Automatic gain control system applied to an audio signal as a function of ambient noise

1. An automatic gain control system applied to a audio signal as a function of the ambient noise, the system comprising:

a variable-gain amplifier receiving said audio signal and suitable to adjust the volume of a signal outputted by a loudspeaker,

an ambient noise estimator module suitable for establishing a current noise value estimated at least from a signal provided by a microphone; and

an automatic gain control module suitable for determining a gain value to be applied by said amplifier to said audio signal;

wherein:

the ambient noise estimator module comprises an MCRA estimator suitable for extracting, from the signal provided by said microphone including real noise, echo of the audio signal and speech if any, a value of the ambient noise component on its own after the speech component has been eliminated; and

the system further comprises a module for estimating the power of the audio signal and suitable for providing the automatic gain control module with a current power value for the audio signal.

2. The system according to claim 1 , wherein the ambient noise estimator module and the module for estimating the power of the audio signal include respective perception filters.

3. The system according to claim 1 , wherein the ambient noise estimator module includes a circuit for canceling the echo from the audio signal present in the signal delivered by the microphone.

4. The system according to claim 1 , wherein the automatic gain control module comprises:

means for initially executing an initialization step 1) with reference to a predetermined minimum threshold for ambient noise;

means for subsequently executing a gain-change step 2); and

means for applying to the audio signal the gain value established at the end of the gain-change step 2);

said initialization step 1) comprising:

a step 1a) of starting the method, consisting in estimating a current noise value and in executing the following step 1b) for determining a reference noise as soon as a current noise value is greater than the noise threshold; and

a step 1b) for determining a reference noise for changing gain, consisting in establishing a reference value for the ambient noise if the current noise value has remained above the ambient noise threshold for a first determined duration, a return being made to initialization step 1) as soon as a current noise value becomes less than the noise threshold;

said gain-change step 2) consisting in:

calculating the difference between the current noise value and the ambient noise reference value;

applying to the gain a variation that is established as a function of said difference when said difference remains greater than a high threshold or less than a low threshold concerning gain change for a second determined duration; and

establishing a new reference value for the ambient noise, and returning to the gain-change step; and

said gain-change step 2) being interrupted with a gain reduction being applied as soon as a current noise value becomes less than the noise threshold.

5. The system according to claim 4 , wherein the starting step 1a) also consists in determining a temporary noise reference value, and wherein the reference noise determination step 1b) is interrupted as soon as the absolute value of the difference between a current noise value and the temporary noise reference value remains greater than a noise dynamic range threshold for the first duration.

6. A system according to claim 4 , wherein the reference noise determination step 1b) also consists in determining a reference value for the audio signal power, and wherein gain-change step 2) is interrupted if the absolute value of the difference between the current power value of the audio signal and the reference power value has remained greater than a power dynamic range threshold for a third determined duration, with a return to gain-change step 2) if the current noise value is greater than the noise threshold, else to initialization step 1).

7. The system according to claim 4 , wherein the ambient noise reference value is equal to the current noise value smoothed by a first order filter.

8. The system according to claim 6 , wherein the audio signal power reference value is equal to the current power value of the audio signal smoothed by a first order filter.

9. Audio equipment including an audio signal source and a microphone, further including an automatic gain control system according to claim 1 .