in November 2006
New Challenges In The Wireless Age
By R. David Read
The RFI 'gremlin' has crept into AV's backyard. = velocity (speed of light in meters per second)
This wireless mic "user" illustrates use of the various forms and types of wireless mics.
Back in the dim, dark age (about 25 years ago), audio, video, telecommunications, control circuits and computer science were distinct and rigidly segregated subjects, all operating on their distinct copper networks. In those days, radio frequency matters were of scant concern to the majority of practitioners in the AV world. In today’s busy, some would say more complicated, world, these previously unrelated disciplines have, by virtue of rampant convergence, become commonplace concerns for AV system designers and integrators.
Beginning with the relatively innocuous occasional interference between competing wireless microphone frequencies and sometimes baffling examples of radio frequency interference (RFI) from broadcast signals creeping into audio systems, the recent proliferation of wireless devices and networks is necessitating that AV/IT integrators become increasingly more knowledgeable about the nature of RF propagation principles.
Broadcast engineers have long held a healthy respect for the harmful interrelations between RF and AV devices, and have proceeded accordingly. AV/IT practitioners, on the other hand, historically have been somewhat blasé when it comes to matters of RFI—until, like the camel’s nose under the tent flap, the gremlin crept into their own backyards.
In the eyes of an audio or video installer, a piece of wire represents a connecting medium between two points in a system. These two points may be confined to a matter of a few inches, or even less, as represented by a trace on a printed circuit board. Or, on the other hand, they may be several hundred feet long and represent a connection between two quite dissimilar components having considerable variances in impedance and/or level considerations. However, in the eyes of Mother Nature, that same AV connection may well appear to be an antenna, i.e., a device designed for the reception of RF signals.
In all cases, the definition as to intent is a function of wavelength. Take, for example, a six-inch length of wire connecting an audio preamplifier to an audio equalizer. For the audio practitioner, this is merely a connection to transmit an audio signal from one device to another. However, if improperly terminated, Mother Nature will gleefully recognize that six-inch conductor as a perfect device for receiving and for subsequent rectification of an RF signal having a wavelength of 450MHz (a popular two-way mobile radio frequency for public-service agencies).
The wavelength of any signal can be calculated using the formula (see Table 1):
f (frequency in Hertz)
Table 1 - Radio Frequency versus Wavelength
With reference to Table 1 [above], and before some readers are compelled to jump up and wave their hands in frantic disagreement, the chart refers to RF frequencies. Those erudite souls well versed in loudspeaker design or audio equalizer ISO center frequencies may, understandably, cite wavelengths relative to loudspeaker horn dimensions and the like. In those cases, reference is being made to audio frequencies (AF) in an acoustical environment, in which case we have modified the speed (velocity) from the speed of light to the much slower speed of acoustical energy under varying conditions of temperature and humidity in various transmission mediums. This only goes to prove that audio calculations are several degrees of magnitude more complex than other aspects of physics.
For AF signals, the formula for wavelength becomes:
= wavelength in feet or meters
c = the velocity of sound in feet or meters per second (dependent on the medium)
f = frequency in Hertz
To offset the detrimental encroachment of atmospheric induced noise and manmade generation of radio frequency, audio line transmission engineers recognized from the earliest construction of telephone circuits that transposition (i.e., twisted pairs) of conductors in any given circuit would reduce induced noise significantly. Also, the notion of employing “balanced” circuits, whereby each conductor was maintained at a similar value relative to ground (earth), would further reduce noise. Not too surprisingly, insofar as much of what we know about audio and video was derived from the telephone industry, lessons learned by pioneers in telephony were carried over into AV practices, with twisted pairs and balanced circuits being two of the fundamental tenets for successful, low-noise AV transmission-line designs.
Balanced vs. Unbalanced Circuits
In earlier times, invariably, transmission circuits were balanced using transformers. This was occasioned, in part, by the need to couple signals to high-impedance vacuum-tube grid circuits.
