HDMI/DVI HDCP handshake problems & how to avoid them

With the advent of affordable 1080p displays, 8-channel 192 kHz sound systems and high-definition A/V sources, consumers are switching to HDMI & DVI uncompressed digital A/V interfaces en masse. Why? Because in many cases these interfaces are the only means to obtain protected content and to maintain pristine quality across long signal chains.

But there's a hitch. HDMI & DVI have a companion high-definition content protection (HDCP) system that sometimes leaves authorized consumers in mute, watching a blank screen, blinking video, or snow while being held hostage by a bug known as the "HDCP handshake problem."

In this article, we'll review the key issues surrounding this problem and introduce you to some rules and tools that'll help you keep your HDMI and DVI design out of trouble.

The Basics
Both HDMI and DVI are HDCP interfaces. HDCP interfaces protect high-value content as it travels between HDCP transmitters and HDCP receivers on its way to presentation devices. HDCP involves legal issues that are beyond the scope of this article " so you should consult your legal department before finalizing your design. For this discussion, we've boiled all the legal requirements down to three basic rules:

1. An HDCP interface must encrypt high-value content when told to do so. In the case of Blu-Ray and HD DVD players, content is encrypted whenever an Image Constraint Token (ICT flag) is true. Once encrypted, content is referred to as "HDCP content."
2. HDCP content must stay encrypted until it reaches the presentation device. The only exceptions are outlined in Exhibit C of the HDCP License Agreement. Exhibit C relaxes this rule slightly for audio, temporary buffering, repeater decrypt/re-encrypt and presentation device processing, such as scaling. The current HDCP license agreement assumes wired point-to-point routing, so for now networked A/V products that require LAN-based or wireless interfaces -- such as digital amplified speakers, video walls and the like - will have to wait for a new agreement from DCP LLC that allows transmission of HDCP content over "proprietary interfaces" incorporating encryption methods such as AES-128 and AES-256.
3. The HDCP interface must allow unprotected non-HDCP content to pass unencrypted.