ISDB-C: Cable Television Transmission for Digital Broadcasting in Japan

Transcription

1 ISDB-C: Cable Television Transmission for Digital Broadcasting in Japan SATOSHI TAGIRI, YOSHIKI YAMAMOTO, AND ASASHI SHIMODAIRA Invited Paper Following the growing digitalization of broadcasting, Integrated Services Digital Broadcasting for Cable (ISDB-C) digital broadcasting over cable TV networks has been increasingly adopted in Japan. Cable TV transmission technologies in Japan can be classified into three types: 1) the transmodulation system, which consists of retransmission of received broadcast signals after converting their modulation; 2) the pass-through system, consisting of retransmission of received broadcast signals without changing their modulation but converting only the frequency as required; and 3) the ReMux system, which consists of first demodulating the received broadcast signal, then combining services before retransmission. In this paper, we describe the multiple TS transmission system and TS division system standardized by the Japan CATV Engineering Association and Japan Cable Laboratories, as well as two-way operation and the headend in the sky service, which is a commercial program distribution service launched in Japan for cable operators. Keywords Cable television, digital broadcasting, MPEG-2 systems, retransmission, transport stream. I. OVERVIEW Japanese cable television (CATV) services have recently evolved into establishments capable of multichannel, two-way communication. By retransmitting these digital TV programs, CATV services had increased their subscriber base to million as of March 2004, accounting for 33.6% of all households, showing that Integrated Services Digital Broadcasting for Cable (ISDB-C) digital broadcasting over CATV networks has been seeing growing takeup. A 64-QAM system was established for the first time in 1996 as a digital transmission system for CATV for retransmission of CS digital broadcasts (124-degree and 128-degree CS). This system consisted of transmission of a single MPEG-2 transport stream (TS) using one carrier The authors are with the Matsushita Electric Industrial Co. Ltd., Osaka , Japan ( tagiri.satoshi@jp.panasonic.com; yamamoto.yoshiki@jp.panasonic.com; shimodaira.asashi@jp.panasonic.com). Digital Object Identifier /JPROC Table 1 Classification of Transmission Systems (hereinafter referred to as a single-ts transmission system); its information rate was about 29 Mb/s using the 64-QAM system with a bandwidth of 6 MHz. Since CS digital broadcasting provides multichannel SDTV broadcasting services, conventional analog video transmissions converted from digital video more or less satisfied retransmission requirements. Consequently, the single-ts transmission system was rarely adopted. BS digital broadcasting, launched in December 2000, on the other hand, provides digital HDTV combined with data services. CATV retransmission after analog conversion cannot provide the same quality of service as original BS digital broadcasting. Since the retransmission of BS digital broadcasting is a useful service provided by CATV, a retransmission scheme using a digital transmission system is essential for CATV. BS digital broadcasting transmits multiple TSs in one transponder at the maximum information rate of 52 Mb/s. To accommodate this transmission scheme, a new digital transmission system (a multiple TS transmission system) for CATV has been developed and standardized. The most important feature of this system is that it enables CATV to transmit multiple TSs over a single CATV carrier. To achieve the ability to handle multiple TSs, the transmission technologies of digital broadcasting of CATV in Japan can be classified into the three types, shown in Table 1. The transmodulation system involves converting the modulation signal of BS or terrestrial broadcasts into the /$ IEEE PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY

