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A new facsimile source coding algorithm, called twodimensional (2-D) delta-mod (Δmod), is presented. The algorithm is based on conventional run length coding. Additional compression is achieved by vertically delta-modulating the horizontal run lengths. The delta-modulation introduces a distortion into the image which can be traded-off versus the rate of transmission. Cumulative error is prevented, as in conventional Δ mod, by including the (run length) quantizer in the feedback loop. Some parametric results are presented 相似文献
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The CCITT has defined Group 3 facsimile apparatus as that which digitally transmits an ISO A4 document over a switched telephone circuit in approximately one minute. Data compression is employed to achieve the reduced transmission time. Study Group XIV of the CCITT has drafted Recommendation T.4 to achieve compatibility between Group 3 facsimile devices. The standard data compression technique specified by T.4 is a one-dimensional coding scheme in which runlengths are encoded using a modified Huffman code (MHC). The recommendation also includes an optional twodimensional compression technique known as the modified READ code (MRC). It is recognized that the switched telephone network is prone to error when transmitting digital data at the standard T.4 data rate of 4800 bits/s. This paper evaluates the error sensitivity of the MHC and MRC when operating over a typical telephone circuit. The error sensitivity analysis is accomplished by means of computer simulation. The error performance of the two coding techniques is analyzed both quantitatively and qualitatively. The quantitative analysis is accomplished using the error sensitivity factor which represents the average number of incorrect pels in the output document caused by a transmission error. The qualitative analysis is based upon viewing actual error-contaminated images generated in the simulation process. Two separate analyses have been performed. First, error sensitivity data for both the MHC and MRC are developed under identical operational conditions, and their relative performance is discussed. In the second part four different techniques for processing the received facsimile signal (MRC), to minimize the subjective effect of transmission errors, are analyzed. 相似文献
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With a facsimile bi-level quantizer, notches often occur along the white and black borders of a document when the output of a photosensor goes up and down around a threshold. The objects of this work are, first, to present a notchless bi-level quantizer, and secondly, to improve coding efficiency by using the quantizer. It is pointed out that quantization is comprised of many important aspects from aesthetic as well as coding efficiency standpoints, and that significant improvement is achieved in coding efficiency through the use of the notchless bi-level quantizer. 相似文献
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