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So far I've mentioned a bit of jargon without really explaining it,
before I describe the configuration necessary to use QFAX, here's
some explanation.
QFAX
-
(in capitals) is a suite of programs, comprising, inter alia,
qfax
, (lower case), a program for sending and receiving G3
facsimile documents using a Class 2.0/Class 2 fax modem.
CCITT
-
International Telegraph and Telephone Consultative Committee. A
international body that standardises communications methods. This
organisation is now known as the ITU-TSS, but is referred to as the CCITT
in some parts of this document.
G3
-
Describes the format of facsimile and the physical method of
transmission. There were earlier standards known as G1 (Group One) and
G2 (Group Two), and there is also a proposal for G4 (Group Four), which
will handle such complexities as ISDN and grey scales; that is if the G4
standard ever emerges in a form that can actually be implemented. G3 is
the method used by just about every fax machine and fax modem is use
today.
G3 also defines methods of encoding facsimile documents, that is the
process of converting a bitmap (the black and white dots) into the
compressed form that is transmitted. This encoding (and hence
compression) is one of the factors that makes G3 six times faster than
G1 and three times faster than G2. G3 supports two types of encoding.
1-D
-
1-D encoding compresses individual pixel lines in a document. It uses
run length encoding, that is a scheme of describing the length of a run
of consecutive dots. The values used to describe a run of a particular
colour have been statistically derived from analysis of typical fax
documents to minimise the size of the resultant G3 encoded file (for
example, white runs are more common (and usually longer) than black
runs). The downside of the efficient packing is that complex algorithms
are required.
2-D
-
2-D encoding uses 1-D to compress a `reference line' and then encodes a
number of subsequent lines as differences from the reference line. This
can result in encoded files that are 30% smaller than 1-D files, (hence
shorter transmission times), but the encoding and decoding algorithms
are very much more complex and slower. There is also the risk that if a
reference line gets corrupted during transmission, then every line that
depends on it will also be corrupt.
Very few existing facsimile machines support 2-D encoding. 2-D is
supported by all QFAX programs.
EIA
-
Electronic Industries Association. A standards body that has (allegedly)
standardised, inter alia, command sets for controlling fax
modems. The earlier Class One command set requires that the computer
software does much of the work. The Class 2.0/2 command set places this
burden on the modem firmware.
When fax is sent using standalone fax machines, the two machines
negotiate capabilities and `agree' on a method they both support. For
example, if you had a 2-D capable machine and tried to fax a 1-D capable
machine, the two machines would negotiate to use 1-D, as this is all the
receiving machine can handle; this is done before the machine scans and
encodes the document.
When you send a fax using a computer and fax modem, you have encoded the
G3 data before the modems start negotiating. Therefore the sending
program, qfax
, cannot adapt the document coding to that of the
receiving machine. In this case, qfax
will report that the remote
does not support the necessary options if your fax encoding surpasses
the capability of the remote machine.
I strongly recommend that you only use 1-D coding unless you know you
are corresponding with a machine (or software) that can handle 2-D.
Due to differences between modem chip set manufactures, fax modems may
require data either with least significant bit (lsb) first or most
significant bit (msb) first. If you send or receive data from your
modem with the wrong bit order, then you (or your respondent) will be
unable to decode it. Your fax modem manual may tell you the setting you
need.
In Class 2 mode. fax modems require you send data to the modem lsb first
but return it to you msb first. This is the QFAX default mode for
Class 2.
In Class 2.0 mode. fax modems default to lsb for send and receive.
This is the QFAX default mode for Class 2.0.
QFAX v2.5 and later can handle both MSB and LSB format fax files,
see section QFAX File Format. QFAX 2.6 will automatically determine
received bit order.
This is unlikely to be an issue for Class 2.0 modems.
QFAX also includes a utility swapqfax
that will reverse the bit
order in a G3 file.
You may have to determine the bit order your modem requires by
experimentation! For example, get a friend to send you a fax. Try to
view it using qfv
. If this fails, reverse the bit order using
swapqfax
. OK now ? If so, change the setting in your
`qfax_dat' file.
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