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AES Section Meeting Reports

Los Angeles - May 27, 2014

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Summary

In May, the LA chapter of the AES hosted a discussion on "Getting the Most from Wireless Microphone Systems" presented by Scott Woolley, Regional Sales Manager for Lectrosonics. The presentation began with an overview of Lectrosonics, where it was highlighted that over 85% of the television and feature film RF microphone systems are made by the company and that their production is located entirely in the United States. These RF systems have automatic frequency selection and use a hybrid analog/digital design with predictive coding to achieve low latency transmission (< 3 ms) with a wide bandwidth (32 Hz - 20 kHz). Lesser known uses of their systems include DSP units that provide automatic digital mixing and routing in courtrooms and distance learning applications. A new device that raised interest at the session was a 4 channel wireless snake with 20 Hz - 20 kHz bandwidth using all digital transmission in the 900 MHz band and achieving up to 1 mile line-of-sight transmission under optimal conditions.

Scott then went on to give an overview of the state of RF microphones in the United States. In 2010 the FCC banned the sale of wireless microphones in the 700 MHz band (698 - 806 MHz). This was a blow many of the manufacturers selling products in this frequency range, and required many audio engineers to purchase new systems. Recently, the FCC opened up discussion around eliminating even more spectrum from use for wireless microphones — possibly leading to everything above 600 MHz being banned. While the FCC has no definite plans around this yet, Scott suggested that those purchasing new systems consider devices operating outside of this range.

The rest of the session was an overview of the common challenges of RF microphone setup.  Scott pointed out that the preamp gain for the microphone lives at the transmitter and improper settings will lead to poor system SNR. The antenna placement and type also play a large role. It is important to place antennas above people to avoid RF absorption issues and to place multiple antennas approximately a wavelength apart. The most common type of antenna is the 1/4 wavelength design which provides a toroidal polar response, but directional configurations like the log-periodic dipole array can provide 6 dB of gain when directed at the transmitter. Scott suggested that planning is a key factor in getting the most out of a wireless microphone system.  He recommended making a table of each of the microphones, their application (drums, vocals, etc.), and their corresponding frequencies and measuring the RF spectrogram in advance. The timeframe in which the spectrogram is taken is important (don't expect measurements near a church on a Tuesday to be the same as on a Sunday), as is averaging over a long period of time or showing waterfall plots, since transmission from competing systems can be intermittent. If you don't have this luxury, a microphone system with auto scanning and high transmit power is important.  For example, a news crew can easily encounter as many as 30 other crews at a site, all competing for spectrum with little time for system setup. Scott also outlined the difference between unlicensed and licensed uses of wireless microphones — broadcast being the only application with licensed spectrum usage. Applications such as churches and other venues operate in unlicensed mode and must not interfere with licensed usage. In each area there are two television bands allocated to wireless microphone use (these can be found through a web search by entering the zip code for a given region).  

The evening ended with a lively Q & A session. First, it was asked what recommendations Scott had for systems including wireless in-ear monitors along with wireless microphones.  In this scenario, Scott replied, it is important to make sure transmitter packs are as far apart as possible — on either side of the body, if possible. Then a discussion on antenna diversity and the technical implementations outlined that the common solution is to monitor the antenna signals and flip the phase before combining the two antennas if signal level falls below a certain threshold. The session ended with a discussion on what the FCC's plans are for the 600+ MHz band. Scott mentioned that in one proposal the FCC was considering using a portion of the auction proceeds to compensate users of wireless microphones that would be forced to replace equipment. However, auction participants shot this down, saying they would take the FCC to court if the proposal were enforced. The FCC is also considering leaving portions of the 600 MHz band open for wireless microphone use or leaving the spectrum open until the buyer makes use of the purchased spectrum. Scott's final recommendation was that those who are looking to purchase wireless microphone that will be in use for more than 5 years should purchase a system the operates below the 600 MHz band. 

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