Advances in facility cables and connectors

Technology rolls on, but broadcast cables and connector paths will always be designed to combat "electrical noise" from EMI, RFI and capacitance variation as this article indicates.

Server based systems in production, post production, news gathering and playout bring increased efficiencies and productivity.

Nonlinear editing and news gathering allow users to access the same material across a data network.

"Digits" are here and over the next decade will increasingly dictate the form of cables and connectors, from camera through to playout.

Technology rolls on, but broadcast cables and connector paths will always be designed to combat "electrical noise" from EMI, RFI and capacitance variation as this article indicates.

The two most important electrical qualities in audio cables are inductance and capacitance, as they dictate characteristic impedance and propagation rate (the speed at which the signal passes through the conductor relative to the speed of light).

Impedance is the total sum of opposition to the flow of energy through the cable and what really enables different cable designs to achieve different sound quality The "balanced audio", hot-cold-screen concept is the first good reason for twisted pair audio cables.

To combat electromagnetically created voltages (mains hum) the twisted pairs will have varying lay lengths so that the magnetic fields in each pair do not align.

A grounded screen can help reduce/eliminate electrostatic hum and is especially important on long cable runs.

Quality and type of shielding here is very important.

For serious abuse the use of thermoplastic rubber (TPR) sheaths assures flexibility with a good degree of abrasion resistance.

FEP and ETFE materials like those from Calmont Wire and Cable offer additional environmental power, temperature and chemical resistant properties.

Oxygen free copper to DIN40500 Part 4 is fast becoming the norm.

The elimination of oxygen by casting and annealing it in an inert gas produces a product with fewer impurities in the "skin" of the conductor where most of the signal will be carried.

Analogue pairs can be quite forgiving of digital signals over shorter runs, but a good digital cable, because of its low capacitance is great for analogue signals.

Digital cables will also "futureproof" an installation while greatly improving signal stability and noise immunity.

The embedding of digital audio with a digital video signal in TV applications requires the use of high quality 75ohm coaxial cables.

Coaxial cables will carry digital audio further than twisted pairs.

However, if you're not embedding, then the cost of baluns or multiplexers can be the deciding factor in using coax.

Neutrik produces AES/EBU baluns which plug directly into the back of broadcast kit to convert 3-pin 110ohm XLRs to a 75ohm interface.

On the other hand, terminating an embedded coax is simple, because generally only one type of BNC or F connector is involved.

Digital cables must be dimensionally stable, because any change in the designed distance between conductors and screen will alter the capacitance - hence the impedance and the resulting return loss.

Choice of insulating material is important because in a screened audio or video cable the insulator is the capacitor dielectric between core and screen and, the lower the capacitance - the less time it will take a signal to reach full amplitude.

For low noise applications the star quad construction is often used.

A quad is a four-wire construction in which diagonally opposed conductors are used to form two pairs.

These cables are manufactured with low capacitance polymers and fillers so that they keep their shape if bent or crushed.

They are very flexible and well shielded because of the spirally applied (not braided) fine copper wire screens over each quad.

TTL Video has just completed the installation of Bryant's STR 32A, eight-star quad reinforced cables for the fixed audio feeds in every UK Premier League football ground.

Another popular cable is the AES/EBU, 110ohm, twisted pair digital microphone cable, suitable for use in RS485 data, DMX or mic applications.

Fine oxygen free strands, very low capacitance insulation, precise lay-up, anti-microphony fillers and a tough jacket make for a handleable yet low noise cable.

Because 100% shielding is required, a combination of tape and braid is used.

This construction works well with the recently introduced Neutrik digital XLR connectors which offer 50 times better EMC screening and can be used for electronically hazardous environments and digital audio to AES3.

In the last few years, 270Mbit/s SDI (Serial Digital Interface) video coaxials like Bryant's BD SD01 range from CDT/Raydex, Belden 1855 and Draka Image 360 have largely superseded the BBC's PSF 1/3M, because of their high velocity of propagation, low attenuation and considerable associated space savings in MCR and OB applications.

Because braid screens are most effective below 10MHz and tape screens above, and digital signals include both high and low frequencies, these cables have a combination of both, but with solid plain copper cores they are still easily terminated with 75ohm BNC connectors.

For multiple video feeds Bryant has a rugged, multiway MIL-C5015, reverse bayonet locking connector with coaxial contacts.

Available as plugs and sockets in both cable and panel varieties, they offer a cost effective and more resilient alternative to looms terminated with individual BNC connectors.

The current installed base of BNCs for analogue video will largely be the 50ohm type.

However, the quarter wavelength of digital video is circa 1500 times smaller than analogue, so reflections caused by BNC connectors in the "digital" chain will affect return loss and attenuation.

Return loss is not just down to cables - impedance mismatches from unsuitable connectors, patch bays etc can lead to signal reflections at high frequencies, so the use of "true" 75ohm BNCs becomes crucial.

Outside the USA, the de-facto standard for video patching was the 50ohm MUSA connector.

For digital signals, new 75ohm "one-piece machined centre conductor" designs have arrived which maintain backwards compatibility to existing MUSA products.

Basic Fourier analysis of a square-wave signal shows that, in the frequency domain of SDI the fundamental plus the odd harmonics are used to regenerate the signal.

A standard rule of thumb is that for reasonable signal fidelity the fundamental plus the harmonics up to the 7th are included in the passband.

