Digital wireless microphone systems have become an invaluable resource in field audio recording. The times continue to change around how these systems work best. Today we will be covering more “tech talk” of the digital wireless world. We hope it gives you some clarity around the work that you may need to be done in that area.
Digital Wireless Systems
Portable wireless microphone systems have been available for many years now. By ‘portable’ I mean mics with small battery powered receivers that are designed to be either plugged directly into a camera or into a portable mixer or recorder for location audio capture. A big problem in recent years is the ever-increasing crowding of the RF spectrum. This makes it harder and harder to find open frequencies to use.
The FCC leaves certain RF frequency bands open to ‘unlicensed’ users of wireless microphones- that means most of us that aren’t paying fees to the FCC for particular frequencies in specific markets. These frequency bands are kind of a ‘wild west’ situation because so many wireless devices are also using these frequencies in addition to microphones. To make matters worse, most ‘portable’ type receivers are much more prone to interference than their stationary counterparts, since they are usually non-diversity (single antenna) designs and have less robust electronics due to the low power battery requirements.
I’m with the band.
One of the most popular frequency bands for wireless microphone use due to its excellent propagation characteristics was the 700 Mhz band. However, the FCC made this entire band illegal for unlicensed microphone use in the U.S. in 2010. I won’t go into the details here, but suffice it to say that Digital Television Broadcasting (DTV) and $19 billion going to the FCC were involved. This made many wireless systems instantaneously obsolete.
In 2016, the FCC will be auctioning off the next most popular band for wireless microphones. This includes the 600 MHz band and may extend down into the middle of the 500 MHz band. The FCC is promising a 3 year transition period before they decide if this band will also become illegal. That’s when we get to throw away thousands of dollars of gear once more. Even if it is not made entirely illegal, chances are this band will become practically unusable due to interference on almost every channel in major urban areas.
I won’t go into details on this or what bands are likely to remain usable for a few more years as that is beyond the scope of this article. I will give some links at the end for more information.
Get the gig(ahertz)
In the last few years, a number of manufacturers have been coming out with portable digital wireless microphone systems. Generally, these operate in the 2.4 GHz spectrum (yes, the same as wi-fi) or the 1.9 DECT band, which is used for cordless telephones and other devices. 2.4 GHz is obviously a very crowded band already. However, most of these devices use some kind of smart technology to sense what channels are occupied and automatically jump around as needed to prevent interference. 2.4 GHz is specified for unlicensed use, and since most wi-fi is operating in this band, the FCC is not likely to change this anytime soon.
There are great advantages to using digital wireless transmission technology, primarily improved audio quality. All analogue wireless systems have to use ‘companding’ to achieve decent dynamic range and lower the noise floor that is inherent to analogue transmission. This causes a number of side effects and definitely changes the quality of the audio. Well-designed digital systems, on the other hand, have an inherently large dynamic range and extremely low noise floor. The audio quality will be much closer to that of a wired microphone.
These units tend to be more expensive than their analogue counterparts, due to the A/D and D/A converters and more complex electronics. However, recently Rode has introduced the ‘Filmmaker’ line of microphones that are relatively inexpensive, even below the cost of many of the better portable analogue systems available.
In the field
After seeing many rave reviews, I recently bought 2 of these systems to test out. I wanted to see if the systems werea viable option for replacing my Sennheiser G2 and G3 systems. The results in actual field use were disappointing. When I tested them at home, they sounded and worked very well. I did get some low-level digital interference (about -50 dB) on one unit. However, switching channels seemed to solve the problem. I then tried to use them for a live streaming broadcast in an office building with many wi-fi transmitters in operation. At this point, one unit exhibited extreme dropouts. Nothing I did seemed to help the situation. I tried changing channels several times, as well as moving the receiver to within 10’ of the transmitter!
I have also had a very disappointing experience with Rode tech support so far. When I first contacted them, they thought I might have a defective unit. I was told a new one would be shipped out immediately with a return kit for me to send mine back. I contacted them again several weeks later and got an apology and a promise it would be sent right out. When I finally did receive a package from them, it only had the microphone capsule. It had no transmitter or receiver. I have been attempting to have further contact with them for over 3 months now with no response whatsoever – not a confidence-enhancing situation!
Sennheiser has their own portable digital wireless systems out as well, the “AVX” series. My guess is that these are much better designed and more robust in their frequency selection capabilities. They also cost more than double what the Rode systems go for. How they would fare in an extreme wi-fi saturation situation is still a big question. Since I have not had my hands on one for testing yet, I must leave this question open-ended. Please comment if you have any experience with these systems, especially in challenging environments.
Ready for prime time?
So, are digital wireless mics for field recording ready for prime-time? Looks like the jury is still out!
Tips on analogue frequencies that will probably remain usable:
Sennheiser on FCC changes:
Shure blog about FCC policies:
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