amps, driver cutter Print Standby/power up Processor driver Power Mains AC Control panel Figure 60.1 Simplified G3 block diagram scanned is collected by a multiple lens assembly and focused on to run lengths in typical documents, different code tables and make up the image sensor array. If the sensors are brought almost into contact codes for the runs longer than 63 elements are used. This is illus- with the paper, the multiple lens is not required and in this case there trated in Figure 60.2 and Table 60.3, where the compression is are holes in the sensor array to allow the LED illumination from 1728:132 or 13:1 for this particular line. behind. Tables 60.4 and 60.5 provides extracts from Modified Huffman The detection of black or white at a particular picture element code tables. depends on the density and the position of the edge of the image with reference to the sensor element. The resolution in G3 is adequate to preserve character shape but there is almost always a difference between the raster images of identical characters even when adjacent. Total scan line 215mm To provide facsimile copy with a similar appearance to computer contains 1728 elements = 8.04 elements/mm generated printer output, the resolution needs to be greater by about 30%. This has led to resolution incompatibility problems for fac- simile and computer interoperation. A4 210mm 60.4.2 Data compression Black run lengths An A4 scanned page of 297mm at 3.85 lines/mm is 1144 facsimile lines. This together with 1728 elements per line gives 1976832 Scan line elements per page, which if sent uncompressed to line at 9.6kbit/s would take 206 seconds, a time which can be significantly reduced by using run length coding. White run lengths 60.4.2.1 Modified Huffman With data compression a code is transmitted to represent the number of black or white bits to be sent, rather than transmitting the 1 or 0 for the appropriate scanning time. The basic G3 compression is Modified Huffman (MH) where, in a limited code table, the most Figure 60.2 G3 resolutio