The interoperability of DTV and consumer electronics
Nov 5, 2006 8:00 AM
HDMI and device control
In the HDMI interface specification there is a single wire that is dedicated to automated device control. This wire acts as the Consumer Electronics Control (CEC) bus. A device tree is built (via auto-discovery) that maps the interconnection of HDMI media devices. Devices have both a physical address (PA) and logical address (LA).
The device that acts as the master is considered the root device, and is given the base PA of 0000 and LA of 0. PAs increment for each device (0, 1, 2, …), while LAs are derived from the level that a device occupies in the tree. For example, if a digital television is the master and is connected to a DVR, the DVR would have a PA of 1 and an LA of 1000, where 1 signifies that it is one level down and connected to the master. If an STB and DVD player are connected to the DVR, then the STB PA is 2 and LA is 1100. The DVD has a PA of 3 and an LA of 1200. Using this methodology, up to 10 devices can communicate with each other.
Clogging the data pipeAs with most digital media applications, transfer speed is essential. Theoretically, implementation of data services across HDMI using CEC is feasible; however, CEC has a limited data rate of 500b/s or less. An alternative method could be sending data during blanking intervals over HDMI. This would require a mechanism to parse the MPEG transport stream, extract data services packets and distribute them over HDMI to ACAP- or OCAP-capable devices.
HDMI 1.3 can support up to 1440 60p video and the emerging uncompressed Dolby TrueHD and DTS-HD audio formats. Content in these formats is too large to fit down any of the traditional content delivery pipes. This presents Internet and terrestrial broadcasters with a channel capacity problem. Cable and satellite operators have the technological means but would have to dedicate and bond multiple channels together to attain a sufficient aggregate bandwidth. This would reduce the number of DTV services that could be carried and would, in turn, diminish revenue.
Advanced codecs may solve the bandwidth problem, but receiving and presentation devices and content production infrastructures must still support the voluminous data requirements. Despite these roadblocks, the consumer electronics industry is still grappling with the viability of true 1080 60p content.
The evolution of HDMISomewhere down the line, there is an opportunity for forward-looking professional broadcast equipment and consumer device manufacturers to address the problem of getting DTV data services over HDMI. To do this, the ATSC, SMPTE, CEA and others need to better coordinate more standards development.
HDMI solves many of the problems inherent in consumer digital media system interconnectivity, but to facilitate the promise of DTV features, it must evolve to include more than just audio and video distribution and simple device control.
Additional Reading:
The HDMI future, Paul McGoldrick, Broadcast Engineering Magazine, Aug, 2006
broadcastengineering.com/infrastructure/broadcasting_hdmi_future/index.html.
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