Guide to DVI and HDMI

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Demand for delivery of DVI and HDMI within the Pro AV discipline continues to grow. The proliferation of DVI and HDMI source, processing, and display equipment, the need to present protected content, and the sometimes arbitrary preference for digital AV solutions are factors driving the trend.
 
Moving native digital signal formats over a metre or two of copper cable requires little or no attention, since this is what they were intended to support. In the case of DVI, the primary intent has been to connect the digital video output of a graphics card with a desktop display. The essence of HDMI’s purpose is to provide a secure connection between set top boxes and other consumer-oriented audiovisual sources to televisions and other forms of presentation equipment. Over the short haul, DVI is an excellent standard that provides robust plug & play performance.
 
But neither DVI nor HDMI signals lend themselves particularly well to transmission over distance. Indeed, they were not developed to do so. The signals occupy significant bandwidth and rely on discrete clock and data channels, making them susceptible to the effects of cable dispersion and skew. The configuration of the electronics at the source’s output port and the sink’s input port are only designed to support short haul transmission over native cables using screened pairs. Successful transmission of DVI or HDMI over longer distances or over non-native cabling requires some form of signal conditioning and/or processing.
There are several approaches that can be taken to move native, uncompressed DVI and HDMI over distances that extend well beyond the reach of standard cables. Solutions for native DVI/HDMI extension include: actively equalised ‘long’ HDMI cables; DVI/HDMI over UTP links (transmitters & receivers) and DVI/HDMI over optical fibre links (transmitters & receivers). Each of these provides us with capabilities that fit best with fairly specific requirements. Here’s a quick rundown of these approaches:
 
Actively equalised HDMI cables can support 1080p/60 to distances of up to 40–45m. They tend to be rather heavy and bulky but can provide a reasonably inexpensive, quick and box-less solution for applications where bulk is not a problem.
 
DVI/HDMI over UTP links rely on electronics at both ends to provide the needed signal conditioning to take advantage of the easily terminated, low cost cable. Power supplies are most often required for both the transmitter and the receiver. Pricing for DVI/HDMI over UTP link solutions vary depending on features and performance, but are certainly inexpensive when compared to fibre solutions. UTP can be much more installation-friendly than bulky native DVI or HDMI cables. This is especially true in commercial settings, and wherever there is a significant density of cables or links, or where it is desirable to dress cables neatly into equipment cabinets.
 
DVI/HDMI over optical fibre links pick up where copper solutions leave off. Distances exceeding 40m or so call for fibre solutions, and will provide consistent and reliable transmission of high quality, uncompressed native DVI or HDMI. Both single-mode and multi-mode fibres can be used, depending on the required distance capability and the mode of signal transmission. Popular techniques to support the required multi-channel transmission include multiple fibres (one per channel), twin fibre, TDM (time division multiplexing) and single fibre, CWDM (coarse wave division multiplexing).
 
Twin fibre TDM solutions can be very cost effective, but are usually limited to lower resolution formats to avoid using expensive broadband optics. 720p or 1080i are about the maximum resolutions that can be supported by low cost optics.
 
Depending on the transmission technique being used, DVI and full colour depth HDMI 1.3 can be carried over distances exceeding 600m of single-core multi-mode and multiple kilometres of single mode fibre. Magenta’s M-HDX links can carry DVI or full colour depth HDMI 1.3, along with separate stereo audio, duplex serial, and USB all on one core of multi-mode fibre.
 
DVI extension is usually straightforward, but successful HDMI implementations can prove to be a bit more elusive because of HDCP. Exceeding manufacturer’s distance specs or using unspecified cables can result in issues that are difficult to troubleshoot and remedy.