Long HDMI Cable Bench Tests - Monster Cable Shootout

Why "Just Buy ANY HDMI Cable" Advice is Wrong… Sort of

I decided to write this article the first time I saw another writer say "HDMI is digital - it either works or it doesn't." Then I saw that statement get repeated over and over. The problem is that HDMI isn't like a digital coax audio cable - it can degrade partially and produce sparkles and snow. We'll illustrate some of this below. It took nearly 6 months to research and prepare for this experiment. I intended to acquire as many HDMI cables as possible and focus on empirical testing of mostly longer lengths to show the differences that abound when you exceed 5 meters. The exercise, I believed, would save many consumers from losing lots of money and time - on a number of levels.

It’s possible that this is one of the more important articles a consumer, custom installer, or retailer who is considering or involved in the installation of longer HDMI cable runs might read this year. Why? Because HDMI is not just a digital cable carrying a fixed and limited amount of bandwidth like that of coax S/PDIF or TOSLINK. Why do I say this? Because HDMI is more complex, as we'll show you, and because we tested nearly 60 INDIVIDUAL CABLES to bring you the real data on just what happens when you ask HDMI to carry different types of data over short and long distances. In short, we hope this article will change the way you look at HDMI cables and understand when it's important to pay particular attention to quality and specifications... It sure changed our view of things, and not in the way you might expect. Read on.

What's So Special About HDMI Cables?

Before we jump into the tests and results, An HDMI cable is actually a bundle of wires - 19 counting the drain wires, and all follow a very rigorous specification in order to deliver the extremely high bitrates that deliver high definition images and audio to your electronics. Here is an example of how one particular geometry of HDMI cable would look if you sliced it open:

Looking at the image above you will see that there are 4 primary paired connections which represent the TDMS (Transition Minimized Differential Signaling) channels plus a drain wire. This can be thought of as the red, green, and blue color information (note that the colors are presented solely for illustrative purposes), multi-channel audio information and also the clock. The multi-channel audio is "easy" to pass along the line and is therefore interspersed among the much denser video information carried across these cables. Because of this, there are no dedicated wires for audio. While the three color/audio TDMS channels run at very high speed, the clock channel has much less demands put on it.

The central pair, in this particular cable configuration, are for DDC (Display Data Channel) clock/data which reads EDID (Extended Display IDentification) information from the display and which also handles the HDCP handshaking. EDID essentially tells the HDMI transmitter what type of signal and display it is dealing with. These wires are absolutely critical since without this information the entire signal is compromised. If HDCP isn't happy - there simply isn't going to be a signal.

The four single wires are a +5V line, a DDC ground, a hot plug detect wire (a signal that is used to monitor hot plug events so that a new HDCP negotiation can be made), and one for CEC. CEC (Consumer Electronics Control) is the two-way single wire control system which allows all the devices connected on an HDMI chain to communicate and automate settings and control functions, reducing the amount of controls and remotes needed to operate the components. In theory this is one of the coolest aspects of HDMI. In reality it is an underutilized, disappointing, buggy feature set that is mostly limited to intra-company products. The 5th solo wire is unused.

Shielding is extremely important with HDMI due to the intense data rates which are traveling through the cable. On the outside of the entire cable lies a PVC jacket. Underneath that is a braided shield which typically has around 85%-90% coverage. Underneath this is a helical-wrapped aluminum mylar material providing 100% coverage. The different shielding works well at different frequencies. For example, braid is better for blocking lower frequency interference (relatively speaking of course), and mylar foil is much better suited for blocking higher frequency interference. In addition to the master cable bundle, each of the TDMS pairs has mylar wrap as does the clock channel.

Due to the complexity of HDMI, it is never terminated in the field with a soldering iron or pin tool as you will see with other connections such as S-video, VGA and component/composite video. One of the few exceptions to this is RapidRun - a product by Impact Acoustics (Cables to Go) which utilizes proprietary interface connections to terminate HDMI and DVI-D "tails" and wall plates in the field. It's cool and, according to our tests, it actually works.

Digital Is Digital, Right?

Unlike the mumbo-jumbo surrounding TOSlink and coax optical cables, HDMI is a little more complex. When the specification is a constantly-moving target, as HDMI has proven to be - and this has been fueled by a willing consumer electronics industry - things get a little hairy. It's important to understand just why, with HDMI, you can't just say "digital is digital, it either works or it doesn't." As we referenced in detail in our article 'HDMI - It's All in the Bitrate,' HDMI has progressed (evolved) from a simple 720p/1080i 8-bit per channel signal to the current 1080p 12-bit (or more) per channel signal it is today. Just what kind of bandwidth is being sent over the cable depends on the source electronics and the capability of the display, but the potential has vastly increased since the debut of HDMI in 2003. As these signals got higher in resolution and bit depth, the amount of bits per second traveling through the HDMI cable increased. When that happened (and most recently culminating in the release of HDMI 1.3) the tolerance requirements for HDMI cables changed as well.

By way of example, with a digital audio coax cable you are dealing with a required maximum bitrate of just over 3 Mbit/s. For a S/PDIF connection, be it TOSlink or digital coax, this is chump change. The specification for that digital connection hasn't changed since ~1997 and the demands made on the cable are far less than the cable's potential capabilities. Now let's look at HDMI. A massive bundle of 15 cables (not including drain wires) - some insulated, all fairly important. In order for HDMI to remain practical, and avoid unnecessary interference, the cable has to be made, at least partially, out of stranded wire, lest 15 solid cables render the cable more of an unbendable, unwieldy stick than a cable. Additionally, the amount of information necessary to transmit the incredibly high resolution video and 8-channels of uncompressed digital audio far exceeds that of the typical DTS or Dolby Digital compressed traffic which meanders through a S/PDIF audio-only connection.

So How Do Cables Differ?

Aside from cost, HDMI cables differ in many ways. Some real obvious differences include the gauge of cable used in construction, stranded versus solid cable pairs (which greatly affects flexibility), and flat versus round. These physical differences are significant, but not nearly as important as whether or not the cables are rated to carry a particular signal a stated distance. Many cables are certified by one or more companies who provide specifications and/or speed ratings to cables. Simplay, for example is a wholly-owned division of HDMI Licensing, LLC that charges large sums of money to test cables and consumer electronics for compatibility and interoperability. Another company coming online is DPL Labs which rates cables on a 1-5 rating. It's a bit redundant since it should be pretty obvious that the Category 1 and 2 specs should be easy enough to certify and uphold. We're more concerned with manufacturer being honest than whether or not some third party certifies them or gives them a sticker. After all, nobody expects every AV receiver to be certified by a third party for exact power ratings (the FCC notwithstanding, but they hardly do more than ensure the product doesn't emit radiation).

Specific problems arise in a couple of areas. First, the equipment required to test HDMI cables is very expensive. We're talking over $200,000 for a basic setup including source generator, scope and calibrated HDMI "probes". This is obviously cost prohibitive for smaller companies to do much more than rebrand someone else's manufactured cables that have already been certified. If they choose the right manufacturer this isn't a problem, but some don't.

You can predict cable integrity and performance fairly accurately by doing the math on the cable geometry and modeling the results. This isn't easy, however (at least not for mere mortals) and we opted to use the measurement methodology instead. For a great (though very technical) article on HDMI cable modeling, please see Eugene Mayevskiy's writeup, which comes from an engineer who worked with Tektronix to deal with these very issues.