Detection and correction of transmission errors in facsimile images

Today, facsimile is recognized as the primary communication tool for both printed and written materials. Most facsimile machines operating on public switched telephone networks use the Group 3 (G3) facsimile compression standards, in which images are entropy-coded. Although the synchronization codeword end of line (EOL) is employed, a transmission error in a codeword may cause the current codeword, the subsequent codewords in the current line, even the codewords in the subsequent lines to be misinterpreted, resulting in a great degradation of the received image. The objective of the proposed error detection and correction approaches is to completely or partially eliminate transmission errors in G3 facsimile images, requiring no extra transmission bit rate and without changing the transmitter and the receiver. The proposed approaches are based on the error checking conditions derived from the relationship between the current line and the previous line as well as the constraints on compressed image data. A corrupted line is detected if any of the error checking conditions is satisfied. When a corrupted line is detected, a sequence of bit inversions and redecoding operations are performed on the current corrupted line and/or its previous lines so that at least one possible (feasible) redecoding solution can be found. Then, the best solution is selected by using some selection criterion. Based on simulation results, the proposed approaches can recover the original or high-quality facsimile images from their corresponding corrupted facsimile images. This shows the feasibility of the proposed approaches.