I have described here the way this is done.
Cable types
Broadcast audio wiring has evolved from the telephone industry, where multiple circuits are required , and the best approach has been to use balanced pair circuits, often bundled into multicores. Balanced pairs are used because the signal is then independent of the grounds at each end, and are resistant to interference from other signal pairs ( crosstalk) and stray RF fields.
For normal, high level signals, an overall screened cable might be used as this gives a compact cable, and with care, an unscreened cable might even be used. The cable must however be of a paired type; unpaired cable, or using the pairs incorrectly will cause interference and crosstalk problems. Normal paired cable is suitable for analogue use.
Digital Audio cable
Digital signals such as RS422 and AES-3 are high-speed data signals, and require a cable suitable for 100 – 120 Ohm use, otherwise the signal will not pass down the cable properly, and will be prone to interference. Using the correct cable will also ensure that long cables will work OK. Why 100-120 Ohms? This comes from the physical properties of the cable, where each metre of cable will add series inductance, and capacitance between the pairs. The ratio of these is known as the Characteristic impedance , and with a proper cable is a constant. Physics says that a typical cable pair of usual thickness and core size happens to have this value. Physics also says that if the cable is not terminated with this impedance at both ends, or at any joins, the signal will be disrupted at each discontinuity causing a reflection.
From a practical standpoint, this means the correct cable should be used, and the source and destination should be properly terminated. The terminations are generally inside the equipment, so all is OK. However trying to send the same signal to two different places by driving two cables from the same source just will not work.
Also looping the cable between several destinations should not work, but in practice can be done if the impedance terminations are removed from the intermediate destinations , apart from the final destination. Even so, this practice is dubious, and should only be considered for short cables. Single Screened Coaxial cable is also used in Broadcast , and operates at normally 75 Ohm with a unbalanced signal core. This type of cable works really well on its own, but is physically large so is not normally used in multi-pairs.
Sometimes a signal is present on 75 Ohm coax, but you want to connect it to a multi-paired cable to a 110 Ohm destination. This should NEVER be done directly. It might seem to work with short cables, but will be prone to interference and will create interference and crosstalk.
It is just about possible to connect together if impedance matching and unbalance to balanced transformers are used, but the signal level will be low, so should only be considered on short cables.
A 75 Ohm source via a coaxial type cable can be directly connected to a single Balanced 110 ohm destination if care is taken. The signal core is connected to the +ve balanced pin. The screen is connected to the -ve balanced pin. A 240 Ohm resistor is connected between the pins: this resistor is across the internal 110 Ohm impedance, and gives the wanted 75 Ohm result.
Finally, the screen should also be connected to the chassis of the balanced input, but this will create a ground loop. Better is to connect the cable screen to the chassis with a capacitor : 0.1 uF ( 100nF ) is often used. The capacitor ensures that at data frequencies a good ground is present, but at audio frequencies the potential ground loop is not present.
Sometimes a signal is present from a 110 ohm balanced source, but you want to connect it to a 75R coaxial destination. Again this should never be done directly. Specially designed impedance matching transformers are available that make the conversion properly. These look like 110 Ohm balanced on one side and 75R unbalanced on the other side. As the signal level of AES-3 balanced is high, it should work with long cables as well. It is possible to make the conversion with a few resistors only; but I would only try this on short cables.
Analogue audio cable
Analogue audio circuits do not need to obey the impedance matching rules, as the maximum audio frequency is far below the point where impedance matching is important. For Broadcast, low impedance drive ( 50 ohm) and high impedance destination ( 10K ohm ) terminations work the best, and multiple destinations can be used with care.
Telephone circuits are a special case, as signals are required in both directions, so both source and destination terminations are then 600 ohms. Interestingly , the cable impedance was never , ever 600 ohms resistive; physics does not allow this with the dimensions of the cables used. The result for long cables is that high frequencies get attenuated more than low frequencies, but with a telephone bandwidth requirement of 3 KHz the cable length is not normally a problem.
For sensitive, low level signals, each pair will be individually screened for best performance, and for microphone circuits ‘ star quad’ cable is sometimes used. This cable has two pairs that are connected at each end as one pair. The twisting and overlapping of the two pairs has been shown to give a superior performance to using a single pair. The cable has great immunity to interference from Mobile telephones and studio lighting for example. The cable is totally unsuitable for Digital signals and should never be used with RS422 or AES-3 type signals.
Connector Types
Traditionally, the well-known 3 pin XLR has been used for balanced audio, with signals X = ground, L = +ve signal ( Line ) and R = -ve signal ( Return) . The connector is robust, but physically large. Traditionally the chassis female connector is used for inputs, and the chassis male connector for outputs. The same connector was adopted for Digital AES-3 circuits.
When many signals are required, the XLR becomes too cumbersome to use, although it is still used on Microphone circuits. Also used is the 5 pin XLR, which can be used for Stereo Analogue circuits.
Multi-way connectors are used , but the actual pins used have never been standardised. BBC used 38 way and 56 way EDACs for many years, and are still in use today. There are some standard pin allocations that are in use, but are not universal.
Military grade multiway connectors are also in use for OB trucks to stage boxes.
D type connectors of 9pin, 15pin, 25 pin and 37 pin are also in use. Usage of the pins has never been standardised , although using the first row ( pins 1,2,3,4 for a 9 way D ) for +ve signals, the last pin ( pin 5 for the 9 way D ) as ground and the second row ( pins 6,7,8,9 for a 9 way D ) for the -ve signals is quite common. There is a recent attempt by the AES to formalise a 25way D and a 50way D connector pin allocation, but it is too late for universal adoption.
