Characteristics of the Japanese Digital BS Broadcasting System

Present condition of digital broadcasting in Japan
Digital satellite television broadcasting began for the first time in the United States in 1994, followed by Europe and Asia around 1996. Japan also started digital CS broadcasting using communication satellites (CS) in 1996. These satellite digital broadcasts provide mainly multi-channel standard definition TV services.
As for the technological development of digital satellite broadcasting using broadcast satellites (BS), the NHK Science & Technical Research Laboratories initiated the research for ISDB (Integrated Services Digital Broadcasting) around 1982. Thorough examinations were carried out from the perspective of efficient use of the limited frequency resources allotted to broadcasting. In the process, a new concept for broadcasting services, not simply replacing conventional analog services with the digital format, has been developed that makes the best use of the merits of digitization; the flexibility of service arrangements inherent to digital signals, the expansion capability for new services in the future, and the community of diverse media.
A report concerning digital BS broadcasting, which is scheduled to start in the year 2000, has been issued by the Telecommunications Technology Council, and technical systems such as transmission/multiplexing systems and video/sound encoding formats finalized. The ARIB (Association of Radio Industries and Businesses), a nongovernment standardization organization, has set the SI (Service Information), and CAS (Conditional Access System)standards for data broadcasting systems.

Digital satellite broadcasting system

Figure 1: Digital BS broadcasting image
Our image of the shape of digital BS broadcasting with these characteristics is shown in Figure 1.
The chief characteristics of digital BS system are as follows;

(1) Large capacity transmission
Trellis Coded (TC) 8PSK modulation has been adopted, which is capable of a maximum 52Mbps capacity on a single 34.5MHz broadband BS broadcast channel. This arrangement enables the simultaneous broadcast of 2 HDTV and four sound broadcasts, in addition to multiple data broadcasting on only one BS channel.

(2) Hierarchical transmission service
In addition to the selection capability of the transmission format (modulation or error correction) corresponding to the contents of the broadcast service, multiple transmission schemes can be used simultaneously by one channel.
Satellite broadcasting has a tendency to be affected by heavy rain or other harsh weather conditions, resulting in a sudden service interruption. Digital broadcasting will be able to provide hierarchical transmission services with simultaneous broadcasting of both a program and a transmission system which makes broadcast interruption unlikely even with weaker signals. These hierarchical transmission services will make possible HDTV broadcasts with high-definition images during usual weather conditions, and basic broadcasting consisting of still picture/reduced video signals in heavy rain, allowing continuous viewing of a program.

(3) Multiplexing TSs on one channel
Figure 2: Example of slot transmission scheme allocation in one channel
In digital BS broadcasting, one channel is time-divided into 48 slots. One slot's transmission capacity is 1.08 Mbps. MPEG TS (Transport Stream) transmission can be carried out by utilizing several of these slots. Each digital BS channel is capable of simultaneous broadcasting that combines four different transmission schemes. An example allocation of TSs and transmission schemes into slots is shown in Figure 2.
Each digital BS channel can, therefore, multiplex multiple TSs. Every digital broadcaster will be allotted individual TSs, and optional service arrangements within each TS can be made by each broadcaster.
To enable flexibility regarding the adjustment of information amounts and the combination of transmission schemes on each TS, TMCC (Transmission & Multiplexing Configuration Control) signals are also included.

Table 1: Video and sound formats
Figure 3: Digital BS broadcasting system structure and each sector's technical characteristics
(4) International coordination encoding system
In consideration of international coordination, we adopted MPEG-2 Video for the video encoding system, and MPEG-2 Audio ACC for the sound encoding system, both having a high-compression efficiency and capable of high-quality sound broadcasting. Multiplexing also uses the MPEG-2 format. Data broadcasting has a multimedia encoding system based on the W3C standard XML. These features are designed to facilitate international digital program exchange.

Structural and technological details of the digital satellite broadcasting system with these characteristics are shown in Figure 3. Video and sound formats using digital BS broadcasting are shown in Table 1.