60.4.4.1 Ecm receiver operation

The receiver knows by the FCS which frames are correctly received. Often, in a thermal paper receiver, the machine will print the copy as received until such time as an error is recognised. It then stops with the data going to memory until corrected data is available. In most practical cases this procedure, and the availability in the receiver of sufficient memory for the next page/partial page, avoids waiting between pages due to printer unavailability.

With the page/partial page correctly received the receiver replies with MCF and the transmitter continues with the next data.

Figure 60.5 shows ECM operation with errors. If errors are de­tected the receiver sends PPR which has an information field of 256 bits, one for each frame of the transmitted block. The receiver sets each bit of PPR to 'O' for a correctly received frame and waits for the transmitter to selectively repeat the unacceptable frames. Pages/partial pages are often corrected with a single retransmission but the cycle is repeated for the then outstanding error frames. When the receiver has all frames correct it finishes printing the page and returns MCF for the next block to follow.

60.4.4.2 Ecm performance

The overhead for G3 ECM is in the transmit direction and is approximately 5% when there are no errors. Normally in non-ECM facsimile transmission a limited number of errors are tolerated due to the image redundancy remaining in the MH and MR coding. The reception at the transmitter of MCF confirms that the document has been satisfactorily received but not necessarily that it is free of errors. There are 2 main user advantages with ECM:

1. The sender is 100% confident that acknowledged pages are complete and received free of error.

2. That as errors are efficiently corrected transmission time (and cost) can be reduced by using higher transmission rates together with MMR coding.

The transmitter has the opportunity from studying the pattern of errors in PPR to reduce the frame size to 64 octets or to change the data rate to maximise throughput. The selective retransmission and large window with minimum turnarounds providing efficient long haul communication even including 3 satellite links.

60.4.5 Printer

There are several methods of printing the facsimile image including electrophotographic (laser or LED), ink jet, thermal transfer and, the most popular, recording on thermal paper.

60.4.5.1 Thermal primers

Heat sensitive paper is passed over a single line thermal head having a heater element for each of the 1728 (A4) picture elements.

A typical method of operation in a thermal printer is at the start of reception to step the paper back from its normal guillotine or tear off position, so that the start of the page is under the thermal head. To avoid having a driver for each element the head may be electri­cally divided into sections for printing in a matrix. The data is latched into a section and printer in 1 to 2.5ms. When all sections of the line are printed the paper is either advanced l/3.85mm for a standard line or l/7.7mm for a fineline.

A standard line may be achieved by printing 2 identical fine resolutions lines. The print energy required is about l/4mJ per black dot, at a temperature on the paper of around 90°C. Print times of between 5ms and 20ms per line are typical, with automatic compen­sation for ambient and head temperature by varying the supply voltage or the print pulse time.

60.4.5.2 Plain paper printers

A number of technologies are available to produce the printed image on plain, office type paper. The advantages of this is that copies feel like other office documents and are generally archival.

Thermal Transfer

The paper is usually in roll form to simplify machine handling. It moves through the machine with, and generally at the same rate, as a thermal transfer film. The thermal head prints in a similar way to normal thermal paper printing and the film is a donor, transferring its image to the plain paper. The thermal transfer film, collected on a take up roll, is used at the same rate as the paper and retains a negative image which may need careful handling for security rea­sons.

The facsimile printer is otherwise identical to a thermal paper printer.

Electrophotographic

A well established technology in office printers uses either a laser or an LED array to discharge a photoconductive drum for offset printing. The latent image on the drum is developed by the attraction of charged normally black toner powder. This image is transferred to a cut sheet and fixed by the combination of heat and pressure at the output rollers.

The additional hardware required for a facsimile machine is bulky, expensive and significantly increases the size and cost. The paper, normal copier type paper, is much cheaper than thermal paper but when allowing for toner, drum replacement and other consumables this saving is much reduced.

InkJet

Ink is a simple way to print on plain paper. The problem has been to keep the ink fluid and flowing until printed and then for it to dry rapidly. This technique is being used in facsimile equipment with a replaceable bubble jet ink cartridge. The cartridge is moved across the paper to print several facsimile lines (typically 16 fine mode lines) at once and the ink bubbled appropriately through a series of apertures, one for each facsimile line. The hardware is not compli­cated or bulky and a range of low cost plain paper machines may emerge for use at about 30 seconds for an A4 page.