Delta modulation

In PCM system, the image intensity is coded by scalar quantisation and the inter-pixel correlation is not exploited.The DM model does just this. In this system the difference between two consecutive pixel intensities is coded by a one-bit (2 reconstruction levels) quantiser. Although the dynamic range of the difference signal is doubled as a result of differencing, the variance of difference signal is significantly reduced due to strong correlation typically present in the intensities of two pixels that are spatially close.

In DM, it is useful to assume that the pixels in an image have been arranged in some sequential manner so that can be expressed as 1-D signal .

If is obtained by reading one row of and then reading the next row, it will preserve some of the spatial correlation present in .

A DM system is shown in Fig (15 ). In figure, represents reconstructed by DM.

To code , the most recently reconstructed is subtracted from .

The difference signal is quantised to if is positive, and otherwise, where is quantisation step size.

At the receiver, is added to to obtain . The reconstructed is also needed at the transmitter, since is used in coding . The dotted line in figure represents a delay and indicates that is computed so that it can be used in coding the next sample .

The equations that govern DM system are

	=

(continued in the next slide)