HDMI: It’s NOT JUST CABLE LENGTH And the role EDID and HDCP encryption will play. BY ROB CARTER The promise of the High-Definition Multimedia Interface (HDMI) is great. installation industry, and the design One cable carries uncompressed digital HD video and audio; what’s not to like? doesn’t scale well. Compounding the But mention HDMI to an AV integrator and you’re likely to get an expletive in problem is the fact that digital control response. A quick search of online forums and industry trade pubs will uncover is relatively new to most of the major a slew of HDMI-related complaints, ranging from annoying switching delays AV distribution players, so the learning and screen flashing to complete audio and video failure. curve has been pretty steep. Though much more complex than Two Reasons For Problems analog, HDMI isn’t nearly as compli- There are two primary reasons for HDMI problems: bandwidth and complex- cated as home automation, Ethernet or ity. Uncompressed HD video requires enormous digital bandwidth, which is any of the myriad wireless protocols. notoriously difficult to push down copper wire. Add popular features, such as Companies with experience in these 1080p resolution and Deep Color, and the problem gets worse. Products are fields are in a position to implement cropping up on the market that attempt to address this issue, some of which HDMI in environments that the de- actually work quite well. But the under-discussed issue is the sheer complex- signers hadn’t anticipated. ity of HDMI. Here, we’ll address the need for the HDMI is a full-duplex digital communications interface. HDMI takes ad- move to HDMI and explain the new vantage of its digital nature and adds several communications mechanisms features that HDMI supports. We’ll that are designed to control and encrypt the content automatically. Unfortu- demystify the “handshaking” that oc- nately, the engineers who designed HDMI weren’t thinking about the custom curs between HDMI sources, repeat- Rob Carter earned a degree in Computer Engineering from Johns Hopkins University, and currently is a firmware developer and HDCP expert at Crestron Electronics, Inc. 78 Sound & Communications www.soundandcommunications.com ers and displays. We’ll also explore the runs the risk of becoming obsolete DK:G*%%IDE7G6C9H6K6>A67A: cause of some common problems seen over the next several years. Now is IDB::I6CNHE:8;DG6CN?D7# in the field. the time for the consumer electronics and custom installation industries to Say Goodbye To Analog embrace the transition. Before we get too involved in the HDMI’s complexity becomes obvi- technical details, there’s an important ous when you compare its cables to question to address: Why do we need those of analog audio and video. Ana- HDMI? As much as a lot of us would log cables typically consist of one to love to stick with tried-and-true analog three wire pairs, depending on the for- distribution, for much of the market, mat, and they simply carry an audio it’s on its way out. Content provid- or video signal. In contrast, the HDMI ers such as television producers and cable consists of 19 wires, which carry movie studios love the fact that HDMI the following digital signals: supports the High-bandwidth Digital • TMDS: The digital audio and vid- Content Protection (HDCP) protocol. eo data is encoded into three Transi- HDCP allows them to encrypt content tion Minimized Differential Signaling while it’s on the wire so it can’t be cop- (TMDS) channels. TMDS is the trans- ied and pirated easily. They’re push- port protocol used by HDMI to reduce ing hard on the consumer electronics transmission problems. Embedded in Full Discount Wholesale industry to move from analog AV to the video data are InfoFrames, which is the premier supplier of HDMI, and they’re getting their way. are data packets that carry informa- Professional Audio, Video, For the unconvinced, here are a few tion about the AV content. The three A/V and Lighting equipment harbingers of analog’s demise: TMDS channels correspond to the to resellers and contractors. • Image Constraint Token: The Blu- three channels of RGB or YCbCr (the ray specification has a built-in time digital equivalent of YPbPr), depend- FDW Offers: bomb that’s yet to go off: the Image ing on the format. Audio and Info- Extensive Inventory Constraint Token (ICT). This disc-spe- Frame data is spread across the three Low Cost Shipping cific flag forces players to downgrade channels during blanking periods in No Minimum Order video on analog outputs to 540p, half the video. These three channels and the 1080i resolution of the current a TMDS clock are carried over four Hundreds of the Top Brands analog maximum. Movie studios are pairs of wires in the cable. Friendly, Experienced