A Guide To HDMI Digital Video Connectivity

A Guide To HDMI Digital Video Connectivity
Joseph D. Cornwall, CTS-D, CTS-I
Technology Evangelist, Legrand N.A.
There are as many ways to connect AV devices as there are imaginations involved in the AV industry.
We often think of DVI-D, HDMI and HDBaseT as mutually exclusive choices, but the truth is that all those
connectivity solutions are transporting essentially identical content. Picking the right solution from the
industry’s menu of possible connections is challenging. Arriving at the right choice is a result of defining
parameters surrounding a digital video link and then a solution that satisfies both current needs and
future scalability. Instead of thinking about resolution and length, we should be analyzing connectivity
choice in terms of meeting image-data payload demands, selecting physical connections that anticipate
future compatibility and meet user expectations for convenience and ruggedness. Specifying those
solutions while meeting budget limitations and bridging to legacy infrastructure is an issue in practical
applied technology. In this article we will look at several digital video connectivity solutions and provide
guidance on when and why you should select each.
Let’s start our exploration with an overview of a few technical items, which may help narrow your
search. Before beginning any project, you should understand your need for scalability and future
compatibility with new devices. This analysis should include a survey of the possible source devices that
will be attached to the system, their aspect ratio and potential image payload.
4K and color space
4K is A/V “shorthand” for video content that offers approximately an 8-million-pixel image and has the
proper name UltraHD (UHDTV or UHD). Your current home HDTV is most likely a 1080p HDTV 16:9
display. This means that it delivers a picture by painting 1080 horizontal lines of picture elements (pixels)
with each line under the previous, much like the text in this article. Each line consists of 1920 pixels
presented side-by-side, like the letters of this sentence. Displays that use this format deliver 1080 lines
of 1920 discrete pixels for a total of 2,073,600 pixels. You could correctly call this a “2K” display for its
(approximate) 2,000 horizontal pixels, or you might think of this as a 2-megapixel image for its
(approximate) 2-million-pixel total.
4K displays and content feature twice as much information along each axis, for a total of four times the
total payload. A typical 4K LCD display delivers a picture by lighting 2160 (1080 x 2) horizontal lines of
pixels. Each line consists of 3840 (1920 x 2) discrete pixels for a total of 8,294,400 pixels. The 4K
moniker comes from the nearly 4,000 horizontal pixel count. As above, you might think of this as an 8megapixel image for its 8-million-pixel total. Another way to look at this is to say that a 4K content
requires four times the image payload of an otherwise identical a 2K image.
Note: there is another 4K “system” that uses 2160 scanning lines of 4096 pixels each. Naturally it is also called
“4K.” This format is primarily used in broadcast and content production and is quite specialized. There is no
distinction made as regards the content of this article and both systems can be considered essentially
interchangeable for connectivity purposes.
There is one more thing you need to know about digital video images and 4K before you can make a
connectivity decision. Not all video content is the same. You must incorporate “color space” in your
thinking to fully appreciate potential system payload demands. While the mathematical model of
representing colors is challenging to understand, its effect is pretty easy to quantify.
Most video content uses a form of compression, known as chroma decimation or chroma subsampling,
where-in an image displays significantly less color information than grayscale detail information. This is
done to make the video signal smaller and easier to transport. Chroma subsampling has been in use
since the very first color TV’s in the 1950’s for both analog and digital content.
Chroma decimation delivers about half as much information in color (chroma) content as it does blackand-white (luma or luminance) content. There is no real perceptual penalty to this technique because
our vision can only perceive about half as much “color” detail as “B&W” detail in everything we see.
This is a physiological limitation. You can identify chroma subsampled content by noting its color space
specification, which is written as a ratio. The most common ratios you will see are 4:2:0, 4:4:4 and
sometimes 4:2:2.