As low-impedance transistor devices became prevalent, it was found that the use of heavy and costly transformers could be dispensed with, in favor of electronically balanced input stages. And, as integrated circuits replaced discrete transistors, the use of transformer-balanced inputs faded to near the point of oblivion.
Despite the obvious advantages of less weight, bulk and cost of electronic balancing circuits, the advantage of isolation and predictable balancing offered by transformers was forfeited. Under these conditions, the value of impedance matching gave way to input bridging principles. It also became popular on the part of some design engineers and manufacturers of primarily low-cost consumer goods to adopt unbalanced input circuits wherein the signal was induced using a single conductor referenced to ground.
For visual reference, consider the reduced size (not to mention performance) of, say, an unbalanced RCA input connector in contrast to a balanced XLR connector. The elimination of transformers undoubtedly saved the manufacturing community untold sums of money; however, it placed the onus of guarding against, and correction of, RFI interference onto the shoulders of the installation and maintenance field technicians.
This illustrates how IM products are a function of non-linear circuits.
Simplified block diagram of a wireless microphone system.
The principles of proper grounding procedures have generated any number of books, white papers, technical-journal and trade-magazine articles, and manufacturers’ technical topic notes, and oftentimes is the subject for lengthy discussions in any forum consisting of more than two AV technicians. My mentor in the audio business many years ago pounded into my head that an understanding of proper grounding was absolutely essential to the understanding of communication installations. His adamant insistence that the fundamentals of grounding were axiomatic rings as true today as they did 30-plus years ago. And, proper grounding is not limited to audio circuits. The same principles apply to video and all other communication systems.
Not only does “bad” grounding create hum, buzz and other detrimental manifestations within a given system by its own right, but it gives rise, if not a downright open invitation, to the susceptibility of RFI in the system. In the role of a technical service manager, I have had my share of experience in assisting others in the resolution of RFI problems. In most instances, these problems could be attributed to inattention on the part of the installer to establishing and maintaining an adequate ground plane.
From the National Electrical Code (NEC) viewpoint, grounding is an essential element in an electrical system and is primarily a safety issue. The NEC is absolutely correct and at no time should a safety ground be ignored or dismantled. On the other hand, if not properly implemented, electronic AV circuits and electrical power distribution system grounds do not always like to coexist.
Generally speaking, the noise induced on electrical distribution neutrals tends to degrade AV system performance. This has given rise to sometimes elaborate “technical ground” schemes that isolate AV systems except at final connection to the building electrical service entry. Properly designed and implemented, such technical ground schemes do minimize system noise and isolate at least one of the elements in AV systems from the introduction of detrimental RFI.
Frequency band allocation for wireless microphone operation (USA).
Unfortunately, far too many equipment manufacturers have succumbed to using “shortcuts” when it comes to implementing ground planes within their products. This practice has been dubbed “the pin 1 problem,” inasmuch as it relates to situations wherein the ground connection at an input is not solidly and directly tied to chassis ground at the input point. It gets its name from the fact that pin 1 of an XLR connector is, by definition and by AES standards, the ground-carrying conductor connection. Again, not only does this “shortcut” create intra-system noise, but gives RF just one more path in which RFI can be induced.
A visual inspection of a suspect piece of equipment almost always will reveal when a pin 1 problem is embodied in the equipment design. Generally, plastic input connectors on the equipment input panel are a dead giveaway that a pin 1 problem is lurking and ready to create havoc.
For more information about the definition of pin 1 problems, refer to AES Standard #48, at www.aes.org/publica tions/standards (free download for AES members; others $25).
Although proper grounding may not solve all noise and RFI problems, the absence of proper grounding is a sure-fire way to open the door to these scurrilous gremlins.
Several excellent publications that discuss the issue of grounding and shielding explore this subject to a much greater degree than we can cover here, including:
In addition, a visit to the website of Audio Systems Group (www.audio systemsgroup.com/publish.htm) will access several related white papers authored by industry expert Jim Brown under the AES Papers subhead.
||• Power and Grounding Sensitive Electronic Equipment, IEEE, New York NY, 1992.
