Note that Super Group 3 faxes use V.34bis modulation that allows a data rate of up to 33.6 kilobits per second.
As well as specifying the resolution (and allowable physical size of the image being faxed), the ITU-T T.4 recommendation specifies two compression methods for decreasing the amount of data that needs to be transmitted between the fax machines to transfer the image. The two methods are:
- Modified Huffman (MH)
- Modified READ (MR)
Modified Huffman (MH) is a codebook-based run-length encoding scheme optimized to efficiently compress whitespace. As most faxes consist mostly of white space, this minimizes the transmission time of most faxes. Each scanned line is compressed independently of its predecessor and successor.
Modified Read (MR) encodes the first scanned line using MH. The next line is compared to the first, the differences determined, and then the differences are encoded and transmitted. This is effective as most lines differ little from their predecessor. This is not continued to the end of the fax transmission, but only for a limited number of lines until the process is reset and a new "first line" encoded with MH is produced. This limited number of lines is to prevent errors propagating throughout the whole fax, as the standard does not provide for error-correction. MR is an optional facility, and some fax machines do not use MR in order to minimize the amount of computation required by the machine. The limited number of lines is two for standard resolution faxes, and four for fine resolution faxes.
The ITU-T T.6 recommendation adds a further compression type of Modified Modified READ (MMR), which simply allows for a greater number of lines to be coded by MR than in T.4. This is because T.6 makes the assumption that the transmission is over a circuit with a low number of line errors such as digital ISDN. In this case, there is no maximum number of lines for which the differences are encoded.
Matsushita Whiteline Skip
A proprietary compression scheme employed on Panasonic fax machines is Matsushita Whiteline Skip (MWS). It can be overlaid on the other compression schemes, but is operative only when two Panasonic machines are communicating with one another. This system detects the blank scanned areas between lines of text, and then compresses several blank scan lines into the data space of a single character.
Group 3 fax machines transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a resolution of 100×200 or 200×200 dots per inch. The transfer rate is 14.4 kilobits per second or higher (but fax machines support speeds beginning with 2400 bits per second). The transferred image formats are called ITU-T (formerly CCITT) fax group 3 or 4.
The most basic fax mode transfers black and white only. The original page is scanned in a resolution of 1728 pixels per line and 1145 lines per page (for A4). The resulting raw data is compressed using a modified Huffman code optimized for written text, achieving average compression factors of around 20. Typically a page needs 10 s for transmission, instead of about three minutes for the same uncompressed raw data of 1728×1145 bits at a speed of 9600 bits per second. The compression method uses a Huffman codebook for run lengths of black and white runs in a single scanned line, and it can also use the fact that two adjacent scanlines are usually quite similar, saving bandwidth by encoding only the differences.
There are different fax classes, including Class 1, Class 2, and Intel CAS.
Fax machines from the 1970s to the 1990s often used direct thermal printers as their printing technology, but since the mid-1990s there has been a transition towards thermal transfer printers, inkjet printers, and laser printers.
One of the advantages of inkjet printing is that inkjets can affordably print in color; therefore, many of the inkjet-based fax machines claim to have color fax capability. There is a standard called ITU-T30e for faxing in color; unfortunately, it is not yet widely supported, so many of the color fax machines can only fax in color to machines from the same manufacturer.
An alternative to a physical fax machine is to make use of computer software which allows people to send and receive faxes using their own computers.
- Margolis, Andrew. 1995. The Fax Modem Sourcebook. Hoboken, NJ: Wiley. ISBN 0471950726.
- Kauffman, Maury. 1998. Internet and Computer Based Faxing, Second Edition: The Complete Guide to Understanding and Building IP and G3 Fax Applications. Manhasset, NY: CMP. ISBN 1578200245.
- McConnell, Kenneth R., Dennis Bodson, and Stephen Urban. 1999. Fax: Facsimile Technology and Systems. Norwood, MA: Artech House Publishers. ISBN 0890069441.
All links retrieved March 26, 2017.
- A Brief History of Facsimile - HFFAX.de
- The Historical Evolution of Fax - Technikum29: Museum of Calculator, Computer and Communication Technology
- FAQ: How can I send a fax from the Internet?
- Group 3 Facsimile Communication - A 1997 essay with technical details on compression and error codes, and call establishment and release
- Scanning & Faxing Tips