What's important and what's not

With the advent of HDMI v1.3, 1.3a, 1.3b and so on, consumers are starting to get really confused about cables and what they need to worry about when selecting a cable product that's going to be compatible with the new specifications.

Before we get too far, it's important to understand a very significant term that relates to digital signals, "bitrate," and what it means. "Bitrate" is the speed that bits are moving through the cable system, from the Satellite HD Receiver, Blu-ray player, media centre, etc to your display device, TV, projector. More commonly advertised and talked about than bitrates in the sale and marketing of HD displays and other devices are resolutions, the more advanced descriptions will express this through the concepts of resolution and color depth. So a lot of times you'll hear bitrate-centric issues discussed more along the lines of 1080p being "better" than 1080i and 1080p with 8-bit colour being less good than 1080p having 12-bit (Deep) colour.

Bitrate is the flow of information being sent from one place to another

But from a digital engineering standpoint everything is bitrate and bitrate is everything. Now that has been asserted and reiterated early on, we can begin our discussion. With respect to high-definition video quality and resolution, 720p or 1080i (the resolutions most commonly outputted by HD Satellite Receivers such as SkyHD, Virgin V+, Freesat HD etc) is the first stopping point in terms of data (bandwidth) sent down your HDMI cable to your display at 742.5 mega-bits per second (Mbit/s). Those resolutions describe 742.5 thousand bits of information traveling through the digital pipeline. And a pipeline is a very good way of thinking about cables and the cable set-up, some cables being larger pipes capable of carrying more than others. A lot of AV receiver chipsets handle these lower HD resolutions with no problems at all. And being quite honest, pretty much all HDMI cables, even ones from the pound shop will pass this level of bitrate for a considerable length.

The next stop on the route to ultimate HD heaven is 1080p. Most people don’t actually realise that there are several grades shall we say of 1080p; 1080p 8-bit (the standard), 1080p 12-bit (deep colour) and in accordance with the version 1.3 spec potentially 1080p 16-bit (yet another advancement for the uncertain future)! Resulting in super confusion for the consumer and pretty much everyone else involved in the industry cable and display manufacturers alike.

1080p although thought of as the perfect format, this was initially limited to the normal 8-bit color depth, yielding a bitrate of 1.485 Gbit/s (1485 Mbit/s). This means that 1080p is twice the bitrate of 720p or 1080i. That should be a big consideration to people looking to install a longer HDMI cable run in their home. Taking this a step further, if you plan to utilise any PC resolutions that exceed 1080p, you're in for even more cable requirements. 1920x1200/60Hz, for example, has a bitrate of 1.54 Gbit/s and 1600x1200/60Hz requires a bitrate of 1.62 Gbit/s. If you're going to utilise HDMI or DVI over long distances you should ensure that the manufacturer has had that cable range and length certified to handle your particular bitrate requirements. Coincidently, all of hdcable.co.uk’s HDMI and DVI-D cables are easily capable of handling up to 2.25 Gbit/s (the highest available bitrate for any products on the market today) at up to 15 metre lengths (without any EQs or active components.) And our super-long HDMI active cables can go up to 40 meters at this bitrate.

HDMI v1.3 - What Were They Thinking?

Before we get into HDMI v1.3 I believe it is important to note the state of things in the consumer electronics industry prior to the introduction of v1.3. At just 1.485 Gbit/s bitrate for 1080p at 8-bit colour, many cable vendors' products fell to pieces after just 5 meters (and believe us, some didn't make it that far.) Most major name cable manufacturers were doing well to label their products, but a lot of imported pieces claimed 1080p compatibility at such ridiculous lengths that it was obvious the companies were not informed. Just as the industry seemed to be catching a hold of this concept and customer installers were properly outfitting their installs with cables that would "stand the test of time" the powers that be over at HDMI Licensing, LLC, a wholly-owned subsidiary of Silicon Image (a principle founder of the HDMI standard), felt concerned about emerging technologies such as DisplayPort from VESA, a competing format that threatened to compete with HDMI. On June 22, 2006 they announced completion of the new spec to the industry to greatly extend HDMI's capabilities - but largely much of the new spec exists only on paper with no devices utillising the stated theoretical potential.

HDMI v1.3 can, per the spec, handle up to 3.4 Gbit/s per channel. Now, don't be confused by the inflated numbers used by the marketing people at HDMI.org - they express the bandwidth as 10.2 Gbit/s - which is simply the bandwidth multiplied by the three color channels (RGB). While Silicon Image and HDMI Licensing, LLC said everyone was going to support 3.4 Gbit/s, they didn't exactly provide a lot of "support" for the first 6 months. For a long time there wasn't enough silicon to produce the transmitter and receiver chips needed to implement the new technology so most manufacturers waited until 2007 to produce consumer electronics products with the v1.3 features. As silicon emerged from Silicon Image and a few other manufacturers in 2007, support was included for up to 2.25 Gbit/s - the bitrate associated with 1080p resolution at 12-bit color per channel. This isn't supportive of the maximum theoretical HDMI 1.3 bitrates, but it is certainly more than 8-bit and less than the 16-bit HDMI v1.3 claims to be able to handle.

