Influencing factors of echo cancellation for smart speakers

1、 What is echo cancellation

Echo cancellation （Acoustic Echo Cancellation,AEC）, It refers to the estimation of echo signal size by adaptive method , This estimate is then subtracted from the received signal to cancel the echo .

2、 The purpose of echo cancellation

The purpose of echo cancellation is to work on the speaker of the speaker （ Play music or voice ） when , From the signal collected from the microphone , Remove the sound signal played by itself . otherwise , When the music is playing , Our sound signal will drown in the sound of music , Unable to continue effective voice control on the speaker .

3、 The principle of echo cancellation

The greater the similarity between the microphone signal and the recovery signal , The better the echo cancellation performance . Therefore, try to ensure the similarity between the microphone signal and the recovery signal , Try to reduce the influence of factors that affect the microphone signal quality .

4、 Influencing factors of echo cancellation

1. The cut is
   The amplitude of the stoping signal will cause the amplitude of the stoping signal to be greater than 0dBFS The signal of is truncated , The effect is that the signal played by the horn is completely distorted , The human voice is accompanied by strong noise . Microphone signal clipping will seriously affect the recognition effect , Therefore, clipping phenomenon must not occur in audio signal processing , Be sure to reduce the power amplifier or gain output if any .

Design principles ：
The speaker plays a full sweep signal at the maximum volume , The loudspeaker shall not break 、 Resonance and other phenomena , The mic signal cannot be truncated . This is also one of the judging principles when specifying the output power of the horn .

2. Harmonic distortion
   Harmonic distortion means that the output signal has more harmonic components than the input signal . Harmonic distortion is caused by the fact that the system is not completely linear . There are many reasons for harmonic distortion , In the entire audio path , From the distortion curve of the loudspeaker unit , To the structural design of the rear cavity of the loudspeaker , Horn front net cover , Microphone receiving hole structure, etc , Will increase the distortion of the microphone signal . The more distorted the mic signal is , The worse the similarity between the microphone signal and the recovery signal , The worse the echo cancellation performance .

Design principles ：

1. Try not to use mini speakers 、 Ultra thin loudspeaker and other loudspeaker units with high resonance frequency , This kind of monomer has large low-frequency distortion ;
2. If the rear cavity structure of the loudspeaker allows , Add passive radiator or guide tube design , Low frequency can be increased , At the same time, the nonlinear vibration of the horn is reduced ;

3. Signal-to-noise ratio
   Ambient noise and circuit noise will reduce the signal-to-noise ratio of the microphone signal , Especially in the case of low volume , The proportion of signal energy decreases , Increase of nonlinear distortion caused by bottom noise , The measured echo cancellation value is very poor . Generally, this noise refers to circuit noise , During the speaker test, the noise level of the test environment will be guaranteed .

Design principles ：

1. Select high sensitivity microphone , Improve signal receiving amplitude , Recommended values [email protected],1kHz And above ;
2. Avoid excessive signal processing in the output circuit , Prevent the introduction of unnecessary signal crosstalk ;

4. Automatic gain control
   Automatic gain control （automatic gain control,AGC） An automatic control method for automatically adjusting the gain of the amplification circuit with the signal strength .
   AGC is a kind of limiting output , It uses the effective combination of linear amplification and compression amplification to adjust the output signal of the hearing aid . When a weak signal is input , The linear amplifier circuit works , Ensure the strength of the output signal ; When the input signal reaches a certain strength , Start the compression and amplification circuit , Reduce the output amplitude . in other words ,AGC The function can automatically control the gain amplitude by changing the input / output compression ratio .

Design principles ：
In the mining circuit ,AGC It shall be placed in front of the mining circuit , To ensure that the signal from the power amplifier to the recovery is consistent with the signal to the loudspeaker .

5. Mechanical noise
   In the category of intelligent projectors or robotic human devices （ Such as Tencent King glory doll speaker ） in , The variables introduced by fan noise and motor noise will cause the bottom noise of the microphone signal to be particularly large , The recognition effect is greatly affected . therefore , In this type of equipment , Pay special attention to the internal fan 、 The distance between the motor and the microphone board , If the structure space allows , The microphone plate can be sealed separately , Prevent internal transmission .
6. Circuit bad signal
   When there are problems in circuit design , Usually, bad signals will be introduced into the recovery signal or microphone signal , Such as pulse 、 Clutter 、 Aliasing , Even lost data . In this case, the hardware circuit design should be modified first , Ensure that the circuit signal is clean , Then carry out acoustic test .
7. The DC offset is too large
   DC offset （DC offset） It refers to the DC component in the AC signal , On the smart speaker , This phenomenon is usually caused by the bias voltage of the analog microphone .

Design principles ：
Except for circuit design reasons , Using a digital microphone can effectively avoid this problem .