Introduction to Data Compression by Khalid Sayood

Author:Khalid Sayood
Language: eng
Format: epub, pdf
ISBN: 9780124160002
Publisher: Elsevier Science
Published: 2013-01-30T05:00:00+00:00

Figure 11.13 A source output sampled and coded using delta modulation.

The reconstruction shown in Figure 11.13 was obtained with a delta modulator using a fixed quantizer. Delta modulation systems that use a fixed step size are often referred to as linear delta modulators. Notice that the reconstructed signal shows one of two behaviors. In regions where the source output is relatively constant, the output alternates up or down by ; these regions are called the granular regions. In the regions where the source output rises or falls fast, the reconstructed output cannot keep up; these regions are called the slope overload regions. If we want to reduce the granular error, we need to make the step size small. However, this will make it more difficult for the reconstruction to follow rapid changes in the input. In other words, it will result in an increase in the overload error. To avoid the overload condition, we need to make the step size large so that the reconstruction can quickly catch up with rapid changes in the input. However, this will increase the granular error.

One way to avoid this impasse is to adapt the step size to the characteristics of the input, as shown in Figure 11.14. In quasi-constant regions, make the step size small in order to reduce the granular error. In regions of rapid change, increase the step size in order to reduce overload error. There are various ways of adapting the delta modulator to the local characteristics of the source output. We describe two of the more popular ways here.

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