We start with a subject - Jim Beecher

Typically, an 8.5x11 inches page is scanned at 200 dpi; thus, resulting in 3.74 Mbits. Transmitting this data over a low-cost 14.4 kbits/s modem would require . ... If, say, the pixels in the image are in the order , then instead of compressing these pixels, one might process the sequence of differentials , where , and . ................
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