Revert to Section 7.3.3
7.3.4 User Requirements
An important aspect of system development leading to DTTB service implementation is the derivation of a clear set of user requirements.
For example, in Region 1, the user requirements are based upon a "market -led", approach to the implementation of DTTB services. The main elements of these requirements as defined in 1995 may be summarised as follows;
7.3.5 Service implementation in an all-digital environment
Alternative distribution systems
A number of distribution systems will be used for the delivery of digital TV services to the general public. These systems such as cable, MMDS, satellite, and optical fibre networks will have their own intrinsic limitations on transmission channel performance. Ultimately, from the standpoint of the viewer, it is important that the received quality be independent of the delivery mediums.
Harmonization and interoperability
Digital technology has the potential for facilitating interoperability among various image systems. Selection of an advanced television system that incorporates attributes needed for interoperability will harmonize interchange of still and moving images from diverse sources. Digital delivery of television signal to the consumer will take place over a number of delivery media, which may differ in capacity. It is desirable that the standards take account of interoperability at the transmission level. It is also desirable that a high level of interoperability be achieved at the receiver level to ensure the maximum flexibility for the processing of differing network and computer-based services, in addition to DTTB.
In the broadcasting environment, interoperability refers to the ease with which the DTTB data stream can be transferred among delivery media such as terrestrial broadcasting, cable, satellite, public network and prerecorded media. In the receiver environment, interoperability refers to the ease with which the receiver can process the differing data streams from television, computer-based graphics, multimedia and other non broadcast sources. Interoperability considers delivery over alternative media (cable, satellite, packet networks), transcoding (with film and format conversion to other video standards), integration with computers and digital technology, interactive systems, the use of headers/descriptors and scalability.
The future benefits of video and other image technologies will be greatly enhanced if universal interchange of all kinds of image and image sequences can be implemented and managed economically. The ultimate beneficiaries are the consumers who will have at their option image information of any kind in a form chosen by them, instantly available, at an affordable price.
Rapid advances in digital semiconductors, digital communication, and digital processing algorithms will make it possible to tailor the video technology to specific applications in terms of picture quality, price, format, and performance. Such a diversity in the video marketplace will be a positive development only if it is easy to move among different formats, applications, industries, and media. The key idea is to facilitate interworking among multiple formats so that market forces can guide the developments of products and services.
Digital representation of signals is the key element in achieving interoperability for images and video. The digital nature of the signal means that all the systems that process the signal have identical material to process. The ease of storing, transporting, and processing digital data is matched by the growing speed, power, and economy of semiconductors.
Once in digital form, signals can be filtered and processed in a predictable and reproducible way so that conversions among formats can be implemented using functions selected based on mathematical theories such as sampling, interpolation and prediction.
Certain specific attributes of image-related systems contribute to interoperability and are described below:
Layered systems and scalability: Interoperability in broadcasting is enhanced by the adoption of standards based on the OSI model, in which there is a clear functional separation between layers. The packet-based approach of MPEG-2 is an excellent example of such an arrangement for video, sound and data. In the case of terrestrial broadcasting, however, consideration must be given to such standards to the possible variations in receiving environment. This may lead to differing levels of priority for data packets, effectively subdividing the channel capacity if a layered approach is employed.
Progressive scanning: Progressive scanning in a raster-based sequence of images simplifies, to some extent, the filtering and interpolation used to convert among formats with different numbers of scan lines, different numbers of samples per line, and different temporal sampling (i.e. picture rate).
Square pixels: For computer graphics, equal geometric spacing among horizontal samples on a line and among samples displaced vertically is desirable for simple rendering of objects that may be transformed after creation.
Provision for headers and descriptors within data: An important area of agreement among advocates of interoperability and harmonization of images and video is the desirability of headers and descriptors embedded within the stream of image data. The purpose of the headers and descriptors is to identify reliably and unambiguously the form of the data. The headers could include information on how the images or images sequences were originated, processed and compressed, thus making the data stream self-identifying and offering flexibility in implementations.
Syntax: Interoperability at the receiver is greatly enhanced by the adoption of common syntax among services. For video, this must take account of colorimetry, transfer characteristics, spatial and temporal sampling structures and sample coding. Flexibility is enhanced if the syntax adopted lends itself to scalability of the image data.
Conditional access: Copyright owners, programme suppliers and service providers require highly secure broadcasted/distribution networks to allow the protection of their programmes throughout the delivery chain up to the final authorized users, thus leading to the need for highly secure end-to-end access control with flexible interfacing to authorization and confirmation systems. Interoperability is enhanced by the adoption of a common system as described in lTU-R Recommendation "Conditional-access broadcasting systems" [Doc. 11/66 period, 1990-1994].
Industries with interests in high resolution images
While the traditional entertainment television industry has been built around a single dominant format (NTSC in 60 Hz countries, PAL and SECAM in 50 Hz countries), non-entertainment industries have generated a number of formats for still and moving images. In some cases, the non-entertainment applications have used conventional television formats, although there has not always been a good match of these capabilities to requirements.
Many now seem to believe that technology has reached a point where many new standards are about to be set, and the opportunity should not be missed to harmonize the various image and video standards. One advantage of interoperability is the ability to create a common technology for use in the large consumer television market as well as other non-entertainment image and video applications.
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