DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

MPEG-2 specifies upper bounds
for picture size at various levels.
Surprisingly, ATSC A/53 specifies
exact values. For example,
MPEG-2 MP@ML allows 704
S AL , but ATSC A/53 does not.
EIA-708-B, Digital Television (DTV)
Closed Captioning.
4.5 684
≈
10.762
286
consumer equipment is guaranteed to implement, any
format outside ATSC’s Table 3. In practice, DTV
decoders conform to MPEG standards, and additionally
conform to the restrictions imposed by the ATSC standards.
The supposed flexibility in the FCC regulations
has forced the consumer electronics industry to institute
a branding program that places “DTV-ready”
stickers on consumer equipment. The sticker supposedly
indicates compliance with ATSC Table 3 and
certain other provisions.
Some consumer HDTV displays use 1080i scanning, but
on a 4:3 display surface (“4:3 glass”). To use such
a display for standard 16:9 aspect ratio HDTV requires
vertical downsampling by a ratio of 4:3. This yields
810 picture lines, rather than 1080; the display format
is denoted 810i.
In the United States, DTV audio is standardized with
Dolby Digital (AC-3) audio coding; this is outside the
MPEG-2 standard. Dolby Digital is capable of “5.1”
channels: left and right (stereo) channels, a front center
channel, left and right surround channels, and a lowfrequency
effects (LFE) channel (the “.1” in the notation)
intended for connection to a “subwoofer.” ATSC
audio consumes a maximum of 512 kb/s.
EIA-708-B is the standard for conveying closed caption
data in a digital television signal (DTVCC).
One or more MPEG-2 program streams, the associated
audio streams, ancillary data, and other data is multiplexed
into a transport stream with a bit rate of about
19.28 Mb/s. That stream is presented to the modulator.
ATSC modulation
The ATSC standardized 8-level digital vestigial sideband
(8-VSB) modulation, transmitting about
10.762 million 3-bit symbols per second. To enable the
receiver to overcome errors introduced in transmission,
two forward error-correction (FEC) schemes are concatenated:
The outer code is Reed-Solomon (R-S), and the
inner code is a simple 3 ⁄ 2 trellis code. Between the R-S
and trellis coding stages, data is interleaved. Synchroni-
CHAPTER 50 DIGITAL TELEVISION BROADCAST STANDARDS 589