麦克风阵列降噪_嘈杂环境如何闹中取静，聊聊音频降噪设计

From the perspective of signal science, noise reduction is a very important research branch, such as digital camera shooting needs noise reduction, headphones also need noise reduction to listen to music, strictly speaking, this is a process of degradation and reconstruction, but the design models to achieve this process are varied, and the topic may have considerable depth, so let’s cite a few simple examples that are common in life today.

Today, we mainly talk about the active noise reduction technology in the audio field, which is different from the digital image in that the noise source of the audio is separated from the signal source, and the noise source comes from the space outside the headphone. so the logic is very simple: we only need to filter the noise in the space, but there are two urgent problems to be solved in the specific design: first, how to ensure the efficiency of the environmental pickup microphone. That is, whether the layout is feedforward or feedback, and the feedforward design microphone faces outward (pay attention to the feed position of noise signal and noise reduction signal):

Feedforward design means that the relative position of the microphone and the noise source is constantly changing, which will form a time difference and pose a greater challenge to the head-wearing design. In addition, it is easy to form wind noise and hear voices when walking. The feedback design is as follows:

The difference between the two pictures is obvious. For noise reduction headphones, when picking up the inner ear signal, the feedback design will inevitably affect the sound source signal, so when many feedback design noise reduction headphones are turned on and off, the music effect will sound obviously different.

The second problem is how to reduce the time delay, because the principle of active noise reduction is that the loudspeaker actively plays the 180-degree inverse signal with the noise signal to achieve cancellation interference, but the noise reduction signal needs to have the process of picking up the noise and generating the inverse signal. this will cause a phase difference between the two, and the greater the phase difference, the worse the interference effect.

Because audio playback itself is a process of digital-to-analog conversion + signal amplification, the noise reduction microphone picks up the ambient noise and carries out analog-to-digital conversion first, and then realizes digital noise reduction in the internal processor. Then, together with the digital audio source, it is fed into the digital-to-analog converter and amplifier to achieve noise reduction output. When adjusting the design, you only need to change the parameters in the chip to verify, and the flexibility is much higher. This also leads to an important function of digital noise reduction-the series of noise reduction can be controlled by adjusting digital parameters, and the environmental adaptability will be significantly stronger. Of course, analog noise reduction can also be achieved by switching different filter capacitors, but the design layout will be much larger, which is relatively difficult to do.

In the case of digital noise reduction, the first problem mentioned above is no longer a problem, while the second problem depends entirely on the speed of the processor. Relatively speaking, the faster the speed, the higher the power consumption, the shorter the life of the wireless design, but with the continuous progress of the manufacturing process, the impact of time delay is getting smaller and smaller, or even almost non-existent.

Moreover, after getting the environmental signal, the pickup microphone of the modern active noise reduction headset can not only be used to create the noise reduction signal, but also realize the synchronous acquisition of the closed playback and the external sound field. that is, wearing headphones with sound-proof and better effect compared to earplugs, at the same time, you can also hear the ringtones picked up by microphones, communicate normally with people around you, and so on. The effect of noise reduction is obviously better than that of simple closing, and the adaptability is wider.

In addition, headphones are basically equipped with microphones to meet the call and voice control functions, so noise reduction should be aimed not only at listening, but also at speaking. At this time, because the sound we make will also be mixed with the ambient noise, we cannot simply use cancellation interference to reduce the noise of the microphone. the easiest way is to put the microphone directly to the mouth, so that our sound waves can vibrate the diaphragm as close as possible, so that it is easy to pick up:

However, this shape is not classy enough, and the beautiful solution is to extract acoustic signals in a fixed direction by beamforming an array microphone. This scheme has a long history and is used by almost all microphones. The main differences are the specific hardware performance, microphone layout and algorithms.

The noise reduction of headphones is also a very basic design is airtight, otherwise the ambient noise will enter the ear through the breathable position, the effect will also be discounted. The types that do the best in this are ear-in earplugs, which are as efficient as blocking their ears with their fingers, but the narrower size of earplugs means that the upper limit of noise reduction design is difficult to reach the height of headphones. and many people can’t adapt to ear-in design, even if it’s shallow. The tightness of the head-mounted noise reduction headphones depends on the material of the earmuffs and the toughness of the head girder. in the case of moderate tightness of the headbeams, the wrapping of the earmuffs should be strong enough, which is actually a balanced design of comfort and noise reduction effect. the small toughness of the head beam means poor comfort, but the head Liang Taisong will lead to air leakage, and it is also necessary to consider that mass-produced products need to adapt to all kinds of head shapes, so the design is not small.

Generally speaking, the spatial noise reduction technology of audio playback has achieved a very high level at least in consumer products, and it should also be difficult to have a large-span improvement space in the future. however, with the growth of the demand for far-field voice control in smart homes, microphone pick-up and noise reduction will have a wider space for development, and at this stage, it should be the cooperative processing between devices, such as talking to mobile phones to control household appliances and so on. But in any case, the application of noise reduction technology in the future life scene will still occupy a very important position.