FAX

Fax mode is commonly used to decode weather maps, this is also referred to as HFFAX mode. It is also possible to decode pictures directly from weather satellites. Since this two modes use different modulation methods, it is necessary to select which mode should be decoded.

You may also want to visit the following web site for more information about FAX mode, including station schedules:

http://www.blackcatsystems.com/radio/fax.html

 

 Two buttons, called HF and SAT, select the mode. Satellite mode has recently been added, so I would be interested in reports of how well this mode works.

 

Next, the correct speed must be selected. The speed is measured in lines per minute (LPM), and possible selections are 60, 90, 120, 240 LPM. For HFFAX, 120 LPM is almost always used.  For POES satellites, 120 LPM is generally used, for GEOS, 240 LPM.

 

The IOC (Index of Cooperation) must be selected. For HF FAX, this is almost always 576, as is also used for POES satellites. GEOS satellites use 288.

 

You may select whether to display gray scale, or line mode (black and white). Gray scale should be used for satellite images (which are also sent via HFFAX stations) and other images where shades of gray are sent. Line mode is useful when weather maps are sent that have no shading information. It is possible to display these in gray mode, but improper tuning and noise may affect the picture quality. In these cases, line mode may provide a better image quality.

 

Finally, is also an AUTO mode, this attempts to use the phasing and start/stop tones sent by HFFAX stations to properly synchronize to the fax signal. When a start tone is detected, a blue circle is displayed under the tuning display (described below), on the left side. MultiMode will now clear the display, lock onto the fax signal, and properly align the image. When a stop tone is detected, a red circle will light up under the tuning display. If auto-save is enabled in the Preferences, the fax image will automatically be saved to disk with a unique filename based on the date and time of day. This mode can be useful for unattended reception of weather fax.

When you start decoding in fax mode, the decoded fax document is displayed in real time in the MultiMode Window. Proper tuning of your receiver is necessary to correctly decode and view the fax. If the image appears negative (white on black) you may want to change from LSB to USB mode, or vice-versa. You need to very carefully tune the radio to achieve proper reception. Note that proper tuning for FAX decoding may NOT mean tuning your radio to the actual frequency of the station, an offset may be necessary. For most fax stations, you must tune 1.9 kHz below the carrier frequency. For example, the USCG in Boston uses a carrier frequency of 6340.5 kHz. Tune your radio to 6338.6 kHz (in USB mode) for proper reception.

To aid this process a tuning display is available (shown above) located in the information window near the buttons. This displays a spectrum of the received signal. Two small tick marks appear on the bottom of this display, the one on the left represents the audio tone for black, the one on the right for white. A typical fax consists mostly of white. So, you can quickly tune in the station by aligning the peak tone (representing white) to the right mark. If the fax also contains a lot of black, you’ll see a second (usually shorter) peak over the left mark.

If you find that the picture is slanted, then the Timing Delta will have to be adjusted. See the section on FAX Calibration at the end of this chapter.

 

Chances are, when you start to decode a fax, you won't be correctly aligned in the horizontal direction. Pressing the << or >> button will slightly shift the decoding to the left or the right. 

When you reach the bottom of the window, no new image information is displayed. You can click the clear button to clear the display, and start decoding new information at top of the display..

The + and - buttons zoom the display out and in. This can be helpful if you’re trying to fit a large fax onto a small monitor. 

It is possible to scroll around the fax window using the scroll bars. The MultiMode Window size may also be changed to suit your display. It may be possible to screw up alignment on a slower computer by moving around too much, this was a problem with the 680x0 version.  (presumably because too much time is spent sloshing pixels around). 

Please note that if insufficient memory is available, the maximum number of shades displayed will be decreased, with only black and white possible as the worst case. Less memory than that will probably cause a crash!

FAX Calibration

This mode is used to calibrate MultiMode to the particular timing of your computer. The timing oscillators in each computer run at a slightly different speed, which means that the audio sampling may not be exactly correct. Any error in sampling will cause FAX images to be skewed and distorted. You only need to calibrate MultiMode once for your machine, the calibration is saved in your MultiMode preferences file.

MultiMode is calibrated for your particular machine by adjusting the Timing Delta value. This is a number, displayed in FAX mode in the status area, where information on the FAX lines per minute, etc is displayed, following the ∆ (delta) symbol. It can be either positive or negative. 

The easiest way to calibrate is to tune into a FAX transmission that included a black sync strip running down the edge of the picture. Adjust the Timing Delta by using the + and - keys to increase or decrease it in steps of 0.1 units, unti the line begins to draw straight vertically. 

If you find that vertical lines slant to the left as they go down, then the Timing Delta must be made more negative. If vertical lines slant to the right as they go down, then the Timing Delta must be made more positive. Once you set this, you should not need to change it, unless you get a new computer, which may have slightly different timing in the audio input circuitry.

Note that when you change the Timing Delta, only NEW parts of the line will be drawn differently, the old part will remain as it was received.

You may want to record the Delta for your machine, so that in case you should lose your Preferences file, you can simply re-enter it, rather than going through the calibration procedure again.