Compression Conundrum

VC-1, AVC battle it out for next-generation codecs
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VC-1, AVC battle it out for next-generation codecs


Acronyms are flying and manufacturers are posturing as the battle for the next generation of coding technology prepares to hit the marketplace. As is often the case, an acronym war may very well presage a format war. The H.264 standard, aka MPEG-4 Part 10 AVC and now generally known as simply AVC or Advanced Video Coding. is a fully defined standard with soon-to-be-shipped products. Microsoft is offering a competing codec technology that is in the process of development toward becoming an SMPTE standard and is known as VC-1 (Video Codec One). Based on the video codec in Windows Media 9, this standard was originally called VC-9, but SMPTE decided to call it VC-1 since it was the first codec to be standardized by the organization.

Matthew Goldman, director of technology for Tandberg Television refers to both of these codecs as ACT, which stands for Advanced Coding Technologies --and that is exactly what each codec is promising. Both are reliably alleged to be at least 50 percent more efficient than MPEG-2, and the result is broadcast-quality standard-definition video coded at below 1.5 Mbps and high-definition video coded at as little as 6 Mbps.

The market for these advanced codecs is potentially huge. The telcos--unburdened by legacy set-top boxes--will be able to use advanced codecs to deliver video to the home via DSL. The next generation of direct-to-home satellite service, with its emphasis on HDTV, will be able to implement these advanced codecs, as will the next generation of high-capacity DVD players.

"Incumbent services cannot abandon MPEG-2 but new services such as the telcos with DSL and satellite delivery of HDTV can take advantage of AVC," said Rob Robinett, CTO of Modulus Video, a Sunnyvale, Calif.-based developer and manufacturer of AVC codecs. "The economics of the satellite business is really changed by AVC. With the availability of AVC, with its huge bandwidth efficiency gains, it makes a very poor business case to justify using MPEG-2 for new services."

Proponents of VC-1 make the identical business case for their codec.


It is standardization that drives acceptance of these new codec technologies. AVC has, in fact, been an international standard for about one year now. A Joint Video Team (JVT) composed of coding experts representing the ISO/IEC and the ITU-T organizations (from the moving picture experts group [MPEG] and the video encoding experts group, respectively), created this advanced coding standard.

These organizations are now poised to ratify extensions to the AVC standard that would result in four new profiles including High Profile directed at broadcast television applications, as distinct from the currently ratified Main Profile. The AVC High Profile is a superset of the Main Profile and adds a few new tools, including the adaptive use of an 8x8 block transform and quantization scaling matrices. The technical work is complete and as of this writing is awaiting formal ratification.

VC-1 is well into its standards process under the auspices of SMPTE. Based on the Windows Media 9 Advanced Profile, which is roughly equivalent to the AVC Main Profile, VC-1 includes tools such as the 8x8 block transform that appear only in the new extensions to AVC.

According to Peter Symes, SMPTE vice president of engineering, "the standardization of VC-1 is somewhat controversial. There are those who think nobody should have the nerve to create a compression standard that is not MPEG. Some felt SMPTE would do nothing but rubber-stamp Microsoft's desires, but that has not been the case."

Symes explained that the SMPTE Technology Committee decided that a reference decoder must be created in software to validate the VC-1 standards document. A third-party vendor divorced from Microsoft was contracted to create this software decoder. Symes referred to this as a "clean room exercise."

This project has resulted in 100,000 lines of code and a 140-page supporting document according to Symes.


ATSC is considering both AVC and VC-1 for use in its proposed E-VSB or Enhanced VSB standard.

"We are reviewing video codecs with some study and evaluation of AVC and Windows Media," said ATSC President Mark Richer. "These would not be in place of MPEG-2, which will remain the codec for primary services. The current effort is for new coding for E-VSB in order to provide some bits for a more robust mode. These codecs could be used to supplement the main program or for new services.

"ATSC is considering whether we adopt one, or both, or neither. It is possible there will be a preliminary decision before the end of the year but that is not guaranteed. The time frame is tough to predict," said Richer.

