Technology in call noise reduction of TWS headsets

Publisher:落寞梦惊Latest update time:2021-10-14 Source: eefocus Reading articles on mobile phones Scan QR code
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As the TWS (True wireless stereo) headset market continues to grow, users' demand for product experience has also upgraded from simple quick connections to higher standards. For example, as of this year, a large number of TWS headsets with clear calls as their selling point have emerged on the market.

 

In order to enable clear voice communication in a very noisy environment, is it possible to generate a solution that combines the signals from the inner ear and the external microphone to achieve an intelligent, environment-adaptive sub-band mixer technology? In fact, some domestic and foreign algorithm companies are committed to this and have achieved certain results.

 

Of course, many solution companies now have call noise reduction solutions that specifically emphasize edge AI (this is one), but in fact they are more about optimizing existing call noise reduction solutions, so let’s remove this part and first look at some basic introductions, that is, what call noise reduction can specifically do.

 

In general, call noise reduction relies on the synchronization of Uplink and Downlink. It is roughly Microphone Array/AEC/NS/EQ/AGC/DRC, and the logical relationship is as follows:

 

 

ADM (Adaptive Directional Microphone Array) Adaptive omnidirectional microphone array

 

This is a digital signal processing technology that creates a directional or noise canceling microphone using only two omnidirectional microphones. ADM automatically changes its directional characteristics to provide optimal noise attenuation in a variety of environments while fully maintaining signal quality. The adaptive process is fast and frequency selective, and can cancel multiple interferences simultaneously.

 

In addition to its good directional characteristics, ADM is less affected by wind noise than traditional acoustic directional microphones. ADM technology allows two types of microphone configurations: "endfire" and "broadside".


In an endfire configuration, the signal source (the user's mouth) is on-axis (the line connecting the two microphones). In a broadside configuration, it's aimed at a line on the horizontal axis.

 

In the end-fire configuration, the ADM has two modes of operation; "far-talk" and "close-talk". In far-talk mode, the ADM acts as an optimal directional microphone, attenuating signals from the back and sides while retaining the signal in front. In close-talk mode, the ADM acts as the best noise-canceling microphone, effectively eliminating distant sounds. The relatively free acoustic design makes the ADM ideal for mobile phones, as it can "softly" switch between far-end speakers and near-end speakers. However, when this type of design is used on headphones, especially TWS headphones, it is more restricted by whether the user wears it correctly. Similar to airpods, the author has observed many people wearing them in "weird" ways on the subway. Some are due to the shape of the user's ears, and some are due to wearing habits, which results in the algorithm not necessarily working under ideal conditions.

 

Acoustic Echo Canceller (AEC)

When part of a signal in a duplex (simultaneous two-way) communication returns to the source signal, it is called "echo." In long-distance analog and almost all digital communication systems, even a small echo signal can cause interference due to the significant round-trip delay.

 

In voice communication terminals, acoustic echo is generated due to the acoustic coupling between the loudspeaker and the microphone. Due to the nonlinear processing applied in the communication channel, such as lossy vocoders and transcoding, the acoustic echo must be processed (cancelled) locally inside the device.

 

Noise Suppressor (NS)

Noise suppression technology reduces stationary and transient noise in single-channel speech signals, improving the signal-to-noise ratio, enhancing speech clarity, and reducing listening fatigue.

 

Of course, there are many specific methods for this part, such as BF (Beamforming) or PF (Post filter) adjustment methods. Generally speaking, AEC, NS, BF, and PF are the core parts of call noise reduction. Of course, the advantages and disadvantages of each algorithm solution provider are different.

 

Automatic Gain Control (AGC) and Dynamic Range Compressor (DRC)

In a typical voice communication system, the level of the voice signal may vary over a wide range due to the different distances between the user and the microphone and due to the characteristics of the communication channel.

 

Dynamic range compression (DRC) is the simplest way to equalize signal levels. Compression reduces the dynamic range of a signal by lowering (compressing) loud speech segments while leaving soft speech segments intact. As a result, the entire signal can be additionally amplified so that soft signals can be heard better.

 

AGC technology digitally increases signal gain (amplifies) when the speech signal is weak, and compresses it when the speech signal is strong. In noisy places, people tend to speak loudly, and this automatically sets the microphone channel gain to a small value, thereby reducing surrounding noise while maintaining the voice of interest at an optimal level. Alternatively, in a quiet environment, people speak relatively quietly, so their voices will be amplified by the algorithm without too much noise.

Reference address:Technology in call noise reduction of TWS headsets

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