In 1987 the ITU-T defined the standard T.6 for the fax machines known as “Group 4″. This standard is completely new and it has no backwards compatibility with “Group 3″ and therefore is not able to work over regular phone lines. The new machines require high speed ISDN lines. The speed of transmission is 64kbit/sec. The resolutions are 200, 300 or 400 dpi. Color faxes are also possible!
The scanning of the image is completely digital, while the previous groups were all analogue. The image is raster (made of dots). During the transmission the data is compressed. The “Group 4″ standard supports two-dimensional image compression – across the line width as well as the line length. These lines can achieve compression ratios of 15:1 for office documents and 20:1 for graphics with a resolution of 400 dpi.
The errors during transmission are NOT corrected. The “broken” lines are either skipped by the recipient or they are printed “as they came”. Usually if the line is good the errors are very rare and they are not critical for the image quality.
There are three classes of “Group 4″ machines. The “Class 1″ machines can only receive and send fax letters. The “Class 2″ machines are capable of receiving “telexes” and “mixed mode” (text and raster) data. The “Class 3″ machines can send and receive letters from any type.
During the 90s and the last 10 years the statistics show that “Group 4″ is still not very popular. It looks like ”Group 3″ machines are still suiting the needs of most people.
More information for the fax standards can be found in Wikipedia: