MEMS Microphones
Mirko Vojnovic
Innovative Technical Program/Project Manager | Expert in Analog, Digital, and Mixed Hardware Systems | 21 Patents Holder
Author: Mirko D. Vojnovic
Credits: CUI Devices, Infineon
ABSTRACT:
MEMS Microphones
MEMS stands for Micro Electro Mechanical Systems, and MEMS microphones are a type of microphone that utilizes a tiny MEMS sensor to convert sound waves into electrical signals. These microphones are known for their small size, low power consumption, and high-quality audio capture.
MEMS microphones are widely used in various applications such as smartphones, laptops, tablets, automotive, wearables, smart home devices, and even industrial applications where precise audio sensing is required.
This article will concentrate on MEMS microphones used in the automotive industry and wearables and briefly mention their use in other applications. Later, we will explore MEMS microphone design, types, and practical design applications.
MEMS Microphones in Automotive Industry:
With their high sensitivity and ability to function in diverse environments, MEMS microphones significantly improve safety, convenience, and overall user experience in modern vehicles.
Here's an overview of their typical usage:
1.?????In-Car Communication:??
2.???? Active Noise Cancellation (ANC):
3.???? Voice Recognition and Control:
4.?????Parking Assistance:
5.?????In-Car Entertainment:
6.?????Emergency Call Systems:
MEMS Microphones in Wearable Devices
MEMS microphones have become integral components in wearable devices, enhancing their functionality and enabling various applications. They enable wearables to capture audio and enhance user experiences through features like voice control, noise cancellation, and health monitoring. As technology continues to evolve, we can expect even more innovative uses of MEMS microphones in wearables in the future.
?Here's a more detailed look at how MEMS microphones are used in wearables, along with some examples:
1.?????Voice Control:
2.?????Noise Cancellation:
3.?????Health Monitoring:
4.?????Communication:
5.?????Environmental Noise Monitoring:
6.?????Entertainment and Gaming:
Other Applications
MEMS microphones find applications in various fields beyond wearables and automotive technologies. Here are some additional uses of MEMS microphones:
Smart Home Devices:
MEMS microphones are integrated into smart speakers, virtual assistants, and home automation systems. They enable voice commands, allowing users to control lights, thermostats, and other smart devices with spoken instructions.
Security Systems:
MEMS microphones are used in security cameras and surveillance systems. They capture audio to enhance video footage, providing valuable context for security personnel. Advanced models can detect specific sounds, such as breaking glass or alarms, triggering appropriate responses.
Medical Devices:
In medical applications, MEMS microphones are utilized in devices like hearing aids and assistive listening devices. They improve sound clarity, enhancing the hearing experience for individuals with hearing impairments.
Industrial Automation:
MEMS microphones are used in industrial settings to monitor equipment and detect abnormalities. They can detect unusual sounds or vibrations in machines, indicating potential issues that require maintenance or repair.
Consumer Electronics:
MEMS microphones are integral components of consumer electronics, such as digital cameras and camcorders. They capture high-quality audio during video recording, ensuring the recorded content is immersive and engaging.
Gaming Accessories:
MEMS microphones are used in gaming peripherals like headsets and microphones. They capture clear voice communications during online gaming, enabling players to strategize and coordinate effectively.
Educational Tools:
MEMS microphones are incorporated into educational devices and language-learning tools. They facilitate interactive learning experiences, allowing students to engage in language practice and pronunciation exercises.
IoT Devices:
MEMS microphones are used in various connected devices in the Internet of Things (IoT) ecosystem. For example, they can be integrated into smart appliances to receive voice commands, making these appliances part of the smart home network.
Navigation Systems:
MEMS microphones are utilized in navigation devices for capturing spoken instructions. They enhance the user experience by providing voice-guided directions, mainly when visual guidance is limited or unsafe, such as while driving.
The versatility of MEMS microphones continues to drive innovation across diverse industries, enhancing functionality and enabling new possibilities in the realm of technology.
MEMS Microphone Basics
MEMS microphones are constructed with a MEMS (Micro-Electro-Mechanical System) component placed on a printed circuit board (PCB) and protected with a mechanical cover. A small hole is fabricated in the case to allow sound into the microphone. It is either designated as top-ported if the hole is in the top cover or bottom-ported if the hole is in the PCB. The MEMS component is often designed with a mechanical diaphragm and backplate, created on a semiconductor die.
