page_title: Fax
section_title: Digital
hierarchical_section_title: Fax / Capabilities / Group / Digital
caption_attribution_description: English: The CCD chip in a fax machine. Only about one quarter of the length is shown. The thin line in the middle consists of photosensitive pixels. The read-out circuit is at left.
context_page_description: Fax, sometimes called telecopying or telefax, is the telephonic transmission of scanned printed material, normally to a telephone number connected to a printer or other output device. The original document is scanned with a fax machine, which processes the contents as a single fixed graphic image, converting it into a bitmap, and then transmitting it through the telephone system in the form of audio-frequency tones. The receiving fax machine interprets the tones and reconstructs the image, printing a paper copy. Early systems used direct conversions of image darkness to audio tone in a continuous or analog manner. Since the 1980s, most machines modulate the transmitted audio frequencies using a digital representation of the page which is compressed to quickly transmit areas which are all-white or all-black.
context_section_description: 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 for transmission of images from satellites.
  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 3 faxes take between 6 and 15 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 conform to the ITU-T Recommendations T.563, T.503, T.521, T.6, T.62, T.70, T.411 to T.417. They are designed to operate over 64 kbit/s digital ISDN circuits. The allowed resolutions, a superset of those in the T.4 recommendation, are specified in the T.6 recommendation.
  Fax Over IP (FoIP) can transmit and receive pre-digitized documents at near-realtime speeds using 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 adapters 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 DPI to 9600 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.
caption: The chip in a fax machine. Only about one quarter of the length is shown. The thin line in the middle consists of photosensitive pixels. The read-out circuit is at left.
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