Japanese Datacasting Coding Scheme BML

Transcription

1 Japanese Datacasting Coding Scheme BML AKIHIRO HORI AND YOSHIHARU DEWA Invited Paper This paper introduces the datacasting coding scheme named Broadcast Markup Language (BML). BML is used for all the digital broadcast such as satellite and terrestrial digital broadcasting in Japan. BML is based on web technology standard; however, some extensions and restrictions are applied for broadcast purposes. These extensions and restrictions are introduced in this paper. Keywords Broadcast Markup Language (BML), data broadcasting, datacasting, digital broadcasting, Extensible Markup Language (XML), Hypertext Markup Language (HTML), hypermedia, markup language, multimedia coding. I. INTRODUCTION In addition to image and sound broadcasting, datacasting is the one of the features of digital broadcasting. A variety of services are provided in datacasting today. A datacasting image example for a baseball game is shown in Fig. 1. In this paper, we would like to introduce the Japanese datacasting coding scheme. The datacasting related standards are specified by the Association of Radio Industries and Businesses (ARIB). The Japanese datacasting standard is named Broadcast Markup Language (BML) and is based on Extensible Markup Language (XML). This is standardized as ARIB STD-B24 [1]. BML is used in all of the datacasting in digital broadcasting system in Japan such as digital satellite and digital terrestrial broadcasting. More than five million BML compliant receivers have already been deployed. Actual operational specifications are discussed among the broadcasters and the receiver manufacturers and operational guidelines are defined for each media. For example, operational guidelines for digital terrestrial sound broadcasting are described in ARIB TR-B13, operational guidelines for digital terrestrial television broadcasting are described in ARIB TR-B14, operational guidelines for digital satellite broadcasting are described in ARIB TR-B15, and operational guidelines for digital satellite sound broadcasting are Manuscript received February 1, 2005; revised August 22, A. Hori is with the Nippon Television Network Corporation, Tokyo , Japan ( akihori@rd.ntv.co.jp). Y. Dewa is with the Sony Corporation Home Electronics Network Company, Tokyo, Japan ( dewa@arch.sony.co.jp). Digital Object Identifier /JPROC Fig. 1. Datacasting example for baseball game. described in ARIB TR-B26. Therefore, it is necessary to understand not only STD-B24 but also related operational guideline to create actual BML content. STD-B24 consists of three parts. Volume 1 is for data coding, Volume 2 is for the XML-based multimedia coding scheme, and Volume 3 is for data transmission specification. The datacasting system structure is shown in Fig. 2. STD-B24 specifies all the interface points of (1) (5) in Fig. 2 in detail. Table 1 shows each interface point s specified part in B24 and outline. II. USER REQUIREMENT FOR DATACASTING A BML study started in March 1999 for satellite digital broadcasting and the first version of STD-B24 version 1.0 was approved in October The user requirements below were considered in designing BML. They are: 1) harmonization with Internet Web content; 2) extensibility; 3) rich content expression; 4) unique presentation. III. OUTLINE OF BML Considering the user requirements above, the following standards are employed as the fundamental basis of BML: XHTML 1.0 [2]; CSS 2 [3]; ECMAScript [4] /$ IEEE 312 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

2 Fig. 2. System structure. Table 1 Each Interface Point s Outline and Specified Section in STD-B24 BML is one of the XML resources. Therefore, it is possible to compensate each other like other XML resources. BML is designed to provide any functions required from any broadcasters or content providers. It is based on the tag sets in XHTML1.0 considering the harmonization with HTML. CSS2 is employed for the screen layout control and the presentation control. ECMAScript is employed as the script language to control the contents. The Document Object Model (DOM) [5] is employed to interface between the script and the tag sets. In addition to the standards above, some extensions are included in BML. These extensions are defined so that BML can be used for future datacasting. We restricted the standard by the operational guideline described in Appendix 2 to Volume 2 (hereinafter denoted as Volume 2-Appendix 2) of STD-B24 and TR-B15 for the digital satellite broadcast to match the actual receiver functionality. This restriction defined in Volume 2-Appendix 2 is the core profile of STD-B24 and is introduced later in this paper. Therefore, it is necessary to read the main body of STD-B24 Volume 2 and Appendix 1 with the restriction of Appendix 2, including the operational guideline defined in TR-B15, to understand actual datacasting standards for satellite digital broadcast. Meanwhile, digital terrestrial broadcasting operational guideline for fixed reception is different from that for mobile reception. For the fixed reception, the relevant datacasting standards are STD-B24 Volume 2-Appendix 1 with the restriction of Appendix 2 and additional operational guidelines specified in TR-B14. In the case of mobile reception, they are STD-B24 Volume 2-Appendix 1 with the restriction of Appendix 4 and additional operational guidelines specified in TR-B14. Terrestrial handheld receivers are equipped with mobile phones. The mobile phone with digital terrestrial broadcast receiver is able to receive not only television, sound, and datacasting but also the data from a telecommunication network like conventional mobile phones. HORI AND DEWA: JAPANESE DATACASTING CODING SCHEME BML 313

