How does a Fax Machine Work?
A fax machine, also known as telefax is one of the devices that you can use to send or receive the exact copies of text, images, drawings etc. Since this technology uses the telephone network, you can send printed documents to any remote fax machine on the network. Although internet based telecommunications are widely becoming popular nowadays, fax machines are still used in offices for sending very important documents. As such, it is a good idea to know the mechanism of this wonderful office equipment.
Components of Fax Machine
A fax machine has following components.
- An image sensor (lens) that uses light to scan the material.
- A rotating plate (drum like) where the material to be scanned is placed.
- A fax modem that converts the scanned data into digital format or coverts the digital format into original format.
- A phone line for the transmision of data
- A printer that produces print outs
The image sensor views the whole document consisting of black and white dots. The capacity of the sensor to focus on the material is expressed as pixels per inch. Higher the pixel rate, better the output. The image sensor is fixed on a hand that can move over the printed material.
In older machines which use drum, the printed document will be placed on the drum facing the sensor. However, modern fax machines (Group 3 and Group 4) do not have drums and use paper feed techniques. While receiving a fax message, a pen like device reproduces the text or image on the paper.
Fax modem both encodes and decodes messages depending on whether you send or receive them. It will assign specific frequency depending on the black or white dots. This digital information is transmitted over telephone lines. Fax machines also use various compression techniques to reduce the amount of date to be sent over telephone lines.
Older fax machines used thermal printers which are cheaper and had long life. However, modern fax machines use inkjet printers and laser printers that give excellent print outs.
Although most fax machines produce print outs in black and white, colour fax machines based on ITU-T30e standard can provide colour print outs using ink-jet printers.
Scanning Process
For an A4 paper size, the scanning is done with a resolution of 1728 pixels per line and 1145 lines per page. This equals to nearly 2 million bits of data that the image sensor scans over a single page. Modern fax machines have light source like fluorescent tube to illuminate the document while its photo sensor (usually CCD) will scan one line at one time. The vertical resolutions often vary (standard is 3.85 lines/mm ) and produce results depending on the requirement.
Compression techniques
Fax machines use different compression methods to reduce the amount of data for transmission. They are:
- Modified Huffman (MH)
- Modified READ (MR)
- Modified Modified Read (MMR) and
- Matsushita Whiteline Skip
These compression techniques look for dots of similar colour (like white or black) and compress them so that fax machines can send the data at much faster rate.
