Fig. 1 Example of application and transmission scheme
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The final draft standard for DTTB (Digital Terrestrial Television Broadcasting)
in Japan is based on the ISDB-T (Terrestrial Integrated Services Digital
Broadcasting) system. The ISDB-T system is applicable to both DTTB and
DTSB (Digital Terrestrial Sound Broadcasting) services in a similar
manner. Some examples of digital terrestrial broadcasting services are
given here.
Outline of ISDB-T
The ISDB-T system can construct transmission signals with various bandwidths
(from a narrow-band signal to a wide-band signal) by combining narrow-band
signals called "OFDM segments" as shown in Figure 1. The frequency bandwidth
of an OFDM segment is about 430 kHz. Program data is divided into data
segments, which are processed individually and transformed into the
transmission signals shown in Figure 1. A Hi-Vision program or several
SDTV programs can be accommodated within 13 segments and the bandwidth
of this signal is 5.6 MHz. DTSB services can be transmitted using 1
or 3 segments. Because the signal structure of each segment is the same,
a DTSB receiver can demodulate and decode the center segment of the
DTTB signal. ISDB-T has several unique features, such as stable mobile
reception and common receivers for DTTB and DTSB.
Future Issues
To implement digital terrestrial broadcasting systems in the congested
frequency environment in Japan, it is essential to make effective use
of frequency. From that viewpoint, a single frequency network (SFN)
is considered to be suitable. In an SFN, several transmitters cover
a wide service area using a single frequency. NHK will continue research
and development of digital terrestrial broadcasting networks to achieve
sufficiently wide coverage, including the development of distribution
schemes of broadcasting signals to the transmitters and stations throughout
the country.
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