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The jargon this far

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.

Encoding caveat

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.

G3 Bit Order

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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