When first introduced, the MPEG-4 compression standard was heralded by many as an MPEG-2 replacement. Over time, a series of profiles and levels have been developed within the MPEG-4 standard that successfully addresses many of the shortcomings of MPEG-2. These include greater compression ratios to facilitate high-definition carriage, resilience to errors introduced by packet-based IP distribution, and the application of MPEG techniques for mobile, hand-held, and PC streaming applications.

Since its introduction, MPEG-4 has been refined to focus on commercial areas with requirements that eclipse the capabilities of MPEG-2. The result is MPEG-4 part 10, more commonly known as H.264 or Advanced Video Coding (AVC). This version of the standard embraces the three shortcomings of MPEG-2 identified above. In this article, we will explore how today's most advanced encoding systems are paving the way for broadcasters to embrace AVC for legacy MPEG-2 infrastructures as well as new installations for contribution and distribution.

In most areas, direct-to-home (DTH) deployments have been based around MPEG-2 - creating an established infrastructure and a significant legacy for which swap-out replacement of MPEG-2 by AVC equipment is not a practical solution. Broadcasters and service providers need to work further within the constraints of MPEG-2 on infrastructure that needs replacement or upgrade to a more efficient implementation. I have always believed that AVC must fit alongside existing MPEG-2 installations as a complementary compression solution and not a total replacement, and there must be a continuous development of core processing algorithms to keep companies at the forefront of MPEG-2 for standard and high-definition applications.

Many early DTH systems now require upgrading due to the number of service hours completed. 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 development in compression efficiency to offer. Traditionally, four strains of MPEG compression systems have been marketed as broadcast products catering to the standard- and high-definition 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 multi-channel architectures. The versatility inherent in such a platform is crucial to address a market whose requirements 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 that MPEG-2 could address. 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 in-roads in these applications, where there is a strong overlap in terms of feature set between the professional and final distribution (i.e. DTH) 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.

Both MPEG-2 and AVC have been used for professional contribution and primary distribution with an emphasis on professional features for higher data rate capability. Of equal importance is support for this professional feature set at the receiver. Complimentary decoders or Integrated Receiver Decoders (IRDs) have always been an issue in professional applications, where the feature set is wide and performance level high. For AVC, the availability of receivers supporting the professional feature set is as much an issue as developing the encoder performance.

The appeal of AVC has led many second-generation Digital Terrestrial Transmission (DTT) platforms to evolve from exclusive MPEG-2 standard definition transmissions. Via next-generation encoders this evolutionary path adds high-definition AVC services alongside MPEG-2 and eventually supports an all-AVC approach embracing standard- and high-definition alongside a wide variety of streamed applications. This evolutionary approach is currently underway for DTT within the United Kingdom and will no doubt be repeated in other regions where MPEG-2 is currently the dominant compression standard.

Many regions, India included, are often 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. Figure 1 shows a video delivery/convergence architecture including the path of streamed variants of linear AVC broadcast content for Internet, IPTV, and mobile applications. Key to this approach is the ability to capture, transcode, and repurpose content depending on the requirements of the target platform.

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 like 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.

As stated earlier, 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 (DTA) 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, they can now deal with a wide variety of input and output formats as shown in figure 2.

The flexibility shown in figure 2 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. By making the second channel a fully featured encoder and adding an up/down converter to the input of the second channel allows a single encoder to produce two encoded outputs, functionality commonly requested by broadcasters and service providers.

The scenario shown in figure 3 shows how two encoder channels can be used to produce both an HD AVC and SD MPEG-2 feed for broadcast applications alongside constant bit-rate SD AVC for IPTV and an output suitable for PC or mobile streaming. The use of 1080p production feeds is another feature that is likely to receive widespread adoption in broadcast chains, introducing into the production environment the benefits of handling source content as progressive scan, high-definition material. Capturing material as progressive scan greatly eases the downstream conversion to emission formats by retaining picture quality that has often been lost when an interlace standard is used as the source master. Even though many end-user displays have 1080p capability, this technique is unlikely to be used for broadcast reception of live content to consumers for some time. However, many broadcasters are preparing production facilities for the introduction of 1080p and the re-use of this feed could well pave the way for a true challenger to Blu-ray via a premium download service.

Rather than viewing AVC as a challenge to MPEG-2, manufacturers have added functionality to their latest generation of encoders to ease the introduction of additional 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 co-exist. Consequently the latest breed of compression encoders is evolving to offer format agility at ingest and emission, and to provide multi-channel and multi-stream capability along with compression performance enhancements for both MPEG-2 and AVC.