Articles and whitepapers

Wiring for Video (3/11/2005)

By Bob Hart, Bryant Broadcast & Data Communications

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, whilst at the same time meeting extended international and European standards.

Prewired faceplate for home installations, with (L to R) D-sub connector, DIN circular and RCA (phono) for VGA, signal/control and audio respectively.

Belden/CDT was the first cable manufacturer to perform 100% 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, of 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 US, 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 achieve 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 minimise 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 the 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 Cat 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% 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 range 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 minimised 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 rigours of a typical installation.

Ultimately the video wiring will probably be RG59 baseband (up to 200 metres) or broadband RG6 coaxial cable tri- or quad-shielded (300 metres). 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 consist of aluminium foil laminated to polyester film, and provides 100% coverage. It improves protection against radiated emission and ingress at audio and radio frequencies. The braid minimises low frequency interference. This combination of shielding methods copes well across the frequency spectrum and strips very easily for ease of installation.

Cable innovations

Another innovation aimed at reducing installation time and labour costs, 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 cable

Banana Peel technology was developed to meet the needs of the installer to save time and decrease labour 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 centre 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 use of a smaller size conduit is possible. In addition, the individual cable components are all instantly identifiable thanks to colour 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 centre 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 fibres 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.

BNC and MUSA connectors

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, then at least run lots of hidden conduit (or 'dark fibre' in the communications installers terminology). It saves an awful lot of chiselling out of plaster later.

...Of course you could go 'wireless', but that's for another day!

Bob Hart is a consultant for Bryant Broadcast & Data Communications, manufacturer and distributor of systems installation equipment for integrators worldwide.

www.bryant-broadcast.co.uk