Demystifying why Xiaomi MIX tried novel acoustic technology?

The Xiaomi MIX concept machine brought you a surprise. This phone has tried a number of novel acoustic technologies, including piezoelectric ceramic vibration systems, ultrasonic fingerprint recognition and ultrasonic distance sensing. Xiaomi's road to innovation is still quite difficult. These technologies also need to be briefed on science, and also correct some media misunderstandings. The piezoelectric ceramic vibration system is not the bone conduction technology mentioned by the media.

Why does Xiaomi try these acoustic techniques that are not well proven?

I don't think Xiaomi is willing to take the initiative, but conceptual design has to require the latest technology to solve the problem. The design of the Xiaomi MIX borderless screen is not the first. In fact, each family is trying to create this concept, but the most amazing thing about MIX is the removal of the top border. This kind of design change may seem like a consumer, and there are still many people who have a bottom border, so that people with obsessive-compulsive disorder feel asymmetrical, but relatively speaking, the industry insiders recognize the design and crafts of Xiaomi MIX. It is still very difficult, and it does require a lot of innovation.

We can take a look at the mobile phone around us. At least there will be 3 devices on the top. The front camera, infrared distance sensor and micro speaker are academically called Receiver. The media is often called "handset", and these three Devices are indispensable, and high-end phones will have a more microphone to reduce noise when noisy the environment. The front camera is convenient for taking pictures, and the infrared distance sensor and the receiver mainly ensure the call and the screen is cut off. The positions of the two devices are not easy to place. In order to cancel the top border, Xiaomi had to do some technical attempts.

Demystifying why Xiaomi MIX tried novel acoustic technology?

Xiaomi MIX first put the front camera from the top to the bottom, so the bottom border can not be canceled temporarily, this design will make the self-timer look troublesome, obviously Xiaomi compromised for "stunning". However, the receiver is actually a bigger problem. The replacement position of this device will affect the most important call function of the mobile phone, and the receiver of the mobile phone mainly generates sound by diaphragm vibration, which requires a long hole in the screen of the mobile phone, but no top border. How to do?

So Xiaomi used a piezoelectric ceramic vibration solution, which is not new, because Sharp's 2011 feature machine uses this technology, including Sharp's smart machine.

This method is the same as the sounding principle of the receiver. It is generated by converting electrical signals into mechanical vibrations. The difference between the two is only the difference of the vibration materials.

The examples of sounding through piezoelectric ceramics can be seen in comparison. Many resonant speakers on the Internet are similar. The vibration of piezoelectric ceramics can drive the glass and even the desktop vibration to produce sound. The air humidifier commonly used in our home, the core device is a piezoelectric ceramic piece. This 1~3MHz piezoelectric ceramic piece can vibrate and emit water mist. Of course, the sound of this frequency can not be heard. At 20 kHz, and as the age continues to decrease, the acoustic online ear test function can be used to measure hearing. This function of piezoelectric ceramics is mainly due to the piezoelectric effect of piezoelectric ceramics, which was discovered by the famous Curie couple in 1880.

Generally speaking, the piezoelectric effect is a polarization phenomenon generated internally when a certain material is deformed by an external force in a certain direction.

The piezoelectric effect has positive and inverse effects. In simple terms, positive effects can generate electricity, and inverse effects can use electricity to generate deformation. When the voltage is applied to the piezoelectric material by the inverse effect, it is deformed. If a controllable alternating current is applied, the deformation we need can be generated, and the deformation can generate vibration to emit sound. In fact, many materials have a piezoelectric effect, but piezoelectric ceramics are relatively cheap and stable.

Demystifying why Xiaomi MIX tried novel acoustic technology?