3 Example 1. Event control tag. In the case of digital satellite sound broadcasting, it is a similar service to digital terrestrial television broadcasting mobile reception, but with slightly different operation. The relevant standards for this are STD-B24 Volume 2-Appendix 1 with the restriction of Appendix 4 and additional operational guidelines specified in TR-B13. In the case of digital satellite sound broadcast (the service started in 2004), the service is targeted to mobile reception and the relevant standards are STD-B24 Volume 2-Appendix 1 with the restriction of Appendix 5 and additional operational guidelines specified in TR-B26. A. Extension of Tag Sets BML includes not only the tag sets defined in XHTML but also the elements defined for the broadcast applications. The major elements that were extended are explained below. 1) Event Control: Event control means that the event message in DSM-CC [6] sent from the station is detected at the receiver side and controls the BML content. This event control enables the receiver to synchronize television image to datacasting and provides new functionalities. It is possible to control the timing of the datacast synchronized to television images dynamically using this event control. In addition, it is possible to signal the end of the program, the data updated in the transmission, and the conditions of the image and sound. The extensions for handling broadcasting service events are described below. bevent element: This element contains beitem elements to be processed in the document. beitem element: This element describes mapping between an individual event and a procedural description. The principal attributes of the beitem element are defined as follows. type attribute This attribute specifies the type of an event. onoccur attribute This attribute specifies character strings describing a procedure that is executed when an event has occurred. subscribe attribute This attribute specifies the availability of the event. The only available value of this attribute is subscribe. When a value other than subscribe or no value is set, the occurrence of this event must be ignored. Example 1 shows a case for the reception of an event message. The event message id is specified by URI: arib:// If the event message occurs, then the function f() specified by the onoccur attribute is executed. The EventMessageFired type attribute can be set to various kinds: ModuleUpdated, which signals the module update in the carousel; Timefired, which signals the timer fired at the scheduled time; CCstatusChanged, which signals the status change of the subtitle language selected or the change of the presentation for the subtitle; and so on. More than one beitem can be defined in a bevent and processed asynchronously. It is possible to use the event message in a live program to enable the linkage between video and datacasting because event message can be sent in real time. The largest difference from Web content is dynamically controlling the content. That is the reason this event control tag is necessary for datacasting. 2) Extension for Monomedia Presentation: The following attributes are for controlling the continuation of monomedia presentations between BML documents. remain attribute This attribute is added to the definition of the object element. The only possible value of this attribute is remain. When this value is specified, the documents share the objects. During transition between documents, the presentation states of the shared objects are retained. The attribute values of the object element and the CSS properties in a document before transition are inherited by the object element after transition. This feature enables continuous playback of video and sound between documents. The same set of the id attributes of the object element for which this attribute has been specified must be applied to both the document before transition and the document after transition. In the case of ordinary Web pages, if video and sound are defined by the object element, the video and sound are interrupted in the page transition. It is no problem in the case of the Web because it is necessary to do so to show the next page. In the case of datacasting, however, the same video and sound may continuously exist in multiple documents even though video and sound are defined by a different object element in each document. Therefore, the remain attribute of the object element is necessary in datacasting. Example 2 shows example BML using the remain attribute. In this case, sample.png is presented continuously before and after the transition of BML documents if the both documents describe the same object element below. 3) Extended Attributes for Controlling Stream Playback: The following attributes are added to the definitions of the object element. streamstatus attribute This attribute specifies the state of a stream. The possible values are stop, play, and pause. Note that the value stop for an audio stream means mute instead of stop. 314 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

