-Cable Television Transmission for Digital Broadcasting in Japan-
(Courtesy Asia-Pacific Broadcasting Union.)
Kimiyuki Oyamada, Takuya Kurakake and Hiroshi Miyazawa*,
(Digital Broadcasting Networks)
*Currently Reception Engineering & Service Center

Integrated Services Digital Broadcasting (ISDB) is an emerging digital broadcasting concept. With ISDB, everything is handled digitally. The three kinds of systems, ISDB-S (Satellite), ISDB-T (Terrestrial) and ISDB-C (Cable) were developed in Japan to provide flexibility, expandability and commonality for the multimedia broadcasting services using each network.
Since digital satellite broadcasting began service using ISDB-S in December of last year in Japan, we would like to introduce the three ISDB systems mentioned above. The ISDB-C system is explained in this article, as a final part in a series of three. This article is a reprint from ABU Technical Review No. 189.


Transmission of Integrated Services Digital Broadcasting services (ISDB) over cable television networks is described. Once ISDB for Satellite (ISDB-S) begins on December 1 2000 and ISDB for Terrestrial (ISDB-T) begins in 2003, cable subscribers should be able to enjoy ISDB for Cable (ISDB-C) services which include them. As re-transmission schemes for the digital broadcasting over cable, the transmodulation and pass-through system have been developed. Transmodulation converts the modulation system into 64-QAM. Pass-through transmits digital broadcasting services over cable without changing their modulation schemes. Using these methods, digital broadcasting services will be implemented by a number of cable television operators in December 2000.

1. Introduction

Cable television in Japan has recently developed toward multi-channel and bi-directional transmission. The number of subscribers reached 17.62 million or 37% of all households as of March 2000. As broadcasting satellite (BS) digital broadcasting and terrestrial digital broadcasting are close at hand, technological development of cable digital broadcasting (ISDB-C) is rapidly advancing starting from the retransmission of these cable television systems.

A digital transmission system in cable televisions that uses the 64-QAM scheme was first established in 1996. This system was standardized for the re-transmission of communication satellite (CS) digital broadcasting. This system transmits single MPEG-2 TS (hereafter single-TS transmission system). It uses 64-QAM of 6 MHz bandwidth and the transmitted information rate is approximately 29 Mbps. However, since CS digital broadcasting serves multiple channels of SDTV, a transmission system that converted digital programs into analog programs was sufficient for retransmission. For this reason, this digital transmission system was not adopted very much.

On the contrary, BS digital broadcasting combines digital HDTV and data broadcasting. It is not possible to offer services equivalent to those available in direct BS reception over cable because digital programs are converted into analog programs for retransmission. Retransmission of BS digital broadcasting is an attractive service for cable television. As such, retransmission by a digital transmission system is an essential service. For this reason, development and standardization of a new digital transmission system, the multiple-TS transmission system, has been carried out. The main feature of this system is that it transmits multiple transport streams (TSs) with a single 64-QAM carrier.

As methods of retransmission for digital broadcasting in cable television systems, the transmodulation and pass-through systems have been developed. The transmodulation system converts a modulation scheme into the 64-QAM scheme. The 64-QAM scheme suitable for the transmission of cable televisions. The first standardized system using 64-QAM was the single-TS transmission system. The multiple-TS transmission system has been expanded based on this single-TS system and both will be explained later.
The pass-through system transmits BS or terrestrial digital broadcasts to cable television without changing their modulation schemes.

2. Single-TS Transmission System

Figure 1 shows the configuration of the single-TS digital transmission system. The specification of the 64-QAM transmission system is summarized in Table 1.

Figure 1: Transmission System of Single-TS Digital Cable Television System

Table 1: Specification of 64-QAM Transmission System
Input signal
MPEG2-TS packets
Frame synchronization
Sync byte inversion for every 8 packets
PRBS (polynomial 1+X14+X15)
Reed-Solomon (204,188)
Byte unit convolutional (Depth: 12)
Given in Figure 3
6 MHz
Symbol rate
5.274 Mbaud
Transmission rate
31.644 Mbps
Information rate
29.162 Mbps

2.1 Transmission Frame Structure

The transmission frame structure is based on the MPEG-2 transport packet structure. A frame consists of 8 TS packets. The first byte of a TS packet is a sync byte (value is 0x47). The sync byte of the first TS packet in the frame is bit-wise inverted for frame synchronization.

To ensure clock recovery, data other than the sync bytes is randomized using pseudo random sequence (PRBS) generated by the polynomial shown in Table 1.

2.2 Reed-Solomon Coding
To each TS packet 16 bytes of Reed-Solomon error correction information are added to make 204-byte RS frame. When 8 error bytes exist in 1 TS packet, a receiver can correct such errors.

2.3 Interleaving
If an error occurs continuously during transmission and errors exist in more than 8 bytes of a TS packet, correction by the Reed-Solomon coding is no longer effective. As a measure for this, information is diffused in time at the transmission side and restored to the original at the reception side, to diffuse any continuous errors in time during transmission. This interleaving operation can effectively correct the error. For RS frames, convolutional interleaving is carried out. However, sync bytes are output at the position as they are without interleaving.

