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Specifying UTP for high resolution video extension
One of the most
popular questions we receive is: “Do I need to use a special UTP cable for high
performance video extension?” With
Magenta’s MultiView technology, the short answer is no, but there are some
important considerations for the systems designer to keep in mind during the
specification phase to ensure optimal system performance, cost and end-user
satisfaction.
CAT5e & CAT6
Although we can trace
UTP cable’s humble ancestry back to telephony and telegraphy, its principal
function today is to provide a structured solution for high-speed data network
connectivity. Data network applications
place certain demands on UTP cabling and in order to provide successful
high-speed network performance, cable bandwidth, crosstalk attenuation and
noise immunity must be optimized. While
trade secrets likely abound within the cable industry regarding special
techniques for optimizing these and other aspects of UTP performance, generally
the needed results are achieved by modifying the physical construction of the multi-pair cable.
Increasing and
carefully maintaining the twist rate of the cable’s individual pairs is one
important method in the cable designer’s arsenal for improving network
bandwidth performance and noise immunity along with assuring better consistency
of the cable’s characteristics. If you
peel back a foot or so of your average garden variety CAT5e (or classic CAT5 if
you can find some) cable, you will see four individual pairs of 24 AWG
wire. Each pair is comprised of two
insulated conductors that are twisted axially about one another. A slightly closer examination may reveal
that each of the pairs is twisted at slightly differing rates. If you inspect a similar specimen of CAT6,
you may notice that the pairs are twisted more tightly than with the CAT5e and
that the twist rate difference between them is more significant. Typically, the CAT6 cable will also be
constructed of heavier 23 AWG conductors.
The reason that the
pairs are twisted at different rates in UTP is that doing so improves crosstalk
attenuation. The principle here is that
with differing twist rates, when the pairs are combined within the overall
cable jacket, they are less likely to conform to one another (or follow one another
around) and are therefore likely to provide less inter-pair coupling surface
which is the fundamental vehicle for crosstalk.
CAT6 is specified to
perform at higher network bandwidths and is often thought to have a “much
broader passband (or higher bandwidth) than CAT5 or CAT5e.” While the analog bandwidth of CAT6 may be
somewhat higher than CAT5 or CAT5e, it’s really more about the predictability
of the cable’s behavior at the upper end of the specified operating frequency
spectrum. It’s also about the reduction
of crosstalk, where in a node to node network application, the powerful
outbound signals carried by the TX pair can couple into the RX pair and impair
reception of the feeble inbound signals causing communications errors. CAT6 is an excellent choice for high speed
network connectivity and it’s usually the only choice considered for new or
reasonably “future resistant” installations – CAT5 and CAT5e are often
considered passé for other than legacy work.
It’s easy to
understand that when a systems designer is tasked with implementing high
resolution UTP video extension, the natural assumption may be to specify the
“best” network cable. Unfortunately,
that’s not an appropriate assumption and in fact it’s often quite the opposite
– less expensive and lower performance network
cables perform equally well or better for UTP video implementations.
To explain: The
somewhat increased top end bandwidth capability of CAT6 is of little benefit in
analog video connectivity. Typically,
the maximum, realistic end to end bandwidth of a UTP based video extension
system is on the order of 100MHz or so. The difference between CAT5e and CAT6
at 100MHz is only about 2dB and as such won’t have a profound impact in image
quality.
So the bandwidth performance is acceptable with
either CAT5e or CAT6 cable, but going back to the improved crosstalk
characteristics of CAT6 that we examined earlier, we’re reminded that the twist
rates are quite different the between the adjacent pairs. Again, this is excellent for data network
applications, but it doesn’t work in favor of high performance analog video
transmission.
The problem is that a
tightly twisted pair is necessarily made from two longer pieces of wire than a
loosely twisted pair. That means that
within a given length of a multi-pair cable, the electrical length of a tightly
twisted pair is greater than a loosely twisted pair and signals will simply
take longer to propagate through it.
This is problematic for multi component video signals where each twisted
pair supports a particular video color channel. The effect of different signal arrival times is referred to as
delay skew.
UTP delay skew
increases linearly with distance and its effect on video increases as the
source resolution is increased. In other
words, lower resolution signals are less affected by delay skew. Magenta has a calculator that helps to
relate cable skew, distance and resolution and it’s available to our partners
upon request by emailing me: cmiller@magenta-research.com
CAT5e and CAT6 cables
from different manufacturers can vary quite materially with respect to the
amount of actual delay skew and this uncertainty can complicate the system
designer’s understanding of how a particular cable may perform in a video
extension application. In actual
practice, very few of the CAT 5e or
CAT6 cable samples that we have tested here at Magenta present skew levels that
are close to the manufacturer’s specification limits.
Fortunately
electronic skew compensation available in MultiView UTP receiver products
enables the successful use of CAT5e or CAT6.
From a technical perspective, skew compensation is a very effective
approach for contending with the inherent, temporal misalignment of the
separate channels and by making it a removable option, a cost benefit can be
realized by those who make use of low skew cable.
In summary, skew
compensation is a rock solid solution for applications that will need to make
use of CAT5e & CAT6. In situations
where alternative cable types can be exploited, low skew UTP cable is a very
attractive, low cost option.
As UTP video
extension has gained popularity and has come to offer such excellent
performance, UTP cable manufacturers have recognized four important points:
1.
High
quality, high performance UTP video transmission is technically viable
2.
CAT5e
& CAT6 cables aren’t optimal for multi-component video transmission
3.
UTP
video distribution and transmission has gained significant popularity
4.