4 Example 2. Remain attribute. Example 3. Attributes for controlling stream playback. streamposition attribute The attribute indicates a current time position during stream playback. When the stream is pause, the current value is constant. When the stream is stop, it is set to zero. When the stream is play, its value is represented with the unit of time defined for each media type. streamlooping attribute This attribute specifies the number of times to loop the stream playback. This attribute is not implemented in the core profile. streamspeednumerator and streamspeeddenominator attributes These attributes specify the speed of stream playback. These attributes are not implemented in the core profile. streamlevel attribute This attribute specifies the volume of the stream. This attribute is not implemented in the core profile. streamstartposition, streamendposition attributes The streamstartposition attribute specifies a start time position to start playing a stored stream. These attributes are not implemented in the core profile. Example 3 shows the audio file is sample.aiff, the play back speed is 1/2, the number of loops is ten times, and the audio level is the volume set by the receiver. Note that the initial streamstatus is stop the audio never starts without changing streamstatus through a script. 4) Extension for Switching On and Off Display of a Whole Document: invisible attribute This attribute is added to the definition of the body element. The only available value for this attribute is invisible. When the invisible attribute is specified, no elements and background of a BML document must be displayed irrespective to the specification of CSS characteristic of each element. When the service has video and audio streams selected in an EPG or other built-in receiver application, these streams must be played without displaying BML document. This attribute controls only the display state of a document. Even if this attribute is specified, an event occurs and a procedural description is executed. This attribute is added to control the screen when pushing the datacasting button to datacasting mode. It is also possible to obtain the same function by changing the visibility property of all the elements to invisible. This kind of invisible control often appears in datacasting, and it is convenient to define this because this attribute controls the whole screen. In the case of the Web, it is meaningless to make whole screen invisible. Therefore, this attribute is not necessary in the Web. B. Extension of Cascading Style Sheet The style sheet is based on CSS2 but it is extended for broadcast purposes as follows. color-index/background-color-index/ outline-color-index property An index color is specified using an index number <color-index> of the color lookup table (CLUT). The index number is an integer greater than or equal to zero. The following extended properties are used to set CLUT. clut property This property specifies the resource of the color map data to be set to the CLUT using a URI. If no clut property is specified explicitly, the receiver s default CLUT setting, which is defined in an operational standard, is retained. resolution property This property specifies the resolution of a text graphic plane. display-aspect-ratio property This property specifies the displayed aspect ratio of a text graphic plane. greyscale-color-index property This property specifies grayscale colors with index colors. The leftmost property indicates a color nearest to the foreground color. For example, in the case of four-step grayscale, two colors (except foreground and background) are specified using this property. This property is use for border color of the fonts. It is necessary to avoid aliasing at the edge of fonts because there is no anti-alias function in the receiver. 1) Properties for Exclusive Control With Remote Control Keys: used-key-list property This property specifies the type of remote control keys to be accepted by a BML browser. For example: When used-key-list is set to basic, a numeric key entry is interpreted as a channel selection with no effects to the operation of the BML document. However, HORI AND DEWA: JAPANESE DATACASTING CODING SCHEME BML 315

