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G3 FACSIMILE COMMUNICATIONS

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 

 directions (two-dimensional

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 

, 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.

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.

Fig. 5-10   Scanlines after Conversion to Run Length

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.

To the line

Image signal

(raw data)

Run length
conversion

MH-coding

Modulation

White run length 5

Black run length 3

White run length 9

Black run length 3

White run length 1708