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WHY RF COORDINATION IS INVALUABLE; A REAL-WORLD EXAMPLE

 

Professional audio engineers have many tools available to aid in operating their wireless equipment, including hardware and software packages.† Some are very expensive; some are free of charge; and some are strictly dedicated to specific brands of wireless gear.† All require technical skill and deep understanding of wireless equipment behavior for maximum benefit.† This becomes more evident as a user increases the number of wireless systems operating simultaneously at a given location.

 

Software-only solutions are convenient and can yield reliable results, but often they are hampered by their use of incorrect or incomplete data.† They require accurate information about the RF environment at the userís location to generate useful results.† Most often, the user defines their location by ZIP code or geographic coordinates and relevant data is acquired through the Internet.† This information is usually based on data published by the FCC for the United States as a publicly accessible database.† Unfortunately, that source of information is often incorrect, incomplete, and/or inadequately utilized for our purpose.


The example here is a musical concert event at the Georgia Dome in downtown Atlanta, Georgia, in early 2012.† The event required the use of 84 channels of wireless audio.† Figure 1 shows a graph of the wireless spectrum to be used by wireless UHF audio equipment, which is shared with broadcast television.† Frequency is plotted along the horizontal axis.† Vertical violet and turquoise bars indicate television channels listed in the FCC database as licensed broadcast television transmitters operating within a 40 mile radius around the venue.† The software assumes that black bars are channels that are free of licensed television signals and available for professional audio equipment.

 

Fig. 1

 

This makes the assignment of frequencies for our audio equipment appear to be simple, and the software should be able to easily generate a clean channel plan by using only frequencies within the black bars.† Unfortunately that would be disastrous, because the information is incomplete and thus incorrect.


What?† This is one of the things that we all pay the FCC for, right?† The science of radio frequency propagation is complex and relies on myriad variables to make predictions.† Geographic topography and atmospheric conditions make each location at each moment in time unique.† License-based predictions cannot anticipate nearby transient transmissions from unlicensed wireless equipment, which generate interference that is never part of the FCC data.† The most reliable way to deal with this is to perform on-site radio frequency spectrum analysis in a timely manner.† This uses precision instruments to collect sets of data that accurately represent the RF environment.

 

Figure 2 shows the previous graph overlaid with RF spectrum analysis data collected at the Georgia Dome on gig day.† Amplitude is plotted vertically.† You can see a correlation between the violet bars and peaks in the yellow spectrum trace, but there are also many peaks covering black bars.† Most of these represent RF signals that are absent from our FCC data and will cause audible interference with any audio equipment attempting to operate on those channels.*

 

Fig. 2

 

This new spectrum analysis information reveals that far less of the wireless spectrum will be usable by our equipment than the FCC data implies.


Figure 3 shows that we have set a threshold, represented by a horizontal red line.† Every place where the yellow trace rises above the line indicates a signal which would cause interference with equipment tuned to that frequency.† White vertical bars are areas of the spectrum that are truly open for reliable operation of our gear.† You can see that the white spaces available to pro audio users in Atlanta are severely limited in comparison to the assumptions made by most wireless coordination software.

 

Fig. 3

 

CleanWirelessAudio.com can handle all of this for you!† This example shows only a small portion of the technical challenges that face wireless equipment users.† Our expertise covers these details and many more.† Please call or email us to add our skills to your production team.

 

* Experienced RF Techs might assume that those peaks spanning across channels 23 and 24, channels 36 and 37, and part of channel 38, are artifacts created by our measurement hardware (known as images) because they do not conform to FCC rules for TV channel allocation, yet exhibit typical DTV signatures. They could easily be dismissed as intermodulation products generated in the front end of our spectrum analyzer. However, tuning our wireless microphone and IEM receivers to those frequencies revealed significant interference and verified that those off-band signals are real enough to be treated as such for our purposes.

 

 

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