Acoustic echo reduction circuit for a “hands-free” device usable with a cell phone

1. A “hands-free” device usable with a cell phone, in particular an independent device suitable for fitting in removable manner in a motor vehicle, the device comprising:

a circuit for picking up acoustic signals, the circuit comprising a microphone ( 18 ) and analog-to-digital converter means ( 38 );

a circuit for playing back audio signals, the circuit comprising digital-to-analog converter means ( 28 ), an amplifier ( 30 ), and a loudspeaker ( 16 );

a telephone network interface circuit ( 20 ), in particular a circuit for wirelessly interfacing a cell phone; and

a circuit for digitally processing audio signals picked up by the microphone for application to the input of the interface means, and for processing audio signals received at the output from the interface means for playing back by the loudspeaker;

the circuit for digitally processing audio signals including means for reducing the acoustic echo that results from interaction of the loudspeaker with the microphone, these means comprising:

an echo cancellation stage ( 40 ) comprising an adaptive linear filter suitable for subtracting from the signal picked up by the microphone a reference signal that is derived from the signal received at the output from the interface means for playing back by the loudspeaker;

a suppression stage ( 42 ) for suppressing the residual echo present at the output from the echo cancellation stage, the suppression stage comprising gain control means; and

a noise reduction stage ( 44 ) for selectively reducing the background noise present in the signal received at the output from the echo suppression stage;

the device being characterized in that it further comprises:

a filtering and digitizing stage ( 54 , 56 ) receiving as input the signal that is applied by the amplifier ( 30 ) to the loudspeaker ( 16 ), and delivering as output a corresponding filtered and digitized signal that is applied as a reference signal to the echo cancellation stage ( 40 ).

2. The device of claim 1 , wherein the stage ( 44 ) for selectively reducing the background noise present in the signal received from the output of the echo cancellation stage comprises means for analyzing the time coherence of the signal, suitable for determining an a priori probability for the presence/absence of speech from respective energy levels in the spectral domain, and for using said probability to estimate a noise spectrum and to derive a de-noised estimate of the received signal.

3. The device of claim 1 , further comprising:

a stage ( 50 ) for selectively reducing the background noise present in the signal received at the output from the interface means for application to the circuit for playing back audio signals.

4. The device of claim 2 , wherein the background noise reduction stage ( 50 ) present upstream in the signal received at the output from the interface means comprises means for analyzing the time coherence of the signal and suitable for determining an a priori probability for the presence/absence of speech from respective energy levels in the spectral domain, and for using said probability to estimate a noise spectrum and to derive a de-noised estimate of the received signal.

5. The device of claim 1 , wherein the adaptive linear filter of the echo cancellation stage ( 40 ) comprises means for implementing an APA type predictive algorithm.

6. The device of claim 1 , wherein the gain control means of the residual echo suppression stage ( 42 ) are selective gain control means operating in different manners as a function of the frequencies of the received signal.

7. The device of claim 1 , wherein the filtering and digitizing stage ( 54 ) comprises lowpass filter means and level matching means.

8. The device of claim 1 , wherein the circuits of the device are incorporated in a common housing ( 14 ) carrying the loudspeaker ( 16 ) and the microphone ( 18 ) that are placed close to each other.