esolution. For example, in cases where black is defined as the smallest amplitude, the black picture signals may not be transmitted. To prevent this, the G2 machine em- ploys phase modulation (PM) technology along with the AM system. The PM system inverts the phase of the largest amplitude occur- ring after the smallest amplitude (Fig. 4-12). (a) indicates the phase of an AM signal while (c) indicates this phase inverted to 180 degrees in comparison with that in (a). (b) indicates the AM-PM signal whose different phases appear alternately. By alternately changing the phases, the black signal (usually the smallest amplitude) is always detected. If the black signal is very short, and if the phases do not change, the receiver is likely to recognize the two white signals (the largest amplitudes) as one continuous white signal, while failing to detect the short black signal. AM signal (0 phase) AM-PM signal AM signal (II phase) Fig. 4-12. PM system comparison used for G2 fax machines. 66 TELECOMMUNICATIONS The transmission system for G3 machines employs a digitized sys- tem as opposed to the analog system used in G1 and G2 machines. Facsimile picture signals are considered redundant if black or white signals are prolonged. Because they are capable of reducing this re- dundancy by coding techniques, G3 machines are standardized to transmit information in one minute. A method of reducing redundancy is through the use of the mod- ified READ coding system. This system is based on the fact that black and white elements on one portion of a scanning line are often similar to that on the previous scanning line. This digital coding sys- tem codes only those black and white element combinations which differ between two neighboring scanning lines. 4.4.1 Stored Switching System When connected to a computer system, the facsimile machine can be used as a terminal to retrieve image information stored in the computer's memory. Through the use of a minicomputer system and OMR (optical mark reader), on which several addresses are re- c