2 Fig. 1. Digitization of broadcasting for CATV. 64-QAM format, which is the most suited for transmission over CATV networks. The above-mentioned multiple TS transmission system is a technique for improving the efficiency of frequency usage by means of a transmodulation scheme. The pass-through system retransmits BS and terrestrial digital broadcast signals over CATV without changing their modulation. The ReMux system, which realizes a combination of services, including the retransmission of digital broadcast signals and in-house-produced programs, has been developed by Japan Cable Laboratories (JCL), and comprises the retransmission of BS digital broadcast programs using the transmodulation system. Fig. 1 shows an overview of digitization of broadcasting for CATV which has so far been developed by JCL. Table 2 64-QAM System II. TRANSMODULATION SYSTEM As mentioned earlier, the transmodulation system uses the 64-QAM system [1] [3] and the multiple TS transmission system [4], [5] realizes digital broadcasting systems for CATV. Table 2 shows an outline of the 64-QAM system. The 64-QAM system is compatible with the analog signals of the existing VSB-AM system, since it is designed to allow transmission of analog TV signals on adjacent channels [6], [7]. The technical conditions for signal quality are shown in Table 3. A. Transmodulation for BS Digital Broadcasting Using the 64-QAM/6 MHz system, the information capacity of one carrier is about 29 Mb/s, as shown in Table 2. This is approximately half the transmission rate per carrier of BS digital broadcasting. Therefore, at least two CATV channels are required to retransmit programs carried by one BS digital broadcasting signal. In addition, considering that the BS digital broadcasting system transmits multiple TSs whose capacities are currently regulated by operation guidelines to less than 29 Mb/s each, 304 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

3 Table 3 Technical Conditions for 64-QAM Signal Quality (Essentials) Table 4 Information Written in the Multiplex Frame Header Fig. 2. Multiplex frame structure. its retransmission using the single-ts transmission system will not always satisfy the requirements of CATV system operators. Since in this case a TS with a low bit-rate occupies one CATV channel, the system might use CATV frequency resources inefficiently. Furthermore, no modification without NIT is permitted to BS broadcast TS. To solve this problem, a new system was developed by expanding the single-ts transmission system to accommodate multiple TSs over one CATV channel. When retransmitting BS digital broadcasting programs, the TSs from each BS digital broadcasting signal is divided into two groups. Each TS group is then transmitted through one CATV channel, or 64-QAM/6 MHz channel, using the multiple TS transmission system. The retransmission of multiple TSs over one CATV channel can be applied to any digital broadcasting programs as well with TS capacity of 29 Mb/s at most so as to efficient transmission through CATV networks. This transmission system introduces multiplex frames for handling multiple TSs, as illustrated in Fig. 2. A multiplex frame consists of 53 slots with the header packet located in the first part. This packet contains the information required for multiplexing/demultiplexing, as shown in Table 4. The set-top box (STB) reads this multiplex frame header information to de-multiplex the desired TS. Current standards allow 15 TSs at most to be multiplexed. Each broadcast TS is uniquely identified by the original_network_id and transport_stream_id (TS_ID) pair. More precisely, each TS packet is identified based on two levels of information: the first level is information on the correspondence between the original_network-id/ts-id pair and relative TS number, and the second level is information on the correspondence between the relative TS number and actual slot. This system requires fewer bits than would be required for directly identifying the TS packet in a slot using the original_network-id and TS-ID pair. The header has the TS packet format, so that multiplexed signals can be processed in the same way as the TS packet array, which enables the same channel coding as with the single-ts transmission system in Fig. 3. The technologies and standards that have been developed for the single-ts transmission system can therefore be applied [8] to minimize the need for new technology development and thus reduce the cost of commercial systems. Multiple TS transmission systems were permitted by ministerial legislation [9] in August 2000 as a digital transmission system for Japanese cable TV broadcasting, followed by industrial standardization at the Japan Cable Television Engineering Association (JCTEA) [10]. The operating specifications [11] of the BS transmodulation system described above were developed by JCL, and the actual system operation was launched by 47 CATV operators in December of the same year. B. Transmodulation for Wide-Band CS Wide-band CS digital broadcasting services, using a communications satellite at 110 east geosynchronous orbit (N-SAT-110), were inaugurated in July To retransmit the wide-band CS digital broadcasting programs over CATV networks, the same transmodulation system as for BS digital broadcasting is used. However, the transmodulation system for BS digital broadcasting programs could not be applied TAGIRI et al.: ISDB-C: CABLE TELEVISION TRANSMISSION FOR DIGITAL BROADCASTING IN JAPAN 305

