onsist of long runs of Os. Therefore, a technique known as run length coding is used where two values, the number of Os to the next coefficient followed by the value itself, are coded. This information is fed to a Variable-Length Coder (VLC) (Ghanbari, 2003) to exploit the statistical redun- dancy still contained within the bitstream. Temporal Redundancy Modern codecs exploy the spatial correlation (local similarity) within a single image and use knowledge of human visual response to reduce the overall bitrate. With moving images Spatial, Temporal, and Statistical Redundancy 104 Trolands 100 Trolands 1 Troland Fig. 3.11 Temporal response of the human visual system. Frequency (Hz) it is possible to exploit not only the spatial information present but also the temporal (motion), as the codecs have access to images from the past, present, and future. The temporal response of the human eye, shown in Figure 3.11, also has much to do with the design of modern codecs. You will note from the figure that the eye is very susceptible to a frequency of around 10 Hz and SO frame rates around this frequency should be avoided. Also note that at fre- quencies around 50 Hz the eye has a very low sensitivity. This is one of the factors governing the frame and field rates in domestic broadcast television. A pair of consecutive frames taken from a video sequence are generally very similar to one another, allowing codec designers to exploit temporal redundancy. If there were no motion at all, a message could be sent to the decoder to redisplay the previous frame. If a small amount of motion were present, only the changes would need be sent. If content could be found in both frames that had simply moved, then the decoder could be signaled to identify the area and how much it had moved. Motion Estimation With these basic premises, a method can be defined that uses the redundancy in coding two consecutive frames of video (ITU-T, 1998). The image is divided into a series of small blocks or macroblocks (e.g., 8 X 8) and then a search is c