When measured at up to 5GHz (to give a close response to the 7th harmonic) the Bryant MUSA combination of U-link and patch panel (two adapters) is typically 0.6dB for the complete signal path.

A new cable to the hybrid audio/video cables portfolio, specifically to substitute for the old BBC PIF8/1 combined video/audio/power cable type is the BD CMQVP.

This incorporates a screened three-core 6A power supply cable, an SDI video cable and a copper lap screened, audio star quad - all in a single jacket.

Each element is individually sheathed to allow the components to be broken out into their separate audio, video and power connectors.

A large established user base in studio and OB installations means that triaxial cables remain the most widely specified.

Bryant still manufactures or repairs a lot of multiconductor hybrid cables like Sony CCZA, Ikegami, JVC and Thomson for connecting professional cameras to CCUs, VTRs etc.

Despite a lack of HDTV transmission to the home across Europe, the high resolution capture of television programmes in digital, especially of sports events, greatly improves sound and picture quality.

SMPTE has developed a specification SMPTE 311 for high definition camera cables.

A composite cable, with two optical fibres for video, two or four power cores and two control/sound conductors, it enables reliable signal transmission in a relatively small, lightweight cable construction.

Fibre usage is increasing and offers great benefit in terms of bandwidth and higher transmission distances.

Optical dispersion (effectively attenuation) reduces as bandwidth increases so with multimode fibres there is a move back to OM3, high bandwidth 50/125um for high frequency transmission.

Fibre optic connectors must maintain alignment during repeated mating cycles and coupling loss can easily result from misalignment, poor end preparation, mismatched fibres and Fresnel reflection.

Tyco has a patented crimping technology for their "LightCrimp Plus" connectors which makes optical termination a simple mechanical process - without the use of adhesives, polishers, or curing processes.

The mating faces of the connectors are factory polished and contain internal index matching gel to cut down dispersion and offer field terminable performance on a par with standard epoxy connectors.

In the OB field, fully sealed ruggedised hermaphroditic "expanded beam" connectors like Fibreco's Fibre Cast system offer one- to four-channel configurations using rugged, deployable mil-spec cables which are far better suited to the rigours of riggers.

These connectors use lenses to first expand the light substantially beyond the diameter of the fibre itself and then refocus the light from the incoming fibre to the receiving fibre.

The optical interfaces are protected by what amount to glass windows and hence if coated with water, dust or mud are easier to clean without causing damage.

Designed for 2000 matings minimum, they require no maintenance.

USB devices can transfer data at up to 12Mbit/s, almost 50 times faster than serial connections, and are increasingly used in the broadcast sector in audio and even digital speakers and microphones.

However, within digital post FireWire is the interface of choice.

FireWire is a high-speed serial input/output technology for connecting digital devices such as camcorders and cameras to desktop and portable computers.

FireWire enables patching at 400Mbit/s bandwidth - more than 30 times faster even than USB.

Bryant broadcast offers "back to back" connectors and patch panels for both USB (A to B), and FireWire/IEEE1394a protocols.

Back in 1994, the BBC R and D Department published a paper at the AES International Convention in Amsterdam entitled "Twisted-pair cables for AES/EBU digital audio signals".

It concluded that Cat5 shielded twisted audio pairs offered the highest performance of a range tested, including coax and cables specifically designed for digital audio.

Cat5 was preferred for its flexibility, consistent performance and use for mixed data applications.

It started a trend resulting in a new range of RJ45 type connectors including the Neutrik Ethercon ruggedised Cat5e connectors, as well as back-to-back panel socket versions.

In the OB sector this led to the "military" style RJ45, based on a MIL-C26482 bayonet connector body.

The connector shell retrofits to a preterminated cable, and the panel connector is a bulkhead back-to-back RJ45 unit.

The cable used with them is a Cat6 over sheathed with a rugged PE jacket to resist abrasion, water and trucks running over them.

The AES/EBU 110mohm +/-20% impedance specification is undoubtedly accelerating the use of UTP (unshielded twisted pair) cables in the convergence of broadcast and computer/control systems.

The latest Cat6 UTP types can achieve a -30dB common mode balance.

Consequently they are capable not only of Gigabit Ethernet transmission but also "multimedia".

New generation insulations like foamed HD polyethylenes offer low capacitance with the necessary crush resistance to avoid capacitance change.

But, be warned: although Cat6 offers an available bandwidth of 200MHz - twice that of Cat 5e and the RJ45 connectors may look alike - at 100MHz the near end crosstalk (NEXT) of an average Cat5e connector is 43dB whereas that of a Cat6 is 54dB.

So a Cat6 connector couples only about an eighth of the noise, pair to pair that a Cat5e connector couples.

Digital signals are in principle more resistant to interference than analogue, and broadcasters can maximise their bandwidth or satellite capacity through digital compression and multiplexing.

As audio and video VCRs, playout carts etc are increasingly replaced with hard disc server devices then multimedia signals are likely to share the same building infrastructure data wiring as other services.

This may still be in its infancy in "copper" terms as the 100m link length of Cat6 is arguably not a match for coax, but better compression and frame rate techniques will lead to more applications like streaming media becoming commonplace.

Then the higher bandwidth that Cat6 and fibre offers will generate smarter hardware and software in a self perpetuating circle.

How about the management of an entire broadcasting infrastructure and operation, from content creation to distribution on a single computer network?.

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