Revert to Section 5.6

The ATSC system was specifically designed to permit an additional digital transmitter to be added to each existing NTSC transmitter in the United States of America, with comparable coverage and minimum disturbance to the existing NTSC service in terms of both area and population coverage. This capability is met and even exceeded.

The system is quite efficient and capable of operating under varying conditions, i.e. clear channel availability or, as implemented in the US, constrained to fit 1 600 additional channels into an already crowded spectrum, and reception with roof-top or portable antennae.

The system was also designed to be immune to multipath and to offer spectrum efficiency and ease of frequency planning.

Signals conforming to the ATSC system can travel on cables and the United States cable industry is just beginning its conversion to digital. The ATSC 16-VSB mode is suited for cable since it can double the capacity on this delivery media. The ATSC has been tested and proven to work reliably over satellite at the same or higher bit rates.

As recalled in the introduction section of this document, the ATSC system was designed to permit an additional digital transmitter to be added to each existing NTSC transmitter in the United States of America with comparable coverage and minimum disturbance to the existing NTSC service in terms of both area and population coverage. Variations can be achieved in the programme formats as mentioned in the relevant Section (SD or HD), and there is a great potential for data-based services utilizing the opportunistic data transmission capability of the system. The system can accommodate fixed (or possibly portable) reception with no resultant loss in payload.

The DVB-T system was essentially designed with built-in flexibility, in order to be able to adapt to all channels: it is capable of coping not only with clear channel but with interleaved planning, and even co-channel operation for the dame programme by different transmitters (single-frequency networks).

It also permits service flexibility, with the possibility of reception by roof-top antennae and also, if desired, of portable reception. Mobile reception is possible for QPSK and also for higher modulation orders, proven by extensive laboratory measurements and field trials under different channel conditions.

The system was also designed to be robust against interference from delayed signals, either echoes from terrain or buildings or signals from distant transmitters in a single frequency network, a new tool which it brings to TV service planning to improve spectrum efficiency which is necessary in the case of particularly crowded spectrum as it is the case in Europe.

The DVB-T compliant signals can also be carried over cables. However, the DVB-T specification is part of a family of specifications covering also satellite (DVB-S) and cable (DVB-C) operation. All use MPEG-2 coding for video and audio and MPEG-2 type of multiplexing. They have common features in the error protection strategy to be used. The main difference is the modulation method

which is specific to the relevant bearer (satellite, cable or terrestrial). The available data capacity is also different, as higher bit rates are offered on cable and satellite. However, transferring programmes from one bearer to another is possible provided that the bit rate is available.

The DVB-T system features a number of selectable parameters, which allows it to accommodate a large range of carrier to noise ratio and channel behaviour, allowing fixed, portable, or mobile reception, with a trade-off in the usable bit rate. Table 1 summarizes the system possibilities. The range of parameters allows the broadcasters to select a mode appropriate to the application foreseen. For instance, a very robust mode (with correspondingly lower payload) is needed to ensure portable reception. A moderately robust mode with a higher payload could be used where the service planning uses interleaved channels. The less robust modes with the highest payloads can be used if a clear channel is available for digital TV broadcasting.

This highlights the DVB-T specific flexibility, which allows the user to tailor the system by using the most appropriate mode among the different possible modes of operation proposed.

Comprehensive discussion of the optimum use of all parameters is complex and would be lengthy. However, the following features should be kept in mind:

• the hierarchical modes when applicable split the channel in two with different (and adjustable) requirements in terms of C/N. This permits different reception conditions for the same or for different programme content;
• the code rate and the modulation scheme can be selected in order to lower down the C/N requirements to the desired form of service;
• the selection of the 2k mode instead of 8k makes mobile reception easier. However, it only permits the implementation of small single frequency networks of transmitters (SFN) as will be explained below.

The ISDB-T system has been designed to have enough flexibility to send not only television or sound programmes as digital signals but also offer multimedia services in which a variety of digital information such as video, sound, text and computer programmes will be integrated. It aims to make use of the advantages provided by terrestrial radio waves so that stable reception can be provided by compact, light and inexpensive mobile receivers in addition to integrated receivers used at home by using BST (Band Segmented Transmission) - OFDM scheme.

Two transmission bandwidth are prescribed, 5.6 MHz and 432 kHz, each oriented to particular types of broadcasting services. The 5.6 MHz bandwidth is mainly for digital broadcasting of television programmes, while the 432 kHz bandwidth is mainly for that of audio programmes. These two modes share all other parameters such as encoding format, multiplexing format, and OFDM carrier interval and frame configuration.

Terrestrial ISDB provides hierarchical transmission features using different carrier modulation schemes (DQPSK, QPSK, 16QAM. 64QAM) and internal encoding rate (I /2, 2/3, 3/4, 5/6, 7/8). This enables part of the band to be allocated to signals for stationary reception and the rest to signals for mobile reception, which means that audio and data broadcasts for automobile and portable receivers can be performed simultaneously with television broadcasts for home use. Each hierarchical level can be set for each BST-Segment having a bandwidth of 432 kHz. Such information can be sent to receivers by TMCC (Transmission and Multiplexing Configuration Control) signal allocated to part of the OFDM carrier.

Because the wide bandwidth and narrow bandwidth in terrestrial ISDB share the same OFDM parameters, the 5.6 MHz wide band can include the 432 kHz narrow-band directly. Consequently, a 432 kHz receiver can receive some 5.6 MHz services, and a 5.6 MHz receiver can receive all services broadcast at 432 kHz. Figure 46 shows the concept of this feature.

Continue to Section 7

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