Difference Between Uniform and Nonuniform Quantization

The key difference between uniform and nonuniform quantization is that uniform quantization has equal step sizes while, in nonuniform quantization, the step sizes are not equal. Another important difference between uniform and nonuniform quantization is that,  in the uniform quantization, some amount of quantization error can happen, but nonuniform quantization reduces the quantization error.

Communication systems send signals from the transmitter to the receiver. These signals are analog signals. Usually, analog signals can affect distortion and interference etc. Therefore, the analog signals are converted into digital signals. This process is called digitization. Generally, the digital signals are clear, accurate and have minimum distortion. Quantization is one step in the digitization process.

CONTENTS

1. Overview and Key Difference
2. What is Quantization
3. What is Uniform Quantization
4. What is Nonuniform Quantization
5. Side by Side Comparison – Uniform vs Nonuniform Quantization in Tabular Form
6. Summary

What is Quantization?

When digitizing, first step is to sample the signal at regular intervals. If the sampling time or the sampling period is Ts then the sampling rate or frequency (fs) is 1/Ts.  For a signal to reproduce exactly, the sampling rate(fs) should be greater than twice the maximum frequency.

Next step is the quantization. It gives a finite discrete value to the samples. The device that performs quantization is the quantizer. It takes the sampled input and generates the quantized output. The quality of the quantizer output depends on the number of quantization levels. Furthermore, the space between two adjacent quantization levels is called step size. In the below diagram, the dash lines represent the quantization levels.

There are two types of quantization depending on the step size. They are uniform quantization and non-uniform quantization. The equation to find the step size (d) is given below. The Xmax is the maximum value of the signal and Xmin is the minimum value of the signal. The L is the number of levels that divide the signal.

What is Uniform Quantization?

Uniform quantization has an equal spacing between the quantization levels. Further, there are two types in uniform quantization. They are mid-tread and mid-rise quantization. Both of these are symmetric about the origin. In the mid-thread quantization, the origin lies in the middle of a tread of the staircase like graph. The quantization levels in mid-thread are odd in number. In the mid-rise quantization, the origin lies in the middle of a rising part of the staircase like graph.  The quantization levels in mid-rise are even in number.

What is Nonuniform Quantization?

In nonuniform quantization, the step size is unequal. After the quantization, the difference between an input value and its quantized value is called the quantization error. As mentioned above, in uniform quantization, the step size is equal. Therefore, some part of the signal might not cover. This can increase quantization error.

 However, in case of nonuniform quantization, the step size changes so it will have a minimum amount of error.  After completing the quantization, the next step is encoding. It defines each quantization level by a binary code.

What is the Difference Between Uniform and Nonuniform Quantization?

Uniform Quantization is the type of quantization in which the quantization levels are uniformly spaced is the Uniform Quantization. Nonuniform Quantization is the type of quantization in which the quantization levels are unequal is the Nonuniform Quantization.

Moreover, Uniform quantization has some amount of quantization error. But, Nonuniform quantization reduces quantization error.

Summary – Uniform vs Nonuniform Quantization

This article discussed the difference between two types of quantization that are uniform and nonuniform. The difference between uniform and nonuniform quantization is that uniform quantization has an equal step size while nonuniform quantization does not have an equal step size.

Reference:

1.Tutorials Point. “Digital Communication Quantization.” , Tutorials Point, 8 Jan. 2018. Available here 

Image Courtesy:

1.’3-bit resolution analog comparison ‘By Hyacinth – Own work, (CC BY-SA 3.0) via Commons Wikimedia