The Missing Link

During most of our recent tests of HDTVs, we've attempted to use them with a Scientific-Atlanta 8300HD cable box supplied by Time Warner connected via an all-digital HDMI (High-Definition Multimedia Interface) hookup. We often end up looking at a screen displaying an imperious message typical of cable-company communications: "Your HDTV does not support HDCP. Please use the YPr Pb component connection to watch television." Sometimes we don't even see that. Thing is, these TVs are claimed by their manufacturers to comply with the HDCP (High-bandwidth Digital Content Protection) spec and do successfully display HDCP-encoded video when it's fed from DVD players equipped with HDMI outputs. We are not amused.

While we try to track down the cause of this "interoperability" problem, a glimpse at how the HDCP system works might offer some hints. There are three main aspects to HDCP technology, all covered by the spec (downloadable from digital-cp.com).

First, there is an authentication system by which an HDCP "transmitter," such as a cable box, determines that an HDCP "receiver," like a television, is licensed to receive the content. During this "handshake" process, the two devices send data about their identities to each other over the HDMI connection and jointly calculate the "shared secrets" used by the HDCP data-encryption system. This scrambles the audio and video data flowing from the transmitter to the receiver via HDMI, preventing any "eavesdropping device" from using the content. If such a device, such as an HDCP-buster box, did somehow pass the authentication-handshake step, there is a permission-revocation system by which incoming content (such as the feed from a cable company) updates a list of blacklisted devices that is supposed to be consulted by the transmitter during the handshake process.

Given these three processes, it's unlikely that anybody has yet deauthorized anything using the HDCP system, much less brand-new TVs. That's because precisely how revocation is supposed to operate through a specific program path (cable, DVD, satellite, etc.) isn't covered in the HDCP spec, in contrast to the other two processes, which are covered in excruciating detail.

That detail is necessary since getting even a single bit wrong in the encryption or authentication systems can throw the whole thing off, perhaps leading to that Scientific-Atlanta error message. But the TVs we've been testing have all operated without problems with DVD-player HDMI outputs, all of which are valid HDCP "transmitters." So the TVs' data-decryption systems seem to operate correctly, as does their half of the handshaking process. My suspicions fall on the authentication phase of Scientific-Atlanta's HDCP processing, which can fail for all sorts of reasons (including bad cables, poor connections, improper HDCP execution, and defective HDMI signals).

If you think there are HDCP interoperability issues now, just wait until high-def disc recorders come out and have HDMI inputs and outputs "protected" by HDCP. The HDMI spec covers not just the flow of audio and video signals but also the control of one component by another. Can you imagine a Blu-ray Disc recorder and a cable box duking it out over control of a system? Armageddon would be more fun.

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