DIGITAL AUDIO AND ANCILLARY DATA SERVICES FOR ATV -­ THE WORK OF THE ATSC SPECIALIST GROUP Graham S. Stubbs Eidak Corporation ABSTRACf that it is important that appropriate em­ phasis should be placed on defining the This paper describes the advice and accompanying sound channels, the features suggestions put forth by the Technology of the ancillary data and control services, Group on Distribution (T3) of the Advanced and the way in which they may be included Television System Committee (ATSC) re­ in the ATV transmission format. garding digital services for Advanced Televi­ sion (A TV). These recommendations were The charter of the ATSC Specialist based on the background work of the Spe­ Group on Digital Services (T3/S3) has been cialist Group (T3/S3) on Digital Services to conduct industry surveys and technical which conducted technical studies, and studies and to develop technical information surveys, and developed the suggestions and and recommendations on the following sub­ recommendations. jects. The Specialist Group commenced its studies in December, 1990. Rapid advances in multichannel com­ posite digital audio coding technology now A Sound & Ancilla:ry Data Services make it possible to plan to provide the con­ sumer (e.g. cable subscriber) with an expand­ Identification of the range of ATV ed audio experience to match wide screen sound channel requirements and high definition TV pictures. This paper how they might be satisfied with re­ summarizes the suggestions adopted by cent rapid advances in the state-of­ ATSC's T3 Group regarding audio and the art digital audio coding. Identifi­ ancillary data services, including the advice cation of the range of desirable that a standard service for A1V should in­ ancillary data services--including clude capacity for a minimum of five audio those already in use and in some channels with composite encoding. cases mandated--and estimates of the data capacity required for each. Particular emphasis was placed on the need for flexible allocation of data to audio B. Conditional Access and ancillary data services on an as-needed basis. Establishment of system attributes and features required to allow condi­ INTRODUCTION tional access to the A TV transmis­ sion in the various media in particu­ The Advanced Television Systems lar Cable Television, with emphasis Committee and other organizations, includ­ on the structure and capacity of the ing the FCC Advisory Committee on Ad­ required control data channel. vanced Television Service, EIA, and NCfA, have recognized that much of the c. Multiplex Structure effort spent on ATV development and stan­ dardization has been put on the develop­ Determination of requirements for ment of vision signal encoding schemes and flexibility in the data multiplex struc 1992 NCTA Technical Papers- 31 ture to permit re-allocation of the "images" which can truly complement the available data capacity for the vari­ high definition pictures in a restricted 6 ous services. MHZ wide HDTV digital channel. Audio techniques developed for motion picture The goal of this work has been to theaters may become applicable to the influence the characteristics of the digital consumers' living room. services (other than video) which are to be provided by the A TV emission system As a discussion vehicle, the Specialist selected for use for terrestrial broadcasting Group prepared a "strawman" proposal for in the United States. (However, it is not TV Audio and Data Services for Simulcast ATSC's intention to affect the ATV testing ATV systems. The "strawman" proposal program already in progress as part of the suggested desirable program audio attri­ FCC selection process.) Specifically, it is butes and listed possible ancillary digital suggested that the final ATV system se­ services, including some of those already in lected for terrestrial broadcasting, cable, use with NTSC such as closed captioning. It and other media follow the guidance de­ was suggested that advantage should be tailed in reference 1 (see Appendix for taken of the opportunity to design a new Executive Summary of the ATSC docu­ generation of ancillary data services into the ment). system from the beginning. Starting in April 1992, the "strawman" proposal was Recognizing that different media circulated for comments and suggestions to (broadcast, cable, DBS, etc.) will use differ­ a wide range of parties with interest in ent modulation methods, possibly carry A TV development and introduction, includ­ different ancillary signals and have other ing the proponents of systems to be tested requirements unique to each medium, in the selection process. By an iterative T3 /S3 has focussed its technical studies to process, the document was refined to reflect facilitate the adoption of voluntary stan­ as much industry consensus as possible, with dards for all media based on the same particular emphasis on flexible allocation of baseband signal representation for video, data