The Tool Desk
Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Pulse density modulation (PDM) represents a signal with a stream of one-bit pulses: the proportion of high bits over time corresponds to the signal’s amplitude. In a common audio use, a digital microphone sends that fast stream to a processor, which filters and decimates it into pulse-code modulation (PCM) samples for ordinary audio processing.
How pulse density modulation represents a signal
In PDM, each bit has one of two levels, but an individual bit is not an amplitude reading. The information is carried by the relative density of high pulses across a period: a higher density represents a higher signal level, and a lower density represents a lower one. Infineon’s PDM_PCM_v2 technical reference describes this distinction from PCM, where sampled amplitude values are quantized into multibit numbers.
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PDM and PCM: different representations
| Aspect | PDM | PCM |
|---|---|---|
| How amplitude is represented | Density of one-bit pulses over time | Quantized multibit sample values |
| Typical audio data pattern | High-rate bitstream, commonly oversampled | Sample values at the selected sample rate |
| What is needed for a conventional PCM workflow | Filtering and decimation to produce PCM samples | Already represented as sample values |
| Key design considerations | PDM clock, channel timing, decimator and filter | Sample rate, sample width and audio data format |
This is a comparison of how the formats encode and deliver audio, not a claim that one is universally better. PDM’s fast bitstream suits a digital-microphone interface; PCM is the multibit sample format commonly used by audio-processing software and systems.
How a PDM microphone signal becomes PCM audio
- Capture: A PDM MEMS microphone converts sound into a clocked one-bit stream. The host peripheral or codec receives the bits at the PDM clock rate.
- Filter: A digital filter processes the oversampled stream to recover the signal’s lower-frequency audio content.
- Decimate: The receiver reduces the sample rate, producing PCM samples for downstream audio processing.
Some processors provide a PDM peripheral with hardware decimation and PCM output. For example, Nordic Semiconductor documents these functions for the nRF5340 PDM interface. Its documented implementation supports up to two microphones as a left/right pair; that capability is specific to this product family, not a universal property of every PDM interface.
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Clock rate and channel details depend on the hardware
PDM audio is oversampled: its bit clock runs faster than the resulting audio sample rate. Microchip’s PDM reception documentation gives one worked example: for a 48 kHz output sampling rate and 64× oversampling, the serial clock is 3.072 MHz. This is an example for those stated parameters, not a universal PDM clock frequency.
For a working design, check the microphone’s supported clock range and channel signaling, then confirm the host peripheral’s clock divider, decimation ratio and filter options. With two microphones on a shared interface, channel selection may depend on clock edges or other interface details. Follow both the microphone and processor documentation rather than assuming that a stereo connection works the same way across devices.
Rank #2
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PDM can also produce an analog output
PDM is not limited to microphone input. In a documented Microchip design, a PDM module generates a pulse stream whose density corresponds to a data-register value. A multistage RC filter smooths that stream into a voltage. This illustrates how filtering can turn a binary pulse sequence into an analog level; the circuit and behavior are implementation-specific. See Microchip’s PDM DAC documentation.
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For an implementation example, NXP’s i.MXRT600 PDM microphone audio-path application note identifies the Knowles SPH0641LM4H-1 as a PDM MEMS microphone. For any microphone or breakout board, verify the current datasheet and board documentation for the PDM output, voltage, clock requirements, pinout and host compatibility. A product example does not by itself establish current availability or compatibility with a particular development board.
Quick Recap
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