Bit depth is what is responsible for the number of colours you can see on a display. Increasing the bit depth of a display product (and matching the source components and cabling to support that display) means that you will incur a much lower chance of experiencing color banding where smooth color gradients cannot be accurately reproduced and break up into bands. To understand how this affects bitrate, let's consider the following:

    * 8-bit color = 2^8 x 3 = 2^24 = 16.7 million colors
    * 12-bit color = 2^12 x 3 = 2^36 = 68.7 billion colors

Increasing the bit depth results in almost exponentially better colour rendition. Now, remember that the old benchmark limit was 1.65 Gbit/s, much more than any home theater enthusiast needed for excellent resolution and performance. After all, at the time, these cutting-edge enthusiasts were only concerned with attaining 1080p at 8-bit color - something that was very new and only available to a handful of elite consumers. A year ago, most of these consumers didn't even know what 1080p was and they almost certainly didn't concern themselves with achieving 12-bit color (let alone 16-bit). Still, 12-bit is nice and will likely result in reduced banding, especially during darker scenes in light-controlled environments.

If you take 1080p resolution at 12-bit colour the maths comes out to 2.2275 Gbit/s. Fortunately for HDMI Licensing, LLC there are now some chipsets out there finally supporting that resolution in mass quantities. What has transpired is a gradual transition whereby consumer electronics manufacturers have the choice of paying a premium (typically up to £1.50 more per chip) for these chips in order to enable Deep Color support and create a value-added product for consumers and dealers.

So the Cable Drama Continues, With a New Twist

Whereas cable manufacturers were starting to come to grips with the fact that most HDMI cables didn't have the metrics to pass 8-bit 1080p over 10 meters without compensating electronics, they are now faced with a new problem. 1080p at 12-bit. The numbers are crunched but the testing phases are still underway as cable manufacturers send in their products for testing to pass the new requirements. Of course many cable manufacturers don't seek certification - in which case buyer beware. There are also great issues with the certification process, but we'll address those later.

Thanks to v1.3 and Deep Colour, distances for passive copper cables have shortened yet again and, in a sense, much has been set back a year until the new effects of this slow transition start to be fully understood. We used to say "Don't put a crappy cable in your wall - it might not pass 1080p." Now we must say "Watch out for 12-bit Deep Color (not to mention any future higher-resolution formats or bitrates) - that same cable might need some electronics to pass it properly." With cables of any length over 3-5 meters it is quickly becoming apparent that active components are playing a major role in signal integrity.

Dealers and custom installers are going to have to bone up on the cable electronics needed to migrate their clients so that they can enjoy the latest technology - and learn what cables can handle this new bandwidth and for how long. Active HDMI cable solutions from respected companies like hdcable.co.uk are now certified to work up to 40 meters at 1080p 12-bit colour. These active solutions are still the most popular method for long-run installation because fiber solutions are so expensive in comparison.

To put it bluntly, you don't want to get caught putting a cable in your wall (or worse, your client's wall) that won't be compatible with current and future (expected) technologies. Undoing a mistake like that could be very expensive, costing you a lot of time and energy.

So what's so special about certification? Nothing, if you're running a 3 foot HDMI from the DVD player to an LCD panel. Just about any old cable will do. However, if you are going to install a cable that is in danger of hitting upon the maximum potential of the format and you're putting that cable into a wall or ceiling - you might want to spend a little extra and buy from a company that guarantees and tests its products.

The interesting thing about certification, however, is that HDMI Licensing, LLC doesn't even account for the real-world situation where chip manufacturers don't fully support 3.4 Gbit/s bitrates. No one we know of supports the theoretical maximums. As a result, the real-world bit-rates of 2.2275 for 1080p at 12-bit color needs to be tested using the same criteria as the 1080i cable that only needs to pass 742.5 Mbit/s. That's right, there is currently no certification for 1080p at 12-bit. Are you scared? You should be.

So far, from what we understand, HDMI certification is largely a very fast and loose (not to mention expensive and apparently profitable) program that does nothing to truly ensure any of the manufacturer's production cables meet or exceed any practical current specification. Why do we say this? Simple. The HDMI testing standard has two categories currently:

Category 1 is for 720p and 1080i cables at 8-bit color. This spec was business as usual before HDMI 1.3 and, unless the connectors fall off, the majority of HDMI cables are going to hit this for at least 4-5 meters without a hiccup.

Category 2 is the super high data intensity 3.4 Gbit/s (non-existent) standard. The only silicone that exists on the market beyond Category 1 is rated to handle up to 2.25Gbit/s. For certification you either test level 1 (duh -easy) or level 2 (meaning you must over-design your product.)

What the HDMI people should have done is inserted a real Category 2 certification for real-world 1080p/60 12-bit color (~2.25Gbit/s) since that bitrate is supported by silicon chipsets. "Category 3" could then take on the theoretical maximums of 3.4Gbit/s for future-ready products such as cables (since no electronics can pass the theoretical maximums at present).

More information on certification can be found in our article titled - What's a "Certified" HDMI Cable?

Conclusion

So where does this all lead? To a grand new adventure. Progress is good, but HDMI v1.3 was a bit of an artificial "nudge" in a direction consumers didn't necessarily feel the need to take. Even so, we believe it will all work out on the end and it did allow cable manufacturers (the serious ones) to quickly realise the need for active solutions to ensure signal stability and integrity for longer cable runs. Without the increased bitrates this may have taken longer and more consumers would be installing cables that wouldn't pass the high bitrate signals now present in an ever-increasing amount of consumer electronics. The key to remember is that you cannot install an HDMI cable in your ceiling or wall that is designed to pass Category 1 specs (742.5 Mbit/s) and assume they will be just fine for features such as 1080p and 12-bit Deep Colour. It won’t. You should seriously consider buying an active HDMI cable  so you can guarantee reliable results and genuine future-proofing.