PX64 Compression Overview

Sampling Frequency. Pictures are sampled at an integer multiple of the video line rate. A sampling clock and network clock are asynchronous.

Figure 16.9 Functional block diagram of the px64 video codec. [From Figure 1/H.261, ITU-T Rec. H.261 (Ref. 20).]

Source Coding Algorithm. Compression is based on interpicture prediction to utilize temporal redundancy, and transform coding of the remaining signal to reduce spatial redundancy. The decoder has motion compensation capability, allowing optional incorporation of this technique in the coder. There is optional forward error correction10 available based on the BCH (511,493) code. The codec can support multipoint operation. Source Coder. The coder operates on non-interleaved pictures occurring 30,000/1001 (approximately 29.97) times per second. The tolerance on the picture frequency is ±50 ppm.

As in Section 16.9.4, pictures are coded as one luminance and two color difference components (Y, Cb, and Cr). Reference should be made to CCIR Rec. 601 (Ref. 23) for their components and codes representing their sampled values. For example:

Black = 16

White = 235

Zero color difference = 128

Peak color difference = 16 and 240

The values given are nominal values and the coding algorithm functions with input values of 1 through 254. Two picture scanning formats have been specified.

For the first format (CIF), the luminance structure is 352 pels per line, 288 lines per picture in an orthogonal arrangement. The color-difference components are sampled at 176 pels per line, 144 lines per picture, orthogonal. Figure 16.10 shows the color-difference samples being sited such that the block boundaries coincide with luminance block boundaries. The picture area covered by these numbers of pels and lines has an aspect ratio of 4:3 and corresponds to the active portion of the local standard video input.

It should be noted that the number of pels per line is compatible with sampling the active portions of the luminance and color-difference signals from 525- or 625-line sources

10FEC coding. Consult Ref. 21 for a discussion of FEC.

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