4 Fig. 3. Relationship between multiple and single TS transmission systems. Fig. 4. The concept of TS dividing. without modifying the TS signal, since wide-band CS digital broadcasting was based on one TS per carrier and the information quantity per TS exceeds the transmission capacity of 64-QAM/6 MHz, which is about 29 Mb/s. Therefore, by partially modifying the BS transmodulation system, the TS division system was introduced, which adopts a dividing method of one TS of the wide-band CS digital broadcasting program into two TSs based on targeted services using one CATV transmission channel for each. In the example shown in Fig. 4, the TS that transmits five TV programs is divided into two groups based on targeted services. In other words, Programs 1, 2, and 3 are assigned to one CATV channel and Programs 4 and 5 are assigned to the other. Common information (PSI, SI, EMM, etc., excluding NIT) of the original programs is transmitted unmodified on both channels. NIT is replaced by a CATV version in the same way as has been applied to the BS digital broadcasting system and is inserted into both CATV channels. Thus, all the wide-band CS digital broadcasting program services can be retransmitted over the CATV network. The transmission system for the TS division system conforms to the multiple TS transmission system described in the section on the transmodulation system for BS digital broadcasting services. A newly defined cable TS division system descriptor describes the transmission parameters, including the frequency of the CATV channel on which the divided TS is transmitted, and broadcast contents contained in the relevant channel. The STB can identify whether the TS division system is being used or not by the presence/absence of the cable TS division system descriptor in NIT. (See Fig. 5.) JCTEA approved the industrial standards for the TS division system and cable TS division system descriptor, after ministerial legislation for this system in July JCL developed the operating specifications of the transmodulation system for wide-band CS digital broadcasting using the above-mentioned system. C. Transmodulation for Terrestrial Digital Broadcasting In response to the launch of terrestrial digital broadcasting at the end of 2003, JCL developed the operating specifications of the transmodulation system for terrestrial digital broadcasting in February The signal transmission rate of terrestrial digital broadcasting is lower than that of a CATV channel, so the signal from a broadcasting station can be retransmitted without 306 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

5 Fig. 5. Cable TS division system descriptor. Fig. 6. Frequency conversion of pass-through system. modification via a CATV network. Because part of the Transmission and Multiplexing Configuration Control (TMCC) in terrestrial broadcasting systems such as the emergency alarm signal needs to be transmitted over the CATV network, the multiple TS system is adopted in the same way as for the BS digital broadcasting system. In conformity to the detailed operation guidelines provided in the operating specifications for the retransmission of terrestrial digital broadcasting, the emergency alarm signal is accommodated in the multiplex frame header of the multiple TS transmission system. In addition, to ensure efficient CATV system operations, a function that commands the STB to scan the frequency from the CATV headend was also standardized. This allows CATV operators to change/add/delete transmission frequencies without requiring CATV subscribers to adjust their STBs. Table 5 Comparison of Pass-Through System Operation III. PASS-THROUGH SYSTEM A. Pass-Through System for BS Digital Broadcasting The simplest system for retransmitting the BS digital broadcasting programs over CATV networks is to transmit the BS IF signals (1 1.3 GHz) from the BS antenna in unmodified form. However, because the 1-GHz band signal cannot be transmitted over CATV networks, it needs to be down-converted at the headend and restored to the original frequency band by using an upconverter in each household (Fig. 3) [12]. The characteristics of this system are as follows. 1) The equipment is inexpensive, which achieves lower initial cost. 2) The same tuner as for direct receiving can be used in conventional TV receivers in the market. 3) More CATV channels are required than for the transmodulation system, because one BS channel needs a bandwidth of 34.5 MHz. TAGIRI et al.: ISDB-C: CABLE TELEVISION TRANSMISSION FOR DIGITAL BROADCASTING IN JAPAN 307

