More and more mobile devices, such as mobile phones, headphones, cameras and MP3s, use advanced noise reduction technology to eliminate background sounds and improve sound quality. For this advanced technology to be implemented, the multiple microphones used must not only have the ability to resist radio frequency and electromagnetic interference, but also have a very small size. The MEMS microphone produced by EPCOS is an ideal solution to meet this requirement. As the smallest microphone on the market, MEMS microphones not only provide excellent audio quality, but also have excellent resistance to electromagnetic interference.
Excellent performance and small footprint
Thanks to Technitrol's MEMS (Micro Electro Mechanical System) microphone technology, a series of highly innovative MEMS products have been launched in the silicon microphone market. The first batch of products to be marketed is the EPCOS T4000 and T4010 series. On them, the design capabilities of advanced MEMS and ASIC technology, in-depth understanding of uses and various patents are gathered.
Now, a digital output microphone, T4030, has been added to the product line. The new product is small in size, with a sensitivity of -26dB FS (full range) and a signal-to-noise ratio of 60dBA. Even if the sound level is 100dB, the distortion is less than 1%. Its frequency response is characterized by high bandwidth and low amplitude fluctuations (see Figure 1). Due to the use of PDM (Pulse Density Modulation) output, the T4030 basically has no response to electromagnetic interference. Power supply noise suppression is -82dB FS. The two-channel transmission can be carried out through a signal line, which makes stereo or multi-microphone applications easier than using analog technology. The power supply voltage is between 1.64V and 2.86V. The power consumption is 650μA, which is less than 10μA in standby mode.
Figure 1: Frequency response curve.
Compact product structure
T4030 has excellent performance and extremely small size. It is most suitable for noise reduction in mobile devices, because in these devices, space occupation is often the first consideration. A good example of this is the Jabra STONE Bluetooth headset developed by GN Netcom. Great Northern Europe is a Danish headset manufacturer whose products are used in mobile phones, contact centers and offices (see Figure 2). The Jabra STONE is an extremely compact earphone, which is characterized by finding the best balance between noise removal and natural sound, providing perfect sound quality. To achieve this effect, only dual microphones are used to capture sound, and only background noise is intelligently filtered. It is for this purpose that manufacturers choose the T4000 series with small size and excellent performance.
Great Northern Europe believes that Jabra STONE is a revolutionary product that will surely subvert consumers' inherent perception of Bluetooth headsets. "It has a unique shape, advanced noise reduction technology, and does not require a cross arm. It is a brand-new product concept," said Anna-R-Rasmuser, vice president of the mobile communication department of Northern Europe.
Figure 2: The Jabra STONE Bluetooth headset produced in Greater Northern Europe.
Advanced noise reduction technology
In mid- to high-end mobile devices, noise cancellation is increasingly becoming a standard requirement. In the receiving path and transmission path, the two noise reduction functions can be combined (see Figure 3). As shown in the figure below, in the receiving path (blue), the microphone 1 is located next to the speaker and receives noise. This noise signal is added to the incoming sound after being inverted. The inverted noise signal cancels out from the surrounding noise signal. In this way, users can understand each other's voice more clearly, even in a noisy environment.
Figure 3: Eliminate noise in the receiving and transmission paths.
In the transmission path (yellow), the microphone 2 faces away from the mouth and receives ambient noise. The microphone 3 is facing the mouth, and it simultaneously receives the surrounding noise and the speaker's voice. Here, by adding the inverted noise signal, the noise is eliminated. At this time, the ambient noise interference from the other party will be reduced.
Using more than three microphones, you can design a more powerful and complex noise reduction system. As more microphones are implanted in small mobile devices, the size of the product has become increasingly important. Figure 4 shows two small T4000 microphones mounted on a circuit board. At the same time, users have higher expectations for sound quality. In the future, the signal-to-noise ratio (SNR) of the microphone will reach 62dB, 55-59dB higher than today's average level. Today, EPCOS T4020 MEMS has excellent performance, the signal-to-noise ratio has reached 62dB, and products with higher signal-to-noise ratio are under development.
Figure 4: The circuit board of the STONE headset with two MEMS microphones mounted on it.
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