PCM vs PDM: Understanding the Key Differences

This page compares PCM vs PDM and highlights the differences between PCM (pulse-code modulation) and PDM (pulse-density modulation).

PCM | Pulse Code Modulation

In this modulation technique, an ADC (Analog to Digital converter) is used to convert an analog waveform into a multi-bit digital code. The analog waveform is first sampled and quantized before the samples are represented in binary form.

PCM modulation converts a CA-CT (Continuous Amplitude Continuous Time) waveform into a DA-DT (Discrete Amplitude Discrete Time) waveform.

The sampling rate should be greater than or equal to the Nyquist rate in order to avoid aliasing.

If there is an n bit quantizer and the sampling rate is Fs, then the bit rate will be:

Rb(bits/sec) = n * Fs

The Signal to Quantization Noise Ratio (SQNR) for PCM with a sinusoidal input is:

SQNR (dB) = 6*n + 1.76, where n is the number of bits of the uniform quantizer.

There are two variants of PCM:

• DPCM (Delta Pulse Code Modulation)
• ADPCM (Adaptive Delta Pulse Code Modulation).

PDM | Pulse Density Modulation

PDM converts a sampled signal to a stream of single bits. It is often referred to as “oversampled 1 bit audio”. In Pulse Density Modulation, specific amplitude levels are mapped using the relative density of pulses.

The following figure depicts a PDM modulated output binary stream. The analog signal is first amplified and sampled at a high sampling rate, and later quantized in the PDM modulator. The reverse processing is carried out in the PDM demodulator.

PDM can be expressed using the following formula:

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

Where:

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

Here, ‘1’ corresponds to a pulse with positive polarity and ‘0’ corresponds to a pulse with negative polarity.

Difference between PCM and PDM

The following table highlights the differences between PCM and PDM techniques.