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4.2.1 Outline of MH coding scheme
Codes having a fewer number of bits are allocated to codes to be swapped
for black and white that appear more frequently when coding is performed.
By this allocation, the data ultimately becomes compressed data, and the
transmission time is reduced. By the MH coding scheme, coding is per-
formed taking only the
direction) into consideration. For this reason, it is called a "one-dimen-
sional coding scheme." Whereas, a scheme where coding is performed in
both the horizontal and
direction) is called the MR coding scheme (two-dimensional coding
scheme) and is distinguished from the one-dimensional coding scheme.
4.2.2 MH coding scheme procedures
The image data read by the reading section is converted to run lengths for
each scanline, and is then coded by MH coding. These processes are nor-
mally performed by the CPU, and the resulting image data is stored to
memory. That data is then sent to the MODEM, where it is modulated and
then transmitted to the telephone line. On the receiving side, the procedure
is completely reversed, and the original data is restored by demodulation.
Image signal
(raw data)
Fig. 5-9 Procedure for Image Data Processing on the Transmitting Side
Let's try actually converting the previously described scanlines.
(1) Run length
The raw data of the image that is read is converted to run lengths.
White run length 5
Of special note at this time, each scanline run must start from a white run.
This rule exists so that the line starts from a white run as there is no means
of notifying that the line starts with a black run or a white run. Accord-
ingly, if the line starts from a black run, a dummy white run (run length 0)
is added at the start of the line before coding is performed.
horizontal scanning
vertical scanning
Run length
MH-coding
conversion
Black run length 3
White run length 9
Fig. 5-10 Scanlines after Conversion to Run Length
G3 FACSIMILE COMMUNICATIONS
direction (one dimensional
directions (two-dimensional
Modulation
To the line
Black run length 3
White run length 1708
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