DTV: A Manufacturer's Perspective, Part 2 — Audio, Metadata, and Network Challenges

by Peter A.Dare, Sony Corporation
Monday, March 30, 1998

DTV-New CHALLENGES

One of the new challenges facing the broadcaster is the multichannel audio capability of the transmission system. Dolby AC3 is the mandated audio format to be transmitted as part of the ATSC standard. AC3 comes in a number of different signal channels, the maximum defined for the ATSC standard is 6 channels AC 3 5. I. This in the "traditional" plant would mean that VTR's, file servers, routers, etc., all would need 3 stereo pairs to handle the six signals. The "NEW" plant will handle audio in some form of precompressed contribution/distribution quality, allowing for compression and decompression a number of time in order to accomondate "voice over," fades, level control, etc. The emission format at a bit rate of 384Kb/s is not designed for multiple compression codecs.

In addition to handling the compressed audio, there will be the issue of "lip sync." This is not just an audio problem. It is an audio/video problem. If the industry was plagued in the past with unequal delay between signal paths, it has not seen anything yet. Delays through video compression equipment could in some cases be in the order of 1 to 2 seconds. VTR's, file servers, and tape streamers will have to handle compressed audio very carefully. The gain through any system must be maintained at 1.0000. There can be no fades at edit points, etc. The AES 3 [the transport layer for the compressed audio] data stream must be faithfully carried as a transparent stream. In the digital domain, this is relatively easy to do if all digital processing devices have a unity gain position. Any analog implementation handling 6 channels or more per source is out of the question from both a cost and practical implementation point of view.

To handle the compressed audio bit streams, much of the equipment currently in place will need replacement or supplementing. Recently, the SMPTE has almost completed work on a recommendation for speaker placement for monitoring the multi-channel sound. There will be further work undertaken to insure that the compressed stream has interface standards in-place to insure interoperability The issue of handling the digital audio in a compressed plant should not be underestimated. The latency and lip sync issues will become the critical issue.

A further challenge will be the issue of handling meta-data and system Information, such as the Electronic Program Guide [EPG]. In the compressed world there is no vertical interval in which to place closed captioning [CC]. CC is carried as a data packet. The question is how and where is it carried.—program related meta-data may exist that requires transmission along with the program. From the emission point of view, this is rather simple. All of this data is either carried as a data packet, as part of the video [as is the case with CC in the ATSC standard], or it is carried as a private data packet within the MPEG transport stream. Sounds simple doesn't it is? The process of getting all the data assembled and put into the right packets that will require some new skill sets and software programs and perhaps separate data storage and signal paths for the metadata content. In some cases incorrect data could mean that you can't tune the TV set to a given channel. In other cases there is a mandate to carry a data packet, but no requirement to put anything in the packet. The bottom line to all of this is that your software supplier and your hardware supplier better understand the issues or you could be left out in the cold.

Within the transition time frame to the mutiplexed DTV service the broadcast industry will be faced with the concepts of video and audio data as files, and with the concept of shared networks. Both of these concepts will require intelligent resource management, and a full understanding of the 7 layer OSI model. [Application Layer, Presentation Layer, Session Layer, Transport layer, Network Layer, Data link Layer, Physical Layer]. This shared network invasion of the traditional TV plant will allow for faster/slower transfer of data with a defined Quality of Service [QoS]. To the TV engineer not familiar with computer networks this may well be a daunting task to implement. Shared networks are not all the same, and there will need to be a well defined model in hand before network design can be undertaken with a full understanding that not all networks will be able to support the familiar and expected isochronous transfer offered by the tradition SD@SDI Physical Layer. Today these concepts are still under development In terms of a large plant design, there are some implementations operating on a limited basis for file transfers between servers. There are no systems in operation that fully exercise the push pull models that will be necessary in a large system. This is not just a hardware issue. In fact, I would submit that the software control of such a system will be the more difficult to implement.

The concepts outlined above will only be successful if there are open system implementations available from a number of manufacturers. Needless to say that network down time in a TV environment is something that can not be tolerated. Fault tolerance in many cases will simply be accomplished through equipment duplication, something that in the past was easily achieved.

The networking topic is a topic that in itself could be the subject of a complete session. The options are just too embarrassing to cover as part of this paper. The requirements of the TV broadcast plant stretch network performance and band widths way beyond the requirements of the computer industry. Although the technology will be based upon computer technology in order to keep the costs down, modifications will be required to accommodate the unique TV application.

CONCLUSIONS.

Manufacturers of both professional and consumer equipment are eager that the role out of the DTV service is welcomed by all who participate in this industry. This will be the largest single event for the TV industry since Color was introduced in the 1960-70. At that time the great mentor for color, General Sarnoff was leading the way. He had good reason to make the roll out a success. He owned a consumer company, a professional company, and a TV network. What else would it take to be successful? Not much!

Thirty years latter the situation is different. Not only is the technology advanced, the model is different. The signal to be transmitted will not be compatible with the existing service. There is no leader. There are more diverse industries trying to place a stake in the sand. In some cases the broadcasters are not sure of the future.

The new DTV service can not be viewed simply as a digitzed version of the analog service. The services to be offered, the technology to be implemented, the available flexibility of the transmitted bit stream, the possible interactive capability, the data services to be offered are all examples of the new way we have to view the DTV service. Broadcasters are becoming data service providers. Yes, there are problems to be overcome. These should not stand in the way of the innovation.

Clearly the new services can not be implemented nor can some of the network capability without comprehensive software packages that don't exist today. This will be an evolving process and with careful planning by the end user and manufacturer, software development will meet the time frames mandated for the DTV introduction.

The broadcasters will have to proceed with a well defined plan, and they will need to work with the consumer and computer industry to support the data services being offered. From time to time there is reported emotional discussions between various industry segments. Are they real? I don't think so. We should spend more time implementing what we believe to be correct rather than debate issues that don't actually have a black and white answer.

Finally, I do believe we have the expertise to implement this extremely complicated infrastructure surrounding DTV. The economics are only a concern if the transition is simply viewed as transition to digital TV. If the flexibility of the transmission pipe line is viewed as an opportunity to deliver other services, then surely the initial investment is more than worth while, providing the industry plans for it. Between the TV sets and computers in the home there is a huge potential for delivering digital services—a bandwidth delivery system never before possible

"Television is not really complicated but it is kept in a state of confusion by the engineers who use abstruse terms to hide their ignorance"

E.C.Johnson-Senator 1949 [Quote provided by Mark Schubin]

Copyright Sony Corporation 1997.