Windows Vista Content Protection - Twenty Questions

[Dominus Atheos]

Windows Vista Blog

A conversation has cropped up since the recent publication of a paper scrutinizing how Windows handles digital rights management, especially for HD video. I've since looped back with Dave Marsh, a Lead Program Manager responsible for Windows' handling of video, to learn from him the implications involved and to learn to what extent the paper's assertions are accurate. The following is an article Dave has put together to address the misconceptions in the paper, followed by answers to what we expect will be the most frequent questions in the minds of our customers. Leave us a comment to let us know what you think. -- Nick

Over the holidays, a paper was distributed that raised questions about the content protection features in Windows Vista. The paper draws sharp conclusions about the implications of those features for our customers. As one of the Lead Program Managers for the technologies in question, I would like to share our views on these questions.

Windows Vista includes content protection infrastructure specifically designed to help ensure that protected commercial audiovisual content, such as newly released HD-DVD or Blu-Ray discs, can be enjoyed on Windows Vista PCs. In many cases this content has policies associated with its use that must be enforced by playback devices. The policies associated with such content are applicable to all types of devices including Windows Vista PCs, computers running non-Windows operating systems, and standalone consumer electronics devices such as DVD players. If the policies required protections that Windows Vista couldn't support, then the content would not be able to play at all on Windows Vista PCs. Clearly that isn't a good scenario for consumers who are looking to enjoy great next generation content experiences on their PCs.

Associating usage policies with commercial content is not new to Windows Vista, or to the industry. In fact, much of the functionality discussed in the paper has been part of previous versions of Windows, and hasn’t resulted in significant consumer problems – as evidenced by the widespread consumer use of digital media in Windows XP. For example:

* Standard definition DVD playback has required selective use of Macrovision ACP on analog television outputs since it was introduced in the 1990s. DVD playback on and in Windows has always supported this.
* The ability to restrict audio outputs (e.g., S/PDIF) for certain types of content has been available since Windows Millennium Edition (ME) and has been available in all subsequent versions of Windows.
* The Certified Output Protection Protocol (COPP) was released over 2 years ago for Windows XP, and provides applications with the ability to detect output types and enable certain protections on video outputs such as HDCP, CGMS-A, and Macrovision ACP.

It's important to emphasize that while Windows Vista has the necessary infrastructure to support commercial content scenarios, this infrastructure is designed to minimize impact on other types of content and other activities on the same PC. For example, if a user were viewing medical imagery concurrently with playback of video which required image constraint, only the commercial video would be constrained -- not the medical image or other things on the user's desktop. Similarly, if someone was listening to commercial audio content while viewing medical imagery, none of the video protection mechanisms would be activated and the displayed images would again be unaffected.

Contrary to claims made in the paper, the content protection mechanisms do not make Windows Vista PCs less reliable than they would be otherwise -- if anything they will have the opposite effect, for example because they will lead to better driver quality control.

The paper implies that Microsoft decides which protections should be active at any given time. This is not the case. The content protection infrastructure in Windows Vista provides a range of à la carte options that allows applications playing back protected content to properly enable the protections required by the policies established for such content by the content owner or service provider. In this way, the PC functions the same as any other consumer electronics device.

With that introduction, here are the top twenty questions, and answers, that aim to address some of the other points raised in the paper.

Dave Marsh - Lead Program Manager for Video

Twenty Questions and Answers

Do these content protection requirements apply equally to the Consumer Electronics industry supplied player devices such as an HD-DVD or Blu-Ray player?

Generally the requirements are equivalent for all devices. For example, an HD-DVD or Blu-Ray disc always requires HDCP protection for DVI/HDMI outputs regardless of the type of device playing the disc. There are some cases, such as DVD-Video, where PCs have slightly different protection requirements than CE devices, but these differences are mainly historical and as dictated by the licenses associated with the systems providing access to the content (e.g., CSS for DVD).

When are Windows Vista's content protection features actually used?

Windows Vista's content protection mechanisms are only used when required by the policy associated with the content being played. For Windows Vista experiences, if the content does not require a particular protection, then that protection mechanism is not used.

Will the playback quality be reduced on some video output types?

Image quality constraints are only active when required by the policy associated with the content being played, and then only apply to that specific content -- not to any other content on the user's desktop. As a practical matter, image constraint will typically result in content being played at no worse than standard definition television resolution. In the case of HD optical media formats such as HD-DVD and Blu-Ray, the constraint requirement is 520K pixels per frame (i.e., roughly 960x540), which is still higher than the native resolution of content distributed in the DVD-Video format. We feel that this is still yields a great user experience, even when using a high definition screen.

Will this affect things like medical imagery applications?

Image constraints only apply to protected content being played and not to the desktop as a whole; therefore, the resolution of other non-protected media, such as medical images, is not affected.

