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Advantages of Pulse Density Modulation | disadvantages of Pulse Density Modulation

This page covers advantages and disadvantages of Pulse Density Modulation (PDM) and its basics. It mentions benefits or advantages of Pulse Density Modulation and drawbacks or disadvantages of Pulse Density Modulation i.e. PDM.

What is Pulse Density Modulation?

Introduction:
It is a system in which sampled signal is represented by stream of single bits. It is referred as "oversampled 1 bit audio".

Pulse Density Modulation (PDM) is used to convert analog signal into binary signal form. We have seen in PCM modulation that specific amplitude levels are mapped to codewords of pulses with different weights. In PDM modulation, specific amplitude levels are represented with relative density of pulses.

Pulse Density Modulation

The figure depicts pulse density modulated output bitstream. As shown '1' corresponds to pulse with positive polarity. Here '0' corresponds to pulse with negative polarity. It can be expressed as follows.
➨ 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'

PDM is basically a high sampling rate, single bit digital system. Let us understand how PDM (Pulse Density Modulation) signal is generated, transmitted and converted back. PDM modulator converts analog audio to PDM 1 bit format. PDM demodulator converts PDM 1 bit signal into analog audio format.

PDM microphone consists of following parts viz. electret microphone element, analog preamplifier, PDM modulator and interface logic. The analog signal is first amplified and sampled at high rate and later quantized in the PDM modulator.

Benefits or advantages of Pulse Density Modulation

Following are the benefits or advantages of Pulse Density Modulation:
➨It uses digital form of signal to represent analog audio. Hence it offers low noise output.
➨It is immune from interfering signals.
➨It is used for low cost implementation in which analog audio is converted into digital form in mono or in stereo over a clock/data pair.
➨Conversion of PDM signal to analog form is simple and requires simple low pass filter.
➨It is possible to achieve extremely high audio performance with appropriate design.

Drawbacks or disadvantages of Pulse Density Modulation

Following are the drawbacks or disadvantages of Pulse Density Modulation:
➨One bit representation is poor compare to multi-point PCM modulation.
➨PDM modulator is overloaded when input level exceeds maximum input level defined. This results into poor noise performance of the modulated signal.
➨Good performance 1 bit A/D and D/A converters are required for PDM modulator and demodulator respectively.
➨It uses process of delta-sigma modulation in order to encode analog signal. Hence it will have same drawbacks as delta modulation >>.



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