Computers graphics cards natively treat the RGB elements of the picture content equally. Therefore,
most computer-originated content doesn’t use chroma subsampling (in other words, it uses a 4:4:4 color
space) and therefore has a much higher data payload than broadcast or Blu-ray content (which uses a
4:2:0 color space). A good example is thinking about high-definition video you recorded on your digital
SLR camera and comparing it to the streaming video you might watch on Netflix. Both content payloads
may be identified as 1080p, but the SLR camera image payload may require twice the bandwidth of the
Netflix image or more!
Another way to understand this data payload comparison is this simple arithmetical trick. 4+2+0=6.
4+2+2=8. 4+4+4=12. 4+4+4+4 (for 4:4:4:4, sometimes called a 32-bit image) =16. A 4:2:0 color space (6)
is half as big as a full RGB color space of 4:4:4 (12) and just under 40% the payload of content with an
RGB signal using a transparency channel 4:4:4:4 (16).
HDMI interconnects rated as “High Speed” will support UltraHD 4K images delivered in a 4:2:0 color
space, which technically has about the same data payload as 2K images in 4:4:4 color space. To support
4:4:4 full bandwidth UltraHD 4K signals, your installation will need an advanced connectivity solution.
This is why it’s so important to invest your effort in a comprehensive survey of the content and source
devices you want to support, both now and over the course of the systems installed lifetime.
When selecting the right connectivity solution for your project you often run into the need to extend
network connectivity to connected AV devices as well as the uncompressed digital video payload. HDMI
standards anticipated this and define a category of connections to address the problem. You can deploy
a special HDMI cable that offers Ethernet extension or an audio return channel. Both of these features
are handled via a single conductor within the HDMI cable assembly, so you can support one or the other
but not both simultaneously. HDMI(e) allows 100Mbps Fast Ethernet to piggyback on the digital video
interconnect on devices designed to support this feature. The connection that supports both features is
the same and there is no performance penalty for selecting a cable with HDMI(e) capability over one
that doesn’t offer this feature. There is seldom a significant difference in cost. It’s good practice to
select an HDMI(e) enabled interconnect whenever possible.
Mini and micro HDMI connectors
In the past, some computers and other devices designed to a small form factor offered a mini-HDMI
connector. These are about half the size of a standard HDMI connection. Additionally, some cell phones
even used a micro-HDMI connector. Both of these formats have diminished in popularity, and they will
likely be used less and less as the industry advances and new connectivity topologies take hold. There is
nothing special about these smaller HDMI connectors other than the size. You can purchase mini-HDMI
and micro-HDMI to standard HDMI connector cables, or use an adapter on any standard cable.
Economy interconnects
Commodity level lines of HDMI cables may be “high-speed” rated, particularly in shorter lengths below
15-feet. This category of performance is acceptable for simple source-to-display connectivity in noncritical home video and desktop applications. An HDMI LLC high-speed rating suggests these cables can
support UHDTV 2160p in a 4:2:0 color space at low refresh rates, but are likely not “intended” for such
demanding applications. Typically sporting a very small wire gauge (30AWG or smaller), it’s definitely
worth demanding a tested brand from an HDMI standards adopter to ensure product performance
matches marketing claims.
High quality interconnects
A performance-optimized quality interconnect for commercial applications will display a UL CL2 rating
for low-voltage in-wall deployment. A dependable, quality HDMI cable will support UltraHD 4:2:0 and
will be HDMI high-speed rated and tested. Very few cables rated as “high speed” can actually deliver
that performance at lengths over 25 feet. Look for interconnects that use a minimum 24AWG signal
conductor to ensure performance at longer lengths.
Super flexible interconnects
Thin, flexible cables can be ideal for short distance temporary patches, short runs to table-top monitors
and even component-to-component patching in an equipment rack. Often high-speed rated and
capable of supporting UltraHD content in a 4:2:0 color space, a quality super flexible interconnect should
also feature HDMI(e) capability for Ethernet or Audio Return Channel applications. When using super
flexible HDMI patch cables it’s important to understand bus power demands in the installation to
prevent hardware handshake problems. Exercise caution if bus-powered signal sensitive selector
switches or extenders are included in the link path.
