The receiving fax machine interprets the tones and reconstructs the image, printing a paper copy.

[1] Early systems used direct conversions of image darkness to audio tone in a continuous or analog manner.

[2] However, they have largely been rendered obsolete by Internet-based technologies such as email and the World Wide Web, but are still used in some medical administration and law enforcement settings.

Around 1900, German physicist Arthur Korn invented the Bildtelegraph, widespread in continental Europe especially following a widely noticed transmission of a wanted-person photograph from Paris to London in 1908,[14] used until the wider distribution of the radiofax.

[15][16][17] Its main competitors were the Bélinographe by Édouard Belin first, then since the 1930s the Hellschreiber, invented in 1929 by German inventor Rudolf Hell, a pioneer in mechanical image scanning and transmission.

[19][20][21] On May 19, 1924, scientists of the AT&T Corporation "by a new process of transmitting pictures by electricity" sent 15 photographs by telephone from Cleveland to New York City, such photos being suitable for newspaper reproduction.

A photograph of President Calvin Coolidge sent from New York to London on November 29, 1924, became the first photo picture reproduced by transoceanic radio facsimile.

Sensing a new and potentially golden opportunity, competitors soon entered the field, but the printer and special paper were expensive luxuries, AM radio transmission was very slow and vulnerable to static, and the newspaper was too small.

After more than ten years of repeated attempts by Finch and others to establish such a service as a viable business, the public, apparently quite content with its cheaper and much more substantial home-delivered daily newspapers, and with conventional spoken radio bulletins to provide any "hot" news, still showed only a passing curiosity about the new medium.

[25] By the late 1940s, radiofax receivers were sufficiently miniaturized to be fitted beneath the dashboard of Western Union's "Telecar" telegram delivery vehicles.

In 1964, Xerox Corporation introduced (and patented) what many consider to be the first commercialized version of the modern fax machine, under the name (LDX) or Long Distance Xerography.

In later years it would be combined with copier equipment to create the hybrid machines we have today that copy, scan and fax.

This audio tone was then transmitted using an acoustic coupler (a speaker, in this case) attached to the microphone of a common telephone handset.

At the receiving end, a handset's speaker was attached to an acoustic coupler (a microphone), and a demodulator converted the varying tone into a variable current that controlled the mechanical movement of a pen or pencil to reproduce the image on a blank sheet of paper on an identical drum rotating at the same rate.

In 1985, Hank Magnuski, founder of GammaLink, produced the first computer fax board, called GammaFax.

[29] Although businesses usually maintain some kind of fax capability, the technology has faced increasing competition from Internet-based alternatives.

In July 2017 the United Kingdom's National Health Service was said to be the world's largest purchaser of fax machines because the digital revolution has largely bypassed it.

[3] There are several indicators of fax capabilities: group, class, data transmission rate, and conformance with ITU-T (formerly CCITT) recommendations.

Since the 1968 Carterfone decision, most fax machines have been designed to connect to standard PSTN lines and telephone numbers.

[52] Modified READ, specified as an optional two-dimensional coding scheme in T.4, encodes the first scanned line using MH.

[48] 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.

This is fed into an arithmetic coder, which adds only a small fraction of a bit to the output sequence if the more probable pixel is then encountered.

The ITU-T T.85 "fax profile" constrains some optional features of the full JBIG standard, such that codecs do not have to keep data about more than the last three pixel rows of an image in memory at any time.

(JBIG implements a similar technique called "typical prediction", if header flag TPBON is set to 1.)

Group 3 fax machines transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a resolution of 204×98 (normal) or 204×196 (fine) dots per square inch.

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 bit/s.

[57] As a precaution, thermal fax paper is typically not accepted in archives or as documentary evidence in some courts of law unless photocopied.

Faxes are received as attached TIFF or PDF files, or in proprietary formats that require the use of the service provider's software.

With the surging popularity of mobile phones, virtual fax machines can now be downloaded as applications for Android and iOS.

These applications make use of the phone's internal camera to scan fax documents for upload or they can import from various cloud services.