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ADV601 Datasheet(PDF) 4 Page - Analog Devices |
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ADV601 Datasheet(HTML) 4 Page - Analog Devices |
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4 / 52 page  ADV601 –4– REV. 0 THE WAVELET KERNEL This block contains a set of filters and decimators that work on the image in both horizontal and vertical directions. Figure 6 illustrates the filter tree structure. The filters apply carefully chosen wavelet basis functions that better correlate to the broad- band nature of images than the sinusoidal waves used in Dis- crete Cosine Transform (DCT) compression schemes (JPEG, MPEG, and H261). An advantage of wavelet-based compression is that the entire image can be filtered without being broken into sub-blocks as required in DCT compression schemes. This full image filtering eliminates the block artifacts seen in DCT compression and offers more graceful image degradation at high compression ratios. The availability of full image sub-band data also makes image processing, scaling, and a number of other system fea- tures possible with little or no computational overhead. The resultant filtered image is made up of components of the original image as is shown in Figure 3 (a modified Mallat Tree). Note that Figure 3 shows how a component of video would be filtered, but in multiple component video luminance and color components are filtered separately. In Figure 4 and Figure 5 an actual image and the Mallat Tree (luminance only) equivalent is shown. It is important to note that while the image has been filtered or transformed into the frequency domain, no compres- sion has occurred. With the image in its filtered state, it is now ready for processing in the second block, the quantizer. BLOCK A IS HIGH PASS IN X AND DECIMATED BY TWO. BLOCK B IS HIGH PASS IN X, HIGH PASS IN Y, AND DECIMATED BY EIGHT. BLOCK C IS HIGH PASS IN X, LOW PASS IN Y, AND DECIMATED BY EIGHT. BLOCK D IS LOW PASS IN X, HIGH PASS IN Y, AND DECIMATED BY EIGHT. BLOCK E IS HIGH PASS IN X, HIGH PASS IN Y, AND DECIMATED BY 32. BLOCK F IS HIGH PASS IN X, LOW PASS IN Y, AND DECIMATED BY 32. BLOCK G IS LOW PASS IN X, HIGH PASS IN Y, AND DECIMATED BY 32. BLOCK H IS HIGH PASS IN X, HIGH PASS IN Y, AND DECIMATED BY 128. BLOCK I IS HIGH PASS IN X, LOW PASS IN Y, AND DECIMATED BY 128. BLOCK J IS LOW PASS IN X, HIGH PASS IN Y, AND DECIMATED BY 128. BLOCK K IS HIGH PASS IN X, HIGH PASS IN Y, AND DECIMATED BY 512. BLOCK L IS HIGH PASS IN X, LOW PASS IN Y, AND DECIMATED BY 512. BLOCK M IS LOW PASS IN X, HIGH PASS IN Y, AND DECIMATED BY 512. BLOCK N IS LOW PASS IN X, LOW PASS IN Y, AND DECIMATED BY 512. N M L K I H J G F E C B D A Figure 3. Modified Mallat Diagram (Block Letters Correspond to Those in Filter Tree) Understanding the structure and function of the wavelet filters and resultant product is the key to obtaining the highest perfor- mance from the ADV601. Consider the following points: • The data in all blocks (except N) for all components are high pass filtered. Therefore, the mean pixel value in those blocks is typically zero and a histogram of the pixel values in these blocks will contain a single “hump” (Laplacian distribution). • The data in most blocks is more likely to contain zeros or strings of zeros than unfiltered image data. • The human visual system is less sensitive to higher frequency blocks than low ones. • Attenuation of the selected blocks in luminance or color com- ponents results in control over sharpness, brightness, contrast and saturation. • High quality filtered/decimated images can be extracted/created without computational overhead. Through leverage of these key points, the ADV601 not only compresses video, but offers a host of application features. Please see the Applying the ADV601 section for details on getting the most out of the ADV601’s sub-band coding architecture in different applications. |
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