Video compression in transition
Sep 1, 2009 12:00 PM, By Ian Trow
MPEG-2 and AVC can coexist in modern facilities.
Figure 4
Select figure to enlarge.
Many early DTH systems now require upgrading simply because they have reached the end of their life cycle. Replacement of these early MPEG-2 systems has led many compression vendors to re-evaluate their stand on video quality and to retract their view that MPEG-2 had no further developments in compression efficiency to offer. Traditionally, four strains of MPEG compression systems have been marketed as broadcast products catering to the SD and HD variants for both MPEG-2 and AVC applications. The benefits of applying techniques learned from AVC, along with the extra processing power available, have yielded a new breed of encoders that not only offer enhanced video quality, but support both MPEG-2 and AVC in dense multichannel architectures. The versatility inherent in such a platform is crucial when addressing a market with requirements that center around legacy system support, the aspirations a provider has for distributing content and a continued desire for greater bit rate efficiency in less rack space.
For contribution and primary distribution applications, where content is exchanged between broadcasters or fed from remote events, similar ties exist to MPEG-2. Professional profiles and levels were developed to extend the range of applications for MPEG-2. The most notable extensions involved enhanced chrominance support through the introduction of 4:2:2, carriage of ancillary data and allowance for milder compression ratios to preserve image quality. In these professional applications, the inertia behind the continued use of MPEG-2 remains strong.
AVC is making inroads in these applications, where there is a strong overlap in terms of feature sets between the professional and final distribution markets. This overlap exists in newsgathering where 4:2:0 chrominance sampling and high compression ratios are in demand for carriage over narrowband satellite, terrestrial and IP links.
The appeal of AVC has led many second-generation Digital Terrestrial Transmission (DTT) platforms to evolve from exclusive MPEG-2 SD transmissions. Through next-generation encoders, this evolutionary path adds HD AVC services alongside MPEG-2 and eventually supports an all-AVC approach that embraces SD and HD alongside a wide variety of streamed applications. This approach is currently under way for DTT within the UK and will no doubt be repeated in other regions where MPEG-2 is currently the dominant compression standard.
The future for broadcast headends
Greenfield broadcasters are in the fortunate position of not having strong legacy ties to MPEG-2; thus, they are able to deploy systems from the ground up that are fully based on AVC. The resulting streamlined systems, based on the latest generation of AVC encoder products, has tremendous advantages, including the ability to offer the greatest bit rate efficiency, the highest channel density with good redundancy provisioning and the ability to address multiple markets by providing simultaneous streams for a wide variety of platforms.
Video quality is retained within systems that allow content to remain within a particular compression standard. Broadcast content can then be manipulated by adjusting parameters such as frame rate, aspect ratio and compressed bit rate to make them appropriate for the target platform. While this approach is commonly used for broadcast MPEG-2 content, AVC-based systems become very attractive due to the reduction in workflow steps, streamlined infrastructure and retention of picture quality.
Regions that do not have a large MPEG-2 legacy infrastructure can deploy AVC directly with obvious benefits. However, with the vast majority of broadcast content existing as MPEG-2, digital turnaround of such material is a common requirement. Decoding MPEG-2 content has often been performed by separate decoders, but increasingly this functionality is integrated within the encoder. High-end encoders not only have the capability to offer significant coding gains for multiple channels, but also they can now deal with a wide variety of input and output formats. (See Figure 3 on page 46.)
This flexibility is further extended by offering dual encoding channels for each video input. Normally, a low-resolution path has been optionally offered alongside the main video encode chain to allow picture-in-picture capability. Making the second channel a fully featured encoder and adding an up/downconverter to the input of the second channel allows a single encoder to produce two encoded outputs. This functionality is commonly requested by broadcasters and service providers. (See Figure 4.)
Rather than viewing AVC as a challenge to MPEG-2, companies are adding functionality to encoders to ease the introduction of more AVC-based services alongside MPEG-2. In the long term, MPEG-2 infrastructure might well be replaced by AVC, but for the moment, the two standards will coexist.
Ian Trow is director of broadcast solutions at Harmonic.
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