The long struggle to set HD² standards has never really reached an end. Existing standards continue to evolve, while new candidate standards undergo the formal ratification process as new media technologies emerge.
The ATSC Standards
Did you know that without the intervention of luck, the SPMTE 1125 line analog HDTV standard adopted in the early 1980s could easily have been adopted by the FCC as the U.S. broadcast standard? We might now be in the same circumstance as Japan, with an obsolete HD² technology. The Eureka MAC system that broadcast HDTV demonstrations of the Olympics in the late 1980s could have caught on in the U.S. as an NTSC-compatible system. Or how about the High Definition System–North America (HDS-NA) developed by Philips or some version of Sarnoff Research Institute’s Advanced Compatible TV (ACTV)?
In 1987, when the Advisory Committee on Advanced Television Systems (ACATS) was formed, an NTSC-compatible HDTV system was the design requirement. A two-channel augmentation methodology facilitated this requirement.
In 1990, the U.S. was a distant third in the race to HDTV. However, General Instrument turned the tables on the HDTV world by demonstrating an all-digital HDTV system. But progress continued as the digital HDTV prototype systems of four consortiums (plus the NHK system) underwent FCC testing. Many speculated that the FCC would not pick a winning system, lest it repeat the embarrassment of the NTSC NBC/CBS and AM stereo debacle. Others thought that regardless of which system won, the other consortiums would take legal action to challenge the decision. As fate would have it, no one system was a clear winner and the FCC urged the consortiums to work together to build one system. Hence, the Grand Alliance was born.
A layered system
The core ATSC DTV standard is a layered system design. Presentation, compression, transport and transmission layers give the standard a flexibility of modular design. This has been particularly important in enabling the standard to be adapted for domestic cable, satellite systems and international broadcasting.
There has been a lot of debate about the presentation layer, which includes 32 different video formats. In fact the battle waged by the computer industry over progressive vs. sequential scanning was so intense that the actual table that specified video formats was dropped from the final document in order to enable final acceptance of the standard in November 1996.
Because MPEG-2 video compression and Dolby AC-3 audio compression are perceptual (lossy) compression algorithms, generations of decoding and re-encoding can lead to artifacts. This renders the standard virtually useless in a production environment.
Preparation of the compressed essence and metadata information is assembled into an MPEG transport stream. Methodologies for program assembly using program map tables (PMT) and program allocation tables (PAT) facilitate multiple program transport streams. Program and System Information Protocol (PSIP) preserves local branding and enhances. Robust delivery is helped by Reed Solomon forward error correction.
Transmission completes the standard by using 8-VSB digital modulation to deliver HDTV in a 6MHz channel.
The standard is described in two ATSC documents: A/52: “Digital Audio Compression (AC-3) Standard and A/53: “ATSC Digital Television Standard”. A companion publication, A54: “Guide to the ATSC Digital Television Standard,” also provides more details.
A flexible standard
Over time, both the scope and number of ATSC standards have grown. A Web accessible presentation by Jerry Whitaker, ATSC vice president of Standards Development, provides details of ATSC organizational structure as well as brief technical overviews of PSIP, EDI, ACAP and other recently adopted standards. Documents now include recommended practices, implementation subcommittee findings, technology group reports and candidate standards. The ASTC site can help technical personnel navigate the maze of standards and terminology with a “Guide To DTV Standards.”
From the outset on the all-digital road, the design goal was not only HDTV and 5.1 audio, but to enable a 19.39bps pipe into the household. Data delivery was always intended to be part of the system. PSIP, DTV Application Software Environment (DASE) and the proposed standard Advanced Common Applications Platform (ACAP) along with other data delivery standards are enabling new enhanced services. Although EPG is the current primary manifestation, these technologies will enable targeted marketing and Interactive TV.
A benefit of the standard’s modular design is that it can be adapted to other industries and for other nations. The SCTE has www.scte.org/documents/pdf/ANSISCTE072000DVS031.pdf adopted cable standards and Direct-to-Home Satellite (DTH) implementation are both specified in ATSC standard A/81. Europe, despite the failure of Eureka HDTV and the EU’s anti-HDTV posture, has finally embraced the 720p HD format and it has been formally sanctioned by the EU.
Enhancements and additions to the ASTC standards is an ongoing process. The 8-VSB transmission methodology is being improved with Enhanced-VSB. Consideration is being given to adding MPEG-4 and VC-1 video coding. The coding standards are intended to get SD and HD MPEG-2 equivalent quality at a lower bit rate while the E-VSB is designed to transmit at a higher symbols rate. A bigger pipe combined with smaller payload will facilitate more economic distribution, more programs and target a wider range of consumer devices.
Next time we will examine the various layers of the ATSC format and production workflows.
ATSC Recommended Practice A/54A: Guide to the ATSC Digital Television Standard - December 2003
ATSC Standards and Activities, Jerry Whitaker
ANSI/SCTE 07 2000 (formerly DVS 031), Digital Video Transmission Standard for Television, SCTE
Guide To DTV Standards