ATSC-EEWORLD

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ATSC

The HDTV system defined by the ATSC digital television standard consists of four separate layers (as shown in Figure 07-05-1) with clear interfaces between the layers.

Figure 07-05-1 American GA HDTV system layered architecture and PS-WP4 reference model

The highest layer in the hierarchical architecture of the American GA HDTV (Grand Alliance HDTV) system is the picture layer, which determines the form of the image, including pixel array, aspect ratio and frame rate. The second layer is the compression (video and audio) layer, which adopts the MPEG-2 image compression standard. The third layer is the transport (packetization) layer, where specific data is included in different compression packets, such as program 1 image, program 2 sound, or auxiliary data, using the MPEG-2 system standard. The last layer is the transmission layer, which determines the modulation and channel coding scheme for data transmission. For terrestrial broadcasting, the standard adopts the 8VSB developed by Zenith, which can achieve a transmission rate of 19.3Mb/s through a 6MHz terrestrial broadcast channel. The standard also includes the 16VSB mode suitable for the high data rate of cable TV systems, which can achieve a transmission rate of 38.6Mb/s in a 6MHz cable channel.

The lower two layers share the responsibility for the transmission of general data, while the upper two layers determine the specific configuration running on top of the general data transmission, such as HDTV or SDTV (standard definition television).

The upper two layers also determine the specific image formats supported by the ATSC standard. There are 18 formats in total (6 for HDTV and 12 for SDTV), 14 of which use progressive scanning.

(1) HDTV, 1920 pixels (H) × 1080 pixels (V), aspect ratio 16:9, frame rate 60 Hz/interlaced scanning, frame rate 30 Hz/progressive scanning, frame rate 24 Hz/progressive scanning;

(2) HDTV, 1280 pixels (H) × 720 pixels (V), aspect ratio 16:9, frame rate 60 Hz/progressive scan, frame rate 30 Hz/progressive scan, frame rate 24 Hz/progressive scan;

(3) SDTV, 704 pixels (H) × 480 pixels (V), aspect ratio 16:9 or 4:3, frame rate 60 Hz/interlaced scanning, frame rate 60 Hz/progressive scanning, frame rate 30 Hz/progressive scanning, frame rate 24 Hz/progressive scanning;

(4) SDTV, 640 pixels (H) × 480 pixels (V), aspect ratio 4:3, frame rate 60Hz/interlaced scanning, frame rate 60Hz/progressive scanning, frame rate 30Hz/progressive scanning, frame rate 24Hz/progressive scanning. Except for one type, all HDTV image formats use progressive scanning. Because the 1920×1080 format is not suitable for progressive scanning at 60 frames/second in a 6MHz channel, it is replaced by interlaced scanning. The 640×480 image format of SDTV is the same as the VGA format of computers, ensuring compatibility with computers. Among all 12 SDTV formats, 9 use progressive scanning, and 3 are retained as interlaced scanning to adapt to existing video systems.

Figure 07-05-2 depicts the hierarchical structure of GA HDTV from the perspective of the protocol stack. GA HDTV is on the left side of the figure, the corresponding reference model established by PS-WP4 is in the middle, and the ISO Open Systems Interconnect (OSI) reference model is on the right.