Canon fax basic 2000 서비스 매뉴얼
5–13
1
2
3
4
5
6
ït
çi
G3 FACSIMILE COMMUNICATIONS
(3) Coding of one scanline
Coding is completed by arranging the MH codes of each run consecutively.
The code sequence for one scanline for which coding has been completed
is called “data.”
Fig. 5-13 MH-coded Scanline
• Information compression ratio of MH coding scheme
In case the scanline is MH-coded, the volume of information of 1,728 bits
of raw data is compressed to 27 bits of coded data.
The compression ratio is calculated as follows:
Information compression ratio = Information size (bits) after compression
/ Information size (bits) before compression = 27/1728 = 1/64
Although the transmission time of the image data is not determined merely
by the size of the data to be transmitted, the effect of data compression is to
reduce the transmission time considerably.
Image Data Unsuitable for Compression
If coding is performed, this does not necessarily mean that the size of the
information of any document can be compressed to reduce the transmis-
sion time. The effect of coding is reduced on documents where short white
runs and short black runs occur alternately (for example, halftone photo-
graphs in newspapers).
Though the following example will hardly occur, let’s consider a scanline
along which black and white runs one bit each exist alternately.
Fig. 5-14 Example of Scanline Unsuitable for Compression
White run length 1
→
6 bits of MH coding “000111”
Black run length 1
→
3 bits of MH coding “010”
For this reason, the size of the information in this scanline is as follows:
Size of information = 6
×
1728/2 + 3
×
1728/2 = 7776 (bits)
MH-coded
scanline
scanline
EOL
Data
1100
10100
10
011000
10
Data
Fill
EOL
Data
EOL
Data
Fill
Total 27 bits
00101101
1728 bits