Pulse-density modulation (PDM) audio defined in a fast primer

November 20, 2025

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Pulse-density modulation (PDM) audio defined in a fast primer

Pulse-density modulation (PDM) is a compact digital audio format utilized in gadgets like MEMS microphones and embedded programs. This compact primer eases you into the necessities of PDM audio.

Let’s start by revisiting a ubiquitous PDM MEMS microphone module primarily based on MP34DT01-M—an omnidirectional digital MEMS audio sensor that continues to function a dependable benchmark in embedded audio design.

Determine 1 A MEMS microphone mounted on a minuscule module detects sound and produces a 1-bit PDM sign. Supply: Writer

When correctly carried out, PDM can digitally encode high-quality audio whereas remaining cost-effective and simple to combine. Because of this, PDM streams at the moment are broadly adopted as the usual knowledge output format for MEMS microphones.

On paper, the anatomy of a PDM microphone boils down to some important constructing blocks like:

MEMS microphone factor, sometimes a capacitive MEMS construction, not like the electret capsules present in analog microphones.
Analog preamplifier boosts the low-level sign from the MEMS factor for additional processing.
PDM modulator converts the analog sign right into a high-frequency, 1-bit pulse-density modulated stream, successfully performing as an built-in ADC.
Digital interface logic handles timing, clock synchronization, and knowledge output to the host system.

Subsequent is the perform block diagram of T3902, a digital MEMS microphone that integrates a microphone factor, impedance converter amplifier, and fourth-order sigma-delta (Σ-Δ) modulator. Its PDM interface allows time multiplexing of two microphones on a single knowledge line, synchronized by a shared clock.

Determine 2 Purposeful block diagram outlines the inner segments of the T3902 digital MEMS microphone. Supply: TDK

The analog sign generated by the MEMS sensing factor in a PDM microphone—generally known as a digital microphone—is first amplified by an inner analog preamplifier. This amplified sign is then sampled at a excessive fee and quantized by the PDM modulator, which mixes the processes of quantization and noise shaping. The result’s a single-bit output stream on the system’s sampling fee.

Noise shaping performs a important position by pushing quantization noise out of the audible frequency vary, concentrating it at larger frequencies the place it may be extra simply filtered out. This ensures comparatively low noise inside the audio band and better noise outdoors it.

The microphone’s interface logic accepts a grasp clock sign from the host system—sometimes a microcontroller (MCU) or a digital sign processor (DSP)—and makes use of it to drive the sampling and bitstream transmission. The grasp clock determines each the sampling fee and the bit transmission fee on the information line.

Every 1-bit pattern is asserted on the information line at both the rising or falling fringe of the grasp clock. Most PDM microphones assist stereo operation through the use of edge-based multiplexing: one microphone transmits knowledge on the rising edge, whereas the opposite transmits on the falling edge.

Through the reverse edge, the information output enters a high-impedance state, permitting each microphones to share a single knowledge line. The PDM receiver is then answerable for demultiplexing the mixed stream and separating the 2 channels.

As a facet notice, the aforesaid microphone module is hardwired to deal with knowledge as legitimate when the clock sign is low.

The magic behind 1-bit audio streams

Now, again within the driveway. PDM is a intelligent option to symbolize a sampled sign utilizing only a stream of single bits. It depends on delta-sigma conversion, often known as sigma-delta, and it’s the core know-how behind many oversampling ADCs and DACs.

At first look, a one-bit stream appears hopelessly noisy. However right here is the trick: by sampling at very excessive charges and making use of noise-shaping methods, most of that noise is pushed out of the audible vary—above 20 kHz—the place it not interferes with the listening expertise. That’s how PDM preserves audio constancy regardless of its minimalist encoding.

There’s a catch, although. You can’t correctly dither a 1-bit stream, which suggests a small quantity of distortion from quantization error is at all times current. Nonetheless, for a lot of functions, the trade-off is price it.