Even with, or perhaps because of this ongoing standards activity, Tandberg Television and Harmonic are bringing to market codecs that support AVC and VC-1 as well as MPEG-2. Modulus Video, however, is offering products that support only AVC.

"It is no small task to optimize and support all these different codecs," said Neil Brydon, director of product marketing for Modulus.

"We are focused on AVC and supporting all the tools that bring efficiency to AVC," said Robinett.

The Modulus executives pointed out that AVC is already a standard. "VC-1 is just entering the standards process, whereas AVC is finished," said Robinett. "VC-1 has not been tested to any extent in the brutal environment of broadcast. And that includes their DRM [Digital Rights Management] technology as well as their compression technology."

"Our customers have moved dramatically toward AVC," Brydon said. "Customers understand the benefits of truly international open standards. They are wary of VC-1, and the SMPTE process is some way off completion."

Jordi Ribas, Director of Technical Strategy, Windows Digital Media Division for Microsoft said, "We were motivated to submit Windows Media to SMPTE for standardization. Our effort, openness and willingness to play by the rules are one reason we are being successful in getting this standard adopted."

Symes agreed that it was fair to say that VC-1, when fully specified and adopted, will be a truly open standard.

Outside the world of broadcast, the two organizations competing for next generation DVD technology (Blu-ray Disc Association and the DVD Forum) have both announced specifications that require support for all three flavors of codec: VC-1, AVC and MPEG-2.

Tandberg Television believes it has come up with a product solution that protects its customers from choosing the wrong side of a format war. Its ICE (Intelligent Compression Engine) platform allows a customer to buy an MPEG-2 codec and activate either AVC or VC-1 in the future.

"You can buy now and run MPEG-2 with the ICE option for either VC-1 or AVC," Goldman said. "With a software license you can, in the future, activate either AVC or VC-1. This gives the customer all-format protection. You can rack the equipment and you're done. You'll have an MPEG-2-only box that can be activated to become either an advanced codec without pulling the box out of the rack."

As for the differences between the two codecs, Goldman said, "from a high-level point of view, they have much in common in regard to coding efficiency The differences are deep down in the coding level, either with how or where a coding tool is applied in the processing chain, or in a few cases, with the choice of a different tool to perform a particular function. Both are excellent and they far exceed the capability of MPEG-2 in bit-rate reduction.

"AVC is more complex and may ultimately offer the potential for higher quality and more efficient coding. VC-1 is less complex and therefore, presumably easier to implement efficiently," Goldman said.

Examples of specific differences between AVC and VC-1 include AVC's use of six-tap filters versus VC-1's use of four-tap filters. AVC uses an entropy scheme known as CABAC (context-adaptive binary arithmetic coding) that is computationally intensive and therefore expensive to implement. VC-1, by contrast, uses high-order entropy coding that is less complex and less expensive to implement.

"We chose smarter algorithms that are less complex," Ribas said. "It is a significant advantage to be less complex. Rather than the kitchen sink approach, we were very careful in our design choices."

Modulus' Robinett, however, believes that CABAC is one of the elements of AVC that will provide video quality superior to VC-1.

"VC-1 is a codec but it is also part of the complete Windows Media platform," Ribas said. "As a codec, it is merely one important piece of an end-to-end solution, which includes a DRM solution blessed by content providers, advanced servers, media players, chips, CE devices, and other key components. This is technology that can be used wherever there is digital content, whether it is the Internet, smart phones, DVD and HD-DVD players and video services transported over DSL, satellite, cable or IP networks. With Windows Media in common, all these devices will be able to talk to each other."

Tandberg Television's Goldman says there's room for both codecs in the marketplace.

"We currently have multiple audio standards such as MPEG Layer II, Dolby AC-3, MPEG AAC and DTS," he said. "The industry supports over five different audio standards and it has been left to the marketplace to decide what is most appropriate for each application. Surely the industry can support two different advanced video codecs as well."