Technology Showcase: HDMI Extenders
May 26, 2008 12:00 PM, By Jay Ankeney
Products to extend HDMI's reach without sacrificing signal quality.
The AV industry is increasingly looking for methods to extend HDMI's reach. This can become very complicated to deal with because the length and integrity of an HDMI connection can depend on the quality of the transmitter at the head end and the receiver at the device or sink end, as well as the quality and content of the cable itself.
The HDMI specification itself does not define a maximum cable length. Instead, HDMI specifies a minimum performance standard, and any cable meeting that specification is considered compliant. Although this can vary greatly, usually a cable of about 5 meters can be manufactured cost effectively to perform at speeds of 75Mhz — which is the equivalent of a 1080i signal, using 28 AWG (American Wire Gauge) wire. Cables using 24 AWG and tighter construction tolerances can reach lengths of 12 meters to 15 meters at speeds of 340MHz, which can successfully handle 1080p signals — including those with increased color depths and/or higher refresh rates.
But you need to throw into the mix the reality that not all HDMI devices conform to the same basic specification in the same way and that some elements of the specification are optional, which means not every manufacturer incorporates them into their gear. The result can sometimes be signal-degrading incompatibilities even with the best of intentions.
In order to deliver the best possible audio, video, and data from an HDMI cable length that would be considered quite short by coaxial standards — sometimes as little as 5 meters — the signal usually has to be regenerated or restored. You can't just amplify the signal because that would simply boost the bad with the good. Additionally, if the goal is to send the HDMI signal to multiple receivers over wire connections, some sort of distribution amplifier matrix will be needed to keep the signal intact.
With HDMI signals transmitting up to 10Gbps of complex information, restoration of the signal must be extremely precise so that — among other factors — the three TMDS channels and clock fire in correct sequence. The signal is usually graphically depicted as an eye-pattern test, earning its name because an ideal result looks like the shape of a human eye with a wide round opening surrounded by symmetrical lines. If the eye pattern starts to close or gets corrupted, the signal needs to be restored. This restoration is best accomplished at the receiving or sink end of the cable.
But the difficulties multiply when extra-long cable runs are required. Active cables or extenders that use fiber-optic and single or dual Cat-5 or Cat-6 cables instead of standard copper can be used to extend HDMI to 50 meters for 1080p and more than 100 meters for 1080i or less.
Several new alternatives to hard-wired extenders have emerged recently using wireless technology. Last January, the WirelessHD Consortium announced the completion of the WirelessHD 1.0 specification, and it has already announced more than 40 early adopter and promoter companies. WirelessHD (also called WiHD) operates in the unlicensed and globally available 60GHz frequency band. It combines uncompressed HD video, multi-channel audio, intelligent format and control data, and Hollywood-approved content protection. Its core technology promotes theoretical data rates as high as 25Gbps as compared to 10.2Gbps for HDMI 1.3, so it could potentially enable even higher resolutions, color depth, and range.
There is also Wireless High-definition Interface or WHDI as a potential alternative to the cost of wire. Driven by Motorola and Amimon, WHDI is a new wireless MIMO (multiple input/multiple output) transmission technology intended to replace cables between video sources and digital TVs or other displays. Because WHDI conforms to worldwide 5GHz spectrum regulations, its range can extend beyond 100ft., can go through walls, and presents a latency of less than 1 millisecond. With WHDI, video data rates of up to 1.5Gbps can be delivered on a single 20MHz channel in the 5GHz unlicensed band, which would be sufficient for uncompressed 1080i and 720p.
With more than 840 consumer entertainment and PC companies having adopted the HDMI specification while wrestling with the above complexities and variations, an exhaustive survey of HDMI extender products would overflow these pages. However, based on recommendations from some of the founding members of the HDMI Working Group and other companies making significant enabling technologies, here is a look at some of the most interesting approaches for extending the reach of HDMI.
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