Key connections

With both audio and video systems presently pushing their respective quality envelopes, the demands on the people, equipment and infrastructures that support them is also growing rapidly. On one side, this manifests itself as a need for operating staff with new skills and greater technical knowledge, while on the other it demands faster and more powerful equipment that is able to support groundbreaking technological advance.

To the consumer, all of this readily relates to an exciting new age of bigger, brighter, sharper images and surround-sound loudspeaker set-ups – all very hi-tech and glamorous. Behind the scenes, while still hi-tech, systems that will not function correctly or staff that are unable to employ them quickly bring things down to earth.

A brief comparison of analogue and digital requirements readily reveals that audio is moving towards a higher channel count and higher resolution, where video has to deliver a higher picture resolution and increased scanning.

In terms of analogue audio, it’s probably true to say that there is not much left in professional markets for cable developers. Analogue systems are still in widespread use in domestic and semi-pro applications, however, including the analogue mixers found in some broadcasting studios – particularly hospital radio and some commercial radio stations. Most high-end broadcasters are either already using digital desks and digital transmission systems, or are currently involved in making the transition from their old analogue set-ups to these digital systems. In many countries, this is being driven by government legislation that places a time frame on the transition to digital transmission and cessation of analogue radio and television broadcast.

The audio chain standard is still 16-bit, 44kHz but high-end recording studios using digital systems such as Pro Tools may use 24-bits at 96kHz, raising the data rates and storage requirements. Cabling has to be able to accommodate increased data rates. Both cinema and domestic A/V systems are increasingly using 5.1-channel systems, and many cinemas are taking this to 7.1 channels and beyond.

All things audio and video

With the advent of digital recording and processing technology the demands on the recording engineer have changed quite drastically. Whereas a typical recording would once have used an analogue signal chain with analogue processing, and the recording media would have been a tape format of some description, most recording now takes place in the digital domain and maintains an entirely digital signal path. Sometimes the analogue-to-digital conversion takes place as early as within the microphone itself, with the end media stored as data.

Digital disciplines require very different skill sets to those required by studios that relied on analogue reel-to-reel machines. Being a recording engineer today is in some respects akin to being an IT technician – modern studios use hard drives and other computer techniques such as networking, and require understanding of file formats, file transfers, conversion and interconnection. From the humblest of hand-held devices to the largest DAW systems with track counts in excess of anything a mere 2-inch tape could ever hope to accommodate, recording equipment itself is now derived from computer technology.

In the analogue domain, issues such as signal-to-noise ratio, frequency response and losses in the signal with every generation pass meant that the dynamic range of the source material had to be carefully controlled. With digital technology, engineers operating equipment with high sample rates and 24-bit dynamic range also have to worry about synchronisation issues between various devices, relevant file formats, project back up and data logging and tracking. With digital technology, ‘problems’ are usually hidden until it’s far too late. Using the wrong kind of cable, or neglecting to set a device as a word clock slave (or master depending on what you are doing), means that errors can be introduced into a project that could make the whole process fall over just when you think that everything is running smoothly. As a result, conversations about block error rates and cyclic redundancy checks and the equipment needed to monitor such are becoming more commonplace. Digital either works or it doesn’t – when it decides not to work you usually get little or no warning of impending disaster.

But with even the most modest project studio using digital technology, there is still a hankering for the ‘magical something’ that engineers from the 1960s and 1970s obtained from valves and transistors. As such, hardware units with valve processing and boxes recreating ‘classic’ analogue processing paths, as well as modules rescued from analogue consoles remain popular. These outboard processors often have their own built-in A/D conversion, otherwise they require external AD/DA converters, meaning that interconnections, again, are digital – even ‘new retro’ has a digital edge.

In professional audio applications, there is now widespread use of ‘embedding techniques’, particularly in OB and fly-away systems. By embedding the AES and Dolby E digital audio streams in primary video coax transmission, significant weight and space saving can be achieved. This, of course, requires embedders/de-embedders and specialist audio monitoring equipment, but there is an overall net saving.

