Principle analysis of active noise cancellation AN

2022-07-28
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What is active noise cancellation? Analysis of ANC technology principle

after the introduction of Apple's airport, Amazon echo and Google home, and the popularity of voice software services such as Amazon's Alexa and Apple's S4 and some thoughts IRI, it is left to the smart home market that the future voice control entrance will have a great imagination. Imagine if you are close to your dream world by controlling the company and all the devices in your home through the airpod that you always carry in your ear. However, with the further popularization of this, we need to solve a problem in hardware, that is, to eliminate the noise when picking up

we believe that the company researching this technology and product will become a new giant in the future

what is active noise cancellation

everyone must be familiar with noise. All the sounds we don't want to hear can be attributed to noise, such as the engine sound when taking a plane, the wheel sound when taking a train, the traffic noise on the road outside, and so on. When listening to music, everyone doesn't like to be disturbed by noise. So, how to reduce the noise interference to us? The traditional method is the closed method, that is, the maximum stress value before the earplug or steel is pulled and broken is called the strength limit or tensile strength. The fully closed earphone is used to cover your ear, so you can't hear the noise. But it's expensive to do this first, because it needs a high cost to make the fully closed earphone comfortable, and the second effect is not good, because the noise will always leak in with the change of wearing posture. So, is there a better way? Active noise cancellation (ANC) is a very popular method recently. The following is an analysis of the technologies used

the principle of ANC technology is not profound. It was taught in high school physics, that is, wave interference. If you still remember the review of the college entrance examination, you must remember that if the amplitude of the two columns is the same and the phase is opposite, the result of their interference is that they cancel each other out and you can't see the wave

after high school physics, let's talk about junior high school physics: it is mentioned in junior high school physics that sound is also a kind of wave, which can spread in various media. In other words, sound waves can also interfere. If we make good use of this feature, we can offset some unwanted sound waves with interference (or at least reduce the amplitude). Using this, we can use the offset method to eliminate the noise

the common active noise cancellation earphones are shown in the figure below. The microphone samples the external noise. After the sampled noise is processed by the active noise cancellation circuit, an "anti" noise with appropriate amplitude and opposite phase to the sampled noise is generated and sent to the loudspeaker of the earphone, so the noise is cancelled in front of the human ear. People with earphones can only hear music, but can not hear the noise

if the system is drawn as a signal flow diagram, it is shown in the figure below. After the microphone samples, the noise enters the signal processing circuit to generate "anti" noise, which is sent into the earphone together with the music. This mode is called feedforward active noise cancellation and then measuring the feedback value of the sensor

however, there is a problem with feedforward noise cancellation. You must have tried blowing into the microphone! Blowing into the microphone can produce a loud sound at the output of the microphone, but you can't hear the sound of blowing away the microphone. Similarly, in the feed-forward active noise cancellation, if there is wind blowing near the sampling microphone, the microphone may produce great anti noise, but in fact, the sound of the wind blowing cannot be heard! In other words, if there is wind blowing (wind noise) at the microphone, the microphone will think that the ambient noise is very loud, but people can not hear any noise. In this way, the false alarm of the microphone will generate more noise

to solve this problem, feedback active noise cancellation can be used, as shown in the following figure

in the feedback active noise cancellation scheme, the microphone samples not the direct ambient noise, but the sound at the headphone output. Obviously, the sound here includes music and ambient noise. So how do you selectively cancel the noise but keep the music? For the n-constant, it is necessary to subtract the music from the sound sampled here, so that the remaining signals are all noise; Then the noise cancellation signal is generated according to the noise. In this way, there will be no wind noise problem. However, there is a new problem here, that is, the bandwidth of the feedback system is usually much smaller than that of the feedforward system, so the active noise cancellation using the feedback mode has limited effect on the cancellation of high-frequency noise

since the two schemes have their own advantages and disadvantages, how about combining them? This is the active noise elimination of the mixed mode, which uses both feedforward and feedback. If the system is properly designed, it can develop its strengths and avoid its weaknesses, that is, it can avoid wind noise and offset high-frequency noise

Bluetooth headsets and anc

with the launch of Apple's airpod, all wireless headsets are becoming more and more popular. Recently, many companies have launched all wireless earphones with ANC. The main challenge of ANC on all wireless headphones is signal synchronization. As mentioned earlier, both feedback and hybrid ANC schemes need to subtract music from the sampled signal to generate noise cancellation signal, which requires high synchronization between music and sampled signal. In wired earphones, this is not difficult, because the music transmission channel is very ideal; However, at present, all wireless headphones mostly use Bluetooth technology to transmit signals, so the synchronization between Bluetooth transmission signals and ANC signals must be carefully designed, and the delay of Bluetooth has become the key point here. In addition, since the earphone in the all wireless scheme has its own battery, the battery capacity cannot be very large from the perspective of user experience, otherwise the weight of the earphone will exceed the standard. Therefore, the power consumption of the ANC circuit must be carefully controlled to ensure good performance and long enough battery life

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