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Wiring
for Video in the Home
By Bob
Hart, In2Connect
We are
in the midst of a digital revolution. Today, many broadcasters,
television production studios, and post-production enterprises have
adopted the high definition, serial digital interface (HD-SDI) as their
format of choice because of its superior quality.
Precision digital video is processed in real time and is extremely
stable, which reduces equipment adjustments and allows copies or
reproductions to retain the same quality as the original. Moreover,
signal degradation is virtually eliminated and noise immunity is greatly
improved. Digital technology not only promises to improve picture and
sound quality, but also paves the way for interactive on-demand TV and
other digital services.
Therefore, broadcast camera and video cables need to deliver top
performance, while at the same time meeting extended international and
European standards.
Belden was the first cable manufacturer to perform 100 percent sweep
testing to 4.5GHz on every reel of SDI cable, and to provide
certification to assure unequalled levels of cable signal integrity and
reliability, with lower return loss (RL) at higher frequencies. The
enhanced testing is currently being performed with three of Belden's
most popular precision video coaxial cables for HDTV/SDI, namely Belden
Brilliance 1505A, 1855A, and 1694A, and, it is claimed, will soon be
expanded to additional products.
This is done to assure broadcasters, leading-edge broadcast equipment
manufacturers and the standards bodies high performance and reliability
as they begin to migrate from the existing 1080i (interlaced) SDI format
to the emerging 1080p (progressive) format, protocols, and equipment,
which require twice the bandwidth of 1080i.
Consistency and Quality
In the 1080i format, the image resolution is 1920x1080 pixels
which, in the United States, is sent at 60 interlaced frames per second.
In the 1080p format, the image resolution is the same but is sent at 60
complete frames per second. Thus, in the progressive format, the
complete image updates every 1/60th of a second and in the interlaced
format, only half of the image updates every 1/60th of a second. As a
result, progressive images result in better image definition and
quality.
Cable manufacturing consistency and quality are both key to achieving
minimal return loss. Good practice requires testing to the third
harmonic of the operating frequency. For serial, single cable 1080p,
that means testing to a frequency of 4.5GHz. This 1080p format is
expected to transmit serial digital video signals at 2.97Gb/s, requiring
an operating frequency of 1.485GHz. This translates to a shorter
wavelength and a greater chance that inconsistently manufactured cables
will choke off the signal with a high level of return loss. This may be
caused by even the slightest irregularity in the cable manufacturing
process. A slightly bent shaft, bad bearing, or uneven cable tension,
for example, can cause reflection and affect the signal wavelength.
Therefore, utmost consistency and control in the manufacturing process
is essential to the reliability of the cable's signal transmission at
high frequencies and to minimize return loss.
This has to be continued into the wired home. It is no longer just a
place to eat, wash up and sleep, but a place to stay connected with the
outside world; a place to receive and route HD video signals from the HD
source to HDTV sets or home cinemas.
Providing Performance
To deliver the performance that delivers the future, wiring of
the smart home for video will involve either Cat 6 or similar
twisted-pair cabling or professionally-screened coaxial cables, which
can perform to the levels that the camera used when recording.
Baseband video signals, video-to-desktop, and CCTV from the front door
will work over UTP (Unshielded Twisted Pair) with suitable
impedance-matching baluns, but in transmitting professional video over
twisted pair, a lot of digital compression will be involved, which
arguably leads to impossible degradation of picture quality. It is
interesting that one of the major problems with baluns and analogue
video is not the high frequency limit, but the low frequency. Very low
frequencies are difficult to get through a transformer and other similar
devices. If using UTP for analogue video, be sure you get performance
data on the baluns across the entire operating range. Broadcast-quality
video requires performance all the way down to DC.
Balance in twisted pair cables is also a critical parameter. The nature
of a “balanced” line means that the two conductors in the twisted pair
are identical. The more identical they are, the easier it is for the
balun to reject noise and interference generated outside the pair.
Ironically, high-end “Category” cables are designed with unidentical
pairs in order to combat pair-to-pair crosstalk. Therefore for a given
length of category cable, the total length of one pair can be greater
than another, and since the signals travel in the cable at a fixed rate
(approximately 90 percent of the speed of light), the arrival times of
the signals at the receiver can be skewed. This can be a big problem
with high-resolution video systems transporting RGB for example. To
compensate for skew, cables, such as the Belden/CDT MediaTwist UTP can
extend video transmission distances out to 400 metres and beyond,
depending on transmission equipment parameters.