In this new intense digital media world we also have another computer phrase to deal with – multimedia. Audio recording is no longer a standalone end product, and most studios need to accommodate the demands of synchronisation to video, streaming techniques and bandwidth considerations for presenting media to disc with a limited amount of data space or bandwidth for online presentation. Liaison with game programmers, web designers and disc authors is as common today as it is with managers and talent. Again, the byword is digital…

Almost without exception, new installations – from shopping malls to concert venues are being constructed with digital video capabilities as part of their infrastructure. These use 270Mb/s data transfer as a minimum and, as digital coax cables can be used to carry analogue video perfectly well, these are the norm for new builds.

Future proofing is always a high priority when considering new cable infrastructure, however. Regardless of the current system’s video capabilities, virtually all-new installations are being prepared for upgrade to 1080p (1080 line, progressive scan) 3G HDTV with cable and patching systems conforming to the SMPTE424M 3GHz specification.

For systems using these standards, it is vital to consider correct impedance matching – the best cable in the world will be ‘strangled’ if poorly designed connectors are used at the ends. To reduce reflection and phase error in HD-SDI and HD-SDI 3G systems, cables and connectors must be manufactured to much tighter mechanical tolerances and are of a more complex design than for analogue, and this results in higher cost. Unfortunately, the vital and significant technological and manufacturing advances, in such basic items as cables, connectors and patch panels are overshadowed by the stunning capabilities of current digital processors, control interfaces and FX equipment, and are of little or no interest to most users. It is important to realise that although hidden from view after installation, the interconnection infrastructure plays just an important role as the sexy software and funky faders. In fact without it, you would be left with a pile of very expensive, but totally inert doorstops and paperweights.

For deployable and installation cables, fibre is now more common, particularly for runs of more than 50m. There are triax/fibre interface boxes available from a number of manufacturers, such as the Lemo Meerkat. Fibre connectors are offered by almost all of the main manufacturers, including Lemo, Canare, Fischer, Neutrik, Fibreco and Stratos.

It is important to note here that what is being recorded and what is being transmitted often differs. It makes sense to record in as high a quality as possible then transmit in whatever bandwidth is available.

Linked in

As with analogue cables, digital interconnection brings its own considerations and measurement parameters. Instead of such terms as capacitive and inductive reactance, we now have to learn about eye patterns and propagation delay in order to ensure correct connection. There are many different cables available for digital and analogue audio and video covering installation applications and temporary use but – happily – it is relatively easy to choose the correct type of cable without having to resort to reading the specifications in detail.

Propagation delay is pretty self-explanatory: the time taken for a signal to pass through an interconnect. Skew is a specific problem that arises with Cat5 when sending RGB video signals – as each pair has a different twist, the length of the four pairs is different and presents timing differences (delays) at the far end. Until recently, this had to be addressed with hardware at the receiving station but low-skew Cat5 cables are now becoming available.

An eye pattern is the oscilloscope display used when evaluating the overall noise, distortion and jitter (timing) performance of a digital interconnect. These key characteristics can be determined by assessing eye’s height, width and degree of over/undershoot.

In terms of camera cable, Furukawa, Belden, Draka and other manufactures offer fibre hybrids conforming to the SMPTE311 specification to carry power and data (bidirectional) through copper cores alongside video over single-mode fibres. Considerations of flexibility versus ruggedness and fire safety mean that there are always compromises to be made; so three distinct design groups are the norm. Flexible PVC jacket constructions are preferred for studio and handheld camera use, less flexible but more rugged and water resistant PUR jackets are suitable for harsh OB and live point-to-point applications. Low fire hazard designs – variously named LFH, NH, FRNC, LSOH – conform to building safety requirements where cable is permanently installed. (The actual regulations and acceptable jacket material compounds are complex and sometimes confusing although there is actually more myth than truth surrounding these. There is an excellent technical article entitled ‘No smoke without Fire’ that can be found on the Canford Audio web site.)

When selecting and fabricating digital cables, the first key considerations are to identify the correct cable for the lengths of run involved and ensure correct termination impedance.

For both temporary and permanent installations, correct cable type for the length of cable run is important because there is little indication that a digital signal is reaching the limit of the decoders before it disappears completely. The difference in cable length between that producing a trivial error rate and one producing an unacceptable mess, can be as little as 15m in several hundred metres of total length. That your cable will not support the length of run is not something you wish to discover after it has been run into a cable duct.