A
market need for an appropriate UTP solution has emerged and is growing
In response to the AV
industry’s acceptance of high performance UTP video signal management
solutions, the cable and wire industry has begun to offer alternative UTP
designs that are optimized for video applications.
Low Skew UTP
“Low skew” cables
have made their way to the marketplace and by now are being offered by several
preeminent manufacturers, including Belden, Mohawk, Gepco and Liberty. These cables are typically constructed by
combining four pairs that are manufactured as identically as possible and as
such, reduce the delay skew values to a fraction of what’s normal for CAT5e or
CAT6 types. Now nearly all great
solutions involve some form of tradeoff and low skew cable is a good example of
this axiom. Using identical pairs
eliminates skew, but of course increases the effects of crosstalk. Fortunately, in multi-component video
transmission, a limited amount of crosstalk between the separate channels does
not cause any difficulty. The crosstalk
effect in low skew UTP is largely frequency dependant and this being the case,
becomes more pronounced toward the upper end of the passband spectrum. The result is that crosstalk events from one
channel to another occur for a fairly short period of time during voltage
transitions and occupy the temporal domain that’s responsible for pulse edges
or in video terms, rapid shifts in contrast.
The effects are not visible even in very high-resolution video
images.
All in all, low skew
cables are an excellent choice for long range, high performance UTP video
extension and the price per foot is competitive with good quality CAT5e.
Naturally however,
because of their higher crosstalk coupling, low skew UTP cables are an
inappropriate choice for data network use and this is a place where the
designer will be rewarded for being attentive….Low skew cable is not for
networks.
Often an expectation
associated with commissioning a structured cable plant is that it can be used
universally for many forms of connectivity and in many instances it can. Because of electronic skew compensation, a
CAT6 cable plant can successfully be used for both network and video
extension. Since horizontal network
segment length is confined to 100m or 328ft, the typical accumulated delay skew
through one run is fairly manageable.
Even highly skewed CAT6 cables yield no more than “45nS per 100m,” a
value representing the limits of the cable manufacturing industry’s
interpretation of the TIA-568 spec. Realistically, the average delay skew value
per 100m is 25nS based on numerous characterizations performed here in our lab.
So, real world distance capabilities are less constrained by skew than the spec
may suggest. Magenta’s AK & XR
series receivers with optional skew compensation can correct for as much as
62nS of delay skew. If CAT6 will be
used for video extension over multiple, max distance (100m) network segments,
then the designer will want to tread carefully since aggregate skew values can
easily mount up to levels exceeding the 62nS correction capability.
In summary at this
point:
1. High
resolution UTP video extension can be successfully achieved with CAT5e, CAT6 or
skew free cable types. For some
applications where high speed network infrastructure cabling must be used and all system cabling must be of the same type, CAT5e or CAT6 will
be the order of the day and as the designer you will want to specify MultiView
hardware with skew compensation facilities.
2. If a
project doesn’t call for a unified structured cabling scheme and low skew cable
can be used, it will provide excellent performance and reduce system cost.
Magenta is here to help
Magenta’s UTP video
solutions can provide high-resolution video transmission over very long runs of
UTP. Our XR-2000 receiver can be used
with any of our transmitters, DAs or switches to carry 1080p/60 or 1920 X 1280
format video over 2000ft or 610m of cable.
It does so without resorting to the use of repeaters or “line
splitters.”
Aside from our 2000ft
solution, other MultiView receivers in our solutions portfolio support 500ft,
600ft and 1200ft operation and provide high-resolution format operation as
well. In applications where long runs
are to be implemented between the source and destination display equipment and
if those runs exceed 100m or 328ft, there is no sound reason to use high speed
data cable for them. UTP run lengths
stretching beyond 100m simply will not be recaptured for future 10/100BASE-T
data network applications. In this
case, it’s very easy to justify using skew free cable to achieve the optimum
performance without sacrificing cost.
High performance,
long distance extension is a benefit whenever a system will be supporting
displays that are situated far from a central source-equipment location. And keeping delicate source equipment
centralized in a secure, environmentally controlled space is a preferred
engineering practice that ensures optimal performance, uptime, system security
and access. By sharing and distributing
from a smaller number of (typically, PC based) content sources, reduced
licensing fees together with the reduction of player hardware and improved
reliability result in lower total cost of ownership.
MultiView equipment
is designed to provide cutting edge system performance and it’s also designed
to be easy to use and simple to integrate with your system. Generally, specifying the appropriate
MultiView extension solution revolves around determining what distances
capabilities are needed and also which auxiliary signals will be required, such
as audio and serial data.
In complicated
systems or those, which require use of specific cable types, Magenta is always
willing to assist. One of the services
we offer to our partners is cable characterization. If you feel concerned about using a particular cable type, we
encourage you to send us a sample and we’ll test it in our lab and provide a
report along with our recommendation.
Here’s a recap with
some quick bullets:
- • MultiView makes high
performance video & auxiliary signal management possible with CAT5, CAT5e,
CAT6 and low skew cable types and ultimately offers the advantage of reducing
total cost of ownership.
- • In addition to
point-to-point extension, the MultiView solutions portfolio provides; point to
multipoint and full matrix capabilities.
- • CAT5, CAT5e and CAT6
introduce delay skew. For some applications
depending on distance and video resolution our skew compensation option will be
necessary.
- • For continual UTP
runs that exceed 100m / 328ft, low skew
cable makes logical sense because runs beyond this length won’t be usable for
standard 10/100BASE-T applications.
Next month, we’ll
explore the topic of video AND audio switching via the Mondo Matrix UTP switch.
Chris Miller
Chief Scientist
Magenta Research
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