Reception of facsimile images

30.11.2013

It often happens that a facsimile image is blurred, distorted or 'torn.' You may find that your fax receiver is defective or lost sensitivity. Before making such conclusions presented below examine the material on the radio waves.

Short waves are transmitted over long distances by reflection from the ionosphere surrounding  the earth. The reflection of radio waves varies depending on the time of day, time of year and  other factors. Therefore, the strength of incoming radio waves constantly changing. That's why  your fax images are fuzzy. Passage of radio waves also depends on the  frequency used. Some frequency signals reflected better, while other signals  frequencies practically not recognized. Therefore, virtually all facsimile broadcast station  uses several frequencies within the range of 2 to 25 MHz for transmission of identical data. You  can select the optimum frequency for best reception.
Multipath propagation of short waves can cause the echo attenuation and  interference.

To receive a clear facsimile image is recommended to select the frequency at which no  observed signal interference. Automatic frequency selection function automatically  selects the frequency at which the signal level is maximum, however, is not always successful for  optimal reception of the image.
So you need to select the optimum frequency for receiving according to the conditions of passage  of the signal depending on the time of day (morning, afternoon, night), season (spring, summer, autumn and  winter) and distance from the station.

Terminology

Scan

Fax receiver receives the fax signal from the air and converts it into an image. In the process of converting the print head starts to print an image point by point, starting from the left edge to the right fax paper, returning to the left edge of the fax paper from the right and typing a new line from left to right.These steps are repeated until, until the signal is received complete image. These movements printhead device called 'scan'.

Main scanning:

Movement of the print head from left to right. In a conventional scan, the print head prints dots at eight 1 mm.

Re-scan:

Paper movement

Scanning line density:

The number of printed lines by moving the paper 1 mm Scanning speed (SCAN): main scanning speed (scanning expressed in minute). E.g., 120 SPM, for example, means 120 scans per minute.

The proportionality coefficient (IOC):

If the document (original) sent from the weather station does not match the size of paper in the fax receiver, the original image is reduced proportionally to the size of fax paper with the length and width of the original document. This value is the coefficient of proportionality.

Record in semitones (HALF):

Using the template points of different density (roughing and fine print) provided mapping clouds in grayscale at photos from meteorological satellite.

Start signal:

This is the first facsimile signal broadcast transmission from the weather station. Signal indicates the beginning of fax transmission. When receiving this signal, the receiver automatically starts to receive fax broadcast signals.

The synchronization signal:

The signal which follows the start signal, and after image transfer, if the signal is successfully synchronized. The receiver uses this signal to automatically adjust the density and the scan rate. This signal is always required for the automatic reception, and receiving a manual reception programmable.

Signal similarity:

Signal following the phase signal and always passed by the end of the transmission image. The receiver uses the signal for phase matching (sync) with manual reception and forced entry.

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