2.4 Byte-To-Symbol Mapping
With the 64-QAM modulation scheme, 6-bit information can be sent with a single symbol. As shown in Figure 2, three bytes (24 bits) are mapped into 4 symbols.

Figure 2: Byte/Symbol Conversion

2.5 Differential Coding And Mapping
After the two MSBs of each symbol are differentially coded, the symbols are mapped into the 64-QAM constellation as shown in Figure 3. In this mapping, rotation-invariant constellation is adopted in which four LSBs become the same values even when the signal point is turned 90, 180 or 270 degrees.

Figure 3: Constellation chart for 64 QAM

2.6 Roll-Off Factor
As seen in Figure 4, bandwidth is limited with a filter that has a 13% roll-off factor. As the symbol rate is 5.274 Mbaud as shown in Table 1, the bandwidth becomes approximately 5.9 MHz after modulation so that the signal can be transmitted within a 6 MHz bandwidth.

Figure 4: Baseband filter characteristics

2.7. Transmission Characteristics
During transmission of digital broadcasts via cable television, interference occurs with adjacent channels in addition to the deterioration of the CN ratio due to thermal noise. Because of this, signal quality after transmission is specified as follows.

(a) CN Ratio
The CN ratio is specified at 31 dB or more (noise bandwidth of 4 MHz), where the bit error rate is 10-4 or less without error correction, and 10-9 or less with error correction.

(b) Adjacent Channel Interference
The QAM signals are frequency-multiplexed together with the analog VSB-AM signals. As digital signals generally have stronger resistance against interference, they are transmitted at a lower power level. If an analog signal is adjacent to the lower frequency of a digital signal, the digital signal is transmitted with its relative level against the analog signal at -18 to -4 dB. On the contrary, if an analog signal is adjacent to the upper frequency of a digital signal, the relative level of the digital signal must be set at -20 to -6 dB. This relationship is illustrated in Figure 5. The power level ranges are different from upper-adjacent and lower-adjacent signals because the frequency spectrum of a VSB-AM signal is asymmetrical.

Figure 5: Carrier level in adjacent transmission of NTSC signal and QAM signal

3. Multiple-TS Transmission System

3.1 Multiplexing Frame Structure
The information rate of 64-QAM/6 MHz is 29.162 Mbps per channel, as shown in Table 1, whereas 52.17 Mbps of information will be transmitted on a BS digital broadcast single carrier. Accordingly, at least 2 cable television channels must be used to retransmit information of a single BS carrier.

Also, with a single-TS transmission system, just a single TS can be transmitted with a single carrier. However, in BS digital broadcasting, multiple TSs are transmitted with a single carrier. If BS digital broadcasting is retransmitted with single TS transmission system, a large number of channels are required as every TS occupies a cable channel even if the TS has small capacity. This results in inefficient use of frequency.

To solve this problem, the single-TS transmission system has been expanded to a system that transmits multiple TSs. Using this system, information on a single BS carrier is divided into two groups in TS units. The two groups are then separately transmitted at 64-QAM/6 MHz.

Multiple-TS transmission system multiplexes multiple TSs using the frame structure shown in Figure 6. The multiplexing frame consists of 53 slots with a header packet as in the first slot. In the header packet, information about multiplexing and demultiplexing, as shown in Table 2, is stored. The receiver reads this information and extracts a desired TS.

Table 2: Key Items of Frame Header Information
Multiplexing frame header PID
Packet-ID to indicate frame header
Slot layout
Specifies method for placing each TS in the slots on the multiplex frame
Multiplexing frame format
Specifies frame length and maximum multiplex TS number
Table of TS-ID and relative TS number
Correspondences between TS-ID and relative TS number
Receiving condition
Information to indicate BS receiving condition at head-end
Emergency alarm instruction
Emergency alarm signal used for start control of a receiver
Table of slot and relative TS number
Information on relative TS number to which packet in slot belongs
Detects transmission error

Since a multiplexed signal is a TS packet stream, it is possible to carry out channel coding identical to that of the single-TS transmission system, as shown in Figure 7. The technology and standards developed so far for the single-TS transmission system can be applied. New technology development could have been minimized and cost reduction achieved.

With present specification, 15 TSs at maximum can be multiplexed. Every TS in ISDB is uniquely distinguished in the pair of original network identification (Original_Network_ID) and TS identification (TS_ID). A TS packet in a slot is identified in two stages: corresponding information between the pair of Original_Network_ID and TS_ID and relative TS number, and corresponding information between relative TS number and slot number. With this method, fewer bits are required for header information than the method that identifies the TS packet in a slot directly by the pair of Original_Network_ID and TS_ID.

In the following, the key items of multiplexing frame header information will be explained.

