Automatic gain control for mobile microphone

A microphone system for use in a mobile telephone, comprising:

• a first automatic gain control unit comprising:
• a first comparator for determining when an input signal level exceeds a high threshold;
• a second comparator for determining when said input signal level falls below a low threshold;
• a third comparator for determining when said the digital gain exceeds a set threshold;
• a first gain controller for generating a first control signal to vary the analog gain of the codec in increments of said first size:
• a second gain controller for generating a control signal to vary the digital gain of the code in increments of said second size, said second size being smaller than said first size;
• and
• wherein said first gain controller is configured to decrease the analog gain of the codec in response to said first comparator determining that said input signal level exceeds said high threshold,
• wherein in response to the input signal level falling below said low threshold in the presence of a speech signal said second gain controller is configured to increase the digital gain of said codec, and
• wherein in response to the input signal level below said low threshold in the presence of a speech signal and the digital gain being above said set threshold, said first and second controllers are configured to increase the analog gain and decrease the digital gain of the codec such that the increase in analog gain compensates for the decrease in digital gain;
• an echo canceller and noise reduction circuitry for canceling echo and reducing noise in the signal output by the first automatic gain control unit;
• and
• an inverse gain control unit for canceling out the gain applied by said first automatic gain control unit after echo cancellation and noise reduction.

A method of controlling the gain applied to an input signal produced by a microphone subject to ambient noise, wherein the input signal is passed to a codec having an analog gain control and a digital gain control comprising:

• continually monitoring the input signal;
• decreasing the gain applied to said input signal in increments of a first size when said input signal exceeds a high threshold;
• and
• when said input signal falls below a low threshold in the presence of a speech-present signal increasing the gain applied to said input signal in increments of a second size being less than said first size, and if the digital gain exceeds said set threshold increasing the analog gain and decreasing the digital gain to compensate for the increased analog gain.