Automatic gain control for mobile microphone

1. An automatic gain control unit for controlling the gain applied to an input signal produced by a microphone subject to ambient noise, wherein said input signal is passed through a codec having analog and digital gain control inputs for varying the analog and digital gain respectively each in increments of a first size and a second size, said second size being smaller than said first size, and said input signal having a signal level, said automatic gain control unit continually monitoring the signal level of said input signal, and comprising:

a first comparator for determining when the input signal level exceeds a high threshold;

a second comparator for determining when the 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 second control signal to vary the digital gain of the codec in increments of said second size;

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, and

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 falling below said low threshold in the presence of a speech signal and the digital gain being above said set threshold, said first and second gain controllers are configured to increase the analog gain and decrease the digital gain of the codec such that the increase in analog gain compensates fir the decrease in digital gain.

2. An automatic gain control unit as claimed in claim 1 , wherein in response to said second comparator determining that said digital gain falls below set threshold, the analog gain is increased by an amount equal to the reduction in digital gain so that the overall gain remains the same.

3. An automatic gain control unit as claimed in claim 1 , further comprising a multiplexer having an input connected to a divider for reducing the digital gain by preset factor and selectable in response to a determination by said third comparator that said digital gain exceeds said set threshold and a determination by said second comparator that said input signal has a level below said low threshold.

4. An automatic gain control unit as claimed in claim 1 , wherein said increment of said first size is 6 dB.

5. An automatic gain control unit as claimed in claim 1 , further comprising a timing arrangement for periodically triggering said automatic gain control circuit.

6. An automatic gain control unit as claimed in claim 5 , wherein said timing arrangement comprises a counter and threshold detector, and wherein said automatic gain control circuit is triggered in response to said counter reaching a predetermined count threshold.

7. An automatic gain control unit as claimed in claim 1 , further comprising an inverse gain control circuit for canceling out the gain applied to said input signal by said first and second gain control circuits after echo and noise suppression.

8. 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.

9. 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.

10. A method as claimed in claim 9 , wherein the increase in analog gain resulting from the digital gain exceeding said third threshold matches the reduction in digital gain so that the overall gain remains the same.

11. A method as claimed in claim 9 , wherein a counter counts up to a predetermined threshold, and periodically triggers the monitoring of said signal when the counter reaches the predetermined threshold.