Introduction

UNIT 5 (60) Facsimile transmission

Write out the words and word combinations which are still unknown to you and learn them.

3. Read and translate the text 11 – 16

4. Put five – seven questions to this part of the text

PART 1 (60.1 – 60.3)

Facsimile, or fax, as it is known for short, is the technique for transmitting a copy of an original page or document to a remote location. It is not taken in the original full sense of an exact copy or reproduction but is a scanned and reproduced copy. The received copy is usually desired as a crisp, clean, good contrast document regardless of the original state, a requirement usually met by mod-era equipment.

The initial invention was by Alexander Bain in 1843 using a pendulum scanner which made contact with characters of raised metal type. The current flowing was passed, by the pendulum in synchronisation at the receiver, through damp chemically impreg­nated paper which then discoloured. This invention operated on telegraph wires complementary to Morse and predating the tele­phone.

One of the early uses of facsimile commercially was for the transmission of photographs for newspapers. In this case it was not just to mark the paper or not, but to mark with a density similar to the original photograph. These were dedicated users with skilled operators, at a time when almost any effort was justified to obtain pictures, not to be compared with today's operation.

Technical problems such as synchronisation, detection of the image at the transmitter and exposure at the recorder, amplification and the transmission by radio and telephone line, were solved step by step.

In parallel with these solutions the search continued for an ideal recording medium with the advantages/disadvantages of photo­graphic, electrolytic, carbon, wax coated, electrostatic, thermal and now plain paper recording, being tested in products and voted on by the users.

Throughout these equipment designs a continuous compromise was necessary concerning resolution and transmission time. The results had to be good but not too good because speed was always of the essence.

Facsimile development, like all communication evolutions, wanted higher bandwidth on existing circuits or faster circuits at the same cost. It may not be too high a claim to make that facsimile developments have pressed network use to its highest efficiency, often exceeding the expectations of the network providers.

The development of weather map transmission equipment from the mid 1940s allowed meteorologists access to International charts, compiled in National Centres, so they could make their own local forecasts. A clear case for the immediate distribution of information in the format preferred by the user. Yesterday's weather forecast is not of much value.

In the mid 1950s page facsimile units for the transmission of whole newspaper pages started to become available. These large format machines, with high resolution to preserve the half tone dot structure, are used to transmit the page layouts for remote printing prior to local distribution. With printing deadlines paramount, high scanning rates and wideband networks encouraged facsimile devel­opment and manufacturing teams to keep pressing at the then known limits.

In the years before 1980 (when the CCITT G3 Recommendations were published) there were really only specialist applications for facsimile. Apart from those mentioned there were uses by govern­ment, military, police, for telegrams, colour transmissions received as separations and as colour prints, cloud photograph receptions from weather satellites, etc. Many of these applications had differ­ent standards and formats, as demanded by the application, and was acceptable since interworking was not necessary.

For document facsimile a different approach was necessary. Interoperation had been the desired aim since 1968 when Group 1 was standardised. No one manufacturer expected to supply the whole requirement and it was expected that business facsimile use, like the telephone, could be to new destinations as yet unknown.

The growth in G3 machines after 1980 was due to the right combination of features:

1. Ease of operation.

2. Adequate quality, convenient size and compatibility with the working environment.

3. Ability to transmit the documents already available in the office.

4. Reasonable operating costs, both line and consumables.

5. Reliable operation.

6. Growing business base.

G3 equipment is stand alone and easy to connect to telephone line and to the power source. Its growth in use has been due to the application of practical CCITT recommendations which reflect the technological and manufacturing possibilities. Coupled with this was the development and availablility of high sensitivity thermal paper, which ensured the realistic equipment purchase price and operating costs.

Дата добавления: 2015-03-31