Demand for delivery of DVI and HDMI within the pro AV discipline
continues to grow. The proliferation of DVI and HDMI source,
processing and display equipment, the need to present protected
content, and the sometimes arbitrary preference for digital AV
solutions are factors driving the trend.
The short haul is easy
Moving native digital signal formats over a metre or two of copper
cable requires little or no attention, since this is what they were
intended to support. In the case of DVI, the primary intent has
been to connect the digital video output of a graphics card with a
desktop display.
The essence of HDMI’s purpose is to provide a secure connection
between set-top boxes and other consumer-oriented audiovisual
sources to televisions and other presentation equipment. Over the
short haul, DVI is an excellent standard that provides robust
plug-and-play performance.
Signal considerations
Neither DVI nor HDMI signals lend themselves particularly well to
transmission over distance. Indeed, they were not developed to do
so. The signals occupy significant bandwidth and rely on discrete
clock and data channels, making them susceptible to the effects of
cable dispersion and skew.
The configuration of the electronics at the source’s output port
and the sink’s input port are only designed to support short-haul
transmission over native cables using screened pairs. Successful
transmission of DVI or HDMI over longer distances or over
non-native cabling requires some form of signal conditioning and/or
processing.
Which design approach?
There are several approaches that can be taken to move native,
uncompressed DVI and HDMI over distances that extend well beyond
the reach of standard cables.
Solutions for native DVI/HDMI extension include:
● Actively equalised “long” HDMI cables
● DVI/HDMI over UTP links (transmitters and receivers)
● DVI/HDMI over optical fibre links (transmitters and
receivers).
Each of these provides capabilities that fit best with fairly
specific requirements. Here is a quick rundown of these
approaches.
Actively equalised HDMI cables can support 1080p/60 to distances of
up to 40–45m. They tend to be rather heavy and bulky but can
provide a reasonably inexpensive, quick and box-less solution for
applications where bulk is not a problem.
DVI/HDMI over UTP links rely on electronics at both ends to
provide the needed signal conditioning to take advantage of the
easily terminated, low-cost cable. Power supplies are most often
required for both the transmitter and the receiver.
System prices for DVI/HDMI over UTP links vary depending on
features and performance, but are certainly inexpensive when
compared to fibre solutions. UTP can be much more
installation-friendly than bulky native DVI or HDMI cables.
This is especially true in commercial settings and wherever there
is a significant density of cables or links and where it is
desirable to dress cables neatly into equipment cabinets.
DVI/HDMI over optical fibre links pick up where copper solutions
leave off. Distances exceeding 40m or so call for fibre solutions
and will provide consistent and reliable transmission of
high-quality, uncompressed native DVI or HDMI.
Both single-mode and multi-mode fibres can be used depending on the
required distance capability and the mode of signal transmission.
Popular techniques to support the required multi-channel
transmission include:
● Multiple fibres, one per channel
● Twin fibre, TDM (time division multiplexing)
● Single fibre, CWDM (coarse wave division multiplexing).
Twin fibre TDM solutions can be very cost-effective but are usually
limited to lower-resolution formats to avoid using expensive
broadband optics. 720p or 1080i are about the maximum resolutions
that can be supported by low-cost optics.
Depending on the transmission technique being used, DVI and full
colour depth HDMI 1.3 can be carried over distances exceeding 600m
of single-core multi-mode and multiple kilometres of single-mode
fibre.
DVI extension is usually straightforward, but successful HDMI
implementations can be more elusive because of HDCP (high-bandwidth
digital content protection). Exceeding manufacturer’s distance
specs or using unspecified cables can result in issues that are
difficult to troubleshoot and remedy.
Author is Chris Miller, chief scientist at
Magenta Research