Editing Fax (section)

=== Group ===

==== Analog ====
Group 1 and 2 faxes are sent in the same manner as a frame of [[analog television]], with each scanned line transmitted as a continuous analog signal. Horizontal resolution depended upon the quality of the scanner, transmission line, and the printer. Analog fax machines are obsolete and no longer manufactured.  ITU-T Recommendations T.2 and T.3 were withdrawn as obsolete in July 1996.

* Group 1 faxes conform to the ITU-T Recommendation T.2. Group 1 faxes take six minutes to transmit a single page, with a vertical resolution of 96 [[scan line]]s per inch. Group 1 fax machines are obsolete and no longer manufactured.
* Group 2 faxes conform to the ITU-T Recommendations T.3 and T.30. Group 2 faxes take three minutes to transmit a single page, with a vertical resolution of 96 scan lines per inch. Group 2 fax machines are almost obsolete, and are no longer manufactured.  Group 2 fax machines can interoperate with Group 3 fax machines.

==== Digital ====
[[File:Dacom DFC-10.jpg|thumb|upright=1.2|The Dacom DFC-10—the first digital fax machine<ref name="etd.ohiolink.edu"/>]]
[[File:Faxchip.jpg|thumb|upright=1.2|The [[Charge-coupled_device|CCD]] single-row image sensor in a fax machine. Only about one quarter of the length is shown. The thin line in the middle consists of photosensitive [[pixel]]s. The read-out circuit is at left.]]
A major breakthrough in the development of the modern facsimile system was the result of digital technology, where the analog signal from scanners was digitized and then compressed, resulting in the ability to transmit high rates of data across standard phone lines. The first digital fax machine was the [[Dacom]] Rapidfax first sold in late 1960s, which incorporated digital data compression technology developed by [[Lockheed Corporation|Lockheed]] for transmission of images from satellites.<ref name="etd.ohiolink.edu"/><ref name="Fax 1971, Pages 112-114"/>

[[Group 4 compression|Group 3 and 4 faxes]] are digital formats and take advantage of digital compression methods to greatly reduce transmission times.

* Group 3 faxes conform to the ITU-T Recommendations T.30 and T.4. Group&nbsp;3 faxes take between 6 and 15&nbsp;seconds to transmit a single page (not including the initial time for the fax machines to handshake and synchronize). The horizontal and vertical resolutions are allowed by the T.4 standard to vary among a set of fixed resolutions:
** Horizontal: 100 scan lines per inch
*** Vertical: 100 scan lines per inch ("Basic")
** Horizontal: 200 or 204 scan lines per inch
*** Vertical: 100 or 98 scan lines per inch ("Standard")
*** Vertical: 200 or 196 scan lines per inch ("Fine")
*** Vertical: 400 or 391 (note not 392) scan lines per inch ("Superfine")
** Horizontal: 300 scan lines per inch
*** Vertical: 300 scan lines per inch
** Horizontal: 400 or 408 scan lines per inch
*** Vertical: 400 or 391 scan lines per inch ("Ultrafine")
* Group 4 faxes are designed to operate over 64&nbsp;kbit/s digital [[ISDN]] circuits. They conform to the ITU-T Recommendations 
** T.563 (Terminal characteristics for Group 4 facsimile apparatus), 
** T.503 (Document application profile for the interchange of Group 4 facsimile documents),
** T.521 (Communication application profile BT0 for document bulk transfer based on the session service),
** T.6 (Facsimile coding schemes and coding control functions for Group 4 facsimile apparatus) specifying resolutions, a superset of the resolutions from T.4,<ref name="T6"/>
** T.62 (Control procedures for teletex and Group 4 facsimile services),
** T.70 (Network-independent basic transport service for the telematic services), and
** T.411 to T.417 (concerned with aspects of the [[Open Document Architecture]]). 

Fax Over IP ([[FoIP]]) can transmit and receive pre-digitized documents at near-realtime{{vague|date=March 2022}} speeds using [[T.38 ITU-T recommendation|ITU-T recommendation T.38]] to send digitised images over an [[IP network]] using [[JPEG]] compression. T.38 is designed to work with [[VoIP]] services and often supported by [[analog telephone adapter]]s used by legacy fax machines that need to connect through a VoIP service. Scanned documents are limited to the amount of time the user takes to load the document in a scanner and for the device to process a digital file. The resolution can vary from as little as 150&nbsp;DPI to 9600&nbsp;DPI or more. This type of faxing is not related to the e-mail–to–fax service that still uses fax modems at least one way.