Are you doing everything possible to maximize the performance of your wireless microphones, instrument and IEM systems? As the airwaves grow more crowded with activity from cellular signals, routers and terrestrial broadcasts (not to mention those pesky Martian game show broadcasts), our MI wireless systems face more competition — and are therefore more susceptible to interference than ever. Here are some tips on squeezing the most out of your RF gear.
No Pain With Da Right Gain
As in all things audio, the key to a happy marriage is proper gain structure. Transmission and reception of wireless audio is only as good as the source signal. Duh. When was the last time you adjusted the gain pot on a handheld wireless mic? Talk into the mic, watch the peak LED and adjust the gain pot until the LED blinks occasionally on loud peaks. If the transmitter does not have a peak LED, you may have to do something radical: Listen to the signal through a mixer back at the shop, when it’s quiet, and not 110 degrees, and there are not 45 people asking you if you really know what all those knobs do. Mics with interchangeable capsules need extra attention, because different capsules require different gain settings.
Some systems allow adjustment of the audio modulation level from the transmitter to the receiver, with a meter and adjustment on the receiver. To avoid modulation distortion, follow the manufacturer’s instructions to set this part of the signal chain. At the other end, match the output of the receiver to the correct input on the mixing console. Some wireless microphone receivers output audio at line level, others at mic level. Still others may be variable across the entire range, which makes it all the more important to start with the proper gain setting on the transmitter.
The same is true in respect to wireless instrument systems. Guitar or bass players who use multiple instruments really should have a transmitter for each one (especially for bassists switching between passive and active instruments). This allows the gain on each pack to be set for a specific set of pickups. Once the input level is dialed in on the transmitter pack, follow the chain: adjust the audio modulation level (if applicable), and then the output of the receiver. Ideally, replacing the wireless transmitter/receiver with a cable will yield no change in the level of the guitar or bass amp (notice I did not say timbre of the amp — which will depend upon the quality of the wireless equipment in use). A large change in the volume of the amp when swapping to a cable indicates that something in the gain structure is not correct.
Bodypacks used for lavalier or headset mics can often be used with 1/4-inch TS cables for an instrument, but that doesn’t necessarily mean the cable is appropriate for use with the high-impedance output of an electric guitar or bass. The instrument’s electronics may not react the same way they would if plugged directly into an amplifier, resulting in a loss of tone. Some manufacturers build specialty cables with active or passive impedance matching to compensate for this problem and ensure that the pickups behave the way they’re supposed to. There’s been an increase in the number of wireless systems incorporating “cable compensation” circuitry in their packs to model the interaction between guitar (or bass), cable and amplifier input, so try that as well.
Setting gain structure in an IEM system is a bit easier because you can send tone from the outputs of the monitor console. A 1 kHz tone at 0 dB sent from the outputs of the mixer should read a corresponding level at the inputs of the transmitter. “Corresponding level” is a bit vague: you’ll need to dig into the spec sheet to verify what a manufacture deems “0 dB.” For most digital mixers, 0 means 0 dBFS (0 dB below Full Scale) — i.e., “no more headroom.” But the 0 dB indication on the transmitter could mean that you still have as much as 20 dB of headroom. Confused? Welcome to our nightmare.
Does This Rack Make My Ears Look Big?
Many wireless units have antenna jacks on the rear panel, and when the units are mounted in a rack, the antenna is inside that rack. This is bad (very technical). Get the antenna out of the rack, by using either a front-mount antenna kit, or remote antenna. This helps maintain “line of sight” between transmitter and receiver, greatly reducing dropouts and dead spots on the stage. Speaking of dead spots, it’s a good idea to walk the stage at sound check and find any serious drops. Possible solutions include varying the broadcast frequency, changing the location of the antenna or nailing the lead singer’s feet to the stage. When using systems that have multiple antennas (diversity systems, for example) try to space the antennas at least a quarter-wavelength apart — roughly four inches for UHF and 16 inches for VHF systems — and at right angles to each other.
While we are on the topic: always use the antenna recommended by the manufacturer. Antenna size is directly proportional to wavelength of the system and lower radio frequencies require longer antennas. Don’t cut the antenna or substitute an antenna from a different system. Extend telescoping antennas to their maximum length.
Autoscan… Then Autoscan
Automatic frequency scanning is a wonderful thing, but it can only avoid the frequencies it finds when the scanning process is performed. Let’s say you do a soundcheck at 3 p.m., and use autoscan for a wireless guitar system. The system finds an open frequency and sets the transmitter and receiver to that frequency. Fast forward to the 10 p.m. showtime. The strip club next door has now opened, and the MC turns on her wireless mic. It happens to be using the same frequency as the wireless guitar. If the guitar wireless is really smart, it may recognize the activity and re-set its broadcast frequency. If not, then all of the fans will hear about Cinnamon’s next dance. Gather information regarding local RF activity ahead of time so you can make intelligent decisions. Does the night manager come in and turn on the wireless router in the business office? You should realize that many Wi-Fi routers use the same 2.4 GHz band as some digital wireless systems. What about venue security? Are their radios in operation at the time of sound check, or will you have RF surprises from hell when doors open?
Not All AA’s Are AA
Follow the manufacturer’s recommendations regarding batteries, especially when it comes to rechargeable batteries. Certain rechargeable batteries (NiCADs in particular) have a poor discharge rate — by the time you get a “low battery” warning, the voltage has dropped to a point where the transmitter is ready to shut down. That’s why almost all wireless packs can use alkaline cells — these discharge in a more linear, predictable manner. Fortunately, many wireless companies are offering rechargeable battery packs for use with their products, which is a much more environmentally friendly solution than disposable batteries. If you must use “disposables,” use last night’s batteries for today’s sound check and change them out prior to the show. Lithium Ion rechargeables fare much better, but be warned that not all Li-ion batteries output maximum voltage when fully charged, so a battery gauge may be inaccurate or the transmitter may not work at al
Choose Your Weapons Carefully
There are three major RF bands for wireless audio use: 2.4 GHz (typically used for “digital wireless”), UHF and VHF. Each has strengths and weaknesses but your decision to use any (or all) of them may be influenced by local RF activity. The increase of activity in the UHF band, and the continued willingness of the FCC to sell UHF spectrum to broadband providers for their exclusive use (until the FCC decides that the sale is no longer valid) means that the audio industry is getting squeezed out of the UHF range (UHF is the “prime real estate” of the RF spectrum). Many people have turned up their noses at VHF in the past, but (you heard it here) I think we’ll see a return to VHF, because this band is being ignored by the Greedy Wireless Broadband Providers. At least for now.
