Misconceptions, misnomers, myths and lies – the world of HDMI is a confusing one to navigate. In part two of this examination of HDMI, Paul Skelton looks at the physical attributes of a certified cable.

In the first part of this examination of high-definition multimedia interface (HDMI), we looked its evolution and how it became one of the more pervasive devices in a home theatre.

From inception (version 1.0) to now (version 1.3c), HDMI has proved itself to be a highly versatile technology that is ideal for most applications.

However, Kordz founder and director David Meyer says the market remains littered with non-compliant products, including those that are wrongly labelled – such as standard-speed cables labelled as high speed.

“This remains an issue, as many vendors rate marketing over integrity when it comes to selling products. So it’s a case of buyer beware – and acquire equipment only from trusted sources.”

Kordz – an Australian manufacturer of audio, video and digital cables and accessories – is the only Australian-based licensed adopter of the HDMI format that specialises in cables.

As technology continues to develop, the limits of the format are continually pushed and redefined.

Fundamentally, for example, there are no length restrictions quoted in the HDMI specification. It is now possible for 1080p to go to unprecedented lengths using the newer active devices such as Category 6 (50-60m), multi-coax (100m) or fibre optic extenders (300m).

But there are bandwidth restrictions, determined by the manufacturer’s own research and development, that in turn dictate maximum lengths.

“At a base level, HDMI Licensing suggests all standard cables should perform at up to 5m in length, but this doesn’t mean they all do,” David says.

“Some better manufacturers have pushed the envelope to produce cables up to 25m, or even longer with active boosting.

“In fact, some brands now have products that join HDMI cables together to make one overall longer length.”

There are two main types to look out for:
• HDMI joiner – a simple passive adapter for joining two cables that does nothing to the signal;
• HDMI extender (also known as a repeater) – an active device that processes the signal and boosts it to drive through a greater length. Some extenders use external power supplies, but the more clever designs draw their power through the 5V DC power line contained in the HDMI cable.

“As of December 2008, the longest passive cable in the world to be certified high speed is a 24-gauge 10m cable that uses a very special – and expensive – metal conductor,” David says.

“The predominantly online retailers who sell 12m+ 26AWG cables and claim high speed with little accountability are making it difficult for manufacturers of these cables.

“According to the law of physics, this can’t be done passively.”

For an installer who wants to get around the issue of length, David suggests using an extender, or running longer Category 6 cabling. One thing you should never do is solder your own HDMI plugs on to the end of a raw or cut cable.

“HDMI is a very exacting specification, and the hand termination of a cable outside of an approved factory will result in a non-approved cable.

“If you can get a site-terminated HDMI cable to work at all, it will most likely not pass any of the higher bandwidth formats like 720p – much less 1080i or 1080p.

“Where a cable needs to be pre-wired into a premises there are a few things you can do to safeguard yourself against the huge costs associated with having to pull out a faulty or damaged cable.

“For example, you can test the cable before pre-wire installation to ensure operation out of the box.

“And always keep the connectors covered and protected during installation. Attach the cable to a draw cord or yellow tongue by taping around the connector and a few inches of cable. This ensures that the connector itself does not take the entire load when pulling through.

“We’ve seen examples where the joints of a cable onto a PCB inside the connector were fractured due to excessive pulling on the connector during installation.

“Make sure you install the cable in such a way that it can be removed again, or used as a draw wire to pull a replacement cable through. I realise this is not always practical, but it could save a lot of headaches later on.”

David says you must remember that an HDMI cable is an advanced piece of electronics technology, comprising a very complex cable structure, PCB assembly and fine multi-pin connectors.

You must treat it with appropriate care and respect – it is not robust, like the coax cables of old.

Excessive tugging on the cable and connectors, rough handling of the cable on insertion and removal with AV devices, and allowing grit and plaster dust in among the connector pins is asking for trouble.

NOT AS THICK AS SOME

Cable packaging often features a measure – 30AWG, 28AWG, 26AWG or 24AWG – to describe the size of the conductors. But what does it all mean?

“AWG, or American Wire Gauge, is a system of determining relative thickness,” David says.

“But it’s a little backwards, in that the higher the number the thinner the cable’s conductors.

“The basis of the system is simple – a 3AWG increase in number represents a halving of the conductor’s cross sectional area. For example, a 21AWG conductor is twice the size of a 24AWG conductor, and a 26AWG is half the size of a 23AWG.

Simple. So in the examples given above, 30AWG is the thinnest and 24AWG the thickest.

“Note that this applies only to the actual conductor size. It’s easy to make the overall cable bigger by just using more PVC on the outside.”

Most short-length HDMI cables use 30-28AWG conductors, but as the length of a cable increases so does the capacitance and resistance, resulting in a loss of bandwidth. It is very difficult to achieve the target standard bandwidth requirements of HDMI – and even tougher to produce the high-speed standard.

“This is especially so for greater lengths, which require a high level of expertise from the manufacturer for success,” David says.

“A larger-gauge cable is normally used, with 24AWG being the most common for lengths of 7-10m or more. However, just because a manufacturer makes long HDMI cables, it does not mean they will work at the full 1080p or beyond.

“To ensure you are getting the full intended performance of HDMI, test your desired cable brand and length in-store with a 1080p source, prior to purchase.”

Ultimately, the performance of an HDMI cable is dictated by its ability to pass the required bandwidth, not by the size.

“Keep this in mind as you go shopping and choose only based on its real performance potential,” David says.

“Several factors determine this potential, the main one being the licensed status of the cable – is it HDMI approved? If not, steer clear.”