KnowHOW: Large-scale wireless microphones systems
KnowHOW: Large-scale wireless microphones systems
With the radio spectrum available for wireless microphones contracting worldwide, Gordon Moore looks at how to manage multiple wireless systems in today’s high RF environment
As churches grow, they tend to incorporate more and more wireless devices into their worship services. Wireless microphones eliminate stage clutter (no cables), in-ear monitors (IEMs) clear loud floor monitors from the stage and intercoms allow the growing technical team to communicate discreetly and better coordinate live worship. Throw in wireless links for mixers, DMX lighting control, Bluetooth pedal boards and you have a complicated soup of radio frequency (RF) devices all competing for spectrum. And this is all in a shrinking spectrum environment.
The radio spectrum available for wireless microphones continues to contract worldwide. The promise of 5G telephone and data networks is creating a demand from much larger user groups for the spectrum needed for these new services. Governments around the world are leasing, selling or auctioning space for billions in currency and under pressure to standardise the spectrum so the cell manufacturers have a simpler, more predictable environment in which to operate.
So, how do you manage 25, 50 or more wireless systems in today’s high RF environment? First, get a full understanding of wireless microphones, intercoms and IEMs and how they work. They are not like light bulbs where you can just turn them on. There are many resources available for understanding RF systems and you should download and study them. You need to understand terms such as intermodulation, third-order intercept, sensitivity, EIRP, RF distribution and cabling needs. Most importantly, find out what changes are scheduled for your part of the world in spectrum allocation. In the US, for example, they took away the 700MHz frequencies in 2009 and then the 600MHz frequencies in 2019. The first announcements of changes coming were in 1995.
Large-scale wireless microphone systems are successful only when the equipment is up to the task. Low-cost wireless systems are designed for simple setups with just a few channels. They do not have the design characteristics required to meet the demand for dozens of channels. What are these characteristics?
Receiver front-end design
High-quality systems have robust front-end designs that are characterised by the ability to reject out-of-channel interference. The front end of the receiver is the first circuit that is fed directly from the antenna. The filtering associated with the front end needs to reject as much of the spectrum as possible outside the frequency of the transmitter it is trying to pick up. Many brands will use SAW filters, pin diode arrays or other switchable circuits that can change to match the tuning range of the receiver. Every high-end manufacturer has effective methods for filtering. There is cost associated with this filtering – you can’t build it with a tough front end and keep the costs low. Good filter stages use more current so the power supply must be bigger.
A high third-order intercept spec is a desirable receiver specification since it measures how well the receiver resists interference caused by multiple interfering frequencies. A good number is –15dBm and an excellent specification is +1dBm or higher.
Spurious rejection measures how well the receiver rejects the entire range of frequencies that can be applied to the receiver by any outside source. Ideally, the manufacturer will have tested the receiver from audio frequencies to microwave frequencies. This number measures how well the first RF section, the IF filters and other sections reject interfering signals. Good is 80dB; excellent is greater than 100dB.
There are other characteristics important in wireless receiver designs but these are two of the most critical. Generally speaking, a unit that is designed well in these regards would also be well-designed elsewhere.
Antenna input – is the antenna detachable and can you put different antennae on your receiver? Low-cost systems have permanent antennas – it is cheaper and easier to pass government testing. But, in a large-scale system, you will want to have an antenna distribution system for maximum effectiveness. SMA or BNC connections are a must.
The transmitters also need to be well-designed and resistant to third-order intermodulation. When you have multiple transmitters gathered onstage, their antennas not only send out a signal, they also can pick up a different signal. A poorly design output stage in a transmitter will allow the incoming signal to heterodyne (or “beat” against the transmitter frequencies oscillator inside the unit). For example, if the transmitter is sending out 550MHz and picking up a 600MHz signal, they will interact and create two new signals – one at 50MHz (600–550) and another at 1,150MHz (550+600). That 50MHz could then combine with another transmission at 500MHz, creating a 550MHz signal that will then interfere with the desired 550MHz original signal. That’s how messy it can get with just three transmitters – imagine the mathematics behind 50.
A well-designed transmitter will reduce this effect by having good filtering on the output stage and keep its “spurious emissions” under control. The more linear the transmitter output stage design is, the more resistant it is to these issues: –60db spurious emissions is good; –80dB is desirable.
