What is active noise cancellation? ANC technical principle analysis

After the launch of Apple Airpod, Amazon Echo and Google Home hardware products, plus the voice software services of Amazon's Alexa and APPLE's Siri, leaving the smart home market, the future voice control portal will have a great imagination. Imagine being controlled by the Airpod, which is always on your ear, to control the company and all the equipment in your home. Is it close to the world of your dreams? However, the further popularization of this problem requires solving a problem on the hardware, that is, noise cancellation when picking up sound.

We believe that companies that research this technology and product will become a new giant in the future.

For noise, everyone must be familiar with it. Anything we don't want to hear can be attributed to noise, such as the sound of the engine when flying, the sound of the wheels when the train is on, the sound of the traffic on the road outside, and so on. When listening to music, everyone does not like to be disturbed by noise. So how do you reduce the noise interference? The traditional method is the closed method, that is, cover your ears with earplugs or fully enclosed headphones, so that no noise can be heard, but it is very expensive to do so, because it is necessary to make the fully enclosed headphones more comfortable. The high cost, the second effect is not good, because there will always be noise leakage as the wearing posture changes. So, is there a better way? Active noise cancellation technology (acTIve noise cancellaTIon, ANC) is a very hot method recently. Let's analyze the technology used in it.

The principle of ANC technology is not esoteric. High school physics has taught it, that is, wave interference. If you still remember the college entrance examination, you must remember that if the two waves have the same amplitude and opposite phases, the result of their interference will cancel each other and you will not see the waves!

After finishing high school physics, let's talk about junior high school physics: in junior high school physics, sound is also a kind of wave that can spread in various media. In other words, sound waves can also interfere. If we take advantage of this feature, we can cancel out some of the sound waves we don't want to hear (or at least reduce the amplitude). Using this, we can use the cancellation method to eliminate the noise.

A common active noise canceling earphone is shown in the figure below. The microphone samples the external noise. After the sampled noise is processed by the active noise canceling circuit, it produces a “reverse” noise with the opposite phase of the sampled noise and the appropriate amplitude. And the speaker is sent into the earphone, so the noise is cancelled in front of the person's ear, and the person with the earphone can only hear the music and can't hear the noise.

If the system is drawn as a signal flow diagram, it is the following figure. The noise enters the signal processing circuit after the microphone is sampled, producing "reverse" noise, which is sent into the earphone together with the music. This mode is called feedforward active noise cancellation.

However, there is a problem with feedforward noise cancellation. Everyone must have tried to blow into the microphone! Blowing the microphone can produce a lot of sound in the place where the microphone is output, but the sound of this blow can't be heard by taking the microphone away. Similarly, in feed-forward active noise cancellation, if there is wind blowing near the sampling microphone, then the microphone may generate a lot of anti-noise, but in fact the sound of this air blow is not heard! In other words, if there is wind blowing at the microphone (wind noise), the microphone will think that the ambient noise is very large, but people can't hear any noise, so the false alarm of the microphone will produce 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, which obviously includes music and ambient noise. So how do you selectively offset the noise but keep the music? It is necessary to subtract the music sampled here from the music, so that the remaining signals are all noise; then the noise cancellation signal is generated based on this noise. In doing so, there will be no wind noise problems. However, there is a new problem here, that is, the feedback system usually has much smaller bandwidth than the feedforward system, so the active noise cancellation using the feedback mode has a limited effect on the cancellation of high frequency noise.

Since the two options have their own advantages and disadvantages, how do you combine them? This is the active noise cancellation in the mixed mode. It uses both feedforward and feedback. If the system is properly designed, it can be used to avoid weaknesses, that is, avoid wind noise and cancel high frequency noise.

With the launch of Apple AirPod, all wireless headsets are getting hotter. Recently, many companies have introduced all-wireless headphones with ANC. The main challenge in doing ANC on all wireless headsets is signal synchronization. As mentioned earlier, both feedback and hybrid ANC schemes require subtracting the sampled signal from the music to produce a noise cancellation signal, which is highly demanding for synchronization between the music and the sampled signal. In wired headsets, this is not difficult, because the music transmission channel is ideal; however, all wireless headsets currently use Bluetooth technology to transmit signals, so the synchronization between the Bluetooth transmission signal and the ANC signal must be carefully designed, and the delay of Bluetooth It has become a key point here. In addition, since the headset itself has a battery in the all-wireless solution, the capacity of the battery may not be large from the perspective of user experience, otherwise the weight of the earphone may exceed the standard, so the power consumption of the ANC circuit must be carefully controlled to ensure good performance and long enough. Battery usage time.