6 Fig. 7. ReMux operating system for digital broadcasting. The bandwidth needed for retransmitting four BS digital broadcasting IF signals is able to carry eight HDTV programs for the transmodulation system, while it is able to carry about 30 SDTV programs using the present pass-through system. Therefore, this pass-through system is suitable for CATV system operators with numerous vacant CATV channels who wish to retransmit the BS IF signals at lower cost. This system has also been standardized by the JCTEA [13]. At present, this pass-through system is not very popular, since the transmodulation system provides better transmission efficiency. (See Fig. 6.) B. Pass-Through System for Terrestrial Digital Broadcasting Terrestrial digital broadcasting uses OFDM with a bandwidth of 6 MHz. The bandwidth of the CATV channel is also 6 MHz, so the OFDM signal may be retransmitted without changing its modulation. It is also possible to retransmit terrestrial digital broadcasting programs using the multiple TS transmission system in the same way as for BS digital broadcasting, which achieves the same frequency efficiency as the pass-through system. There are two types of pass-through system: 1) same-frequency pass-through operating system; 2) frequency-converted pass-through operating system. (See Table 5.) IV. THE REMUX SYSTEM The ReMux system realizes digital transmissions based on the same 64-QAM system as the transmodulation system. The difference is that the digital broadcasting TS received at the CATV headend is retransmitted without any change to the transmodulation system, while in the ReMux system, new TSs are created from the received TSs. To satisfy the demands of CATV operators for provision of autonomously edited/controlled digital broadcast programs to their subscribers, JCL collated the requests of CATV operators before reviewing the basic operating requirements, after which they established the ReMux Operating Specifications for Digital Broadcasting in October Fig. 7 shows an operating system using these specifications that allows CATV operators to use the content from SKYPerfecTV! and in-house-produced programs and to provide digital broadcasting services based on their own compilations. The headend equipment is composed of three subsystems: the SI/EPG subsystem, the MUX subsystem and the CA subsystem, whose interfaces are defined as standard interfaces to achieve multivendor conditions for these subsystems. For the CAS subsystem, C-CASs complying with the JCL specifications are provided by three vendors, so that CATV operators can select a CAS vendor for their C-CAS. The C-CAS is capable of controlling (based on ARIB STD-B25) viewing under a monthly flat-rate contract on a service basis, 308 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

7 Table 6 Examples of Services That Can Be Provided According to the ReMux Operating Specifications entitlement, management, and messaging (EMM) for transmitting s, power supply control, parental control, etc., to individual STBs. Examples of services that can be provided according to these ReMux operating specifications are shown in Table 6. V. HITS SERVICE A. i-hits Digital Broadcast ReMux This is a system designed to realize a ReMux system at lower cost by using the i-hits (headend in the sky) service, which comprises commercial program distribution to CATV operators (consisting of compilation, digitization, and multiplexing of programs, in addition to electronic program guide (EPG) information and their distribution to CATV stations via an FSS (fixed satellite service) link. By using this system, the head-end equipment can be simplified, using only PSI conversion equipment instead of the SI/EPG and MUX subsystems needed for ReMux. B. JC-HITS Transmodulation This system involves transmitting the program distribution service for CATV operators by JC-HITS using the transmodulation system. JC-HITS has three sets of C-CAS equipment in its digital headend center, which provide programs to CATV operators, who distribute programs, EPG information, etc., to CATV operators via an FSS link simultaneously by using multiple CASs. The transmodulation system makes it possible to configure headend equipment to a similar size to that for BS digital broadcasts or wide-band CS digital broadcasting. This system is also capable of providing value-added services based on pay per view (PPV), which are not specified for the ReMux system. TAGIRI et al.: ISDB-C: CABLE TELEVISION TRANSMISSION FOR DIGITAL BROADCASTING IN JAPAN 309

