Selecting the Right HDMI Solution for Your Custom Installation
Long, long ago, in the day of polyester leisure suites, Saturday Night Fever , and the Hustle, Sony founder Akio Morita commented "This is the product that will satisfy those young people who want to listen to music all day. They'll take it everywhere with them…" Morita was, of course, referring to the brand new Sony Walkman, a technology that would forever change the way we, as a society, relate to the recorded arts.
Since that fateful time we have witnessed the evolution of portable entertainment. This evolution has taken our music from delicate and limiting analogue tapes to powerful and adaptable digital discs. The digital standards associated with the compact disc prompted adventurous engineers to envision the magic of digital applied to images. From this vision was born the Moving Picture Experts Group, a nickname given to a family of International Standards used for coding audio-visual information in a digital compressed format.
MPEG, of course, provided the motivation for the creation of MP3. From this came the myriad of MP3 players, iPods and video iPods we take for granted today. The miracle of MPEG also allowed the practical introduction to the marketplace of improved resolution video formats such as HDTV. Beyond this, the bandwidth savings of digital audio and video have caused a proliferation of narrowcast streaming entertainment options, digital satellite and cable channels galore, and now the introduction of HD Radio! Our analogue world has turned digital in just two short decades!
All of this is the digital revolution . Digital is everywhere in today's home theater environment. But, despite this proliferation of digital technologies and content, most digital devices still rely on analogue interfaces. The use of analog in this digital environment is the weak link in the chain affecting the ultimate quality of the content. Maintaining the integrity of the digital signal from device-to-device and location-to-location is the key to providing the highest quality multi-media experience. A digital interface standard does just that.
The digital infrastructure challenge applies to more than just home entertainment. Commercial applications such as the delivery of video, audio, still pictures, presentations and control commands from a single server to multiple locations in large buildings is a central issue in the BICSI realm. Digital signage, a broad term encompassing a variety of different applications and technologies, refers to electronically controlled signs that can be updated without the cost and expense of changing the physical sign itself. Thus, changes can be made quickly and cost effectively, and targeted messages can be distributed to hundreds or thousands of displays at once. Digital signage takes the form of scrolling message boards, plasma monitors, large projection screens, and other emerging display types such as organic LED (OLED). In addition, the creative use of existing display technologies has led to innovations like polarized projection film, which allows high-contrast digital signage images to be projected directly onto store windows. Once again, a digital interface standard is the key to the unhindered evolution of this important technology.
"The nicest thing about standards is that there are so many of them to choose from, "says Ken Olsen, founder of Digital Equipment Corp. How can a single installation accommodate such flexibility? The solution comes in four letters; H-D-M-I. HDMI combines both uncompressed digital audio and digital video into a single compact interface that supports bandwidth speeds of up to 4.95 Gbps (now 10.2 Gbps with version 1.3). Instead of using separate cables for audio and video transmission and system control, High Definition Multimedia Interface demands only a single cable for a high-resolution, all digital signal transmission. HDMI is an outstanding connectivity solution and it's no wonder its popularity is spreading. However, HDMI technology isn't without its challenges. HDMI cables are known for having a dependable length limitation of about 5 meters. Copper cables longer than 5 meters in length often just don't work. And when it's time to create an infrastructure to seamlessly transport all this digital treasure from room-to-room or floor-to-floor in a residential or commercial structural, the lack of properly designed CL2 and CMP solutions create real stumbling blocks. What to do?
The issues associated with the installation of digital transportation infrastructure relate to problems of implementation, not encryption. HDMI Type A is backward-compatible with the single-link Digital Visual Interface (DVI-D), the standard used on modern computer monitors and graphics cards and the de facto standard of digital signage. This means that a DVI source can drive an HDMI monitor, or vice versa, by means of a suitable adapter or cable, but the audio and remote control features of HDMI will not be available. Additionally, without support for High-Bandwidth Digital Content Protection (HDCP) on both ends, the video quality and resolution may be artificially downgraded by the signal source to prevent the end user from viewing, or more specifically copying, restricted content. While nearly all HDMI connections support HDCP, many DVI connections do not. Type B HDMI is similarly backward-compatible with dual-link DVI-D. Clearly HDMI is the right standard. But what is the right connection?
There are three ways to address infrastructure installation of a digital media pipeline. These are shielded, purpose-designed copper conduits, unshielded twisted wire pairs driven by transformers, or optical fibers driven by active transcoding devices. Which solution is the right solution is entirely dependent upon budget, environment, ancillary gear and scale. Let's take a look at each of the three.
