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VoIP Voice and Fax Signal Processing by Sivannarayana Nagireddi

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14.2 FAX IMAGE CODING SCHEMES

An optical scanner scans the document and generates a series of electrical signals corresponding to the picture elements on the scan line. On a standard size (8.5″ × 11″) document, the scanner gives 1728 bits of data or pixels in the horizontal direction and 1145 lines of information in the vertical direction (while using the lowest vertical resolution that is 3.85 lines per mm). This procedure produces (1728) × (1145) = 1,978,560 bits (approximately 2 million bits per page) of information from one page. Without any compression and coding techniques, it would take minimum of 207 seconds to transmit the entire one page data at 9600 bps rate [URL (Maine)].

The method that G3 fax machines use to reduce data is called coding. In voice, voice compression or codecs are used for the compression operation. In fax, the coding name is used for compression. Fax coding performs data compression, and voice codecs perform signal compression. The ITU recommendations T.4 and T.6 are used for coding the fax data. T.4 implementation comprises image information, encoding, and decoding operations that are required for a facsimile device. It also considers image size and resolution. T.4 and T.30 complement each other in a G3 facsimile device. The main parts of T.4 are the run-length coding/decoding schemes, the bilevel compression schemes, and the color/grayscale modes. Bilevel scan lines are composed of white and black areas, and groups of black and white pixels are placed ...

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