Compressed digital video for business TV applications.

The most important technical development enabling rapid growth and expansion of business television (BTV) is the advent of affordable, high quality, compressed digital video (CD-Video) transmission.

CD-Video is appealing for business television because it allows satellite transponders to carry more than one program at a time. A compression scheme that achieves multiple channels simultaneously in a transponder dramatically expands the satellite time available for business television networks.

Several competing hardware manufacturers are now promoting their compressed video products for business television networks.

Two-way videoconferencing uses video compression to transmit an "adequate" talking head or graphics image through a small bandwidth. This significantly reduces the transmission cost of "full motion, full color" videoconferencing. What is new is application of this powerful signal processing technique to BTV.

For BTV, the goals in applying CD-Video are very different.

The image viewed by the user must continue to look like "plain old television" -- anything less will be completely unacceptable. BTV requires high production value television as we are accustomed to seeing at home. And, BTV productions often include plenty of action, vivid color and special effects.

So, the compression technique must not visibly degrade the image, and significant compression artifacts are not acceptable. Image quality instead of bandwidth efficiency is the primary consideration

Applications of CD-Video

CD-Video can benefit existing and future business and educational television users in many ways.

* The spectrum efficiency of CD-Vido can solve the traffic contention problems most users have experienced for several years in scheduling available full transponder time. More programs will be scheduled on shorter notice, and international transmissions will be greatly simplified.

* An industry home-base satellite can become a reality for business television with CD-Video. Today's BTV users are spread across virtually every Ku-Band satellite in orbit to minimize the satellite transponder scheduling contention problem. Affordable cross-networking and third-party programming can be more easily justified when hundreds of BTV networks operate on the same satellite.

* Few satellite data networking companies now encourage their customers to use VSATs for frequent video transmissions. The full transponder time required for video on the same satellite used for data is simply too difficult to schedule during normal business hours. CD-Video will solve this problem and make it easier to justify future VSAT networks on the basis of combined data and video services.

* The efficiency of CD-Video transmission allows the network designer to significantly reduce the required antenna size by transmitting fewer signals per transponder. Many BTV networks will be able to justify expansions since the installation cost will be significantly reduced.

Video signals contain a large amount of redundant information that can be eliminated without degrading the picture. The presence of this redundancy makes video compression possible since the video is compressed by eliminating unnecessary picture information.

Two characteristics of the video signal contribute to the redundant information -- redundancy within a video frame and redundancy between video frames.

Within a frame, there may be large areas of one color or intensity. This information may be encoded so that the color information is transmitted only once for the entire area, reducing the amount of data that must be transmitted. This is known as intraframe compression.

A video signal consists of individual frames, or pictures, transmitted at a rate of 30 frames per second. Typically, very little of the picture actually changes from one frame to the next, so the data can be compressed by transmitting only the changes between frames. This is known as interframe compression. Video compression systems take advantage of one or both of these redundancy characteristics to compress the video at the transmitter and decompress the video at the receiver. A typical compression system operates by digitizing the video and then digitally processing the video to eliminate the redundancy. At the receiving end, the data is received and more digital processing is needed to reconstruct the video.

Until recently, the electronic hardware needed to process the large amount of video data was very complicated and expensive. However, low-cost silicon processor chips have now become fast enough and powerful enough to handle the compression algorithms.

Uncompressed digital video typically requires a data rate of 100 Mb/s. The compression techniques that are being developed will allow reconstruction of the television signal from only 3 to 10 Mb/s.

A compressed digital receiving system for BTV must offer several key capabilities. The system must have an encoder that can receive analog video as an input, digitize and compress the video, and output the digital information to a modulator that drives the uplink.

The receiver must receive and demodulate the digital signal, decompress the digital video, and convert the audio and video back to standard analogi signals for display.

Finally, the system must provide a conditional access feature that allows only authorzed viewers to receive the program and prevents unauthorized viewers from decoding the signal.

Three U.S. companies are developing products that meet the requirements for a complete CD-Video system for business and educational television. Compression Labs is offering its SpectrumSaver product with 1991 availability and a data rate of only 3.3 Mb/s. General Instruments is offering its DigiCipher system that promises compatibility with HDTV. And Scientific-Atlanta is building a system around its existing B-MAC audio and conditional access technology to offer a degree of compatibility with existing analog B-MAC systems.

The preceding article was condensed from a papaer written for the International Teleconferencing Association (ITCA) Yearbook. The ITCA is a professional organization established to promote recognition, research and application of teleconferencing systems and services. BTVI was the winner of the 1991 Communications News Award for the outstanding paper published in the ITCA Yearbook. The winner is selected by Communications News magazine, and the award was presented June 25, 1991 at the annual ITCA convention in Washington, D.C.