Plenum HDMI interconnects
A plenum space is any enclosed space in a buildings used for airflow. Some wires can be run in a plenum
space if they are properly rated in accordance with applicable electrical codes. The materials used in
cables to be placed in plenum spaces are designed to meet rigorous fire safety test standards in
accordance with NFPA 262 and outlined in NFPA 9. Many HDMI plenum-rated interconnects are
standard speed rated and not intended to support UltraHD or 4K content, but some products at the
premium end of the product spectrum may have a high speed rating. Plenum HDMI interconnects are
often specified for use in projects in schools and public buildings.
HDMI cables with gripping connectors
Although the weight of an interconnect or patch cord should never be supported by the connector
alone, there are times when the additional security of a “locking” connector comes in handy. Designs
are available with proprietary locking, or gripping, connectors. The two are not the same. A gripping
connector delivers about 3 times greater grip between the plug and the jack, but will disconnect if
yanked. Keep in mind that if you trip over a “locked” connector its really easy to pull the jack out of the
device and do permanent (and expensive!) damage. A gripping connector is a better choice as it
prevents slippage while still allowing connector release in the event of a catastrophic jerk!
Digital video over Coax
Sometimes there is already wire in a building that’s in good shape and can be repurposed during a
retrofit renovation. Most boardrooms, educational facilities and houses of worship were wired a
decade or more ago using quality RG6 quad-shield coaxial cable. Quite often there are five of them and
they were used for RGBHV analog video connectivity. Wouldn’t it be great if you could just run an HDMI
signal over those wires that are already in place? You can.
HDMI and DVI-D solutions (and even some DisplayPort) devices that enable the distribution of
uncompressed digital content to as many sixteen displays over a single coaxial cable are readily
available. HDMI-over-coax solutions may be an ideal choice for a small scale digital signage system or an
easy upgrade during a system renovation. Rarely designed to support UltraHD 4K or 3D video content,
adapting HDMI to existing coaxial cable works well with standard high definition content from just about
any device you can imagine, including workstations and laptop computers at distances up to 300-feet
and more.
HDMI over Cat5
When an installation is taking place in commercial space we may decide to leverage the category cables
that are typically associated with local area networks (LANs). In this scenario we use the same kind of
twisted-pair (also known as a “category” cable) to carry digital video as would be used to connect a
network switch to a device like a desktop workstation. Using Cat cables makes installing the system
easier, and sometimes more cost effective, by leveraging the scale of economy associated with
purchasing and installing large quantities of wire. Since tens of thousands of feet of Cat cable may be
installed in a school, office building, or house of worship it’s often less expensive and more efficient to
pull more category cable for the A/V system, too. As with all things associated with digital video, there’s
more than one way to get from here to there. In fact, there are at least three.
IPTV is a method of encrypting video content into a format that can then be streamed over the actual
network including wireless access points. There are significant benefits to using an IPTV design, such as
freedom to access content anywhere in the network (and even beyond) and the number of end-points
in the design can be very high. At the extreme end, this is how you enjoy YouTube videos – streaming
video. But there is a corresponding downside – IPTV is resource intensive and can add a tremendous
burden to the LAN. Most schools and universities, business and churches my find that their LAN simply
can’t support the additional payload. In the context of a new project, however, allowances can be made
for a virtual LAN (VLAN) to support A/V connectivity.
Short range HDMI over Cat5 extenders don’t leverage the LAN, but work by amplifying and equalizing
the signal so it can move through the UTP category cable directly from source to sink. This simple
technology is similar to the HDMI-over-coax solution above in that it provides a cost-effective method of
delivering standard HD 1080p content up to 160 feet (50 meters) over a cabling infrastructure that is
probably already there, or is being run during the project to support other systems.
The challenge with an HDMI extender that relies on equalization and amplification is that its effective
length can change with varying content. For example, a device might support 1080p content up to 65
feet over Cat5e, or 130 feet over Cat6. That same device might support lower resolution content such as
720p even farther, as the carriage distance of this type of solution is inversely proportional to the size of
the digital video payload. Understanding and living within the limitations of this type of technology
demands a very good survey of system sources, sinks and anticipated evolution.
