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G3 FACSIMILE COMMUNICATIONS
are mounted on a MODEM, and communications is performed using the
fastest transmission speed held by the two MODEMs on the transmitting
side and the receiving side.
3.2.2 Image data compression methods
The image compression method is also called the "coding scheme." When
image data of the same size is compressed, the size of the data after com-
pression changes depending on the coding scheme that is applied. In other
words, the transmission time changes as the size of the data to be actually
transmitted changes depending on which compression method was used to
compress the data. The transmission time can be reduced by using a highly
efficient compression method to reduce the size of the data.
3.2.3 Minimum transmission time
Time is required for executing processing in reading of documents and
printing of received images. This time differs according to the machine
type as it is determined according to mutual mechanical and electrical per-
formance. On G3 facsimiles, the minimum time required to process each
single line at transmission and reception is defined as the "minimum trans-
mission time" (MTT) for expressing the processing capability of that
machine.
If the transmitting side transmits without taking into consideration the pro-
cessing capability of the receiving side, recording on the receiving side
will not be able to keep up with reception, and communications will not be
possible if, for example, the receiving side machine has a low processing
capability. In other words, to ensure successful communications between
facsimiles, the faster side must perform processing in keeping with the
machine on the slower side. For this reason, during communications, both
the transmitting and receiving side G3 facsimiles inform each of other of
the MTT to prevent data shorter than the MTT of the slower of the two
machines from being sent to prevent this kind of trouble. When a receiving
side machine having an MTT of 20 ms communicates with a transmitting
side machine having an MTT of 10 ms, the transmitting side inserts a fill
after the data to adjust the length to 20 ms or more when the transmission
time of a single line including the EOL is less than 20 ms, that is the MTT
of the receiving side.
5–6
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