Intermodulation Distortion

The word technicians cringe to… intermodulation. I will break it down and make it short and easy to understand for some of us.

Intermodulation (IM) or Intermodulation distortion (IMD) is what happens when two or more frequencies interact in a non-linear system (microphone system) and harmonics of the main frequencies interacting are produced.

Each frequency has its own harmonics, so the multiples of a frequency are its harmonics. These harmonics are usually lower in amplitude the higher they go on the RF spectrum. So, when you have multiple microphones in use, we usually get miracles happening on stage… everytime the backing vocalists come close to each other the lead vocalist’s microphone drops out. Why? We need the ghost busters.

No, we don’t need ghost busters. This is what is happening when that happens. Remember the harmonics I mentioned earlier? They are doing their job. We have different types of harmonic distortion, and they are categorised according to how far they are from the main/fundamental frequencies (the backing vocalist microphones). We have 3rd order, 5th order, 7th order and 2nd order intermodulation, I have stated them from highest to lowest in amplitude and destructive potential. When the two transmitters interact, they produce harmonics and one of them (usually 3rd order IMD) falls on the lead vocalist’s microphone frequency. When this happens, there is what we call constructive interference (when two of the same frequencies meet in phase and double in amplitude).

When this happens, the doubling in amplitude will overload the receiver circuitry and this will result in a dropout. Working with multiple RF channels, (e.g. 60 – 80 channels of RF) you don’t want unnecessary spectrum cluttering because you run out of plotting space for your other microphone and in-ear systems, also, you don’t want random dropouts everywhere on a large setup.

Proper frequency coordination is needed to avoid IMD interference…that’s where specialists like RF Junky come in.


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