Microphones are essential components in audio recording and communication systems. They can be categorized into two main types: analog microphones and digital microphones. Understanding their differences helps in choosing the right type for specific applications.
Analog Microphones
Analog microphones typically consist of a diaphragm, backplate, JFET (junction field-effect transistor), and a shielding case. The diaphragm is a thin metal-coated film, while the backplate is made of electret material, which retains a permanent charge after high-voltage polarization.
When sound waves cause the diaphragm to vibrate, the distance between the diaphragm and the backplate changes, generating voltage variations that translate into an audio signal. A JFET is used for impedance conversion and signal amplification, and in some high-sensitivity microphones, an operational amplifier is used to further enhance sensitivity.
Digital Microphones
Digital microphones include a diaphragm, backplate, a digital microphone chip, and a shielding case. The digital microphone chip comprises several key components:
- Buffer stage: Performs impedance conversion.
- Amplifier stage: Amplifies the signal.
- Low-pass filter: Removes high-frequency signals to prevent aliasing.
- Analog-to-digital converter (ADC): Converts the amplified analog signal into a Pulse Density Modulation (PDM) signal.
Most digital microphones use an oversampled 1-bit delta-sigma ADC.
Silicon MEMS Microphones
Silicon MEMS microphones can be either analog or digital. They consist of a semiconductor-manufactured diaphragm, backplate, and supporting structure. A charge pump applies the necessary polarization voltage to the backplate.
To improve noise immunity, small filter capacitors are added between the microphone's power and ground connections.
Comparison of Microphone Types
The following factors distinguish different types of microphones:
- Analog MEMS microphones: Require biasing circuits and typically use differential output to minimize interference.
- Digital MEMS microphones: Have integrated ADCs and provide direct digital output.
PDM Signals and Digital Microphone Interface
PDM signals are converted into PCM signals for processing. According to the Nyquist theorem, the sampling rate must be at least twice the highest frequency of the analog signal. Higher bit depth improves accuracy but increases circuit complexity and cost.
Most digital microphones utilize a 1-bit delta-sigma ADC, oversampling the analog signal. Oversampling shifts quantization noise away from the desired audio frequency, enabling high precision with simpler anti-aliasing filters.
Digital Microphone Pin Configuration
Typical digital microphones have five pins:
- VDD (Power supply)
- GND (Ground)
- CLK (Clock)
- DAT (Data)
- L/R (Channel selection for stereo applications)
Digital microphones require an external clock signal (1.024–3.074 MHz). Upon receiving the clock signal, they switch from power-saving mode to active mode. The PDM signal is then processed by a decimation filter to produce a lower-frequency, higher-bit-depth PCM signal.
Both analog and digital microphones have their unique strengths. Analog microphones are widely used due to their simplicity, while digital microphones offer advantages in noise immunity and integration with digital systems. The choice depends on the specific application requirements.
