Отчет мсэ-r bt. 2140-1 (05/2009)



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1.5 ATSC


1.5.1 Overview of the ATSC Digital Television System

The ATSC Digital Television (DTV) standard ushered in a new era in television broadcasting. The impact of DTV is more significant than simply moving from an analog system to a digital system. Rather, DTV permits a level of flexibility wholly unattainable with analog broadcasting. The ATSC Digital Television Standard describes a system designed to transmit high quality video and audio and ancillary data within a single 6 MHz terrestrial television broadcast channel. The design emphasis on quality resulted in the advent of digital HDTV and multi channel surround-sound. The ATSC system pioneered a layered architecture that separates picture formats, compression coding, data transport and digital transmission as shown in Fig. 15.

FIGURE 15


A block diagram of the system is provided in Fig. 16.

FIGURE 16


1.5.1.1 Video Formats


The source video formats for the ATSC standard were carefully selected for their interoperability characteristics with film (wide aspect ratio and 24 fps), computers (square pixels and progressive scanning), and legacy television systems (480 lines and ITU-601 sampling), as illustrated in Fig. 3. In addition, the HDTV formats and the square pixel SDTV format are related by simple 3:2 ratios, allowing high quality, yet economical conversion among these formats. ATSC system.

1.5.1.2 Video Compression

The ATSC DTV Standard specifies the MPEG-2 video stream syntax (Main Profile at High Level) for the coding of video. The ATSC DTV Standard defines the video formats for HDTV and SDTV (Table 2).

TABLE 2

Digital Television Standard Video Formats*



Vertical lines

Pixels

Aspect ratio

Picture rate

1080

1920

16:9

60I, 30P, 24P

720

1280

16:9

60P, 30P, 24P

480

704

16:9 and 4:3

60P, 60I, 30P, 24P

480

640

4:3

60P, 60I, 30P, 24P

*Note that both 60.00 Hz and 59.94 (60x1000/1001) Hz picture
rates are allowed. Dual rates are allowed also at the picture rates
of 30 Hz and 24 Hz.

ATSC consumer receivers are designed to decode all HDTV and SDTV streams providing program service providers with maximum flexibility.

ATSC also provides the ability to utilize Advanced Video Coding (AVC) within an ATSC DTV transmission. Part 1 of ATSC A/72, “Video System and Characteristics of AVC in the ATSC Digital Television System," and "Part 2 “AVC Video Transport Subsystem Characteristics". The standard details the methodology to utilize Advanced Video Coding (AVC) within an ATSC DTV transmission. AVC which was developed by the ITU-T Video Coding Experts Group together with the ISO/IEC Moving Picture Experts Group is also known as H.264 and MPEG-4 Part 10. The A/72 Standard defines constraints with respect to AVC, compression format restraints, low delay and still picture modes, and bit stream specifications.

1.5.1.3 Audio Compression

The ATSC DTV Standard utilizes “Digital Audio Compression (AC-3)” for the coding of audio as based upon the ATSC A/52 Standard.



1.5.1.4 Transport

Transport defines the methodology of dividing each bit stream into “packets” of information. The ATSC system employs the MPEG-2 transport stream syntax for the packetization and multiplexing of video, audio, and data signals for digital broadcasting systems.



The ATSC A/65 Program and System Information Protocol (PSIP) describes the information at the system and event levels for all virtual channels (channel numbers are not tied directly to the actual RF channel frequency) carried in a particular TS. Additionally, information for analog channels as well as digital channels from other Transport Streams may be incorporated.

There are two main categories of information in the ATSC PSIP Standard (A65), system information and program data. System information allows navigation and access of the channels within the DTV transport stream, and the program data provides necessary information for efficient browsing and event selection. Some tables announce future events and some are used to locate the digital streams that make up an event. The PSIP data are carried via a collection of hierarchically arranged tables, repeated in the packet stream at frequent intervals.

1.5.1.5 RF Transmission


“RF Transmission” refers to channel coding and modulation. The channel coder takes the packetized digital bit stream, reformats it and adds additional information that assists the receiver in extracting the original data from the received signal, which due to transmission impairments may contain errors. In order to protect against both burst and random errors, the packet data is interleaved before transmission and Reed-Solomon [isn’t a reference needed?] forward error correcting codes are added. The modulation (or physical layer) uses the digital bit stream information to modulate a carrier for the transmitted signal. The basic modulation system offers two modes: an 8-VSB mode and a 16-VSB mode. The 8-VSB mode was designed for spectral efficiency, maximizing the data throughput with a low receiver carrier-to-noise (C/N) threshold requirement, high immunity to both co-channel and adjacent channel interference, and high robustness to transmission errors. The attributes of 8-VSB allow DTV channels to co-exist in a crowded spectrum environment that contains both analog and digital television signals. In addition, the lower power requirements (typically, 12 dB lower than analog NTSC) of 8-VSB allow ATSC DTV stations to exist on channels where analog stations cannot due to interference constraints. The spectral efficiency and power requirement characteristics of 8-VSB are essential to the conversion of terrestrial broadcast transmission from analog to digital since new spectrum is not allotted during the transition phase.


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