Television broadcast service based on ATSC 3.0 technology, or NEXTGEN TV, is better than ATSC-1 in virtually all respects. But the ATSC 3.0 transition is challenging because, unlike the original DTV transition, broadcasters do not have access to additional unused RF channels on which to transmit ATSC 3.0 signals while also simulcasting their ATSC-1 transmissions. Without extra channels available for simulcasting signals using the new standard, the practical method that has emerged for the transition to ATSC 3.0 consists of stations combining all their ATSC-1 services on as few RF channels as possible via channel-sharing arrangements in order to free up one or more RF TV channels that can then be used for ATSC 3.0 transmissions. However, the goal of freeing up enough spectrum for a compelling and complete ATSC 3.0 service offering (i.e., enough “wow factor” to lure consumers) is in tension with the equally strong desire to avoid sacrificing the quantity or quality of existing ATSC-1 service, either by eliminating program channels or degrading video/audio quality by reducing bit rates. And that’s a problem, because combining all the current ATSC-1 services as closely together as possible in the minimum number of RF channels generally results in freeing up zero, one or maybe two channels for ATSC 3.0, depending on the specific market and stations participating. Using more efficient video compression would be helpful in minimizing ATSC-1’s bandwidth needs if it was possible. And it is, with a few caveats.
MPEG-2 compression, the video compression technology used in ATSC-1, was developed in the early 1990s and significant improvements in efficiency have been achieved in commercial MPEG-2 products over the last 25+ years. While the first HD compression encoders for ATSC-1 used essentially as much of the 19.3 Mbps capacity of the ATSC-1 RF channel as possible to produce a single acceptable HD program, more contemporary MPEG-2 compression engines developed over the past five years or so can encode multiple HD programs in a single TV channel. At this point, while additional modest improvements may be squeezed out using the latest techniques, experts advise that potential improvements for MPEG-2 have mostly been exploited in this now-mature technology.
Additional spectrum efficiency is also possible through the use of statistical multiplexing, a tool for combining multiple video streams more efficiently whereby a fixed bandwidth channel is shared among multiple services by allocating bandwidth for each stream according to each stream’s real-time needs. Compared to allocating a constant bandwidth for each video program, typical bandwidth savings of 20% to 40% are claimed for statistical multiplexing arrangements of multiple video streams. The state-of-the-art in the MPEG-2 world is at a performance level which might be surprising. With the caveat that perceived acceptability will be dependent on the type and complexity of the content and somewhat subjective overall, encoder/multiplexer solutions are now available that can pack as many as 2 HD plus 4 SD signals or 1 HD plus 7 SD signals in one 6 MHz ATSC-1 channel, and in some cases more than that.
However, there is also another tool that can used to minimize ATSC-1 spectrum usage—using MPEG-4 video compression.
Some quick points about MPEG-4:
- MPEG-4 compression, also referred to as AVC or H.264, is roughly twice as efficient as MPEG-2 for video compression.
- ATSC developed standards for using MPEG-4 video coding and transport in ATSC-1 transmissions (originally in 2008 and updated in 2015).
- The FCC rules require that the broadcaster’s primary channel is encoded using MPEG-2, but this compression constraint does not apply to multicast channels and diginets.
- Most TV sales today are smart TVs, including all 4K UHD TVs, and an increasing number of U.S. consumers have access to a smart TV in their homes.
- Virtually all smart TVs will decode MPEG-4 video from broadband sources due to the prevalence of video on the Internet compressed using MPEG-4.
- While comprehensive data and studies are not available, anecdotal and informal reports suggest that over-the-air broadcasts with MPEG-4 encoded video will be receivable on most and possibly almost all recent smart TVs, although manufacturers cannot guarantee this capability since reception of MPEG-4 signals transmitted over-the-air has not been part of their testing profile.
- A number of broadcasters around the U.S. have experimented with or have begun deployment of MPEG-4 diginets within their ATSC-1 multiplexes.
An action that recently took place in the broadcast standards world should help convince TV set manufacturers to ensure that over-the-air reception of MPEG-4-encoded signals is as reliable as MPEG-2 in future TV sets, regardless of whether they are ATSC 3.0-capable or not. TV set manufacturers are driven by technical standards to define and have confidence in what they expect their products can and can’t do. As noted earlier, MPEG-4 coded signals in ATSC-1 transmission channels are decodable on most or all recent smart TVs. However, while ATSC did adopt a version of the MPEG-4 standard beginning in 2008, it was anticipated that MPEG-4 signals would be announced and signaled as being present in ATSC-1 multiplexes in a somewhat different way than what is successfully being done currently in the marketplace and works on most receivers. In effect, this meant that while MPEG-4 broadcasts are somewhat fortuitously decodable on many smart TVs, they are not in complete compliance with the ATSC DTV Standard. The ATSC standards community recognized this and began a new project formally proposed by NAB in the first quarter of 2022 to bring the ATSC-1 standard into compliance with MPEG-4 implementations that are known to work in the field with legacy receivers. This work resulted in the balloting and adoption of a new ATSC Candidate Standard in late September 2022: A/53 Part3:2013 Amendment No. 1 “AVC Signal Transport.” The Candidate Standard phase of the standardization process is an explicit call for implementation and technical feedback and normally leads to a full Standard if no issues are uncovered or when any issues that are discovered are sorted out and fixed. Formally documenting MPEG-4 in the ATSC DTV Standard to match marketplace implementations gives all manufacturers confidence that they should include it as part of the set of fully tested features for broadcast reception in all new TV sets.
The second part of this series looks at some theoretical examples as to how the use of MPEG-4 compression in the ATSC-1 DTV service can free up more RF channels for ATSC 3.0 transmissions.