Cable return loss is another electrical component that can affect both
video quality and transmission distance. Poor cable return loss can
cause picture quality problems such as ghosting, poor pixel alignment,
and picture sharpness. These problems can be minimized by cables such as
the Belden/CDT VideoTwist. The UTP pairs in this product have a patented
bonded-pair construction. The conductors of each pair are bonded
together so they cannot separate, resulting in precise
conductor-to-conductor spacing and precise impedance
characteristics—even after the rigors of a typical installation.
Ultimately the video wiring will probably be RG59 Baseband (up to 200
meters) or broadband RG6 coaxial cable tri- or quad-shielded (300
meters). Your structured wiring system could have quite a few video
outlets. To get to each outlet, the incoming signal is split, and
splitting means losses. A two-way splitter for example, has a loss of
5dB, a four-way splitter loses 8dB, so most systems will involve a video
amplifier near to the video source.
Even coaxial cables create losses, so for HD, SVHS video, etc, we would
recommend cables such as the Belden Duobond Plus versions of 75 ohm RG6
or RG59, which have a three-shield construction consisting of an
overlapped foil tape, surrounded by a braid screen and a further outer
layer of foil to create the effect of a cable within a metal conduit.
The foil tape typically consists of aluminium foil laminated to
polyester film, and provides 100 percent coverage. It improves
protection against radiated emission and ingress at audio and radio
frequencies. The braid minimises low frequency interference. This
combination of shielding copes well across the frequency spectrum and
strips very easily for ease of installation.
Banana Peel
Another innovation aimed at reducing installation time and labor
cost is the recently introduced range of mini high-resolution component
video cables in a “Banana Peel” composite cable configuration. These are
designed for high-resolution VGA on large screens, HDTV, hi-resolution
CAD, animation, editing, and special effects.
Banana Peel technology was developed to meet the needs of the installer
by saving time and decreasing labor costs. By eliminating the overall
bundle's jacket, a whole step in the termination process can be removed.
Bundled coax cables are usually very stiff, but the Banana Peel RGBs
overcome this by allowing individual cables to be “peeled-off” a center
spline and terminated. The elimination of the overall jacket reduces the
diameter of the composite, so the cable's overall bend radius is
improved and the use of a smaller size conduit is possible. In addition,
the individual cable components are all instantly identifiable thanks to
color coding and print legends. Banana Peel Hi-Resolution Composite
Video cables Series 1281 is an enhanced version of traditional RGB
cables and features 25 AWG solid copper center conductors for lower
attenuation and easier termination, flexible PVC jackets, and
high-frequency Beldfoil™ and lapped wire shields.
Distributed video in the truly smart home is unlikely to be installed in
isolation from audio, telephony, high-speed Internet access, networked
computers, security systems, lighting, and energy management. You can
set-up a video network that allows you to share one VCR or DVD among all
the televisions in your house. With structured wiring you can also set
up computer network and share expensive equipment such as printers and
scanners.
A number of cable makers are offering composite, overall sheathed cables
which combine UTP data with video coax and even optical fibers for those
long runs where attenuation would overcome broadband. The most commonly
used connector in video wiring is the 75 ohm BNC connector. At lower
bandwidths up to 300MHz, the connector impedance has negligible effect,
so the bulk of installed video coax will be 50 ohm, but with digital
video the 75 ohm BNC or MUSA connector is advisable.
Conclusion
The right amount and type of pre-wiring is key to the successful
installation of a smart audio/video system for the years to come. It is
far safer to pull in more than you think you need and to use the best
cabling available as of today, as technically it will soon be
superseded!
Ten years ago, who would have thought of installing anything other than
TV drop lead? Always keep the potential future use of your installation
in mind. Whatever you think you want now, you will have changed your
mind tomorrow as something better emerges. If you do not know exactly
what you want in a video system, then at least run lots of hidden
conduit (or “dark fiber” in the communications installers terminology)
in your home. It will save your from an awful lot of demolition later. |