Do things such as HFS (Hardware Functionality Scan) affect the ability of the open-source community to write a driver?

No. HFS uses additional chip characteristics other than those needed to write a driver. HFS requirements should not prevent the disclosure of all the information needed to write drivers.

Will the Windows Vista content protection board robustness recommendations increase the cost of graphics cards and reduce the number of build options?

Everything was moving to be integrated on the one chip anyway and this is independent of content protection recommendations. Given that cost (particularly chip cost) is most heavily influenced by volume, it is actually better to avoid making things optional through the use of external chips. It is a happy side effect that this technology trend also reduces the number of vulnerable tracks on the board.

Will Windows Vista content protection features increase CPU resource consumption?

Yes. However, the use of additional CPU cycles is inevitable, as the PC provides consumers with additional functionality. Windows Vista's content protection features were developed to carefully balance the need to provide robust protection from commercial content while still enabling great new experiences such as HD-DVD or Blu-Ray playback.

Aren't there already output content protection features in Windows XP?

Yes. Output content protections are not new requirements for commercial content. The CSS content protection system for DVD-video discs requires output protections such as Macrovision ACP and limiting the resolution on component video outputs to standard definition. Windows XP has supported these requirements for some time.

Is content protection something that is tied to High Definition video?

While HD content has some unique content protection requirements, many of the requirements apply to commercial content generally, independent of resolution.

What about S/PDIF audio connections?

Windows Vista does not require S/PDIF to be turned off, but Windows Vista continues to support the ability to turn it off for certain content -- a capability that has been present on the Windows platform for many years. Additionally, in order to support the requirements of some types of content, Windows Vista supports the ability to constrain the quality of the audio component of that content. Similar to image constraint for video, this quality constraint only applies to the audio from content whose policy requires the constraint, not to any other audio being played concurrently on the system. As a practical matter, these audio restrictions are not widely used today.

Will Component (YPbPr) video outputs be disabled by Windows Vista's content protection?

Similar to S/PDIF, Windows Vista does not require component video outputs to be disabled, but rather enables the enforcement of the usage policy set by content owners or service providers, including with respect to output restrictions and image constraint.

Will echo cancellation work less well for premium content?

We believe that Windows Vista provides applications with access to sufficient information to successfully build high quality echo cancellation functionality.

Will it mean that there will no longer be unified graphics drivers?

The Windows Vista content protection requirements for graphics drivers will not lead to movement away from unified drivers. In fact, all graphics drivers shipped with Windows Vista are unified drivers.

Will Windows Vista audio content protection mean that HDMI outputs can't be shown as S/PDIF outputs?

It is better if they show as different codec types, as it allows the difference to be reflected in the UI, thus providing the user help with their configuration and creating a better user experience. The user wants to know the difference between HDMI and S/PDIF, as they are different physical connectors.

What is revocation and where is it used?

Renewal and revocation mechanisms are an important part of providing robust protection for commercial audiovisual content. In the rare event that a revocation is required, Microsoft will work with the affected IHV to ensure that a new driver is made available, ideally in advance of the actual revocation. Revocation only impacts a graphics driver's ability to receive certain commercial audiovisual content; otherwise, the revoked driver will continue to function normally.

Does this complicate the process of writing graphics drivers?

Adding new functionality usually introduces new complexity. In this case, additional complexity is added to the graphics driver, but that complexity comes with the direct consumer benefit of new scenarios such as HD-DVD or Blu-Ray playback.

Will the 'tilt bit' mechanism cause problems even when the driver is not under attack from a hacker, e.g., when there are voltage spikes?

It is pure speculation to say that things like voltage fluctuations might cause a driver to think it is under attack from a hacker. It is up to a graphics IHV to determine what they regard as an attack. Even if such an event did cause playback to stop, the user could just press 'play' again and carry on watching the movie (after the driver has re-initialized, which takes about a second). Again, it is important to note that this could only occur in the case of watching the highest-grade premium content, such as HD-DVD or Blu-Ray. In practice I doubt it would ever actually happen.

Does Windows Vista's use of OMAC-authenticated communication impact graphics driver performance?

The authenticated communication mechanisms used for Protected Video Path in Windows Vista are only actively used while commercial content is playing. This means that while there is a performance impact, it is limited to the scenarios where it is required to provide robust protection for commercial content.

Do content protection requirements mean that graphics chips have to provide hardware acceleration for video decode?

No. The Windows Vista content protection requirements do not require that graphics hardware include hardware acceleration for decode for many years, but such support is highly recommended to improve the user experience for HD content.

Will the video and audio content protection mechanisms affect gaming on the PC?

The Windows Vista content protection features were design for commercial audiovisual content and are typically not used in game applications. A game author would have to specifically request these features for them to impact game performance.