The RJ45, with its four pair allocation is also in use.
Apparatus Room and Jackfield use
Again borrowed from the telephone industry, it is common in a big installation to bring all multi-way cables into an apparatus room, and terminate on connector blocks, often these days on IDC technology Krone blocks. Jumper cables link the multi-way cables together at these Krone blocks.
This idea allows the installation to be tested at the Krone blocks, and changes made at a later date when required.
Jackfields allow signals to be easily monitored (the original idea came from the telephone industry) and signal flows changed by ‘ overplugging’ circuits. The standard gauge PO jack is still in use (invented on the 1930’s?) together with a miniature version that became common in multitrack studios. Jackfields and jack plugs need to be kept spotlessly clean otherwise intermittent connections are made. ( As telephone circuits use 100 Volt ringing, this also served the purpose of cleaning the connections!) Broadcast engineers routinely twist the plug when inserting the plug to ensure good connections. Jackfields are just about suitable for digital audio AES-3, but there has been a clear trend towards gold plated pin-patch systems in modern installations. RJ45 patch bays are also used, borrowing from the computer industry.
Fibre and Coax
Most audio systems use balanced pair cables, because the cables can be taken via multiways and Krone blocks. Coax is not so common, although Audio for Video and the whole Consumer audio world is using unbalanced coax cables. Fibre and Coax is often used for Multichannel signals where the better bandwidth and achievable distances become important.
Multiplexers and Multichannel Signals
All cable types and signals described so far are still in use, and will remain so.
But there is a requirement to pass more than one signal down a particular cable, and these are generally known as ‘ multiplexers’ . Multiplexers are these days always digital. The most common multiplexed signal is the AES-3 circuit, as it passes two channels of linear PCM audio, and with encoding, up to eight channels. ADAT multiplexes 8 signals. There are current experiments with enhancing the AES-3 to pass 4 and 8 linear PCM signals, but these are not in use today. MADI has been around for a while, and can pass up to 64 channels of linear PCM at 48KHz sample rate down one Coax or Fibre. SDI video exploits the video blanking periods to squeeze in up to 16 audio linear PC M signals. ATM, Super MAC and other solutions also exist. One common aspect of all these technologies is that they are point-point systems. If both the sender and receiver use identical equipment it is likely to work. A big departure from this is Ethernet, where the hardware interface , taken from the Computer industry is very standard, but many different protocols are currently in use, so that one brand cannot communicate to another brand at all. There are attempts ( the AES X192 project for example ) to standardise this.
The old idea of fixed wiring is redundant with Ethernet solutions; the problem is now of standardisation, and software configuration. I will leave discussion of this to a different article!
About Mike Law:
Mike Law started out with an early passion for electronics and a summer job allowed him to work at UK mixer manufacturer Alice, where he learnt about broadcast audio, and 3 years at Birmingham University where he learnt about PA systems, lighting and putting on bands, culminating in an honours degree in Electronic Engineering.
Mike worked at Alice, designing audio mixing consoles until redundancy allowed him to set up his own UK company with the help of two Swiss recording studios, with the aim of producing a digitally controlled music console. The ideas behind that console is the basis of much digitally controlled analogue audio in use today.
His company, BCD Audio continues to this day, and Mike has become a specialist in designing digital and analogue audio systems, and has kept up with the times, and can now talk DSP and VHDL when he can find someone to converse with.
Mike is an associate of the IPS, a member of the AES, and is on the SC-02-02 committee, and is a member of the IET.
About BCD Audio:
Established 25 years ago to provide high quality audio systems for radio and television broadcasting, BCD Audio’s in-house skills were immediately sought after by prestige clients including the BBC and ITV companies. Today, BCD Audio’s designs are heard and seen all over the world, keeping broadcasters on air 24/7.
The product range has grown from pure analogue engineering to fully embrace today’s digital world, from standalone black box problem solvers, to dedicated rack mount units and full Eurocard infrastructure systems. BCD Audio can also deliver bespoke audio solutions to a client’s exacting requirements.
At the forefront of technological advances, BCD designers use the latest digital systems, often implemented with FPGAs or processors with network control.
BCD solutions now often involve video too, and the in house design skills include the processing of SD/HD video, with a bias towards audio embedding, de-embedding and DSP.
In-house manufacturing has also evolved; BCD have their own surface-mount assembly plant, and are capable of producing units in-house with fine-pitch components on multi-layer boards.
Recent projects include an automatic audio-triggered webcast system for the UK Parliament, which has evolved into a radio station webcast system, a 1U HD video capable audio mixer, and a fully digital audio monitor unit. BCD Audio rack- based systems have been specified in recent OB truck, radio station and TV studio presenter audio monitoring designs.
Mike Law concludes “We welcome enquiries from broadcast and installation companies, and if the solution is not already on the shelf, we will be happy to design and build it!”
Contacts for further information
Mike Law, CEO
BCD Audio (Acrone Ltd),
5 Bristol Way,
Stoke Gardens,
Slough, Berkshire,
SL1 3QE, United Kingdom
Telephone: +44 (0)1753 579524
Fax: +44 (0)1753 577981
email: sales@bcd-audio.com
web: www.bcd-audio.co.uk
Mel Noonan
Stylus Media Consultants
Broadcast Industry authoring & communication
office: +44 (0) 1525 756 097
email: stylusmediamel@gmail.com
300DPI CMYK photo attached: Mike Law of BCD Audio
Higher res photos by request to Stylus Media Consultants