The MEMS diaphragm is placed very close to a backplate to form a capacitor. When sound waves hit the diaphragm, it moves, thereby changing the capacitance between the diaphragm and the backplate. This change in capacitance is then converted into an electrical signal, representing the sound the microphone is capturing.
MEMS microphones typically contain a second semiconductor die, which functions as an audio preamplifier, converting the changing capacitance of the MEMS to an electrical signal.
The output of the audio preamplifier is provided to the user if an analog output signal is desired. If a digital output signal is desired, then an analog-to-digital converter (ADC) is included on the same die as the audio preamplifier.
A common format used for digital encoding in MEMS microphones is pulse density modulation (PDM), which allows communication with only a clock and a single data line. Decoding of the digital signal at the receiver is simplified due to the single-bit encoding of the data.
Digital I2S outputs are a third option that includes an internal decimation filter, allowing for processing to be completed in the microphone. That means the microphone can connect directly to a digital signal processor (DSP) or microcontroller, eliminating the need for an ADC or codec in many applications.
Types of MEMS Microphones
The internal design of MEMS microphones determines whether they have analog or digital output interfaces. The digital output ones can be further divided into Pulse Density Modulation (PDM) or Inter-IC Sound (I2S) types.
The decision to employ MEMS microphones with analog or digital output signals often depends upon how the output signal will be used.
An analog output signal is convenient if it is connected to the input of an amplifier for analog processing within the host system. Examples of conventional analog applications are a simple loudspeaker or radio communication system. MEMS microphones with analog outputs also tend to have lower power consumption than those with digital outputs due to the absence of the ADC.
A digital output signal from a MEMS microphone is advantageous when the signal will be applied to digital circuitry, typically a microcontroller or digital signal processor (DSP). The digital output signal can also be beneficial if the conductors between the microphone and the host circuit are in an electrically noisy environment. The digital output signals will exhibit better electrical noise immunity than traditional analog signals.
The paragraphs below will explain each type in more detail and discuss its connection to the rest of the circuitry.
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Analog MEMS Microphone Interfaces
MEMS microphones with analog outputs allow for a straightforward interface to the host circuit, as shown in the figures below. It should be noted that the microphone's analog output signal is driven by an amplifier internal to the microphone. Therefore, it is already at a reasonable signal level with a low output impedance.
The DC blocking capacitor (C1) is employed so the DC input voltage of the host circuit does not need to be matched to the DC output voltage of the MEMS microphone. The pole frequency created by the combination of C1 and R1 forms a High-Pass filter. It must be set low enough to pass the desired audio frequency signals to the host circuit with an acceptable attenuation level.
For example, for the audio frequency range above 20 Hz, we set the filter’s 3dB point such that f3dB = 1/(2*π*R1*C1) < 20 Hz.
1.?????? MEMS microphone connection to an external inverting Op-Amp with dual (+Vcc, -Vcc) power supply is shown below.
2.?????? MEMS microphone connection to an external inverting Op-Amp with a single (+Vcc) power supply is shown below.
Digital MEMS Microphones
Two popular output options are the digital protocols of Pulse Density Modulation (PDM) and Inter-IC Sound interface (I2S).
These interfaces each possess their own distinct characteristics, and both have advantages and drawbacks that engineers need to understand for proper design implementation. As with all engineering decisions, choosing between these two protocols will require a designer to examine the technologies and understand which protocol works better under the conditions of each application. The key considerations that will need to be taken into account when comparing the two technologies are the following:
PDM's good noise immunity and bit error tolerance can make this appealing for many applications where audio quality is a priority.
In contrast, the ease of installation, reduced overall footprint, and lower component count that I2S enables will have merits in situations where the product size or price tag is among the primary concerns. The I2S interface will deliver better signal integrity over longer distances, so it can also be applicable for implementations where the microphone and the processing circuitry cannot be close to one another on the board.
However, this cannot be taken to an extreme as I2S was not designed specifically for transmission over cables or other transmission devices, and many devices will need proper impedance matching. Further research into the available parts, demands of the application, and data rates expected will be required on a case-by-case basis.
Pulse Density Modulation (PDM) Microphones
The output signals from MEMS microphones with a digital interface are often encoded with pulse density modulation (PDM). With PDM, the analog signal voltage is converted into a single-bit digital stream containing a corresponding density of logic-high signals. Some of the advantages of PDM include electrical noise immunity, bit error tolerance, and a simple hardware interface.
