Mary S. Moore
The basic block construction of a facsimile apparatus is shown in Fig. 1(a). The classical and modern means used to achieve the functions of each block are shown in the schematic diagrams (b) and (c) of Fig. 1.
(figure available in print form)
In the photo-electric conversion block at the transmitter and the recording block at the receiver, the older drum scanning system, in which the original or recording paper was wrapped around a drum and scanned, has advanced to the flat-bed scanning system, in which scanning is performed while the original remains flat. This improvement has made operation much simpler. Also, in place of mechanical scanning, electronic scanning by using a solid-state image sensor or recording head was introduced. As a result this, much higher speed and reliability were achieved.
For the processing and transmission of signals, recent developments have been proceeding from analog to digital technologies. In previous analog transmission systems, amplitude modulation (AM) or frequency modulation (FM) was widely used. Also, vestigial sideband (VSB) transmission was partially used as the band compression technology. When such analog transmission systems are used over the normal telephone line, they require about three to six minutes to send a single A4-size document. The quality of the receiving document suffered greatly because of the telephone line transmission characteristics. Today’s high-speed transmission method uses modern digital-processing technology to perform redundancy reduction, encoding, and conversion to binary signals. These signals were then transmitted by using a data transmission modem, resulting in a transmission time for an A4 page of one minute or less.
As scanning systems supported by advancements in semiconductor technology move to solid-state electronics, and as hardware and software continue to progress, these new developments contribute to the improved cost performance of facsimile apparatus.
For facsimile transmission, the public-switched telephone network is most popular in many countries, but in Japan a special public facsimile communications network has also been made practical, thus allowing low transmission costs. As public data networks and future ISDN’s become practical, high-speed digital transmission lines will likely be used for even faster and higher-resolution transmissions.
Photo-electric Reading and Writing Technique
are the two processes involved in facsimile transmission. When the fax machine cord is plug into an AC power line, and the switch is
on
, the document is ready to be placed in the transmitter for duplication. The synchronous motor that keep the drums rotating at a constant speed, allows the scanner and recorder to move line by line picking up the image of the original document in the form of light impulses. These synchronized impulses pass through a precision optical system and hit the photo-electric cell where they are converted to an analog or digital signals which are coded and transmitted over ordinary telephone lines. The synchronized impulses sends information which cause the rotation of the scanner and recorder to go at the same speed, allowing both pictures to start at the edge of paper. This is call synchronization of timing. At the receiving end, the process is reversed and the electronic signals are coverted back into printed matter to produce a facsimile, or exact copy, of the original.
Many kinds of recording methods are used, such as spark recording, electrolytic recording, ink-jet recording, electrostatic recording, electrophotographic recording, and thermal recording. These recording methods all have their own merits and demerits in the areas of recording speed, resolution, reproduction of halftones, sensitivity to environmental conditions, and economy. The methods in widest use at present are thermal recording and electrostatic recording. One reason these systems have come into such broad use is their good adaptability to solid-state scanning technology and digital signal-processing technology.
The thermal recording method is shown in Fig. 3. The recording paper construction is shown in Fig. 3(a), a layer of color-developing emulsion being coated on a base paper. The color-developing layer includes a dispersion of two components, A and B, which chemically react with each other in the presence of heat, thus developing the color. These components remain separated under normal temperatures so that no color is produced.
(figure available in print form)
The recording head uses a row of heating resistors spaced at minute intervals on an insulating board. Various means, including thin-film, thick-film, and semiconductor technology, are used to fabricate the thermal head, but thin-film technology is used most widely.
Thermal recording is a type of direct recording, which does not require the additional steps of developing and fixing. Therefore, its recording construction is simpler, and maintenance is easier than in electrostatic recording. On the other hand, electrostatic recording has higher speed and higher resolution than thermal recording, and its hard copies have superior preservative qualities. Thermal reading is the most widely used for document facsimile in offices, but there is also a strong demand for electrostatic recording, especially for the transmission of legal documents.
One of the greatest obstacles to the expansion of the facsimile market has been the problem of incompatibility. Incompatibility between facsimile machines causes distortion, compression or expansion of copy, and sometimes complete inability to communicate. There are numerous factors involved with this problem of incompatibility; some of the more common ones are the use of both FM and AM modes and variations in transmission speed and resolution.
Part of the responsibility for this lack of compatibility was with the manufacturers who had exhibited little desire to conform to an industry standard. In addition, for many years the technology of facsimile was still developing so that many within the industry feared that standardization would come at the expense of further advancement and improvement in the field. Each company had been hoping that by introducing a more advanced line of machines, it could dominate the market. thereby creating a de facto standard. In recent years, however, the situation has improved quite markedly, with manufacturers offering a variety of speeds on a single machine and with the introduction of international (standards) guidelines provided by the CCITT.
The CCITT (Consultative Committee for International Telephone and Telegraph) is a committee of representatives from member countries of the United Nations that has been set up to study telecommunications equipment and to recommend standards in design and operation. Although subcommittees meet continuously, a plenary session is held once every four years, at which time subcommittee recommendations are considered for ratification.
All machines that were manufactured over 30 years during this time were divided by the CCITT into four groups: Group I and II machines employing slower transmission speeds. Group I includes analog machines which can transmit and 8 1/2” x 11” page in four or six minutes, and Group II includes analog machines which can transmit an 8 1/2” x 11” page in two or three minutes. Group III includes digital machines using data compression to transmit an 8 1/2” x 11” page in one minute or less. Group IV machines have transmission speeds that are six times faster than those of Group III; however, the telecommunications bandwidth required by Group IV facsimile machines has only recently become available and still is so costly that few users take advantage of its high speed.
Now that most of the facsimile units can communicate with one another, technological advancements in the field have increased and manufacturers are producing what the users want: less costly, high-speed models that can fall back and communicate with slower machines. These models are also able to communicate with word processors and serve as printers for remote computers and data processing terminals.