Now that ATSC is about to embark on its status as the primary TV transmission method in the United States, let's take a look at a number of changes to the original standard, both implemented and under development.
E-VSB provides for enhanced transmission
A new version of 8-VSB called Enhanced 8-VSB (E-VSB) enables optional enhanced services, designed to have higher immunity to certain channel impairments than the main service. E-VSB adds FEC coding layers to the data in the form of Reed-Solomon coding and trellis coding. The enhanced data is delivered at a reduced information rate selected by the broadcaster from a set of specified options, including half-rate and quarter-rate coding.
As with any transmission system, there is a trade-off between data rate and noise margin. For example, a half-rate system requires 2n bits of channel space to deliver n bits of data. Thus, the addition of enhanced services will subtract from the total data rate by an amount that includes additional overhead — a factor not unique to any particular enhancement method. (See Figure 1.)
In the fall of 2005, tests conducted in real-world conditions confirmed that the E-VSB bit stream delivers an expected 6dB margin improvement compared with the normal stream. Additionally, the tests confirmed that the presence of E-VSB did not adversely affect the normal stream reception of legacy receivers.
ATSC-M/H allows vehicular TV reception
E-VSB, however, did not improve performance under severe multipath conditions, such as in moving vehicles. For that, a change to the equalizer training sequence would be needed, and with that, an addition to the standard would be required. Last year, ATSC launched a process to develop a standard that will enable broadcasters to deliver television content and data to mobile and handheld devices via their DTV broadcast signal. Designed to facilitate the use of the DTV broadcast channels to provide new services directly to small receivers moving at a high rate of speed, broadcasters will be able to allocate a portion of their 19.39Mb/s signal to mobile and handheld while continuing to transmit primary video services in the main channel.
A request for protocol (RFP) was issued last year outlining the new service and its intended applications. The requirements for the ATSC-M/H standard include service area and reliability comparable to 8-VSB, and reliability comparable to, or exceeding, cell phones and similar devices. In addition, the system must support mobile reception up to at least 75mph. Both real-time and non-real-time modes (NRT), e.g., offline downloading, must be supported, and video resolutions should include 480p for mobile and common intermediate format (CIF, 352 × 288) for handheld applications.
The ATSC is discussing a layered approach that includes conditional-access system (CAS) and digital rights management (DRM) elements. The system must also be backward compatible with existing ATSC receivers. (See Figure 2.)
Several proposals were received by ATSC, including solutions for specific elements of the system, as well as three integrated proposals from Samsung and Rohde & Schwarz, LG and Harris, and Thomson and Micronas. The first two of these systems (A-VSB and MPH respectively) have already garnered considerable industry attention, having been demonstrated at NAB2007 and CES 2008. In-vehicle demonstrations of VSB and both A-VSB and MPH provided ample evidence of the viability of the new service.
The A-VSB proposal incorporates three new independent elements: a supplementary reference signal (SRS), a Scalable Turbo Stream (STS) and support for SFNs. The SRS adds an equalizer training sequence to the transport stream adaptation field so that a receiver equipped with this new equalizer can track rapid multipath fading. The STS provides additional protection for the video.
Public details of the MPH system are still sketchy, with most information confidential within ATSC. Company materials, however, describe scenarios using 1.1Mb/s of payload in a 4.4Mb/s channel (i.e. ¼-rate coding), as well as service with a total allocated bandwidth of no more than 3Mb/s. This would provide “one quality mobile channel and one secondary channel of slightly less quality,” according to company materials Again, it is believed that an additional equalizer training sequence is key to successful mobile reception.
An Independent Demonstration of Viability (IDOV) process has been established by ATSC to show that the three integrated systems under consideration can reliably progress toward implementation. IDOV, a Design for Six Sigma (DFSS) discipline to ensure the optimal function of products and services, was endorsed by the Open Mobile Video Coalition (OMVC), a new consortium formed with the intent of accelerating the development of mobile digital broadcast television. The OMVC alliance encompasses U.S. commercial and public broadcasters operating more than 420 commercial television stations.
The first phase of the IDOV activity took place in February in San?Francisco, and an interim report was expected the first week of April. Given the not-coincidental timing with the analog cutoff, a standards war is somewhat unlikely, so OMVC will likely endorse one proponent or suggest a merging if there's a close call. The OMVC is hoping to launch mobile digital television services by 2009, and officials at ATSC say they are shooting for a candidate standard by first quarter 2009.
FCC updates its rules
It is not currently known whether the ATSC-M/H standard will require FCC rules changes; however, revisions to the ATSC standards have often resulted in updates to FCC Part 73, usually as incorporation by reference.
Last December, the FCC incorporated into its rules the latest version of the DTV transmission standard. This version, known as ATSC A/53:2007, differs from the previously used standard, A/53-B, in several respects.
First, A/53:2007 includes the specifications for E-VSB for terrestrial broadcast. It further describes the coding constraints that apply to the use of the MPEG-2 systems specification in the DTV system, including mandatory main and optional enhanced services.
A/53:2007 also improves the AFD specifications by revising and clarifying the relevant standards. AFD enables DTV receivers to automatically switch aspect ratios as signaled in the bit stream. Currently, broadcasters are not required to carry AFD, but these clarifications are hoped to encourage its use.
ATSC features under discussion
The ATSC Advanced Common Application Platform (ACAP) is an interactive TV (iTV) standard for terrestrial broadcast, published in 2005. Using Java and JavaTV middleware, ACAP (ATSC A/101) provides a single method of delivering interactive applications to terrestrial and cable set-top boxes by harmonizing the ATSC DTV Application Software Environment (DASE) standard and the CableLabs OpenCable Applications Platform (OCAP) specification.
Enhanced AC-3 (E-AC-3) was incorporated into the ATSC A/52-B audio standard in 2005. Developed to add capabilities to multichannel audio playback, the standard allows for up to 7.1 channels, increased compression quality and extra sample rates. Although not backward compatible with existing AC-3 decoders, an E-AC-3 decoder is required to be able to downconvert the output to an AC-3 stream.
NRT video delivery is a method of delivering content to viewers that is gaining appeal to content owners and providers. Essentially a method for streaming video to viewer storage platforms, businesses can be built that transfer content automatically and efficiently. NRT could be deployed today, with mostly off-the-shelf standards and existing ATSC services, or with ATSC-M/H when it is approved.
The ATSC Planning Committee is also in the process of developing requirements for ATSC 2.0, which is envisioned as a complete suite of next-generation services for the conventional fixed DTV receiver viewing environment. ATSC 2.0 might include interactive services (ACAP), NRT and enhancements for fixed indoor reception, such as new codecs. All services, however, are planned to have backward compatibility with current ATSC receivers.
There are also other video modes being discussed by the ATSC. The possibility of 1080p60 video is gaining more interest, as are methods for delivery of 3-D video to the home. It's hard to believe that the ATSC as a committee is 25 years old this year. Given the rate of technological development, there's every indication that the group will continue to make possible new television businesses far into the future.
Aldo Cugnini is a consultant in the digital television industry.
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