FAX: Thou Shalt Make an Image
Nils Schiffhauer DK8OK has an update on radio facsimile (FAX) transmissions
Nils Schiffhauer DK8OK has an update on radio facsimile (FAX) transmissions, one of the most enduring modes of short wave utility signals, which you can receive and decode with little effort and to great effect.
Do you remember? Around 25 years ago or so, you may have received weather and press facsimile (Fax) broadcasts from such exotic locations as Diego Garcia, Khabarovsk, Casey Base and Molodezhnaya in Antarctica or Beijing. Alas, these stations have faded, but there still are many other Fax pictures to collect.
On 12th December 1922, the American inventor Charles Francis Jenkins (1867-1934) was the first to successfully demonstrate transmitting pictures by means of radio waves.
His technique, called ‘Radio-FAX’, is still popular today. You can receive signals from Hawaii to Japan and from New Zealand to Seoul – daily and with free software.
The key idea was to scan the original line by line, thereby translating the brightness fluctuations into electricity and thus modulating a transmitter. At the reception end, things work the other way around, translating different tones into the transmitted picture.
By means of this technique, news agencies would distribute their pictures worldwide, via landline, sea cable, HF or even satellite. With the advent of the Internet, FAX gave way to file transfer.
Nowadays, only the Japanese news agency KYODO transmits some text and pictures via HF FAX from stations in Japan; Singapore does so too, to supply some cruise vessels with a fresh daily (Fig. 1).
Apart from this, there are still a number of meteorological services disseminating weather charts, especially for ships at sea, with a power between one and 20kW.
Free software makes decoding the signals a breeze and renders reception very comfortable. Thanks to the unique combination of robust modulation and the powerful pattern recognition of our eye-brain system, you can also read the results under conditions of poor reception quality.
Usually, all Fax charts (plus some text in English, Japanese, Thai, Russian or Korean) are transmitted in black and white. More rarely, images from weather satellites are also broadcast in grey (‘halftone’).
For this purpose, FM is used, with a centre audio frequency of 1,900Hz: 1,500 Hz corresponds to white, and 2,300Hz represents black. Line drawings, such as weather maps, can thus be interpreted as frequency shift keying (FSK), with an 800Hz shift.
What else do you need to know? The important thing is how many lines are sent per minute. For weather maps, this is almost always two lines per minute (120 rpm). However, occasionally, a rate of one (60 rpm) or 1.5 (90 rpm) is in use.
The abbreviation ‘rpm’ stands for ‘revolutions per minute’ and refers to the mechanical method of FAX transmission, in which the original was mounted on a drum.
The ‘Index of Cooperation (IOC)’ determines the aspect ratio of the FAX image. This ratio is usually 576 and is calculated from the diameter of the drum, usually 152mm, multiplied by the number of lines per mm.
At the beginning of the transmission, standardized tones are transmitted. This enables the software to automatically recognize the correct values for RPM and IOC.
Fig. 2 shows a complete FAX transmission with automatic picture transmission (APT) tones and – along with other things – a grey ‘wedge’ at the end.
One full transmission cycle can take between a few minutes and more than half an hour.
Where to Listen
The most reliable and up-to-date source for weather FAX stations and their associated frequencies is a module of the MWLIST resource (Fig. 3).
This website uses your PC’s time to display (in bold) any currently active schedules. From time to time, you have to click the refresh button of your browser to ‘re-match’ your PC time to the schedules.
The worldwide FAX schedules from the US National Weather Service are free of charge and very useful:
Any standard short wave receiver can be used to receive a FAX. Set it to USB and subtract 1.9kHz from the listed centre frequency, so that, after demodulation, the frequency pair 1500Hz / black and 2.300 Hz / white is present.
I usually work in RTTY / CW (USB) and set the BFO tone to 1,900Hz. The optimum receive bandwidth is around 1,600Hz, while a higher bandwidth degrades the signal-to-noise ratio accordingly.
A lower bandwidth may ‘thin-out’ the lines.
In practice, ‘soft’ filters (Fig. 4) have proven to be the best: They produce lines of a rich black.
Many Decoders available
Almost every multimode software also offers FAX reception - from FLDIGI and MixW4 to MultiPSK.
In addition to the professional software W-Code from Wavecom, I like to use Sorcerer. Above all, I am working with the powerful and comfortable program SeaTTY (49 US-$) by Sergei Podstrigailo (Fig. 5):
There are decoders for the Mac, and even for tablets and smartphones. My recommendation: Just start with some free software such as FLDIGI or Sorcerer and familiarize yourself with FAX reception based on strong European signals such as the German Weather Service / Pinneberg at 3855, 7880 and 13882.5kHz.
If you think this it fun and want to get deeper into this subject, I should not be surprised if you did not end up using SeaTTY.
Virtually all Weather FAX stations work according to more or less fixed schedules. Occasionally, however, extra transmissions are pushed in or some others disappear.