5 Example 4. Navigational properties. Fig. 3. Focus behavior. when used-key-list is set to numeric-tuning and the input element has the focus, a numeric key entry is interpreted as a numeric entry to the input element, not as a channel selection. 2) Properties for Describing Remote Control Operations: The following properties provide the extension necessary to describe focus movement operations with a remote controller. nav-index, nav-up, nav-down, nav-left and nav-right property These properties are used to control the direction of focusing. These are used with the tags for showing focus such as div or object element and control the movement of focus when users push the button of the remote controller. Example 4 shows an example of these navigational properties. Fig. 3 shows the state transition when a user pushes the button of the remote controller. The squares show the objects such as the still picture defined by the object element. Using nav-index properties, it is possible to control the direction of the focus between the object elements up, down, left, and right. In the case of tab-index element defined by XHTML the focus direction is defined by the occurrence order of the elements of the documents and it is not simple to define the operation shown in Fig. 3. Especially in the point id, when a user pushes the down button, the direction to go is defined to be the upper left object. Otherwise, it is difficult to decide the direction to go at the receiver side. This property enables BML content to work exactly how a content creator wants. C. Script Extensions The script is used to change the tags or the style sheet to control BML content dynamically. Script written in EC- MAScript provides this control. BML content is controlled by the DOM methods. Some attributes and properties are added to BML but basically this is similar to Web page script. The script must process required functions for broadcast. This process mainly depends on the receiver s function and cannot be processed with programs. Receivers are equipped with the functions. It is possible to use these functions without explicitly defining them. Sophisticated BML content cannot be produced without using some of these functions. Since Version 3.2, all the tag sets in STD-B24 completely conform to the XHTML standard. Therefore, it is possible to produce compatible BML content for the Web. These functions, however, have no compatibility with Web content. Some typical functions are introduced below. 1) Binary Table Functions: The binary table object is added to the built-in object in the script. This works as a matrix, and 1 bit is used for separating the data and the type of stored value is represented by the character. Therefore, it is very efficient to store the data. And it is suitable to store large-scale matrix data. Built-in functions to read and write binary table and search data are available in the script. It is therefore possible to control binary tables by these functions. 2) Nonvolatile Memory Functions: Receivers are normally equipped with nonvolatile memory. Functions to read and write the memory are defined. These functions are not for disk files but to some blocks in the receiver memory. The usage of this nonvolatile memory depends on the broadcaster. Each broadcaster is allowed to write and read only assigned blocks in the nonvolatile memory. 3) External Character Functions: This function is added to define external characters in the document. This function loads external character data coded with a dynamically redefinable character set (DRCS). D. STD-B24 Volume 2-Appendix 2 (Core Profile) As described before, the actual operation of datacasting for satellite and terrestrial digital broadcast is a subset of STD- B24. These restrictions are summarized in Volume 2-Appendix 2 for fixed reception and in Appendix 4 for mobile reception (satellite mobile reception is Appendix 5). This restriction is applied to the tag sets, style sheet, and scripts described above. This section shows principal Appendix 2 restriction. 1) Tag Sets Restriction: A variety of restrictions are applied to the tag set. It is limited to a small set of elements to minimize the size of the browser in the receiver. The 16 elements used in Appendix 2 are div, p, span, br, a, input, object, meta, title, script, style, body, head, bml, bevent, and beitem for satellite operation. Attributes are also limited but details are not described in this paper. In addition, the following restrictions are applied to tag sets scripting. All top elements which are displayed must have a containing box for their positioning. Namely, only a child element div or p can be set in body element. 316 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

6 bml element consists of one head element and one body element. Both elements are mandatory and head must appear before body. title, meta, script, style and bevent elements appear only once according to this sequence in head element. head is mandatory but other elements are optional. All of the child elements of script and style must be in CDATA section. bevent element must contain more than one beitem element. body element must contain div or p element. div element must contain boxed elements. p element must be PCDATA text or normal flow element. Boxed element is with positioning and either div, input, object, or p element. Normal flow element is without positioning and is either a, br, or span element. 2) Style Sheet Restrictions: The style sheet is also restricted. Details are not described here but it is limited to minimum operation required. The following restrictions are applied to keep the unique presentation. The Boxed element must be specified with absolute positioning. The nav-index property must be set to all focused object for remote controller. 