60.2 Facsimile types

Excluding the document facsimile equipment, all other types per­form specialist functions and are exclusively separate transmit and receive units, for transmission in one direction. Document facsimile types are almost exclusively transceivers.

Table 60.1 summarises the characteristics of the different types of facsimile equipment.

60.2.1 Photofax equipment

It is used for the transmission of full tone photographs of size normally less than A4. Often the received picture is printed on photographic paper. The important requirements are not image size but correct aspect ratio, which is preserved by using the same index of co-operation.

Some scanners are designed to transmit colour separations from 35mm colour negatives using sequential transmission. Photofax equipment may be used, one transmitter to one receiver for example for sending news photographs from field locations, or one transmit­ter broadcasting on a wirephoto distribution network to many re­ceivers. The transmission is simplex; there is no equipment confirmation of reception.

Analogue transmission is used and may be by PSTN, speech band private circuit, high frequency (h.f.) or long wave radio. There are several higher speed proprietary standards for equipment on broad­cast distribution networks. Received image may be taken into an editorial workstation for cropping, manipulation and onward pro­cessing before printing.

Moves towards a digital standard for compressed images pro­gresses slowly, relying on the CCITT/ISO JPEG work. Interwork­ing may be possible at the application layer, but there seems no agreement on the communication layers at present.

Indications are that compression around 1 bit/picture element (8:1) could be achieved for monochrome images.

60.2.2 Weatherfax equipment

These equipments are usually operated on private circuits or radio networks, on a simplex broadcast basis. The analogue transmission involved limits transmission speed and this method of operation is being superseded by point to multipoint computer data transfer. Providing the raw data to the local processor allows action replays of changing conditions, which are not easily appreciated from the printed meteorological charts. The existing network base continues in operation supplementing the data network.

60.2.3 Pagefax equipment

In this area there are no standards, the equipment being tailored to suit individual newspaper requirements. Compression and trans­mission characteristics, reproduction ratio, and image processing are all selected to suit production requirements in the remote print­ing of the full newspaper pages.

The savings in transportation costs and time, by using this equip­ment, funds sophisticated high speed transmission developments, including the transmission of colour separation with sequential receptions on the same receive mechanism.

60.2.4 Mobile equipment

There have been several attempts at mobile facsimile, some using narrow format (100cm) machines. The most popular method was to split the radio speech channel to keep the mobile units in speech contact during facsimile operation, normally transmitting in the direction towards the mobile terminal. These units were costly for their mobile environment and suffered as a result of needing spe­cialised radio equipment.

With the use of the TACS cellular radio system normal G3 equipment can operate when a telephone type interface is available. The Cellular Line Interface connection can be sophisticated and allows automatic answering and call origination by the G3 machine. Calls made between the mobile and normal PSTN connections provide access to the large G3 installed base.

Opinions differ about operations above 4.8kbit/s. Some believe in operating as fast as possible, while the connection is good, whilst others prefer to rely on the improved signal/noise performance at lower modem rates. Most opinions however recommend the use of CCITT ECM.

The question of facsimile use on GSM is another matter because the system codes the voice and there is no synchronous transparent channel available for facsimile. The topic is well studied but no clear proposal has emerged.