• Morrison, Ralph. Grounding and Shielding Techniques in Instrumentation (3rd Edition), John Wiley & Sons. New York NY, 1986.
• Morrison, Ralph and Lewis Warren H., Grounding and Shielding in Facilities, John Wiley & Sons, New York NY, 1990.
Frequency band allocations for UHF wireless microphone operation (worldwide).
Wireless Mic Operation
In all probability, one of the first RF devices that will be encountered by an AV technician is the now seemingly ubiquitous wireless microphone. They come in all sizes, shapes and operating frequencies; however, all are comprised essentially of two elements: a microphone element connected to an RF transmitter and an RF receiver with a suitable output for connection to an audio system (i.e., mixer, etc.). And, their primary purpose is to replace some 25 feet of microphone cable.
Once confined for use by singular soloists in live-performance venues, they were then adopted by the TV industry to cut down on stage clutter (no messy cables to clutter up the visuals). Now, like clothes hangers in a dark closet that breed unseen, wireless microphones seem to be everywhere. Accounts abound of situations in theatrical performances, churches and even schools where upwards of 20 wireless microphones are used on a routine basis.
Pat Brown of Syn-Aud-Con tells us that, in an impromptu survey of attendees at a training conference, he asked, “How many channels of wireless have you ever used?” The record, thus far, goes to an unnamed party who reported using 80 channels. Wow! Except for some extraordinary circumstances, one has to question the sanity of those who succumb to the “pop-culture” that demands multiple wireless microphones where the need clearly doesn’t require such proliferation. Not only is it expensive and an ungodly nightmare to operate, but such systems can lay themselves wide open to any number of RFI problems.
Not only are wireless microphones susceptible to external interference from broadcast and other RF sources, they inherently have the capability of generating their own inter-system interferences. Without going into a lengthy discourse here, suffice it to say that inter-modulation distortion is a real-world problem when multiple transmitters, and receivers, are asked to co-exist in a limited geographical ocation.
As mentioned, wireless microphones are often the AV designer or technician’s first, sometimes uncomfortable, sometimes disastrous, introduction to the RF world. Fortunately, there is a lot more information available today than there was when “radio microphones” first began creeping into the AV designer’s tool kit. It would be highly recommended that, before a system designer considers using multiple wireless microphone systems, whatever the application, he study publications such as the one that Shure Inc. makes available at www.shure.com/wireless, or Lectrosonics’ equally excellent white paper available at www.lectro sonics. com/wg/wg2000.htm.
In addition, most reputable wireless microphone manufacturers will provide frequency allocation services for specific applications and fixed-location geographical areas. The introduction of “self-tuning” systems that will seek out vacant channels, and the gradual shift to recently introduced spread spectrum modulation-based systems, also are alleviating some of the problems attendant to the use of wireless microphones.
RF Pollution Proliferation
In addition to some of the already identified sources of RFI, the adoption of wireless devices seems unending. Computers and the increasing use of WiFi networks, the seemingly everywhere imaginable observance of cell-phone users and internal self-clocking DSP devices, are all joining the ranks of previously recognized generators of RF “hash.” Traditionally, such recognized RF generators have included stage lighting systems and electrical motor controls. Nowadays, horror stories abound about AV conference system “crashes” attributable to introducing Blackberry and certain types of cellular communication devices into an otherwise pristine environment.
Hence, it behooves today’s designer, installer or operator, increasingly being confronted by the prospect of RFI disruptions from, until recently, unimaginable sources, to be aware of how these “alien” RF products can impact their AV systems.
Consequently, the AV practitioner must be all the more aware and much more knowledgeable about RF principles than ever before. Knowledge is power and the more one knows and understands about RF propagation, the better equipped he will be to identify and track down the gremlins of RFI before the camel protrudes its nose further into the AV tent.
Contributing Editor R. David Read has worked in both the RF and AF aspects of AV.