Pulse Density Modulation (PDM): Advantages and Disadvantages

This page explores the pros and cons of Pulse Density Modulation (PDM) and explains its fundamental concepts.

What is Pulse Density Modulation?

Introduction: Pulse Density Modulation (PDM) is a technique where a sampled signal is represented by a stream of single bits. It’s often described as “oversampled 1-bit audio.” PDM essentially converts an analog signal into a binary signal.

Unlike Pulse Code Modulation (PCM), which maps specific amplitude levels to codewords with different weights, PDM represents amplitude levels through the relative density of pulses.

The figure above illustrates a pulse density modulated output bitstream. A ‘1’ corresponds to a pulse with positive polarity, while a ‘0’ corresponds to a pulse with negative polarity.

This can be expressed mathematically as:

x[n] = - A (-1) a[n]

Where:

• x[n] is the bipolar stream with values of either -A or +A
• a[n] is the binary stream with values of either ‘0’ or ‘1’

PDM is essentially a high-sampling-rate, single-bit digital system. Let’s break down how a PDM signal is generated, transmitted, and converted back:

• A PDM modulator converts analog audio into PDM’s 1-bit format.
• A PDM demodulator converts the PDM 1-bit signal back into analog audio.

A PDM microphone typically consists of:

• An electret microphone element
• An analog preamplifier
• A PDM modulator
• Interface logic

The analog signal is first amplified and sampled at a high rate before being quantized in the PDM modulator.

Benefits (Advantages) of Pulse Density Modulation

Here are the key advantages of using Pulse Density Modulation:

• Low Noise: Uses a digital form of signal to represent analog audio, leading to a lower noise output.
• Immunity to Interference: Less susceptible to interfering signals.
• Low-Cost Implementation: Suitable for cost-effective implementations where analog audio is converted into digital form (mono or stereo) over a clock/data pair.
• Simple Analog Conversion: Conversion from PDM signal back to analog is straightforward, requiring only a simple low-pass filter.
• High Audio Performance: With appropriate design, extremely high audio performance is achievable.

Drawbacks (Disadvantages) of Pulse Density Modulation

Here are the main drawbacks of using Pulse Density Modulation:

• Poor Representation: The one-bit representation is less detailed compared to multi-point PCM modulation.
• Modulator Overload: The PDM modulator can become overloaded if the input level exceeds the defined maximum, resulting in poor noise performance.
• Converter Requirements: Good performance 1-bit A/D and D/A converters are needed for both the PDM modulator and demodulator.
• Delta Modulation Drawbacks: PDM uses delta-sigma modulation to encode the analog signal, inheriting the same drawbacks as delta modulation.