From the above analysis, it can be understood that this technology is to replace a diaphragm material, originally a small receiver, and now the entire ceramic body or screen can vibrate to produce sound. But don't mistake this sound quality better. The piezoelectric ceramic acoustic system has a very low frequency, and the nonlinear effect is strong when the body vibration is driven, so the sound quality is actually lower than the receiver. Of course, the receiver is not so good, basically can only produce more than 400Hz sound, but compared to the low bit rate codec system of mobile calls, it is also a sloppy. Xiaomi tried this kind of acoustic technology very boldly, not only increased the difficulty of ceramic processing (Yesterday and Feng Rui Capital's friends talked about piezoelectric ceramics is too difficult to cut), basically gave up the call quality pursuit, Xiaomi had to do this Compromise again.

To clarify here, this technology is not what the media calls "bone conduction" technology.

We know that the human ear hears the sound mainly through two ways, one is air conduction, and the other is bone conduction. As the name suggests, air conduction is the way in which sound waves enter the external auditory canal through the air to cause the tympanic membrane to vibrate and then transmit to the inner ear. Bone conduction is the way to transmit sound waves through the human skull, inner ear lymph, spiral, auditory nerve, auditory center, etc. Because of the bone conduction effect, our ears hear that the sound they make is not the same as the sound heard by others. Try comparing the sound recorded by the computer with the sound you hear yourself. The advantage of bone conduction sound is the privacy and anti-interference of the sound. There are some bone conduction headphones on the market. Google Glass 1 has also used bone conduction technology, but from the sound quality, the current bone conduction technology Still can't compare with traditional sound technology. Obviously, Xiaome's piezoelectric ceramic acoustic technology has nothing to do with bone conduction. Of course, when it is close to the face, it can be considered that some vibration is transmitted to the inner ear through the bone conduction.

Demystifying why Xiaomi MIX tried novel acoustic technology?

Let's talk about the ultrasonic distance sensor used by Xiaomi MIX. This technology will be understood by everyone. The reversing radar in our car is actually ultrasonic distance sensing, but not real radar. This technology is not too difficult, mainly to replace the infrared distance sensor. The inconvenience of the infrared distance sensor is that it cannot be blocked. How to open a small hole (free transparent glass is not needed, mainly light transmission), so it cannot be placed under the screen, but it is obviously unreasonable to put the bottom frame, so it is lost. The requirement to turn off the screen. Ultrasonic distance sensing just meets the needs of Xiaomi, although the relative infrared is still a little worse, but the demand for mobile phones is still not much problem.

Demystifying why Xiaomi MIX tried novel acoustic technology?

Finally, under the ultrasonic fingerprint sensing technology, Xiaomi MIX finally no longer needs to open the hole for fingerprint recognition, and even can directly unlock the fingerprint on the screen of the mobile phone. But unfortunately, Xiaomi MIX put the ultrasonic fingerprint recognition sensor on the back of the phone, which weakens the cool function of some ultrasonic fingerprint recognition.

Of course, Xiaomi MIX still allows users to experience the convenience of ultrasonic fingerprint recognition. We know that traditional capacitive scanning fingerprints can only draw 2D information of the finger plane through contact. Therefore, when there are water, sweat, stains, oil stains, hand cream, etc. on the finger, the recognition accuracy is greatly reduced, and it is difficult for the user to unlock the mobile phone. In an emergency, you have to wipe your fingers and try to unlock them, which is especially bad for women. The biggest feature of the ultrasonic fingerprint recognition technology is that it is not affected by water, grease, hand cream, etc. It can be hidden under glass and plastic, that is, the user only needs to put his hand on the screen of the mobile phone to unlock. In addition, the technology also supports glove touch operation, user-defined touch gestures, multi-identity biometric recognition, and quick activation of split screens.

Demystifying why Xiaomi MIX tried novel acoustic technology?

In addition, Xiaomi MIX still retains the 3.5MM headphone jack, which is not consistent with Apple's strategy. In the end, the design of Xiaomi MIX is still in line with the current demand for dazzling screens in China, and looks forward to the better performance of Xiaomi.
The author of this article is Chen Xiaoliang, Ph.D. in Engineering, and founder of Sonzhi Technology.

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