The Fax Machine – Translation and transmission
Alexander Bain patented the first facsimile machine design
in 1843. In 1850 Frederick Bakewell had obtained a patent for what he called an
image telegraph, both of these early attempts were not viable for large scale
use due to the fact that they produce poor quality image because they were not
properly synchronised. However in 1861 Giovanni Caselli invented the
Pantelegraph, and this became the first commercial telefax service between
Paris and Lyon. Early facsimile machines, or fax machines, as we know them
today involved a rotating drum. The document to be sent was attached to the
drum facing outwards where a small photo sensor attached to a moveable arm would
move across the paper as the drum rotated. This process is s similar to how a
lathe works. Modern fax machines have a bar of photo sensors that enable whole
lines to be read at once, while the document is fed through rollers instead of
being attached to a drum.
The sensor reads the paper as it passes by, picks up white
or black for a given area and then the information is electronically encoded as
binary. White dots are 0’s and black dots are 1’s. The data is then
sequentially transmitted via the phone lines to the recipient fax machine,
where it is decoded and read. The information is then applied to the paper in
the form of black dots via the print head. Fax machines also come in a number
of exclusive groups in which they only communicate to others of the same type.
These are broken down into four different categories. Groups I and II are
extremely low detail, much of the information read and encoded is lost. Group
III fax machines are the standard office variety, they are more sensitive and
will not lose much detail during the encoding process. Group IV fax machines
are the top of the line variety, they are able to recognise and encode a vastly
larger amount of detail then the other groups. Group IV’s also need their own
special form of phone line in order to communicate the data to the fax machine
at the other end.
A single line of a document can be represented bin 1728
bits, a standard group III fax contains 1145 lines and therefore, there is a
little under two million bits of information in a single document. To reduce
the volume of information that needs to be transmitted the data can be
compressed using a number of compression techniques. These techniques are the
Modified Huffman, Modified Read and Modified Read Read. These compression
algorithms function by looking for groups of black of white bits, this
drastically reduces the time in which a document can be transmitted. A fax
machine can transmit at 14400bps or bits per second, however depending on the
noise of the line in which the information is being transmitted this can go as
low as 2400bps. This lowering of the speed of communication ensures that the
information transmitted is not lost.
With the advent of the World Wide Web, it has become
possible to send documents using fax machines connected to the internet. This
requires the information to be encoded differently, using TCP/IP (Transmission Control Protocol/Internet Protocol) which involves
reading the whole document, encoding it, compressing it and then transmitting
it or RTP (Real-Time Protocol) which allows a documents information to be read,
encoded, compressed and sent as soon as it is ready. No matter how a fax
machine sends its data, it must know where to send it. For fax machines on the
phone lines, a special phone number is required but fax machines connected to
the internet only require an IP address.
In the tutorial of week 2 we were given a task to come up
with our own encoding and decoding processes for a human fax machine, and while
ours wasn’t as complicated as a normal fax machine it was on the right track.
We decided to encode the information to be sent as a variable number of
sequential taps that represented a coordinate on the piece of paper. Once
coordinates had been sent, lines were to be drawn between them. This worked
reasonably well, but we lost a lot of detail because we could not encode enough
information without taking too much time.
References:
Chris Woodford, 2
January 2016, Fax Machines, viewed 13 August 2016, <http://www.explainthatstuff.com/faxmachines.html>
Marshall Brain, 2 October 2001, How Fax Machines Work, HowStuffWorks.com, viewed 13 August 2016, <http://electronics.howstuffworks.com/gadgets/fax/fax-machine.htm>
O'Hearn, B 1992, 'WHAT TO LOOK FOR WHEN BUYING A FAX', Journal
of Accountancy, vol. 174, no. 3, pp. 79-81.
Tebbs, RG 1999, 'Real-Time IP Facsimile: Protocol and
Gateway Requirements', Bell Labs Technical Journal (John Wiley & Sons,
Inc.), vol. 4, no. 2, p. 128.
Wikipedia, 13 July 2016, Alexander
Bain, Wikipedia.org, viewed 13 August 2016, < https://en.wikipedia.org/wiki/Alexander_Bain_(inventor)#Facsimile_machine>