3) Script Language Restrictions: The following restrictions are applied in script language. Number must be 32 b. Note that any number object must also be 32 b. Float must not be supported. Math built-in object must not be supported. EUC-JP must be used as a character-coding scheme of character string data. The run-time interruption of a script character string must not be supported. The eval() function of the global object is not supported. E. Transmission of BML Documents DSM-CC [6] is employed to transmit a BML document and monomedia referred by a BML document. Data carousel system is easier to control the module and needs less overhead than object carousel system in reception. Each module transmitted by data carousel is set in accordance with the entity format defined in HTTP/1.1 of the Internet Engineering Task Force (IETF) RFC 2068 [7]. This entity consists of an entity body to store a BML document and monomedia and an entity header to describe metadata such as file size. The Multipurpose Internet Mail Extension (MIME) multipart type is used to package multi-bml documents or a set of mono-media in one module. Unlike MIME defined by RFC 2045 [8], it is not necessary to convert binary data into text format in entity format defined in HTTP/1.1. Therefore, binary data such as still picture data can be stored as they are without any change to the format. IV. CONCLUSION The BML coding scheme for multimedia datacasting is introduced in this paper. There are several datacasting coding schemes in the world. BML is widely used in Japan for most of datacasting services, and more than 5 million TV sets have already implemented the BML browser. The BML standard has been commonly used for different services, and STD-B24 is now Version 4.3 in order to reflect various requirements of these services. It also has been revised when unforeseen problems or ambiguous expressions were found. STD-B24 is a well-refined standard now. A possible revision to include a recordable receiver function is now under study. REFERENCES [1] Data coding and transmission specifications for digital broadcasting, ARIB STD-B24, Association of Radio Industries and Businesses (ARIB), [2] XHTML 1.0: The Extensible HyperText Markup Language, a reformulation of HTML 4.0 in XML 1.0, W3C Recommendation, Jan [3] Cascading style sheets, Level 2 CSS2 specification, W3C Recommendation, May 12, [4] Standardizing information and communication systems standard, 2d ed., ECMAScript Language Specification ECMA-262, Aug [5] Document object model (DOM) Level 2 core specification version 1.0, W3C Recommendation, Nov. 13, [6] Generic coding of moving pictures and associated audio information Part 6: Digital storage media command and control, ISO/IEC , Standards Org./Int. Electrotech. Comm.. [7] R. Fielding et al., Hypertext Transfer Protocol HTTP/1.1, MIT/LCS, Univ. California, Irvine, RFC 2068, Jan [8] N. Freed and N. Borenstein et al., Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies RFC 2045, Innosoft, First Virtual, Nov [9] W. Kameyama, T. Hanamura, and Y. Dewa et al., Digital Broadcast Textbook Revised Version vol. 1 MPEG-1/2/4-H.264/AVC (in Japanese). Tokyo, Japan: Impress, 2004, ch. 8, pp Akihiro Hori received the B.S., M.S., and Ph.D. degrees in electronics engineering from University of Electrocommunications, Tokyo, Japan. He was with Hitachi Central Research Laboratory, where he developed a fiber-optics system and telecommunication equipment. He is currently a Chief Engineer of the Technology Research and Development Department, Engineering and Technology Division, Nippon Television Network Corporation (NTV), Tokyo. He developed the world s first digital watermarking system and progressive scan television system. Dr. Hori is a Fellow of the Society of Motion Picture Television Engineers (SMPTE) and Chairman of ARIB Datacasting Coding Working Group and the Datacasting Program Exchange Working Group. Yoshiharu Dewa received the B.S. and M.S. degrees in information engineering from Hokkaido University, Japan. In 1993, he joined Pioneer Corporation Corporative Research and Development Laboratory, and researched multimedia encoding. In 1998, he joined Sony Corporation, where he worked on music contents distribution by communicate satellite broadcast. Through a series of standardization of international standards for multimedia encoding schemes [SC29WG12(MHEG)], he currently researches broadcasting and networks. Mr. Dewa is a member of the Association of Radio Industries and Businesses (ARIB) data broadcasting working group, a chairman of application execution engine task group, and a subchairman of the presentation engine task group. He is also a member of the Advanced Integrated Document Investigate Committee, Japanese Standards Association; a member of the C#/CLI Japan Industrial Standards Drafting Committee, Information Processing Society of Japan/Information Technology Standards Commission of Japan; and a member of Japan Society for Software Science and Technology. HORI AND DEWA: JAPANESE DATACASTING CODING SCHEME BML 317