Sales Pros waiting for more widespread HDMI Exceptional Customer Service and Blu-ray adoption before shipping Data Display Channel Blind Drop Ship Capability discs that implement this limitation, • DDC: The Data Display Channel and reportedly have decided not to (DDC) is a two-way communications do so until 2010 or 2012.1 interface between the source and the • Content provider support: SkyHD, downstream repeater or display device. a popular European satellite service, Originally, this channel was provided is already shipping HDMI-only set-top to communicate device capability in- boxes. In the United States, the MPAA formation, which is encoded in a struc- is petitioning the FCC for permission to ture known as Extended Display Iden- block certain movies from being trans- tification Data (EDID). HDMI devices mitted on set-top box analog outputs.2 use the EDID to indicate what audio and video formats they support. This Feature Support is discussed in more detail later. The • Feature support: The latest and DDC interface is also used to set up greatest features are supported only and maintain HDCP encryption. by HDMI. The aforementioned 1080p • Hot Plug Detect: The downstream and Deep Color formats, for instance, device, or sink, indicates its presence aren’t available over analog, and they to the source with the Hot Plug De- never will be. Early adopters, the tect (HPD) signal. The sink can toggle bread-and-butter for much of the cus- the Hot Plug Detect signal to reset the tom installation industry, will need to HDMI connection, which resets the move to HDMI. HDCP session and triggers an EDID Any analog distribution system that read. carries commercially created content There are more control signals asso- September 2008 79 have in different ways. Combining the EDIDs can be a complicated issue, so it’s worth researching how a given switch handles EDIDs before installation. Consider a simple system you might design for a financial services firm. The client has a 1080p Deep Color projector with a surround sound pro- cessor in the boardroom and a 720p LCD with integrated speakers at the trading desk. The 1080p projector also supports 720p but, obviously, the cus- Figure 1. Figure 2. ciated with HDMI, but they are beyond The EDID exists in a memory chip tomer would prefer 1080p when pos- the scope of this article. on the display or AV processing device. sible. How should the switch combine Every HDMI installation consists of The content source reads the EDID the EDIDs? at least one content source, such as a over the DDC interface and analyzes Some devices on the market simply cable box or Blu-ray player, and a sink, it to decide what formats to send. It copy the EDID from the first output, as such as a TV or projector. Most custom is the responsibility of the content depicted in Figure 1. In our scenario, installations will also involve at least source to send only formats that the 1080p Deep Color video and surround- one repeater, which is a device that ac- downstream devices can support. HD sound audio will be sent to the trad- cepts and retransmits HDMI content. players, such as Blu-ray players, must ing desk LCD, which supports neither. Repeaters include simple devices, such include a video scaler to re-scale the At best, this will result in no audio or as switches and distribution amplifiers, video on the disc to match the capa- video and, at worst, in damage to the as well as more feature-rich devices, bilities of the television or projector. LCD display or speakers. such as audio and video processors. Likewise, the source must provide a supported audio format. Smarter Switch EDID In the simplest installations with A slightly smarter switch may take a • EDID: HDMI display and AV pro- one television and an audio proces- “best common” approach and generate cessing devices use the EDID to ad- sor, the EDID protocol works reason- an EDID that advertises formats that vertise their capabilities. For instance, ably well. Multi-room installations, on both rooms support. This scenario is a television may use the EDID to indi- the other hand, quickly can become shown in Figure 2. The merged EDID cate support for the standard HD reso- much more complicated. Several tele- will limit the content to 720p video and lutions plus 1080p and Deep Color. An- visions may be connected to several stereo audio. Now, both rooms support other TV may max out at 720p/1080i sources through one or more HDMI the content, but the client’s lavish resolution. One audio processor may switches. The switches are responsible boardroom isn’t getting the topnotch support Dolby TrueHD, whereas an- for collecting the televisions’ EDIDs content that justifies the expense. Ex- other only supports standard Dolby.