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:
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.
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).
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:
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.
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).
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.
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.
Next (Synchronous AM)