As FAX reception on HF follows the usual laws of propagation, you can easily match schedules to propagation by using propagation software like Propman 2000.
This clearly shows the best channel, depending on the time of day (Fig. 6). Incidentally, FAX stations are also an excellent general propagation indicator, thanks to their usually regular broadcasts.
The connection between receiver and PC is via the soundcard of the PC (if you have an analogue receiver) or via a Virtual Audio Cable (VAC) if you operate an SDR.
There are three types of synchronization errors (Fig. 7):
Slant is due to an inadequate synchronization of the ‘drum speed’ between the transmitter and the receiver.
In principle, you can correct the sampling rate of the soundcard of the PC, for which each FAX software offers more or less convenient possibilities. A distinction must be made between a general deviation of the soundcard from the standard value and an individual deviation from station to station. The received weather chart remains legible with a light or medium ‘skew’.
Incorrect start of the line (‘phase’) occurs only when you start your decoder manually. This is sometimes necessary, in case of weak and fading DX stations. For the correction of this error, there is almost always a corresponding function in each individual FAX software.
Stairs occur when the synchronization between receiver, software and the operating system of the computer does not work properly. Then the picture may get simply unreadable. As a remedy, you should first try a PC restart, then lower the processor load of the computer (for example, by switching an SDR to a narrower RF range) or use other software. Unfortunately, stairs cannot be corrected later with FAX software.
You could also try to solve this annoying problem by bypassing your sound card. This is done by the KiwiSDR, which communicates with the computer by IP.
It also has a FAX decoder and a convenient on-board schedule of (many) FAX transmissions.
You can use remotely-located KiwiSDRs on a worldwide scale or locally, just try your own way.
In some cases of deficient transmission quality, the transmitter can be to blame: Meteo Thailand, for example, often simply ‘breaks off’ after the lettering ‘FAX CHART’, and the stations in Seoul and Taipei also seem to need an update.
Additionally, there sometimes occur impairments on the HF path. Low signal-to-noise ratios result in noisy pictures. However, all in all, the human eye-brain combination, with its striking pattern recognition ability, still renders charts with weak signals and strong interference reasonably readable (Fig. 8).
Actually, the halftone representation is the right thing for black and white photos, but because of the usual short wave interference only very few photos are transmitted; an example is shown in Fig. 9.
QSL Cards: Always Nice
On top of that, multipath reception can greatly affect readability, especially in the case of strong signals. The reason becomes clear when one considers the transfer concept again: at 120rpm, one line is written in 500ms.
The image-width is on the scale 1:1, usually between 1,500 and 2,500 pixels. At 2,000 pixels / line, the resolution is up to 0.25 ms.
In principle, this means that two paths with a difference of less than 30 km can be separated. After decoding, this effect is shown by multiple lines. They are particularly likely to occur at stations, which are less than 1,000 km away and at sunrise or sunset, as the ionospheric layers regroup (Fig. 10).
All FAX stations look forward to receiving reports and confirming them with QSL cards (Fig. 11). Many reception confirmations will now routinely land in electronic format in your e-mail inbox.
Stations Received in 2018
FAX schedules are as reliable as tax increases. Table 1 contains a list of stations, I received in Germany in 2018, with some additional information. For official schedules, please see the sources given in the main text.
Australian Weather FAX, Charleville (VMC) and Wiluna (VMW) stations, each 1kW.
Canadian Coast Guard, MTCS Sydney, VCO, 5 kW.
Canadian Coast Guard Iqaluit, VFF, only active May to (late) October.
Magallanes Radio, CBM, 1 kW. Very weak signal.
Valparaiso Radio, Chile, CBV, 1 kW. Very weak signal.
German Weather Service, Pinneberg, stations DDH3 and DDK3 / 6, each 10 to 20 kW.
Hellenic National Meteorological Service, SVJ4, 8 kW.
Kagoshima Fishery Radio, JFX.
Kyodo News Agency, JJC, 5-15 kW. News for subscribers aboard ships.
Tokyo Meteo, Japan, stations JMH / 2/4, 5 kW.
New Zealand MetService, ZKLF, 5kW.
Murmansk Meteo, shift 1,000 Hz and inverted.
Kyodo News Agency, Relay Singapore, Station 9VF252, 10 kW. News for subscribers aboard ships.
Seoul Meteo, HLL2, 3 kW.
Bangkok Meteo, HSW64, 3kW. Apparently decrepit transmitter with defective modulation, which leads to a noisy playback even with a good signal (see Figure 14).
Joint Operational Meteorology and Oceanography Centre, Northwood Station, GYA, 10kW
USCG Boston, USA/Massachusetts, NMF, 4kW.
USCG Kodiak, USA/Alaska, NOJ, 4 kW.
USCG New Orleans, USA/Louisiana NMG, 4kW.
USCG Pt. Reyes, USA / California, NMC, 4 kW.
National Weather Service Honolulu, KVM70, 4kW.
This article was featured in the November 2018 issue of Radio User