HDTV HDMI/DVI CAT-5 Extender System
One
of the biggest complaints from consumers installing front projectors is the cable
length limitation encountered with HDMI and DVI interfaces. As HDMI begins to
make its way into the consumer electronics market complimenting the already established
DVI interface, the need for longer cables become necessary, especially with the
increasing popularity of front projection systems. It is not uncommon to find
users asking for a long DVI or HDMI cable to solve this distance problem only
to find that the video interface becomes unreliable. Unfortunately, long DVI/HDMI
cables compromise signal integrity and eventually fall out of the DVI specification
resulting in sparkles on the screen or a complete loss of picture. To solve this
problem, Gefen Incorporated has introduced several new products that extend the
length of DVI/HDMI signals. Gefen offers a variety of DVI/HDMI solutions including
switchers, extenders, splitters and distribution amplifiers. The new HDTV Extender
is designed to extend the HDMI interface, but is completely capable of supporting
DVI signals as well when used with an adapter. Futhermore, the interface is also
compatible with HDCP copy protection. Our focus will be on DVI since our products
currently use this interface format. However, since more products are now showing
up with the HDMI interface, we decided to review the HDMI version to show that
it is fully backwards compatible with current DVI products.
If you are only intersted in DVI, Gefen also sells the DVI CAT-5 Extender that has DVI connectors instead of the HDMI connectors. Our tests utilized Accell Corporation's DVI-to-HDMI adapter along with their DVI-to-HDMI cable. The HDTV Extender supports 480P, 720p, 1080i, 1080p and other resolutions up to 1920 x 1200. The single link design supports pixel rates as high as 165 megapixels per second. In addition to 720p and 1080i signals, we ran a variety of VESA compliant timing signals to our projector with no problems.
The HDTV Extender package consists of a transmitter, a receiver, an AC power supply and an HDMI cable. The two interface boxes each have an HDMI connector and a pair of RJ-45 jacks. A standard pair of CAT-5 cables are used to transfer a single link DVI signal up to 150 feet from the source. Although Gefen does recommend using shielded cable for better noise immunity when running longer distances, non-shielded cable may work just fine in many cases. The AC power adapter plugs into the transmitter and the receiver gets its power from the transmitter through one of the CAT-5 cables. The transmitter and receiver are small and identical in size, each measuring only 3.4" W x 3.25" D x 1.25" H. The package we received also included a six-foot HDMI-to-HDMI cable.
The
Problem and the Solution
DVI and HDMI use Transistion Minimized Differential
Signaling (TMDS) to send data over twisted pair cables. Differential signaling
is an effective way of sending signals over long distances, but as distances increase,
bandwidth decreases. The signal integrity is a function of the transmitter, the
cable (both length and quality) and the sink or receiver used in the setup. All
three play a role in the signal performance and can be measured using the "Eye-Diagram"
on a high-speed storage oscilloscope. Most of the DVI transmitters chips we've
seen specifiy a cable length of up to 5 meters using quality twisted pair wires.
Based on our experience, DVI reliability can vary widely depending on the setup.
We have been successfully using a 20 meter AudioQuest DV-1 cable in many applications,
but we have encountered some problems with using a Bravo D2 DVD player. With that
said, we would advise users to minimize the DVI cable length whenever possible.
The HDTV Extender design leverages from proprietary high-speed communications technology that allows high-speed data to be reliably transmitted over standard CAT-5 cables. The HDTV CAT-5 Extender is a system that can transmit a DVI/HDMI signal up to a distance of 150 feet using two CAT-5 cables. We used unshielded CAT-5 to run approximately 50 feet and didn't encounter any problems with our setup.
Installation
The installation of the HDTV Extender is simple compared to the fiber optic based
system we reviewed from Gefen . We were interested in running our own inexpensive
CAT-5 cables for testing the HDTV Extender. After all, the great benefit of this
product is the fact that easy to find CAT-5 cables can be installed in the walls.
Our installation
required 50 feet between our DVDO iScan HD processor and our Optoma H77 DLP projector.
The 1000-foot spool
of unshielded CAT-5 cable was a mere $65 at the local electronics store. This
comes out to less that 7 cents per foot for a total of $7 for cable (not including
connectors). The Gefen manual didn't say much about the cabling so we assumed
that a straight-through CAT-5 cable was required. We were correct and didn't have
any problems with our custom cable length. For those do-it-yourselfers who want
to build their own cables, I would recommend using a simple CAT-5 cable tester
to ensure that the wiring is correct. It's possible to damage the Gefen unit with
improper wiring since power is sent over the CAT-5 cable link. LEDs on both the
transmitter and receiver let you know that power is getting to both units.
Performance
Our
setup consisted of a Samsung SIR-T165 set-top box drive connected to a DVDO iScan
HD video processor. The DVI output of the iScan HD is connected to the Gefen HDTV
Extender transmitter box. The receiver unit is then plugged into our Optoma H77
high-definition DLP projector. High definition content sent from the Samsung unit
is passed through the iScan processor. The Samsung has the capability to provide
us with a DVI signal from terrestrial high-definition broadcasts or from our FireWire™
(iLink™) connection to our D-Theater VCR. With this setup we can play our
D-VHS tapes and digitally link them to the H77. The
link to the projector was very reliable and provided us with a wonderful picture.
Conclusion
Gefen has once again solved a practical problem faced by many consumers installing
a projector into their home theater system. Unreliable HDMI and DVI interfaces
due to long cables have forced many users to look for alternatives. The HDTV Extender
provides a reliable HDMI/DVI connection for any display located as far as 150
feet from the source with the use of inexpensive CAT-5 cabling. In addition, full
HDCP copy protection is supported, making it attractive for today's new video
systems. While the system costs $500, it is actually less expensive than many
high-end cables that sometimes give marginal performance. The robust design ensures
a good signal to the display.
- Kevin Nakano
