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G3 FACSIMILE COMMUNICATIONS
This scheme is particularly effective in coding documents containing lots
of text broadly spread vertically or horizontally (that is, documents con-
taining few shifts of changing pixels).
By the MH coding scheme, 3-bit, 4-bit and at maximum 13-bit coded data
must be used to express a single run length. However, with the MR coding
scheme, the run length can be expressed just by coded data of one bit if the
previous run length is the same as the current run length.
4.3.2 Combination of MH and MR schemes
Actually, a combination of the MH and MR coding schemes are used as
the MR coding scheme. In other words, both the MH code table and MR
code table are used for the following reason.
(1) By the MR coding scheme, coding must be started from completely
(2) When a document is transmitted by only the MR coding scheme, the
(2) above is an important issue, and is a disadvantage in the MR coding
scheme. The MR coding scheme is a means for complementing this disad-
vantage. A number of scanlines in one page are combined and MH coding
is carried out on every leading scanline. MR coding is performed on subse-
quent scanlines. The combined number of scanlines is referred to as
5–18
Fig. 5-15 Example of Two Consecutive Scanlines
describing the initial line. Otherwise, subsequent lines cannot be
coded using the data that indicates the presence of changes.
receiving side facsimile sometimes cannot correctly decode the scan-
line due to an error occurring midway during transmission. Because of
the nature of the MR coding scheme, this transmission error will affect
to the end of the following scanlines.
Previous line
Present line
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