ABOUT COMPANDING

All analog radio mics use some form of companding, a term used to describe the compression and re-expansion of an audio signal. The incoming audio signal is compressed in the transmitter, used to modulate the RF carrier, and again transmitted. Upon reaching the receiver, the audio is detected (separated from the RF signal) and expanded to its original amplitude. Although it seems quite simple, the process requires that all compression and expansion rates be precisely matched.

The use of companding in wireless mics allows greater dynamic range without the need for increased RF carrier deviation. This pleases both the sound mixers and the Federal Communication Commission.

It should be noted that the compression portion of these companding circuits is independent from the externally adjustable compressor limiter (affecting microphone gain) found on most wireless equipment.

However, because the compressor limiters are externally adjustable on the transmitter, poor dynamic range and/or distortion can occur as a result of misinformed use. This user adjustment is made via the mic gain control and should be set so that the loudest expected portion of audio just activates the limiter — which is generally indicated by meter deflection or a lit LED.

Normal audio should remain low enough so as not to trigger the built-in limiter. Desired increases in gain can be achieved by raising the input level at the mixing panel or recorder, rather than by increasing the mic gain on the transmitter. A few minutes spent properly adjusting mic gain and limiter with regard to vocal attributes of the talent will greatly improve recordings.

Lavalier microphones should be checked periodically. Their tiny elements fall prey to the same afflictions as condenser microphones. Many lavalier power supplies have active circuitry and need to have battery and ground contacts checked.

All microphones should be checked regularly for reliable ground connections. Similarly, connector and contact surfaces should be inspected and cleaned. Routine inspection, cleaning, and calibration is your best assurance to continued, trouble-free operation of your microphones.

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