Now that the Audio Engineering Society has adopted the "pin 2 is hot" standard, the question of what to do with pin 1 is being addressed. A recommended practices document is being created covering interconnection of professional audio equipment. How and where to connect pin 1 is too complex to be issued as a standard; thus only a recommended practice is being developed. The recommended practices may affect manufacturers who choose to follow them.

Many shield-wiring practices exist in the audio industry today. The majority of available literature on the subject prescribes clear solutions to any wiring problem, yet problems are rampant due to inconsistent variations on the well-documented ideal. Two clear groups have developed on either side of a hard-to-straddle fence -- the balanced world and the unbalanced world.

Over the years, the declining cost of professional audio equipment has facilitated its use in more and more home studio environments. As home studios incorporate professional, balanced equipment into their systems, the unbalanced and balanced worlds collide. Home studios adding balanced equipment to their traditionally unbalanced gear also add connectivity problems. Professional users never consider unbalanced gear, yet still have connectivity problems.

The performance of any interconnection system is dependent on input/output (I/O) circuit topologies (specific balanced or unbalanced schemes), printed circuit board layout, cables and connector-wiring practices. Only wiring practices, both in the cable and in the box, are covered here. The I/O circuit topologies are assumed ideal for this discussion to focus on other interconnection issues.

The Audio Engineering Society recommendation will address a simple issue, the absurdity that one can not buy several pieces of pro audio equipment from different manufacturers, buy off-the-shelf cables, hook it all up, and have it work hum and buzz-free. Almost never is this the case. Transformer isolation and other interface solutions are the best solutions for balanced/unbalanced interconnections, though they are too costly for many systems. Even fully balanced systems can require isolation transformers to achieve acceptable performance. Some consider isolation transformers the only solution. These superior solutions are not covered here.

Another common solution to hum and buzz problems involves disconnecting one end of the shield, even though one can not buy off-the-shelf cables with the shield disconnected at one end. The best end to disconnect is unimportant in this discussion. A one-end-only shield connection increases the possibility of radio frequency (RF) interference since the shield may act as an antenna. The fact that many modern day installers still follow the one-end-only rule with consistent success indicates that acceptable solutions to RF issues exist, though the increasing use of digital technology increases the possibility of future RF problems. Many successfully and consistently reduce RF interference by providing an RF path through a small capacitor connected from the lifted end of the shield to the chassis.

The details of noise-free interconnections and proper grounding and shielding are well covered in other literature. They are not revisited here. Readers are encouraged to review the References listed for further information. Most of these materials have been applicable in the audio industry for well over 60 years, though they have not been implemented or embraced by many.

Balanced vs. Unbalanced Shields

For the ensuing discussion, the term shield is qualified with the description balanced or unbalanced. An unbalanced return conductor physically resembles a shield and provides shielding for electric fields, but magnetic fields are not shielded. Though this is also true for balanced shields, the twisted-pair construction of balanced cables provides much greater immunity to magnetic field interference. Unbalanced cable shields also carry signal in the form of return current, further alienating unbalanced shields from "true" shields. Shield is defined by Ott [1] as "... a metallic partition placed between two regions of space. It is used to control the propagation of electric and magnetic fields from one place to another." Balanced interconnection provides the superior interface of the two.

The "Pin 1" Problem

Many audio manufacturers, consciously or unconsciously, connect balanced shields to audio signal ground; pin 1 for 3-pin (XLR-type) connectors, the sleeve on 1/4" (6.35mm) jacks. Any currents induced into the shield modulate the ground where the shield is terminated. This also modulates the signal referenced to that ground. Normally great pains are taken by circuit designers to ensure "clean and quiet" audio signal grounds. It is surprising that the practice of draining noisy shield currents to audio signal ground is so widespread. Amazingly enough, acceptable performance in some systems is achievable, further providing confidence for the manufacturer to continue this improper practice -- unfortunately for the unwitting user. The hum and buzz problems inherent in balanced systems with signal-grounded shields has given balanced equipment a bad reputation. This has created great confusion and apprehension among users, system designers as well as equipment designers.

Similar to the "pin 2 is hot" issue, manufacturers have created the need for users to solve this design inconsistency. Until manufacturers provide a proper form of interconnection uniformity, users will have to continue their struggle for hum-free systems, incorporating previously unthinkable practices.

The Absolute, Best Right Way to Do It

Clearly, the available literature prescribes balanced interconnection as the absolute best way to interconnect audio equipment. The use of entirely balanced interconnection with both ends of the shield connected to chassis ground at the point of entry provides the best available performance.

The reasons for this are clear and have been well-documented for over 60 years. Using this scheme, with high-quality I/O stages, guarantees hum-free results. This scheme differs from current practices in that most manufacturers connect balanced shields to signal ground, and most users alter their system wiring so only one end of the shield is connected. Due to these varied manufacturer and user design structures, an all-encompassing recommendation with proper coverage of both balanced and unbalanced interconnection is essential.

Conceptually, it is easiest to think of shields as an extension of the interconnected units' boxes (see Figure 1). Usually, metallic boxes are used to surround audio electronics. This metal "shell" functions as a shield, keeping electromagnetic fields in and out of the enclosure. For safety reasons, the enclosures in professional installations are required by law to connect to the system's earth ground (which in many systems is not the planet Earth -- an airplane is a good example).

A Speculative Evolution of Balanced and Unbalanced Systems

One may ask, if the balanced solution is best, why isn't all equipment designed this way? Well, reality takes hold; unbalanced happens.

Back in the early days of telephone and AC power distribution a specific class of engineers evolved. They learned that telephone and AC power lines, due to their inherently long runs, must be balanced to achieve acceptable performance. (To this day, many telephone systems are still balanced and unshielded.) In the 1950s, hi-fi engineers developed systems that did not necessitate long runs, and used unbalanced interconnection. The less expensive nature of unbalanced interconnection also contributed to its use in hi-fi. These two classes of engineers evolved with different mind-sets, one exclusively balanced, the other exclusively unbalanced. The differing design experience of these engineers helped form the familiar balanced and unbalanced audio worlds of today.

Now add spice to the pot with the continued price decrease and praise devoted to balanced, "professional" audio interconnections with the desire for better audio performance at home, and one sees the current trend of merging balanced and unbalanced systems arise. Home studio owners, previously on the unbalanced side of the fence, dream to jump but unfortunately straddle the fence, getting snagged on the fence's ground barbs when connecting their new balanced equipment (Figure 2).

Figure 2: Home studio owner trying to jump the balanced-unbalanced fence.

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