images will not appear jerky or distorted, since they are given priority in the network.
But a real benefit of ATM is data transmission. As multimedia applications, such as CD based training, and voice and video over the Internet continue to explode, ATM provides the optimal flexibility and "investment protection" for your data requirements both today and into the future.
For more information please contact:
Evans Associates
210 South Main Street
Thiensville, Wisconsin 53092
(262)242-6000
fax: (262)242-6045
http://www.evansassoc.com
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