capacity to audio and ancillary data audio, data and control to maximize inter­ services. operability and to provide a simple com­ mon interface to the consumer A TV re­ The following summarizes some of ceiver. Work by T3 /S3 on Conditional the principal suggestions as to the charac­ Access is reported in References 2 and 5. teristics of the audio and digital services which should be provided by an ATV emis­ WORK ON AUDIO AND ANCILLARY sion system adopted for use in the United DATA SERVICES States, both for terrestrial broadcast and by alternate media. Certain services are iden­ Advances in audio bit rate coding tified which the final distributor of pro­ technology have occurred extremely rapidly gramming may provide in a standardized in the past two years, now making it poss­ manner. Standardization of services is ible to encode multiple audio channels necessary so that receiver manufacturers using data rates previously allocated to a may produce receivers that can receive single audio channel. It is now technically these services. A number of capabilities are possible to create multiple channel audio 32 -1992 NCTA Technical Papers identified which every receiver should be added in a compatible m&-mer by means of required to provide. the flexible allocation of data capacity. The key is to provide a way to identify new MAJOR PRINCIPLES types of allocated data services, so that new or upgraded receivers may make use of the Flexible Allocation new data types, while older receivers simply ignore them. The new data types could The number and type of audio and offer new kinds of audio services (perhaps data services which an A TV system should with more audio channels) and new kinds of deliver will vary significantly depending on information services. Some of the addition­ what services are available to accompany al data types could be intended for private the picture, and the needs of the particular or commercial reception, with the data service area for the distribution medium, capacity being sold by the final program e.g. broadcast, cable. etc. In order to pro­ distributor to provide a supplemental rev­ vide a maximum level of utility, many audio enue source. channels and data services might be need­ ed. A fixed provision for many audio Multi-Channel Audio channels and data services would require, however, that a significant portion of the The A TV system should be capable available transmission capacity be reserved of delivering multi-channel sound appropri­ for these services. This would unnecessar­ ate to the wider, higher definition picture. ily constrain the video quality, which would Consistent with demonstrated psychoacous­ suffer if its available data rate were re­ tic principles, the preferred channel assign­ stricted. Since much of the programming ment is: Left, Center, Right, Left Surround, may not require many audio channels or and Right Surround. A significant advan­ data services, a fixed allocation of transmis­ tage of three front channels (rather than sion capacity to these services would be two as in present TV stereo) is stabilization inefficient. Therefore, the A TV transmis­ of the audio dialogue image in the vicinity sion system should be capable of a flexible of the TV screen. allocation of data to audio and data servic­ [Note: This is consistent with the CCIR es on an as-needed basis, with the remain­ Task Group 10/1 Draft Recommendation ing data available to the video system. At on Multi-Channel Sound (Reference 3), and the point of emission or final distribution, the SMPTE Film Sound Sub-Committee a choice may be made based on the avail­ Report](Reference 4). An optional low able services and the needs of the intended bandwidth channel for low-frequency en­ audience as to which services are to be hancement (subwoofer channel) may also delivered via the final distribution medium. be provided. Monophonic and two-channel stereophonic transmission modes of opera­ Extensibility tion should also be provided, and will offer the most efficient method of delivery for It is not possible to enVIsion and mono or two-channel stereo programming. provide for all potential audio and data services which might be useful components Even with the use of advanced low of the A TV service in all media. It is bit-rate audio coding technologies, provision feasible to allow new digital services to be of five high quality independently coded 1992 NCTA Technical Papers- 33 discrete channels would require significant When surround information is rele­ data capacity. Current estimates of re­ vant and available, the 3/2 composite cod­ quired bit rates per individual audio chan­ ing mode is preferred. Three front chan­ nel are: nels and two surround channels are provid­ ed. High Quality 128 kbits/sec When surround information is not Medium Quality 96 kbits/sec ·available, the 3/0 mode is sufficient. Three Low Quality 64 kbits/sec front channels are provided; the 3/2 mode may also be used.