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H.261 Datasheet(PDF) 9 Page - List of Unclassifed Manufacturers

 Part No. H.261 Description LINE TRANSMISSION OF NON-TELEPHONE SIGNALS Download 31 Pages Scroll/Zoom 100% Maker ETC1 [List of Unclassifed Manufacturers] Homepage Logo

 9 page 6 Recommendation H.261 (03/93) The filter is switched on/off for all six blocks in a macroblock according to the macroblock type (see 4.2.3, MTYPE). 3.2.4 Transformer Transmitted blocks are first processed by a separable two-dimensional discrete cosine transform of size 8 by 8. The output from the inverse transform ranges from –256 to +255 after clipping to be represented with 9 bits. The transfer function of the inverse transform is given by: f x y C u C v F u v x u y v u v ( ) = ( ) ( ) ( ) cos[ ( ) ] cos[ ( ) ] , , / / 1 4 0 7 0 7 2 1 16 2 1 16 = = ∑ ∑ + + π π with u, v, x, y = 0, 1, 2, . . ., 7 where x,y = spatial coordinates in the pel domain, u,v = coordinates in the transform domain, C u ( ) 1 / 2 = for u = 0; otherwise 1, C v ( ) 1/ 2 = for v = 0; otherwise 1. NOTE – Within the block being transformed, x = 0 and y = 0 refer to the pel nearest the left and top edges of the picture, respectively. The arithmetic procedures for computing the transforms are not defined, but the inverse one should meet the error tolerance specified in Annex A. 3.2.5 Quantization The number of quantizers is 1 for the INTRA dc coefficient and 31 for all other coefficients. Within a macroblock the same quantizer is used for all coefficients except the INTRA dc one. The decision levels are not defined. The INTRA dc coefficient is nominally the transform value linearly quantized with a stepsize of 8 and no dead-zone. Each of the other 31 quantizers is also nominally linear but with a central dead-zone around zero and with a step size of an even value in the range 2 to 62. The reconstruction levels are as defined in 4.2.4. NOTE – For the smaller quantization step sizes, the full dynamic range of the transform coefficients cannot be represented. 3.2.6 Clipping of reconstructed picture To prevent quantization distortion of transform coefficient amplitudes causing arithmetic overflow in the encoder and decoder loops, clipping functions are inserted. The clipping function is applied to the reconstructed picture which is formed by summing the prediction and the prediction error as modified by the coding process. This clipper operates on resulting pel values less than 0 or greater than 255, changing them to 0 and 255, respectively. 3.3 Coding control Several parameters may be varied to control the rate of generation of coded video data. These include processing prior to the source coder, the quantizer, block significance criterion and temporal sub-sampling. The proportions of such measures in the overall control strategy are not subject to recommendation. When invoked, temporal sub-sampling is performed by discarding complete pictures. 3.4 Forced updating This function is achieved by forcing the use of the INTRA mode of the coding algorithm. The update pattern is not defined. For control of accumulation of inverse transform mismatch error a macroblock should be forcibly updated at least once per every 132 times it is transmitted.