PDM signals look more like longitudinal wave than the stereotypical transverse wave associated with audio, but they are a digital representation of an analog signal.
As shown in the figure above, the density of the high bits increases as the analog signal amplitude increases, and correspondingly, the digital signal stays at its low value for greater lengths of time when representing the lower end of the analog signal amplitude. That produces a signal that yields many of the benefits of a digital signal while still being directly correlated with the analog signal. Creating this PDM signal requires higher than usual sampling rates (rates above 3 MHz), as the digital pulses must occur many times more often than the oscillation of the analog signal they represent.
Due to the digital nature of the signal, PDM is much more resilient to electrically noisy environments than an analog signal and has an increased bit error tolerance when there is signal degradation. The high-frequency signal does create distance limitations as increased capacitance on longer transmission lines may cause unwanted attenuation and an accompanying degradation in audio quality. These signals also require further processing by an external DSP or microcontroller with an appropriate codec to decimate or downsample the PDM signal to a lower sample rate. Its simple underlying concept means that PDM devices only require two signals, tend to be less expensive, have smaller footprints, and use less power. However, the additional circuitry necessary to process the signal coming from the PDM device may offset these advantages.
PDM Microphone Interface Connections:
The figure below shows how a single digital microphone with PDM output can be connected to a host circuit. Connecting the "Select" pin to either Vdd or Gnd in the figure will determine if the data is asserted on the rising or falling edge of the clock signal.
Note: Consult the data-sheet to see how this selection works for your particular device.
The diagram below shows how two microphones can be connected to the host circuit using a shared clock and shared data line. This configuration is often employed when implementing stereo microphones.
Note: One microphone asserts data on the CLK rising edge, while the other asserts data on the CLK falling edge.
Inter-IC Sound (I2S) Microphones
Originally emerging back in the mid-1980s, I2S is another popular interface option, though it is only recently becoming more integrated into smaller devices such as microphones. While the name invokes a relationship with I2C protocols, the similarity is purely coincidental. As with PDM, it is a dual-channel interface, but that is where the similarity ends.
The I2S protocol is a three-wire serial protocol with a clock, data, and “word select” line. In this case, “word select” indicates the channel, right or left, with which data currently being transmitted is associated. Unlike PDM, I2S is an entirely digital signal that does not need to be encoded or decoded. There is no universally required data transmission speed, however, the minimum speed is dependent on the data being transmitted and its precision. If the audio sample rate is the industry standard of 44.1 kHz with 16 bits of precision, then a stereo channel will need a clock speed of 1.411MHz. Any change in precision will also change the minimum transmission bandwidth.
The formula is:?
Sample Frequency * Data Precision * Channel Number = Bandwidth
In the example above:??
44,100 Hz * 16 bits * 2 channels = 1.411 MHz
While PDM mandates an external codec to bring its sample rate down, I2S utilizes an internal codec through its built-in filter. Therefore, the data rate of the audio signal is already at an acceptable level when it arrives at the DSP.
This eliminates the need for additional components within the design for processing or conditioning the captured audio data.
Based on this, I2S is likely to be the best path to follow in relation to price-sensitive products that are wholly self-contained and where energy-efficient battery-powered operation is a prerequisite. This will be equally valid if the product they are being integrated into needs to be as compact as possible (wearables, handheld equipment, etc.).
The ease of installation, reduced overall footprint, and lower component count that I2S enables will have merits in situations where the product size or its price tag proves to be among the main concerns. It should also be noted that the I2S interface will deliver better signal integrity over longer distances, so it can also be applicable for implementations where the microphone and the processing circuitry cannot be close to one another on the board.
Lastly, when comparing the resource requirements of PDM and I2S, it is important to note if there are already DSP capabilities integrated into the design. If so, the three signal lines and larger power consumption of an I2S device may be more resource-intensive than a PDM device that can utilize the DSP capabilities already available on the board.?
I2S Microphone Interface Connections:
A typical I2S MEMS microphone has the following pins:
VDD - voltage supply
GND – ground pin
BCLK - the bit clock, also known as the data clock or just 'clock.'
DATA - the data output from the microphone.