Diving into PDM conversion and reconstruction, we start with the direct sampling of an analog sign at a excessive fee—sometimes a number of megahertz or extra. This produces a pulse-density modulation stream, the place the density of 1s and 0s displays the amplitude of the unique sign.

Determine 3 An instance that renders a single cycle of a sine wave as a digital sign utilizing pulse density modulation. Supply: Writer

Naturally, the encoding depends on 1-bit delta-sigma modulation: a course of that makes use of a one-bit quantizer to output both a 1 or a 0 relying on the instantaneous amplitude. A 1 represents a sign pushed totally excessive, whereas a 0 corresponds to completely low.

And, as a result of the audio frequencies of curiosity are a lot decrease than the PDM’s sampling fee, reconstruction is simple. Passing the PDM stream by means of a low-pass filter (LPF) successfully restores the analog waveform. This works as a result of the delta-sigma modulator shapes quantization noise into larger frequencies, which the low-pass filter attenuates, preserving the specified low-frequency content material.

Inside digital audio: Codecs at a look

It goes with out saying that in digital audio programs, PCM, I²S, PWM, and PDM every serve distinct roles tailor-made to particular wants. Pulse code modulation (PCM) stays essentially the most broadly used format for representing audio alerts as discrete amplitude samples. Inter-IC Sound (I²S) excels in exact, low-latency audio knowledge transmission and helps versatile stereo and multichannel configurations, making it a well-liked selection for inter-device communication.

Although not sometimes used for audio sign illustration, pulse width modulation (PWM) performs a significant position in audio amplification—particularly in Class D amplifiers—by encoding amplitude by means of obligation cycle variation, enabling environment friendly speaker management with minimal energy loss.

On a facet notice, you possibly can convert a PCM sign to PDM by first growing its pattern fee (interpolation), then lowering its decision to a single bit. Conversely, a PDM sign might be transformed again to PCM by lowering its sampling fee (decimation) and growing its phrase size. In each circumstances, the ratio of the PDM bit fee to the PCM pattern fee is called the oversampling ratio (OSR).

Crisp audio for makers: PDM to energy simplified

Cheerfully compact and maker-friendly PDM enter Class D audio energy amplifier ICs simplify the trail from microphone to speaker. By accepting digital PDM alerts straight—typically from MEMS mics—they scale down each complexity and element rely. Their environment friendly Class D structure retains the ability draw low and warmth minimal, which is good for battery-powered builds.

That’s to say, audio ICs like MAX98358 require minimal exterior parts, making prototyping a pleasure. With filterless Class D output and built-in options, they simplify audio design, releasing makers to give attention to creativity slightly than complexity.

Sidewalk: For these desperate to experiment, ample instance code is offered on-line for SoCs just like the ESP32-S3, which use a sigma-delta driver to provide modulated output on a GPIO pin. Then with a passive or lively low-pass filter, this output might be formed into clear, smart analog sign.

Properly, the blueprint under exhibits an lively low-pass filter utilizing the Sallen & Key topology, arguably the only lively two-pole filter configuration you’ll find.

Determine 4 Circuit blueprint outlines a easy lively low-pass filter. Supply: Writer

Echoes and endings

As normal, I really feel there may be a lot extra to cowl, however let’s soar to a fast wrap-up.

Whether or not you might be decoding microphone specs or sketching out a sign chain, understanding PDM is a quiet superpower. It’s not nearly 1-bit streams; it’s about how digital sound travels, transforms, and finds its voice in your design. If this primer helped demystify the fundamentals, you might be already one step nearer to constructing smarter, cleaner audio programs.

Let’s preserve listening, studying, and simplifying.

T. Ok. Hareendran is a self-taught electronics fanatic with a robust ardour for modern circuit design and hands-on know-how. He develops each experimental and sensible digital tasks, documenting and sharing his work to assist fellow tinkerers and learners. Past the workbench, he dedicates time to technical writing and {hardware} evaluations to contribute meaningfully to the maker neighborhood.