Short range extenders are less expensive than HDBaseT or IPTV solutions. The cost savings might make
up for a slightly less-than-predictable performance in some installations where the content is well
defined and the video payload is limited.
HDBaseT is the new IEEE 1911™ standard that enables the optimized delivery of uncompressed, UltraHD
digital media. HDBaseT features its powerful 5Play™ feature set, including transport of ultra-highdefinition digital video & audio, 100BaseT Ethernet, USB 2.0, up to 100W of Power (through POH, similar
to 4-pair POE) and control signals all through a single 100m/328ft Cat6 cable. Generally speaking, most
HDBaseT solutions don’t support the uppermost level of 4:4:4:4 RGB 2160p 4K content (read 32 bit
video). Like high speed rated HDMI cables, HDBaseT is best for 4:2:0 content.
HDBaseT doesn’t travel over the LAN. Although it uses an Ethernet jack (RJ-45) and category cable,
HDBaseT is based on a different protocol that standard Ethernet equipment can’t work with.
HDBaseT solutions are also available in an HDBaseT Lite for projects that aren’t limited by the Lite
version’s 50-meter length limitation or constrained feature set consisting of audio, HD video, and
embedded control signals. Naturally the HDBaseT Lite solutions also come with a lighter price tag, all
the better to fit into that stretched budget!
The one thing that should be considered for any HDBaseT installation is the use of a shielded or
equivalent non-continuous shielded Cat6 or Cat6a cable. HDBaseT is very unforgiving of alien crosstalk
distortions, so the signal must be very well protected from environmental RFI and EMI.
Active HDMI interconnects
It’s been said by a number of industry experts that active cabling is the future of A/V electronics. In
particular, if we want to limit the physical size, thickness, weight and stiffness of cables then we need to
employ the power of a chipset. A silicon chip, powered by the source or sink devices themselves, can do
a lot to limit attenuation, crosstalk and group velocity distortions that limit the effective length and
physical size of passive copper links. In this way we can run a longer link over a lighter cable that might
only support a run of a few meters in a passive application. The benefit of embedding chip technology in
cables is the reduction of expensive copper used in production, reduced overall form factor, longer
reach and lower power consumption than active solutions like HDBaseT.
Available in lengths up to 100feet, active HDMI cables are often CL3-rated for in-wall installation and will
support content payloads up to 1080p, including 3D video. Active HDMI cables are a good choice when
you need to get a longer run, but there isn’t much room for big, thick cables in the cabinet or through
the conduit. Active cables also mean that you can avoid having a physical device at each end, as would
be the case with HDMI-over-Cat or HDMI-over-Coax solutions. Attention must be paid to bus power
demands when deploying active cables or active optical cables, however.
Active Optical HDMI interconnects are a variation on the theme above. In this case, the actual digital
video payload is converted from electrons to photons and sent over a set of fiber optic links, thereby
delivering the very best performance. Often active optical HDMI cables are plenum rated for use where
the standard copper active HDMI cable can’t go. Like active copper cables, active optical HDMI cables
are often rated to support 4:2:0 UltraHD resolution, including 3D and deep color payloads but not 4:4:4
32 bit payloads. This is a limitation of the chip set used in the cable.
RapidRun Optical Cables
This changes everything. RapidRun Optical isn’t just using fiber optics to transport the signal. It’s using
VCSEL lasers to provide the absolute highest performance connectivity in the industry. Unlike an active
optical HDMI cable, RapidRun Optical doesn’t use a hybrid interconnect assembly. There is no copper
between the source and the sink. This means perfect isolation, perfect immunity from RFI and EMI and
perfect performance to the very limits of 4:4:4:4 32-bit RGB UltraHD video at lengths to an astounding
1,000 feet (330 meters)!
RapidRun Optical is a Plenum-rated solution that can support not only HDMI, but DisplayPort or DVI-D
too. Its six channels of clear glass can deliver upwards of 20 Gbps of thru-put per channel, making this
one of the most powerful point-to-point video connections on the market. Select RapidRun Optical
when you need the best performance and the best scalability the industry has to offer!
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