Optical interfaces comes in two "flavors" - multimode or single mode. Single mode fiber is designed for transmission of signals via laser and is not commonly used in this type of application. That leaves multimode. Multimode operates in the infrared spectrum and consists of the following:
- Transmitter - Encodes the light signals from electrical signals
- Optical Fiber - Conducts the light signals over a distance
- Optical Regenerator - boosts signals for long distances
- Optical Receiver - Decodes the light signals back to electrical signals
There are very strong benefits associated with a fiber optic installation. First, the source and receiving products are electrically isolated so electromagnetic noise and radio frequency interference have no ingress in the length of the transmission path. The only noise you'll get at the output is the noise that was there at the input! Additionally, the attenuation of light through a properly designed and installed fiber connection is minimal so an optical interface can effectively span very long distances and carry tremendous amounts of dense, complex data. This sounds about ideal for a multimedia interface - and when budget is of no concern it is ideal.
There-in lays the downfall of fiber. Fiber can be expensive to install, expensive to maintain, and expensive to interface - especially considering its demand for transmitters and/or receivers at each node. Even more than this, fiber solutions don't necessarily support HDCP. With Digital Rights Management being a major issue in the entertainment industry today, the lack of HDCP compliance could be a source for serious compromise as various HDCP conditions are implemented and enforced.
Transportation of HDMI over un-shielded twisted pair wire, such as CAT5e solves some of the problems associated with optical solution. While a UTP (unshielded twisted pair) solution can carry signals for a long distance, they cannot compete with the hundreds of meters possible with the best optical networks. Most UTP configurations are effective to a maximum of 150 feet or so. Still, this is often more than enough for a residential or light commercial installation. Because of the density of the HDMI digital signal, a UTP solution requires a pair of delicate CAT5e cables between the source and receiving location. A typical UPT solution consists of a transmitter, a receiver, and an AC power supply. Standard HDMI cables are used to connect the transmitter and receiver to the components, as is the case with the optical solution.
Advantages of a UTP conduit include the avoidance of ground loops as in the optical network. The nature of transformer coupled devices also limits common mode noise, so again RFI and EMI ingress are thwarted. Using this type of connection you'll get a quiet, wide bandwidth pipeline from source to load, provided the CAT5e is properly handled during installation. And because UTP can carry the proper "handshake" voltages, UTP solutions are almost universally compliant with HDCP.
Disadvantages of the UTP solution are delicacy and cost. The performance of CAT5e can be easily compromised if the cable is kinked, stretched or otherwise mishandled. Add to this the fact that most UTP systems designed for high quality digital audio and video sell for several hundreds of dollars or more. While this can be a fraction of the cost of an optical solution, it is typically several times the cost of a straight cable configuration.
One additional pitfall, the transmitter and receiver components may demand either awkward surface mounting or deep or large in-wall placement, which may not always be accommodated in a retrofit situation.
This brings us to the purpose-designed interconnect. Properly manufactured interconnects do not require costly and awkward transcoding to connect to source and load components. Further, a dedicated HDMI interconnect can be made to be compliant with CL2 or CMP NEC requirements for near universal applicability.
A reported problem with HDMI is the maximum cable length. As with all cables, signal attenuation may become too great beyond a certain length. Standard HDMI copper cables use 28 AWG conductors, leading to the aforementioned limit of about 5 meters (approximately 16 feet). The HDMI Web site, however, disputes the 5 meter limit. "HDMI technology has been designed to use standard copper cable construction at long lengths. In order to allow cable manufacturers to improve their products through the use of new technologies, HDMI specifies the required performance of a cable but does not specify a maximum cable length. Cable manufacturers are expected to sell reasonably priced copper cables at lengths of up to 15 meters."
One proven way to increase functional length is to increase the size of the copper cables, effectively decreasing impedance. 24 AWG wire is considered superior to 28 AWG. Additionally, amplifiers and signal repeaters can string several HDMI cables together.
For the system engineer or project manager, the advantages of purpose designed custom-installation interconnects are strong. HDCP compliance is almost certainly ensured. Installation is straight-forward. Lengths to 60 feet or so are easily accommodated, and lengths to 150 feet are possible with active compensation. This compensation can be built into an active wall-plate or be designed to be housed at a central, accessible location.
Because copper cables are a thoroughly understood technology these interconnects can be built of rugged materials with physical properties that prevent damage during the installation process. And, because these materials are used in many other assemblies, the cost of purpose-designed interconnects can be less than either optical or UTP solutions of the same length.
The Digital Revolution isn't over. In fact, it may be just starting. In the coming years we'll see a proliferation of new applications, new devices and new ways to consume music, movies, entertainment, news and data. Portability will transcend battery operated Walkman and iPod devices as the demand for broadband distribution of audio, video and control protocols becomes a functional part of our residential and commercial structures. Wireless solutions are, by their very nature, of limited applicability. A new generation of structured wiring solutions will evolve to harness the power unleashed by this revolution of media. Careful consideration now will allow you to properly prepare for the possibilities yet to be delivered by the promise of digital. Understanding the proper application of fiber, UTP and purpose-designed interconnects will make your life a little easier in the future.
Special thanks to Impact Acoustics.