 Hello, and welcome to this short introduction to microphones. In this video, I'm going to cover some general aspects about the principles on which microphones are based, as well as a comparison between several microphone types. Generally speaking, a microphone is a transducer that converts sound pressure into an electrical signal. The main components of a microphone are a capacitor with a fixed plate, the back plate, and a movable one called the membrane. Acoustic waves make the membrane move, or define the distance between the plates, while the total charge is constant. This generates a change in the capacitance, which is reflected to a variation of the voltage measured at the output. This signal represents the output of the microphone, and it's proportional to the sound pressure impinging the membrane. The capacitive principle we just went through is very common throughout microphones types, but a first distinction can be made based on the technology used to build them. We are going to compare the two most known and adopted families of microphones, traditional ECM versus MEMS microphones. ECM microphones are composed of discrete components, and they usually include a variable capacitance, usually based on a permanent electrode, a transistor used to amplify and buffer the variations in capacitor voltage. ECM microphones output an analog signal, which must be further processed and filtered to be used. On the other side, a MEMS microphone is a dual-dye device consisting of two components, the sensor and the integrated circuit. The sensor uses MEMS technology, and it is basically a silicon capacitor. The integrated circuit can perform several operations, depending on the type of the microphone. It always implements a first analog stage to amplify and filter the capacitor output. It can include an analog-to-digital conversion stage. For this reason, depending on the features implemented in the integrated circuit, MEMS microphone output can be either analog or digital, as the microelectronics offers a wide range of MEMS microphones, both analog and digital. For a detailed explanation about MEMS microphones, you can refer to application note 4426, a tutorial for MEMS microphone, or refer to this YouTube video. A different way to categorize microphones is based on the kind of signal they produce as output. The output of an analog microphone is an electric signal whose voltage is proportional to the sound pressure. ECM microphone and analog MEMS microphone both belong to this family, but there are significant differences between them in terms of acoustic performances, side, and power consumption. We will go into those differences in the following slides. On the other hand, the kind of operations they require in order to be integrated in an application are very similar. The signal produced by an analog microphone requires a first step of analog preprocessing, consisting in a set of operations needed to filter and amplify the signal. This is usually implemented with additional discrete components, like operational amplifiers or passives. Big care must be taken in all the phases for possible noise which can be added to the analog path. Work-to-digital conversion may be needed as well if digital signal processing will be implemented in the product. This is performed by a dedicated ADC, which must meet stringent acoustic requirements and be specific for audio applications. At the end of the process, a PCM signal is available for the application purposes. This signal is usually acquired using an I2S or a TDM interface. On the other side, a digital microphone moves the complexity of the analog front end as well as the digital conversion inside the component itself, providing the user with a digital signal which is far more easy to handle and transmit. The output of a digital microphone can be directly acquired by the host. The most common format used in digital microphone is PDM, or Pulse Density Modulation. It consists of a stream of pits in which the relative density of the pulses corresponds to the analog signal sampled. This format is simple to obtain from a Sigma Delta 1-bit ADC and can be easily transmitted using two signals, a clock line and a data line. In order to acquire a PDM stream, the host must have a serial interface and provide a clock to the microphone. An additional stage for PDM to PCM conversion, which can be performed in software or with specific hardware interfaces, is needed to obtain the stream in PCM format. We will refer to this as a PDM microphone, and we will go in details on this format in the following video. Another option which is more rarely used as output in digital microphones is PCM. Pulse Code Modulation is the well-known digital format in which each sample is represented by a number of pits running at a standard audio frequency. This is the standard for non-compressed audio processing. There are a few digital microphones on the market which are able to perform PDM to PCM conversion themselves and provide the user directly with a PCM signal ready to be used. Since PCM streams are usually transmitted using an I2S interface, we will refer to this kind of microphone as I2S microphone. PCM's analog microphones are widely used in a hearable device, like headsets, earbuds, through wireless stereo devices, due to the lower power consumption and the minor latency of a full analog processing, enabling functionalities like active noise cancellation. Also mobile phones were traditionally using analog microphones due to the lower power consumption and the legacy with ECM. Even if you can see a trend for them to switch to digital MEMS now. Acoustic presence detectors, due to their low complexity, often adopt analog microphones and a full discreet implementation. On the other side, digital MEMS microphones are widely used in personal computers, smart speakers and the Internet of Things in general, where PDM interface reduces interferences in case of long cables or electrical noise environments. For the same reason, automotive applications are rapidly adopting digital microphones for their hands-free applications. ST Microelectronics offers a wide range of MEMS microphones. The MP23-APS-1 is an analog bottom-port single-ended microphone. It's the perfect companion of earbuds, active noise-canceling headset or application based on noise-canceling algorithms. MP23-DP-01-HP is a bottom-port digital multi-mode high-performance microphone with PDM interface. The multi-mode operation, leveraging dynamics which between low-power and normal mode, makes this microphone the proper candidate also for low-power applications. MP34-DT05-DT05-A and DT06-J represents this ST offer in terms of top-port digital microphones with the best performances for audio fielding. In this table, we will resume a set of pro and cons for microphone categories. Analog MEMS, PDM MEMS, I2S and ECM microphones. In terms of power consumption, we can see how MEMS, both analog and PDM, are the best performers. In terms of dimensions, ECM microphones are much bigger than MEMS, and they are not easily integrated in hearable or mobile devices. About signal management, digital microphones are much more easy to handle, and they don't need analog front-end or specific care in the signal management. In terms of pin-out, I2S has the bigger number of signals to be managed and routed, resulting in a more complex layout. About acoustic performances, analog and PDM MEMS microphones offer the best and more advanced audio result. Given the advantages offered by PDM microphones in terms of acoustic performances, ease of adoption, low-power consumption, digital output, and digital interface, from now on, we are going to focus on the adoption of PDM digital microphones. You can refer to those links to have an idea of the ST microelectronics offer in terms of MEMS microphones.