8 Fig. 8. Example of configuration of two-way digital broadcasting system. VI. TWO-WAY OPERATION In addition to digital broadcasting services, provision of various services using two-way capability is required via CATV networks. JCL has been developing the operating specifications needed for constructing a two-way platform that is compatible with current digital broadcasting services. An example of a two-way digital broadcasting system is illustrated in Fig. 8. Its operation covers STB maintenance function for two-way STBs incorporating a cable modem, PPV function and two-way applications. Each of the functions provided by the two-way digital broadcasting system assumes the following two-way services. 1) Maintenance/management function Maintenance-related services performed by the center to monitor the operating conditions of STB through collection of information on STBs; and management services consisting of the provision of viewing data by collecting information on viewing conditions from STBs. 2) PPV function PPV programs transmitted from the ReMux head-end; or PPV program service using the JC-HITS service. 3) Two-way application function Portal services, walled-garden, Internet access and Web mail services provided by the operator to be used from the STB. VII. CONCLUSION We have discussed CATV transmission technologies for digital broadcasting with the central focus on those standardized by JCL. These technologies were first introduced for retransmission of BS digital broadcasting and, more recently, have been applied to the retransmission of terrestrial digital broadcasting and other services, thus accelerating the digitalization of cable television services. Currently, for cable television services in Japan, systems are being considered that will be able to integrate various services centering on digital broadcasting, including those utilizing its bidirectional capabilities. Further technological developments are expected in the days ahead which will rapidly bring these systems. REFERENCES [1] Digital Multi-Program Systems for Television, Sound and Data Services for Cable Distribution, ITU-T Recommendation J.83. [2] Postal Administration Ministerial Ordinance, (in Japanese) no. 74, [3] Japan Cable Television Engineering Association Standard (in Japanese), JCTEA STD-002, Japan Cable Television Engineering Association (JCTEA). 310 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

9 [4] T. Kurakake et al., A Method of Multiplexing MPEG-TSs in Cable TV Systems, Inst. Image Inf. Television Eng. (ITE) Tech. Rep. vol. 23, no. 48, pp. 7 12, [5] Time division multiplexing of multiple MPEG-2 transport streams over cable television systems, ITU-T Recommendation J.183. [6] T. Noda et al., Study of system requirements for digital cable television with experiments, J. ITE, vol. 51, no. 9, pp , [7] M. Maeda et al., Adjacent channel interference between 64 QAM signals discussed by telecommunication technology council and analog modulation signals cable television conference working group reports No. 2, (in Japanese) Inst. Television Eng. Jpn. vol. 20, no. 35, pp , [8] N. Nakamura et al., Experiment results of multiple MPEG-2 transport streams transmission in cable TV network, in ITE Winter Annu. Conf , p [9] Postal Administration Ministerial Ordinance, (in Japanese) no. 51, [10] Multiplexing for digital cable television (in Japanese), JCTEA STD , Japan Cable Television Engineering Association. [11] The operating specifications of the trans-modulation system for BS (in Japanese), JCL SPEC , Japan Cable Laboratories (JCL). [12] The Operating specifications of the trans-modulation system for 110-degree CS (in Japanese), JCL SPEC , JCL. [13] JCL, JCL SPEC : The Operating Specifications of the Trans-Modulation System for Digital Terrestrial (in Japanese). [14] T. Shibata et al., Frequency downconversion transmission method of BS digital signals for small and medium-sized cable television systems, ITE Tech. Rep. vol. 23, no. 48, pp , [15] BS digital broadcasting pass-thru system for cable television, JCTEA STD , Japan Cable Television Engineering Association. [16] T. Kurakake, Transmission Technology of Digital Broadcasting on CATV, (in Japanese) NHK STRL R&D, no. 70, Nov Satoshi Tagiri received the B.S and M.S degrees in applied physics from Osaka City University, Osaka, Japan, in 1987 and 1989, respectively. Since 1989, he has been with AVC Networks Development Center, Matsushita Electric Industrial Co. Ltd. Osaka, where he is currently Staff Engineer. He has been working with cable television transmission technology for digital broadcasting. Yoshiki Yamamoto received the B.E degree in electrical engineering from Mie University in Since 1991, he has been with AVC Networks Development Center, Matsushita Electric Industrial Co. Ltd., Osaka, where he is Staff Engineer. He has been involved with the development of digital broadcasting systems. Asashi Shimodaira received the B.E degree in mechanical engineering from Waseda University in Since 1993, he has been with AVC Networks Development Center, Matsushita Electric Industrial Co. Ltd., Osaka, where he is Staff Engineer. He has been working on cable television transmission technology for digital broadcasting. TAGIRI et al.: ISDB-C: CABLE TELEVISION TRANSMISSION FOR DIGITAL BROADCASTING IN JAPAN 311