Most pre-fabricated systems are rigged with cabling laid out on the workshop floor where it is exposed to possible damage by being trodden on. Although poor practice, little damage is caused to solid insulator cables but even the slightest crushing does untold, and unseen, damage to gas-filled dielectric digital cables. The decoders and error correcting circuitry built into equipment is designed to account for source signal errors but are increasingly being called upon to deal with system infrastructure signal degradation, caused by poorly selected components and ‘casual’ installation quality, leaving little capacity for real errors when they occur.

In temporary set-ups for live sound and broadcast, cables should be flexible and rugged, and in-line connectors should be used to guard against prevailing weather conditions. Placing stage boxes in suitable places can help prevent damage, and there are now field repair kits for fibre cable – this has not always been the case and has been a definite disadvantage when compared with copper cable.

Among the most frequently occurring problems and mistakes when selecting and installing digital cable is the minimum bend radius. Attempts to turn cable more tightly than is recommended will invariably result in crushed dielectrics (the insulation between the conductors). Both coaxial and twisted pair cables will suffer from impedance dips and reflections as a result.

Tie-wrapping coax cables can also cause damage to dielectric in a cable. Too tight and the cable is crushed, changing the impedance and causing reflections. If over tightened cable ties are placed at regular intervals it is possible to set up a resonance or ‘ringing’ within a cable, seriously affecting the bit error rate. As it is sometimes difficult to change old habits and ensure irregular spacing of loosely tightened traditional cable ties, several alternative tie types are becoming common. The simple Velcro type is effective but sometimes difficult to fit. With a more traditional fitting method however the Millepede ‘Mille-tie’ cannot be over-tightened as the patented combination of stretchiness and the ‘slip back’ latching system will always achieve the perfect tension.

Although not developed for audio or video applications, it is impossible to contemplate a large-scale installation without encountering Cat5/Cat6. Used to build data network infrastructures, these are categories of cable (hence the term ‘cat’) offering alternative performance characteristics.

Cat5 is an unshielded twisted pair with 100 ohms impedance and electrical characteristics supporting transmission at frequencies up to 100MHz. It is intended for use with 10Base-T, 100Base-T4, 100Base-T2, and 100Base-TX Ethernet. Cat5e – enhanced Cat5 – is similar to Cat5 but offers improved specifications for NEXT (Near End Cross Talk), PSELFEXT (Power Sum Equal Level Far End Cross Talk) and attenuation. This may be used for 10Base-T, 100Base-T4, 100Base-T2, 100BaseTX and 1000Base-T Ethernet. Cat6 more than doubles the Cat5 bandwidth to 250MHz, and is backward compatible with lower category grades as it supports the same Ethernet standards as Cat5e. Cat7 is a proposed standard to support transmission at frequencies up to 600MHz over 100 ohms twisted pair.

Baluns (transformers) are used at each end of a Cat5 run to send analogue/digital audio and video, comms and remote control over structured wiring common in buildings – the same that is commonly pre-installed in buildings to support a computer LAN (local area network). A network supporting 1000Base-T can accommodate data traffic speeds of up to 1,000Mb/s (1Tb/s). There is also now a multitude of solutions on the market that can be used over Cat5e and by broadcasters and A/V professionals to ease projects for live events and permanent installations. The cable may also be a deployable type such as Canford Audio’s Cat5E-F design that permits the cable to be constantly rigged and de-rigged on location as necessary. With this in mind, it can be also be used by broadcasters and A/V contractors for various alternative uses – either as a core part of the activity or as an aid to the main objective by using either active or passive baluns at each end of the cable run.

Likely applications for these networks are broad, and include multichannel audio distribution, analogue audio and video distribution, HDMI connections, VGA and DVI distribution, CCTV, remote control and KVM extenders.

It must be noted, though, that under certain conditions performance over Cat5e may not be quite as expected. Possible limiting factors include keeping A/V signals on different cable to computer LAN Ethernet traffic, and areas of excessive RF or sources of high electro-magnetic interference.

The early days of digital audio’s introduction centred on questions of quality and comparisons with established analogue systems’ performance and operation. The broad acceptance and wide implementation of digital systems in applications as widely spread as broadcast, audio-for-video postproduction, shopping malls and the internet have shifted our attention to the practical matters of specification, installation, operation and maintenance, however. It’s not the prettiest aspect of digital audio or video, but correct cabling and interconnection is every bit as important as any other.