High definition optical discs and their associated copy protection acronyms


The next generation optical disc formats, like standard DVD, are encrypted on disc, but unlike standard DVD, also require protected paths from the player to the display. With this comes a confusing new group of terms, many of which betray the idiosyncrasies of the next generation content protection requirements.

The confusion has become obvious, especially when otherwise very technically literate websites start producing Blu-ray and HD DVD articles but confuse the various copy protection mechanisms. I’ve seen the confusion become quite widespread among users and now tech journalists. Let’s go over how this all works…

At the most basic what we have is an optical drive, the associated A/V decoding and processing circuitry (or software), and various outputs of both analog and digital nature. What happens is that an AACS key is exchanged, the exact processes isn’t important but suffice to say it is more complicated then standard DVD. Once this key is authenticated the data on the disc can be read by the player.


Now what happens to this data is up to one key element:
How is the player connected to the display?

  • 1. Is it connected via a digital link (DVI or HDMI)?
    • If digital, does it have HDCP and has it handshaked properly?
      • If no — stop immediately and warn the user. 
      • If yes — continue at full resolution.
  • 2. Is it connected via a high definition capable analog connection (Component or VGA)?
    • If yes, is the DOT set? If no, continue to the next step
    • Is the ICT set? If so — downsample the image to no more then 960×540.
  • 3. Is it connected via a legacy standard definition analog connection? (why you’d use this for an HD optical disc is a bit silly, but it needs to be addressed)
    • If so, is the DOT set? If yes, warn the user and disable any playback.
    • If no, is the player Macrovision enabled? If so enable Macrovision and allow playback.

A quick FAQ with definitions of terms

What is HDCP?
High-bandwidth Digital Copy Protection: a standard of content protection that encrypts each pixel as it moves from a PC or set-top box to digital displays. It can be used with a DVI or HDMI interface for connecting digital A/V components.

Why do I need HDCP?
Blu-ray and HD DVD all REQUIRE a secure connection when using DVI or HDMI. This means it must support HDCP. If you have a DVI or HDMI connection but one of the links in the connection chain isn’t HDCP enabled, you’re flat out of luck.

Does HDCP protect the content on the disc?
The content itself is not "HDCP protected." The content is protected by AACS using an Advanced Encryption Standard (AES) key on the optical media.
HDCP is a "down the wire" encryption mechanism for digital video and audio transport. HDCP is made to protect the link between the player and the monitor/TV and not the file system of the media.

Does HDCP prevent analog output?
HDCP is about digital output. It has NOTHING to do with whether analog output is possible and/or whether the output is full 1080p resolution.

What has control over the analog output?
The control over analog output is part of the AACS specifications. There are two flags that the movie studio may choose to set when the disc is authored: 

1. The ICT (Image Constraint Token) can be enabled to downsample the 1080p content to 540p. The ICT has not been used by any studio to date and it is rumored that most won’t until 2010, when the start of the "analog sunset" will begin. This means that the ICT is basically a way to start coaxing consumers with legacy HDTVs to migrate to a HDTV with a proper digital connection when 2010 comes.

2. There is also a DOT (Digital Only Token) flag that is not currently valid to use, but is in the specifications for future use. The DOT’s purpose appears to be to facilitate a full "sunset" of the analog outputs in 2013.


In summary

So let’s be clear. HDCP is NOT a flag, it is NOT copy protection on the optical disc. It is one of the approved protection mechanisms for audio and video TRANSPORT. It has nothing to do with the content itself. That being said, HDCP must be present with DVI or HDMI to satisfy the AACS security. This is to provide protection when the movie is transmitted from the player to the display, insuring you can’t insert a capture device between the player and the display to capture the raw digital data.

I think what most people get HDCP confused with is the other highly talked about acronym: ICT. The ICT is a flag a studio may set, though none have yet. It is the infamous flag that filters the image down to a quarter of the resolution when using an analog connection. The ICT is about analog connections and trying to discourage their use — it has no bearing on digital connections. If you have a digital connection with HDCP you have met all the AACS security checks and are allowed full 1080p resolution.

One final disclaimer

This write up is about how the consumer electronics players and current Windows XP software players work. Vista’s content protection mechanisms are considered a valid digital protection scheme by the AACS. So it is possible with Vista’s core level content protection schema that slightly different variations of the rules I laid out above will be allowed for Vista PCs when Vista native player software is available. For example: a DVI connected PC monitor without HDCP under Vista might be allowed to display the movie, unlike consumer devices which won’t allow playback at all, but it will be at quarter HD resolution, note that this is not the ICT (which is for analog output), this would be an idiosyncrasy of Vista’s content protection.