features of facsimile is the ability to operate in an unattended mode. Unattended systems coupled to ordinary tele- phone networks can send/receive hundreds of pages automatically, using auto-answer phone interfacing. Some units even allow off-hour transmission on an unattended basis via automatic telephone dialing attachments. Positive feedback provides the sender with confirma- tion that the document has been transmitted and copied at the re- mote location. Internationally standardized facsimile machines are divided into three groups in terms of transmission speeds: Group 1 machines, which are capable of transmitting information in six minutes; Group 2 machines, in three minutes; and Group 3 machines, in one minute. The main factor which differentiates the respective group machines is their transmission methods. Since the facsimile picture signals include signals of frequencies lower than the voice band (300 Hz - 3400 Hz), it is necessary to modulate the facsimile picture signals so they can be transmitted via a telephone line. For G1 machines, frequency modulation (FM) is used. The FM system gives a different frequency to each black and white picture element to be transmitted, as illustrated in Fig. 4-11. FM signal AM signal Fig. 4-11. AM and FM facsimile signals. TERMINAL TECHNOLOGY 65 The duration of each high or low frequency carrier is proportionate to the length of the black and white picture element. The AM system used in G2 machines defines the white and black picture elements as the largest and smallest amplitudes of a carrier or vice versa. The standardized G1 FM system is capable of transmitting a vol- ume of picture elements equivalent to 180 scanning lines per minute. It is technologically possible to make the transmission time shorter, but this results in decreased resolution and thus lowers the quality of transmitted information. However, because G2 machines are required to transmit two times more information (picture elements equivalent to 360 scanning lines per minute) than G1s, they decrease r