August 2020
HDMI, HDBaseT, or SDI to Your Projectors? Part 1 - HDMI
HDMI - simple, plug-and-play, ubiquitous. Your inexpensive switcher or presentation computer supports it, and so does your projector. So, it seems to make sense to use the common interface on both ends to connect your presentation system to your projectors, doesn’t it?
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HDMI simple, plug-and-play, ubiquitous. 

Your inexpensive switcher or presentation computer supports it, and so does your projector.  So, it seems to make sense to use the common interface on both ends to connect your presentation system to your projectors, doesn’t it?

Here's the rest of the story.

There are myriad capabilities which HDMI supports to make complex home theaters less complicated to operate. It also includes support for 4K HDR and WCG formats.  However, extending this short-range (“peer-to-peer” or “point-to-point”) P2P system for applications beyond its design parameters is ripe with opportunities for failure at the most inconvenient of times – usually during a live presentation!

 Issue 1 - Distance Limitations

 Long HDMI cables are VERY, very expensive, and surprisingly stiff.

HDMI is designed for short distance connections between consumer devices in the home, carrying audio, video, bi-directional control, and importantly – protection of the signals (HDCP) from copying through device identification.  “Field termination” of connectors on bulk cable is expensive, tedious, and prone to failure, so pre-built cables are the default – and can become very, very expensive at length. 

Fiber is reliable but can be VERY expensive.

HDMI can be extended via fiber, most often as an integrated HDMI-fiber-HDMI one-way cable powered by the end-point devices.  These active cables typically are not suitable for the stresses of pulling through conduits but are excellent as one-off deployments for a single temporary display or a temporary feed to an adjacent building. 

“Extenders” are moderately expensive but can be problematic.

HDMI extenders range from passive “baluns” to relatively expensive media converters/amplifiers. These devices do not decode the content, but instead change only the physical layer to something which supports greater distances. Each has a benefit, and they must be selected very carefully for the application.  The reliability of these devices is hit or miss.

Issue 2 - Reliability

HDMI is designed primarily for point-to-point transmission.

This means one source to one display.  Various devices exist to passively select multiple HDMI signals, but the glitchy results are certainly not something you would want to be displayed on large format IMAG. 

Active devices may include input buffering to allow relatively or completely clean switching between multiple sources.  Buffering introduces latency which is not desirable for live event large format display.  These devices may also block content, which is HDCP protected, especially if the outputs are in a professional format like SDI. Normal workarounds to bypass HDCP are illegal and are also prone to random failures requiring a power-cycle to re-establish the signal.

HDMI “splitters” are active distribution devices.  When using a splitter to distribute to multiple destinations, there can be some confusion with the source device as to what the supported capabilities are of the display which is sending return information. The sending device is looking for a handshake to determine the resolution capability of the receive device (EDID) and if the receiver is authorized for HDCP.  Which one of the receive devices provides that information and will be appropriate for all the receive devices?   

Splitters are often in the extreme low-cost range and the same device can be sold under a dozen different brands changing almost monthly.  When issues do arise, often a power-cycling of these devices at the most inconvenient of times is necessary to temporarily regain functionality.

HDMI - The Conclusion

In short, HDMI works very well for its intended purpose like short jumpers from certain professional equipment to nearby displays.  No matter how convenient, it is NOT intended for use in fixed installations outside of the home.  The shortcomings in the extended use of this format unfortunately play out all too often right in front of your largest gatherings!


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