Information Technology Reference
In-Depth Information
15.11 Fax transmission
Facsimile (fax) transmission involves the transmission of images over a telephone line using
a modem. A stand-alone fax consists of:
•
An image scanner.
•
A graphics printer.
•
A transmission/reception modem.
The fax scans an A4 image with 1142 scan lines (3.85 lines per millimetre) and 1728 pixels
per line. The EIA and ITU originally produced the RS-328 standard for the transmission of
analogue voltage levels to represent different brightness. The ITU recommendations are
known as Group I and Group II standards. The Group III standard defines the transmission of
faxes using digital transmission with 1142
1728 pixels of black or white. Group IV is an
extension to Group III but allows different gray scales and also colour (unfortunately it re-
quires a high bit rate. )
×
1728) scanned elements. If each element
is scanned for black and white, then, at 9600 bps, it would take over 205 s to transmit. Using
RLE (run length encoding) coding can drastically reduced this transmission time.
An A4 scan would consist of 1 976 832 (1142
×
15.11.1 Modified Huffman coding
Group III compression uses modified Huffman code to compress the transmitted bit stream.
It uses a table of codes in which the most frequent run lengths are coded with a short code.
Typically, documents contain long runs of white or black. A compression ratio of over 10:1
is easily achievable (thus a single-page document can be sent in under 20 s, for a 9600 bps
transmission rate). Table 15.5 shows some code runs of white and Table 15.6 shows some
codes for runs of black. The transmitted code always starts on white code. The codes range
from 0 to 63. Values from 64 to 2560 use two codes. The first gives the multiple of 64 fol-
lowed by the normally coded remainder.
Table 15.5
White run length coding
Run length
Coding
Run length
Coding
Run length
Coding
0
00110101
1
000111
2
0111
3
1000
4
1011
5
1100
6
1110
7
1111
8
10011
9
10100
10
00111
11
01000
12
001000
13
000011
14
110100
15
110101
16
101010
17
101011
18
0100111
19
0001100
61
00110010
62
00110011
63
00110100
EOL
00000000001
For example, if the data to be encoded is:
16 white, 4 black, 16 white, 2 black, 63 white, 10 black, 63 white
it would be coded as:
101010 011 101010 11 00110100 0000100 00110100