Figure 6: Frame Structure Figure 7: Relationship between single and multiple TS transmission systems

3.2 Multiplexing Frame Header Information

(a) Multiplexing Frame Header PID

The first 4 bytes of the multiplexing frame header have a structure similar to the MPEG-2 TS packet header. The first byte is a sync byte with a value of 0x47. Also, value of the second byte, which is called a multiplexing frame header PID, is set to 0x2f. The header packet can be identified from other TS packets in the slots of a multiplexing frame as this value of 0x47 is not used as PIDs of TS packets.

(b) Multiplexing Frame Header SYNC
A 13-bit pattern (0x1A86) is used as the multiplexing frame sync. Whole bits are reversed for every two multiplex frames. The header packet is also distinguished by this multiplexing frame sync. Using both a multiplexing frame sync and a multiplexing frame header PID, frame synchronization is ensured.

(c) Slot Layout
The slot layout identifies the layout of TS packets in the slots. Presently, we only define the case where TS packets that belong to a certain TS are placed in the same slots in every multiplexing frame.

(d) Multiplexing Frame Format
The multiplexing frame format is used to identify the multiplexing frame length and the maximum number of multiplexed TSs. At present, only one format with a multiplexing frame length of 53 and 15 maximum multiplexed TSs is specified. The present format (53, 15) is chosen for efficient transmission of TSs broadcasts for BS.

(e) Table of TS_ID and Relatives Number
Each TS is indirectly distinguished using a relative TS number. This table shows the correspondence between a pair of Original_Network_ID and TS_ID, and a relative TS number.

(f) Receiving Condition
BS digital broadcasting system has a mechanism called hierarchical service, in which multiple modulation schemes are applied simultaneously and a user can choose data of robust modulation when reception is poorer due to heavy rainfall or other conditions. To enable hierarchical service to be retransmitted via cable television, it is necessary for the receiver to detect the condition of reception at the head-end. As such, the receiving condition of each TS is described in the header information.

(g) Emergency Alarm Instruction
Emergency alarm instruction is used to identify the start control signal of the receiver.

(h) Table of Relatives Number and Slot Number
The relative TS numbers of the TS packets allocated into 52 slots, except the first slot for the header packet, are identified with this table.

(i) CRC
Since an error in the multiplexing frame header information has significant influence on the reception, CRC (cyclic redundancy check) is added to detect any errors. As defined in ITU-T Recommendation H.222.0, the value of CRC has zero register output when 184 bytes of a multiplexing frame header excluding first 4 bytes are input into the register of a decoder.

4. Pass-Through System for ISDB-S

One of the simplest systems for retransmitting BS digital broadcasts is to transmit the signals (1-1.3GHz) of the BS reception antenna output as is. However, as a 1GHz-band signal cannot be transmitted with cable television, the signals are converted into lower frequencies at the head-end and transmitted via cable. The signals are restored to the original 1GHz-band signals at each subscriber with a frequency-up-converter (Figure 8). This system has following characteristics.

  • Equipment costs less and initial installation cost is inexpensive.
  • Same receiver for direct reception can be used.
  • Compared with multiple-TS transmission system, many cable channels are required.

Approximately 174MHz bandwidth including the guard band is required for the transmission of 4 carriers of BS digital broadcasts. In the multiple-TS transmission system, 8 channels are required. This pass-through system requires as many as 29 channels, however. Therefore, this system is suitable for facilities that have many unused channels and retransmission is possible without much extra cost.

Figure 8: Frequency conversion in "pass-through system"

5. Pass-Trough System for ISDB-T

For terrestrial digital broadcasts, the Orthogonal Frequency Division Multiplex (OFDM) scheme with a 6 MHz bandwidth will be used. Since the bandwidth of a cable television channel is also 6 MHz, it is possible to transmit OFDM signals as they are without changing their modulation scheme. OFDM was a new modulation scheme for cable television. Thus, the establishment of new standards was required. For the transmission of OFDM signals via cable, adjacent channel interference with the signals of other modulation scheme had to be avoided. The relationship at the carrier level for the adjacent transmission of a VSB-AM signal (NTSC) and an OFDM signal is shown in Figure 9. Also, the relationship at the carrier level for the adjacent transmission of a 64-QAM signal and an OFDM signal is shown in Figure 10. As a result of experiments, it has been confirmed that OFDM signals can be transmitted without interfering with other signals by setting the transmission level of the OFDM signals at a proper level in cable facilities under the current specification standard.

It is possible to retransmit terrestrial digital broadcasts via a transmodulation system. The number of required cable channels is equivalent to the channels needed for the pass-through system.

Figure 9: Carrier level in adjacent transmission of NTSC and OFDM signals

Figure 10: Carrier level in adjacent transmission of 64QAM and OFDM signals

6. Conclusions

Development of equipment that employs the multiple-TS transmodulation system is rapidly progressing at manufacturers. To insure compatibility among the equipment, Japan Cable Laboratories has established standard specifications for the multiple-TS system. Manufacturers also have already commercialized low-cost equipment that employs the pass-through system for BS digital broadcasts. When BS digital broadcasting starts in December 2000, digital broadcasting will be implemented by a number of cable television operators.

Copyright 2001 NHK (Japan Broadcasting Corporation) All rights reserved. Unauthorized copy of the pages is prohibited.

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