cocoaModem
cocoaModem HF-FAX Receiver

Kok Chen, W7AY [w7ay (at) arrl (dot) net]
Last updated: Mar 31, 2006


Index (User's Manual - HF-FAX Receiver)

General Information
Aural Monitor
Accessibility (Incremental Speak and Voice Assist)
Macros
RTTY Interfaces
PSK Interface
MFSK Interface
Hellschreiber Interface
CW Interface
ASCII Interface
SITOR-B Receiver
HF-FAX Receiver
Synchronous AM Receiver
Versions
Part II


HF-FAX Receiver

cocoaModem supports an interface to receive HF FAX weather charts. Fig 1 shows the HF-FAX interface:

hffax
Figure 1 - HF-FAX Interface


HF FAX is transmitted as an FM signal, with a frequency deviation of 400 Hz. By convention, the receiver is set to USB mode with the audio signal centered at 1900 Hz. Black level is 1500 Hz and the white level is 2300 Hz.

Weather faxes are transmitted with an IOC of 576 (a scan line of about 1809 pixels for a 1:1 aspect ratio) and 120 lpm (2 scan lines per second).

Some world wide marine FAX stations are listed here. Schedules and frequencies of the U.S. NWS broadcasts can be found here. A detailed schedule for the Coast Guard station NMF (Boston area) can be found here, and a detailed schedule for NMC (SF area) can be found here.

HF FAX frequencies are usually specified at the center frequency of the FM signal. If you are receiving with a USB receiver, tune1.9 kHz lower in frequency than the published frequency. That will place the center of the signal at 1900 Hz.

cocoaModem should produce an image once the input device is set up in the Config Panel and the input is switched to the active state. The image can be manually paused by clicking on the Pause button, shown in detail in Figure 2.

manual
Figure 2 - HF-FAX Controls


The New button resets the input to the top of the image.

If the Auto checkbox is selected, cocoaModem will automatically pause when it detects a Stop signal (a 450 Hz subcarrier) and resets the input to the top when it detects a Start signal (a 300 Hz subcarrier). See Figure 4 to see what the Start and Stop patterns look like on an image - cocoaModem removes most of the Start and Stop patterns when Auto is engaged, but you can view them by disengaging the Auto checkbox.


Saving Charts

When it detects a Stop signal, cocoaModem can save the image to a PDF file if the Fax folder in the Config Panel is set to point to a folder on your disk (see Figure 3).

Folder
Figure 3 - FAX Folder


To set up the Fax folder, click on the Browse button to select the folder that you want to use. If you don't want any image (typically 1/2 a megabyte per weather chart) to be saved, simply click on the Clear button to clear the folder field.

The PDF file can be converted to other formats by opening the file from the Preview application in MacOS X. With Preview, you can save to JPEG, JPEG2k, GIF, PNG or TIFF, and other formats.

The indicator in Figure 2 shows green when a signal is being received, and turns red when it is paused. It turns yellow between the time a Stop signal is received and when the new image starts.

You can also manually save the image in the view by clicking on the Save button in Figure 2. Since cocoaModem has no idea where the "page breaks" are when it is not in Auto mode, manual saving of the image view will save the entire view (1600 scan lines). The file will be saved into that folder that was previously set the Fax folder (Figure 3). If the field in Figure 3 is empty, cocoaModem will save the file into your home directory.

When you save a file, the filename will take on the format that looks like:

fax 2006-03-31 0524.pdf

The filename has the year, month and day and time in UTC.


Horizontal Position

When set to Auto mode, cocoaModem will also look for a horizontal sync signal that is sometimes transmitted by HF FAX stations.

sync
Figure 4 - FAX Sync Signals


Figure 4 shows a typical HF FAX signal. Right at the top, you can see the 450 Hz Stop signal. That is followed by noise that cocoaModem receives when the station is quiescent between charts.

The transmission resumes with the 300 Hz Start signal. In the case of this signal from NMC, right below the Start signal you can also see a long horizontal black bar, with a short white gap. This is the horizontal sync signal.

Not all stations transmit this horizontal sync signal. The U.S. Coast Guard stations NMF, NMG, NMC and NOJ all appear to transmit this horizontal sync.

If cocoaModem detects a pattern right after the Start signal that it identifies as a horizontal sync, it will redraw the image following it so that the sync appears at the left edge of the image.

If the station does not transmit such a horizontal sync signal, or you'd caught a signal further down the chart after the horizontal signal has gone away, cocoaModem will not be able to try to realign the image.

You can manually change the horizontal position of the image by clicking on the chart image. The location that you click on will become the new left edge of the image.

cocoaModem maintains a raw copy of the image in the background, to allow positioning, clock adjustments and image control to be applied interactively. The HF FAX mode of cocoamodem uses a substantial amount of memory. If you don't intend to use the mode, be sure to remove the HF-FAX interface in your Preferences.


Clock Adjustment

cocoaModem derives pixel timing from the A/D converter of the audio device that you are using. FAX images will show appreciable skew when the local clock is different from the transmitter clock by just a few parts per million. Even if the transmitting stations clock is perfectly accurate, it is rare for an A/D converter clock to maintain this kind of absolute accuracy. The resampling period in cocoaModem is not fixed but can be manually adjusted to within one part per million using the clock adjustment interface at the bottom left of the HF-FAX interface (shown in Figure 5 below).

ppm
Figure 5 - Clock Adjustment


Image Controls

On the bottom right of the HF-FAX interface are controls for adjusting the black level and contrast and for selecting the image size.

image
Figure 6 - Image Controls

The black level sider has the same effect as retuning the VFO knob on your transceiver, but it is applied to the entire image (including the past) and not just to the future if you had used the VFO knob to adjust the black level. The contrast slider changes the dynamic range between black and white parts of the image.

Bandwidth Selection

There is a popup menu just to the left of the image control which allows you to adjust the bandwidth of the input passband (see Figure 7 below). If you hear no interfering signal close by, you can select the Wide position to get a slightly sharper image. If you see an interfering signal in the waterfall, you can select the Narrow position, which gives a softer but cleaner output when the FAX signal is weak.

Bandwidth
Figure 7 - Bandwidth Menu



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