The first two factors are something you will have little control over – that will be a basis for your system brand of choice but you cannot change them. The system design, however, is completely in your hands. How you design and install your wireless system will determine the measure of your success.
Antenna proximity. The closer your antenna can be to the performers’ transmitters, the higher the reliability of the signal and the better they will perform in a tough RF environment. Wireless systems are all about the signal-to-noise ratio. The signal is the radio signal you want. The noise is all the other radio signals on the carrier frequency. If the desired signal is better than the noise by at least 6dB (generally speaking), the signal is considered effective. The easy way to have a good S/N ratio is to get the antennas close. It is that simple.
So, what is better? Set the receivers close to the antenna with a short cable and long audio cables? Or set the receivers close to the mixer and have long antenna cables? In pre-digital days, the answer was almost always the first but, sometimes, the signal loss over cable was greater than the cable loss in the antenna run. Often, having the receivers in the sound booth was important in case there were changes to be made.
The new modern wireless receivers in large-scale systems today now have Dante or other digital transport ports and easy remote control of the receivers via the network. This makes placing the receiver very close to the stage desirable because you will be able to control all aspects of the system remotely with the manufacturer’s software.
The next key of the physical installation is having an effective antenna distribution system. An antenna multi-coupler has a pair of inputs for two antennas and outputs for multiple receivers so they can share a single pair of antennas onstage. Good units have strong filtering and prevent intermodulation products. They are not cheap but they clean up your rack and prevent the “antenna farm” – a stage bristling with multiple antennas. I was at a public performance recently where the wireless receivers were all placed at the foot of the stage – each with its own pair. It looked very much like a chrome spike farm with over 40 antennas poking up in front. Very distracting and – unfortunately – the wireless performed badly as well. A clean antenna distribution system will provide better reliability, cleaner sightlines and better overall system performance.
Use low loss RF cabling for your antenna system. Read the specifications carefully. RG58 cable is fairly high loss and can cause much of your gain from close antenna placement to be lost in the cable. RG6 video cable can be found in low loss versions – but the impedance will be wrong at 75Ω. While much better than RG58, RG6 will lose a few decibels of signal strength over a long run, more than a proper RG8 or Belden 9913F which is the proper 50Ω impedance required by wireless microphones. Keep your antenna cables away from AC power lines, lighting controls, data cables and other RF sources.
Finally – and most important – learn good frequency coordination. As stated earlier in this article, wireless systems are not like light bulbs where you just plug them in and they work. Frequency coordination, the proper selection of frequencies according to the physical characteristics of the wireless systems involved, is a complex mathematical exercise. Just touching on the basics will take more space than this magazine could allow. The important thing is that you understand that it is important and must be done. Don’t ignore it. So, how do we do it? There are a few easy solutions.
Nearly every high-end wireless manufacturer now offers a wireless coordination program. Sennheiser, Lectrosonics, Shure and many others all offer sophisticated control software for coordinating large-scale systems. You can scan the available spectrum (which will also reveal TV stations and other sources in the area) and then allow the software to select the frequencies in a carefully coordinated pattern that assures your best-possible reception.
It is important that you have all RF sources taken into account. Do you use walkie talkies? Make sure those frequencies are loaded into the software. What about wireless DMX lighting controls? What frequencies are they on? Enter them into the software. IEMs? Make them a part of the calculation. Special tip – plan for spare frequencies. Large-scale systems are a bit like the stacking game Jenga. The larger the system, the more “fragile” it can be. If someone walks in and fires up a transmitter that is not coordinated into the whole system, they can cause problems for multiple channels. Change one, and all might need to be changed. That’s why coordination software should be acquired and learned. Most of these programs are free and available online.
Finally, ask for professional help. Many high-end manufacturers have either factory direct support or well-trained support from dealers. Don’t be afraid to ask for help when designing and coordinating your new large-scale system. Professional assistance can take away a lot of pain from your efforts and yield higher reliability in the long term.
Don’t be afraid of a large-scale system. Be prepared to spend more per unit. Be prepared to spend more time planning (don’t just order 50 systems and then think you’ll work it out in a day or two). Do it right and you will have a clean stage, wonderful sound and a long-term investment that should last for years. Mix well and be Blessed!