SEL – selects whether data will be output as a left or right channel. By default, this pin is low, so it will transmit on the left channel mono when the WS signal is low. If you connect this pin to high, the microphone will transmit on the right channel when the WS signal is high. When using two microphones, by connecting the SEL pin on one of them to low and the other to high, we decide which microphone will be left and which will be right.
WS - the left/right clock, also known as word select, tells the microphone when to start transmitting. When the WS is low, the left channel microphone, whose SEL pin is set to low, will transmit. When the WS is high, the right channel microphone, whose SEL pin is set to high, will transmit.
The figure below shows how a single digital microphone with I2S output can be connected to a host circuit. The SEL pin is connected to the ground, so it will be configured as mono, outputting data when WS is set to low (LEFT Channel).
The figure below shows how two digital microphones with I2S output can be connected to a host circuit. The microphone with SEL set to low will be the LEFT channel, and the microphone whose SEL pin is set to Vdd will be the RIGHT channel.
MEMS Microphone Arrays and Sound Directionality
Due to their design, MEMS microphones are omnidirectional, which means they pick up sounds equally from any direction. However, many microphone applications have a specific source for the desired sound. Therefore, particular designs require "listening" to sounds from that direction and "ignoring" sounds from other directions.
Microphone arrays can be designed to create a directional response, also known as beam-forming, that filters out unwanted noise and processes sound from a more desired direction(s).
For the signal processing to be effective, the microphones employed in the array must have tightly matched performance specifications. The sensitivity of the microphones is the primary parameter that is required to be well-matched in the array. Due to semiconductor manufacturing processes, MEMS microphones are readily available with tightly matched sensitivity tolerances, making them the ideal choice for microphone arrays.
Here, we will cover the basics of MEMS microphone arrays, including their working principles, common configurations, and typical applications.
Broadside Microphone Arrays
Broadside Microphone Arrays are one or two-dimensional arrays of microphones placed perpendicular to the source of the desired sound, with the signals from each microphone summed to produce the desired electrical signal. Sounds originating from a direction perpendicular to the array will arrive at the microphones at the same time and thus will add constructively, producing a higher signal level. Sounds originating from a direction other than perpendicular to the array will arrive at the microphones with differing time delays. When summed, the signals with differing time delays will produce a lower-level signal.
An audio interface to a computer monitor or TV screen is a good application of a Broadside Microphone Array. The array would be constructed in the same plane as the display since the user is located directly in front of the screen.
Endfire Microphone Arrays
An Endfire Microphone Array is constructed by arranging a line of microphones in the direction of the desired sound source, where the desired sound arrives at each microphone with a different time delay. The processing circuit for each microphone can employ an electronic time delay to compensate for the audio time delay of the microphones. Endfire Microphone Arrays are similar to broadside microphone arrays in that the signals from the desired direction sum in a constructive manner, but signals from directions other than the desired direction sum to a lower value.
While both Broadside and Endfire Microphone Arrays enhance sound capture along the desired axis and attenuate other noise sources, the Broadside Array captures sound equally well both in front of and behind the microphone array. An Endfire Array will only capture sound in front of the array. It will attenuate noise from behind the array as well as all other directions. It is also required to be physically oriented towards the sound of interest. A handheld microphone is a good application for this topology where the device can be pointed directly at the person who is talking (or singing) and capture only that signal.
Additional MEMS Microphone Array Applications
MEMS microphone arrays can also be employed to determine the direction of a sound relative to the array. In one typical implementation of this application, microphones are placed on the perimeter of a circle or a sphere. Electronic signal processing is used to identify the desired signal from each microphone, while the relative time delay of the desired signal between each microphone is used to determine the source of the sound relative to the microphone array.
A typical application for sound location microphone arrays is gunshot detection for the police and military. The digital signal processing (DSP) circuits associated with the microphone array can differentiate the characteristics of a gunshot from other noises and then determine the direction from which the gun was fired.
CONCLUSION:
MEMS microphones, characterized by their miniature size and exceptional sensitivity, are at the forefront of technological innovation. These micro-electro-mechanical systems find versatile applications, transforming industries such as automotive, wearables, and smart homes. In vehicles, they enhance communication and safety, while in wearables, they enable hands-free convenience. Furthermore, MEMS microphones contribute to medical devices, smartphones, computers, security systems, industrial automation, and gaming peripherals, redefining user experiences and driving advancements in various domains. Their adaptability and precision continue to shape the future of interactive technology.