Distance-Based Automatic Gain Control with Continuous Proximity-Effect Compensation

This paper presents an Automatic Gain Control (AGC) that derives the gain from the sound source to microphone distance, utilizing a distance sensor. The concept is motivated by the fact that the microphone output level varies inversely with the distance to a spherical sound source. It is applicable to frequently arising situations, in which a speaker does not maintain a desired distance to a microphone. In addition, for pressure-gradient microphones, as the sound source to microphone distance decreases, the bass response of the microphone increases, a characteristic known as proximity-effect. Knowledge of the sound-source to microphone distance permits to accurately compensate for both the spectral changes and the distance-related signal level changes. In particular, such a distance-based AGC can normalize these signal level changes without deteriorating the signal quality, as opposed to conventional AGCs that introduce distortion, pumping, and breathing. Provided an accurate distance sensor, gain changes can take effect instantaneously and don't need to be gated by attack and release time. Likewise, spectral changes due to undesired proximity-effect variations can be adaptively corrected using precise inverse filtering derived from continuous distance measurements, sound arrival angles, and microphone directivity, no longer requiring inadequate static settings on the microphone for proximity-effect compensation.