Issue link: http://digital.copcomm.com/i/1351516
18 S P R I N G 2 0 2 1 I C A S Q U A R T E R L Y Mixers today are often using wireless paths to the recorder exclusively. There are more emerging wireless services on the set. Take your production into a city and there are literally thousands of RF emitters in range of your antennas. Yikes! How do we manage to get through the day? Luckily, knowing some scientific principles can help. So can knowing how to apply them to your situation. First, let's talk about what radio is. Albert Einstein put it this way: "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat." There is something that replaces the cat though, it's a collection of resonant circuits which generate, b y G . J o h n G a r r e t t C A S RF in the Twenty-First Century emit, are susceptible to, and try to filter out all the non-cats except yours. An electromagnetic wave is generated at a specific center frequency with modulation characteristics that let it carry information. Your receiver is designed to just listen for signals in the same band, and those resonant circuits are tuned to the same frequency as the transmitter. Depending on the design, there are filters of one sort or another to help keep spurious signals, images, and other undesired artifacts out of the information you're trying to receive. Also, depending on the design, you can fool the receiver by having two transmitters nearby tuned too close to each other such that a mixing product can be generated inside the receiver, causing interference. Check this link for a good explanation of how this works with most wireless mics: https://www.electronics -notes.com/articles/radio/rf-mixer/rf-mixing-basics.php Sometimes mixing is a normal part of receiver behavior, like when the carrier frequency is mixed with an intermediate frequency (IF) oscillator or two and the remaining difference signal is the desired audio. When another near-frequency transmitter is close by, this can result in unwanted intermodulation (mixing) distortion (IMD). This is why there are frequency coordination apps, and minimum frequency spacing requirements when using multiple radios in the same band. The other maddening source of this is when two transmitted signals combine so that the sum or difference in the frequencies lands on one of the intermediate frequency oscillators (often 10.1 MHz and 455 kHz). That's going to show up in the audio. So, how can we avoid some of these problems? One important law to be aware of is the inverse square law which says that your radio signal strength diminishes at a rate of 1/r2. That means if you double the distance from the transmitter, there is 1/4 power at the receiver, triple = 1/9, and so on. There are several good illustrations of this on YouTube and Let's take a look at the beast that gives us freedom, traps us, mystifies, and frustrates. No, not our spouses, the use of RF in production sound! A lot has happened in the last five years. While the FCC has auctioned off a lot of the crowded spectrum we use every day, huge leaps in technology have helped us overcome this setback. However, the laws of physics don't change, which is why having a good understanding of what happens between the mic and the recorder can help.