# RF Wireless World

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## PCM vs PDM | Difference between PCM and PDM

This page compares PCM vs PDM and mentions difference between PCM (pulse-code modulation) and PDM (pulse-density modulation).

### PCM | Pulse Code Modulation

In this modulation technique ADC (Analog to Digital converter) is used to which converts analog waveform to multi-bit digital code. Analog waveform is first sampled and quantized before samples are represented into binary form.

PCM modulation converts CA-CT (Continuous Amplitude Continuous Time) waveform into DA-DT (Discrete Amplitude Discrete Time) waveform. Sampling rate should be greater than or equal to nyquist rate in order to avoid aliasing.

If there is a n bit quantizer and sampling rate is Fs then bit rate will be
➨Rb(bits/sec) = n * Fs
Signal to Quantization Noise Ratio for PCM for sinusoidal input is
➨SQNR (dB) = 6*n + 1.76, where n is bit of uniform quantizer.

There are two variants of PCM viz. DPCM (Delta Pulse Code Modulation) and ADPCM (Adaptive Delta Pulse Code Modulation).

### PDM | Pulse Density Modulation

PDM converts sampled signal to stream of single bits. It is referred as "oversampled 1 bit audio". In Pulse Density Modulation, specific amplitude levels are mapped using relative density of pulses. Following figure depicts PDM modulated output binary stream. The analog signal is first amplified and sampled at high sampling rate and later quantized in the PDM modulator. The reverse processing are carried out in PDM demodulator.

PDM technique can be expressed using following formula.
➨ x[n] = - A (-1)a[n]
Where,
x[n] is bipolar stream with either -A or +A
a[n] is binary stream with either '0' or '1'
Here '1' corresponds to pulse with positive polarity and '0' corresponds to pulse with negative polarity.

### Difference between PCM and PDM

Following table mentions difference between PCM and PDM techniques.

Parameters PCM PDM
Full Name Pulse Code Modulation Pulse Density Modulation
How it works Amplitude levels are mapped to codewords of pulses with different weights. Amplitude levels are mapped with relative density of pulses.
Performance Better performance due to multi-point representation. Poor performance due to single bit representation.
Overload condition Overload appears when modulating signal changes between samplings, by an amount greater than the size of the step. PDM modulator part is overloaded when input level exceeds maximum input level defined. This results into poor noise performance of the modulated signal.
Transmission Bandwidth Larger Smaller
Application It is used as standard form for digital audio waveforms in desktop PCs, CDs, digital telephony, satellite communication etc. It is used to deliver audio from microphones to signal processor in smartphones or mobile phones.
Complexity Complex as it uses multi-bits Simple as it uses only 1 bit to convey audio

PCM >>, DPCM >> and ADPCM >> techniques.

### Modulation types

BPSK -This page describes BPSK modulation technique with equation and constellation diagram.
QPSK -This page describes QPSK modulation technique with equation and constellation diagram.