Articles and whitepapers

Technology: Practical Tips for Distributing HDMI Around the Home (3/9/2010)

By Adrian Hicks, Habitech

At the CEDIA UK 2008 a major change took residential installers by storm - Sky withdrew all warehouse stock of Sky HD boxes with the component connectors, and so the only method of distribution was to use the dreaded HDMI connector. Until then, we were happy with component - you knew where you were with three coax cables and if there was a problem, fault finding was simply checking for continuity and swapping cables to achieve a picture.

Now we were left with the dreaded flat connector and a whole load of rumours about keys, encryption and compatibility problems between HDMI versions and/or different manufacturers' equipment. Could we use converters back to component or could these be magically 'blacklisted' when you bought a new Blu-ray disk? In some cases, installers had based their cabling schematics around installing video cabling, but most had already adopted video over CAT cable for the main backbone. What to do?

How HDMI works

The design of HDMI (the connector) and HDCP (the software encryption behind it) was originally based on a single source (e.g. Blu-ray) sending video to a single display down a short cable. On plugging the HDMI cable in, the screen and the source exchange information on which formats of signal they support. This is their Extended Device Identification Data (EDID), and typically includes the display resolution and which audio format the display can accept. In the simple original model, this would then be:

The EDID typically includes the display resolution and which audio format the display can accept.

The lowest common format is 720p, 2-channel audio. The two devices then exchange agreed encryption keys which the source uses to encrypt the data so that it cannot be understood whilst being transmitted, but the display can decode the information as a video signal and display it. These keys are then re-exchanged (typically every 2 seconds) to prevent the encryption keys being learned over time.

The whole-house challenge

This military-grade encryption is all well and good, but in its original form bore no thought to whole-house video distribution. In our installations, we don't have a simple two-box solution. We want all of our sources to be centrally located. For example, if I have recorded a film on my Sky HD box, I want to be able to watch it upstairs in bed or pause my movie and watch it in the kitchen whilst making dinner. I want my Blu-ray to route through the matrix in my rack so I can watch it in every room, but I don't want to go to the garage to insert the disc, so I need a return path to my matrix from my home cinema.

Most of all, I need to route this information all around large houses. A standard Category 1 HDMI cable will struggle to transmit the signal over 5m and is near-impossible to site-terminate, so any damage during the installation to the cable or connector renders the cable unusable.

Standard 5m Category 1 HDMI cable.

The last two years have seen the rapid emergence of many products which are designed to accommodate or work around the limits of the HDMI standard. In the early years these worked with varying levels of success, primarily because the product manufacturers, including the larger players, did not adhere completely to the standard. Typically, these new products perform three functions:
1) Splitting and matrix switching, allowing multiple sources and displays.
2) Transmission over distance using inexpensive cables which can be easily installed and terminated.
3) Converting to and from analogue video to accommodate legacy video products.

Splitters and matrix switchers

In the analogue world, the intelligence of these products was in the circuitry to split and join video signals. In the world of HDMI and HDCP, this changes to the challenge of fooling both sources and displays that they are in the typical two-box system. This involves the high-speed generation of digital keys in both directions, and the negotiation and renegotiation of formats whilst also switching the signals. In the case of one source being sent to multiple displays, this also raises the issue of the different capabilities of the displays. Consider the following example:

Example of a TV having a different format to that of a Blu-ray player and another display/receiver.

If the home owner is watching a movie with surround sound in the cinema, and the kitchen TV is switched to the same source, the matrix will negotiate between the TV and the Blu-ray. Since the Blu-ray can only send one format of video and audio signal, the matrix will switch the video format to 720p (so the movie is lower resolution in the cinema) and the audio to stereo. With the older matrix chipsets, this would also be accompanied by the loss of display for about a second while the negotiation takes place.

The latter problem has been cured in the latest generation of matrix switchers by manufacturers such as Vision HD and Crestron, as the matrix switcher regenerates the keys internally for all of the displays rather than retransmitting them. The former problem of incompatible formats is the responsibility of the installer to clearly understand the capabilities of the screens/AV receivers that they recommend - all displays in the system must be a common format (ideally 1080p). Also, some screens now have the capability to receive and process surround sound and so do not cause a drop from 5.1 to 2.0 audio when connected.

The Vision HD MX0408-310matrix switch allows up to four HDMI audio/video devices to be independently switched to eight HDMI monitors, HDTVs, or projectors.

Converters for cheap and long transmission

The standard cable for transmission of HDMI throughout the house is CAT5 or CAT6 cable, although 5-wire video cables and, more recently, a single coax solution, are coming on stream. There is a multitude of different transmitter and receivers which can be used as standalone products, but also many matrix switchers which have the transmitters built-in. Typically, these offer 50m distance with the option to repeat the signal mid-run. Fibre optics are an oft-discussed high-bandwidth holy grail, but they are not simple to run, or within many installers' capabilities.

Assuming that installers use CAT cable, it is imperative that they understand the limitations of the medium being used. Although the cable is the least expensive component, it is the most critical because it is installed in walls and cannot be easily replaced, so it is worth considering whether the saving of GBP30 a drum by buying copper-clad aluminium (CCA) CAT5 is worth it. CCA cable is not as good a conductor, and cheaper cable typically has uneven twists, resulting in different-length cores. For data such as email, this makes no difference, but with three high-speed video signals, a 3m difference will cause the signals to be out of sync – and the dreaded blue screen will result. In order to minimise this problem, products such as the HNC Pro cable by SCP have pure copper cores with balanced twists, specifically optimised for HDMI traffic, but equally suited to data.

The SCP HNC Pro cable has pure copper cores and balanced twists.

Recommended practices

I recommend the following best practices for installing these products:

* Minimise the number of connections in the path from the matrix to the screen since patch cables and panels introduce copper on copper connections, and punch downs attenuate the signal and introduce noise.
* Draw in a minimum of 50% spare capacity of cable - this allows for breaks whilst pulling, but more importantly, allows for future additions such as networking at the screen.
* Use CAT6 if you have the option - it is a higher-bandwidth cable.
* Plan and know where the cables are run within the house. Where possible, keep away from sources of electrical noise - it is still an unshielded cable.
* Consider locations for repeaters for long runs.

Following best practices will help to avoid this situation.

Despite all of this, if, at the final hurdle when you are standing in front of a blue screen and have no way of seeing whether this is due to keys, encoding, cabling or formats, I recommend the following:

* Go back to basics - check the CAT cables with a good cable tester.
* Try operating the sources when set to a lower resolution - this drastically reduces the bandwidth required.
* Invest in a signal generator and a test monitor. When you are lugging a 65" screen on and off the wall it is no fun, but with a 7" test monitor you can capture the EDID of the client's screen and test each outlet before the expensive kit hits site.

Video input adaptors

As HDMI has been (reluctantly) adopted as the standard for residential video installation, the final part of the puzzle is the inclusion of legacy products such as video cameras, computer video signals and older Sky HD boxes. These can all be incorporated with converters with a variety of signal scaling. Please bear in mind that they cannot convert a standard definition picture to HD, but they do allow the entire video domain to be switched and transmitted as HDMI.

A component to HDMI converter.

Conclusion

In all honesty, neither installers nor distributors wanted to adopt HDMI, and certainly not with the learning curve we have all endured. However, this curve is flattening out and the products are becoming more and more stable. With the right components and practices in place, the installer should be well-equipped to produce a system which delivers all of the digital benefits of HDMI, with the minimum of grief.

Adrian Hicks is the Technical Director of Habitech Limited, trade distributor of residential home entertainment and automation technology.

www.habitech.co.uk