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Microphone Month 5

Why are all wireless mic signals compressed and then expanded?

This has to do with the dynamic range limitations of the wireless signal, rather than the microphone or audio itself. Professional audio applications require a dynamic range of more than 100dB to achieve acceptable performance, yet even the best analog wireless link has its bandwidth limited so that it can achieve no more than about 70dB. That’s because the receiver decreases the amplitude of some frequencies relative to others when it demodulates the FM signal.

In order to compensate for this, analog wireless systems employ a process known as “companding” to hide the dynamic range limitations of analog FM demodulation. The transmitter takes the audio signal from the input and compresses it the same way a compressor does on a mixing console (most systems use a fixed 2:1 ratio). Then the transmitter modulates the signal with a radio frequency (RF) signal. The receiver snatches the modulated RF signal from the air, demodulates it, and expands it back to its original dynamic range (using a 1:2 ratio).

Although companding improves the dynamic range, it also creates distortions (“artifacts”) that are often audible. Different manufacturers apply different techniques to remedy this. For example, Shure’s Audio Reference Companding scheme utilizes level-dependent compression rather than a fixed ratio. At low signal levels, no compression is applied, which eliminates artifacts. Audio Reference Companding utilizes a “soft-knee” type of compression, where the onset of compression occurs gradually, allowing the wireless system to avoid companding until it is absolutely necessary.

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