Hey Habr.
It's already the 21st century, and it would seem that you can transfer data in HD quality even to Mars. However, a lot of interesting devices are still working on the radio and you can hear a lot of interesting signals.
Of course, it is unrealistic to consider all of them, let's try to choose the most interesting ones, those that can be accepted and decoded on our own using a computer. To receive signals, we will use the Dutch online receiver , MultiPSK decoder and Virtual Audio Cable.
For convenience of consideration, we will present the signals in increasing frequency. I won’t consider broadcasting stations, it’s boring and banal, those who wish can listen to Radio China in AM on their own. And we will move on to more interesting signals.
Time Signals
At a frequency of 77.5 kHz (long wave band), the exact time signals of the German station DCF77 are transmitted. They already had , so we can only briefly repeat that this is a signal of simple structure in amplitude modulation - “1” and “0” are encoded with different durations, as a result, a 58-bit code is received in one minute.
130-140KHz - power grid telemetry
At these frequencies, according to , control signals for the German power grid are transmitted.
The signal is quite strong, and according to reviews, it is accepted even in Australia. You can decode it in MultiPSK if you set the parameters as shown in the screenshot.
At the output, we will get data packets, their structure, of course, is unknown, those who wish can experiment and do analysis at their leisure. Technically, the signal itself is very simple, the method is called FSK (Frequency Shift Keying) and consists in the formation of a bit sequence by changing the transmission frequency. The same signal, in the form of a spectrum - bits can even be counted manually.
Weather teletype
On the spectrum above, very close, at a frequency of 147 kHz, another signal is visible. This is the (also German) DWD (Deutscher Wetterdienst) station, transmitting weather reports for ships. In addition to this frequency, signals are also transmitted at 11039 and 14467 kHz.
The result of decoding is shown in the screenshot.
The principle of teletype coding is the same as FSK, text coding is of interest here. It is 5-bit, using , and has almost 100 years of history.
It seems that a similar code was used on punched tapes, but weather teletypes have been sent out somewhere since the 60s, and as you can see, they still work. Of course, on a real ship, the signal is not decoded using a computer - there are special receivers that record the signal and display it on the screen.
In general, even in the presence of satellite communications and the Internet, data transmission in this way is still a simple, reliable and cheap means. Although, of course, we can assume that someday these systems will go down in history and be replaced by fully digital services. So those who wish to receive such a signal should not delay too much.
Meteofax
Another legacy signal with almost the same long history. In this signal, the image is transmitted to analogue form at a speed of 120 lines per minute (there are other values, for example, 60 or 240 LPM), frequency modulation is used to encode brightness - the brightness of each point in the image is proportional to the change in frequency. Such a simple scheme made it possible to transmit images back in those days when few people heard about “digital signals”.
Popular in the European part and easy to receive is the already mentioned German station DWD (Deutche Wetterdienst), which transmits messages at frequencies 3855, 7880 and 13882 kHz. Another organization whose faxes are easy to receive is the British Joint Operational Meteorology and Oceanography Centre, they transmit signals on the frequencies 2618, 4610, 6834, 8040, 11086, 12390 and 18261 kHz.
USB receiver mode must be used to receive HF Fax signals, and MultiPSK can be used for decoding. The result of receiving via the websdr receiver is shown in the figure:
This picture was taken right at the time of writing the text. It can be seen by the way that the vertical lines have moved out - the protocol is analog, and the synchronization accuracy is critical here, even small sound delays cause the image to shift. When using a "real" receiver, this effect will not occur.
Of course, as in the case of a weather teletype, no one on ships decodes faxes using a computer - there are specialized receivers (an example of a picture from the beginning of the article) that do all the work automatically.
STANAG 4285
Let's now consider a more modern short-wave data transmission standard - the Stanag 4285 modem. This format was developed for NATO, and exists in various versions. It is based on phase modulation, the signal parameters can vary, as you can see from the table, the speed can be from 75 to 2400 bps. This may not seem like much, but considering the transmission medium - short waves, with their fading and interference, this is quite a good result.
The MultiPSK program can decode STANAG, but in 95% of cases the result of decoding will be only “garbage” - the format itself provides only a lower-level bit-by-bit protocol, and the data itself can be encrypted or have some kind of its own format. Some signals, however, can be decoded, for example, the recording below at a frequency of 8453 kHz. I didn’t manage to decode at least some signal through the websdr receiver, apparently, the online transmission still violates the data structure. Those who wish can download the file from a real receiver using the link . The results of decoding in MultiPSK are shown in the screenshot below. As you can see, the speed for this recording is 600bps, apparently a text file is transmitted as the content.
Interestingly, as you can see in the panorama, there are really a lot of such signals on the air:
Of course, not all of them are possible, they belong to STANAG - there are other protocols on similar principles. For example, we can analyze the signal .
As in the case of other considered signals, specialized devices are used for real reception and transmission. For example, for the modem shown in the photo Declared speed is from 75 to 9600bps with a signal bandwidth of 3KHz.
The speed of 9600, of course, is not very impressive, but given that signals can be transmitted even from the jungle or from a ship in the ocean, and without paying anything for traffic to the telecom operator, this is not so bad.
By the way, let's take a closer look at the panorama above. On the left we see... that's right, good old Morse code. So let's move on to the next signal.
Morse code (CW)
At a frequency of 8423 kHz, we hear exactly it. The art of listening to Morse code is now almost lost, so we will use MultiPSK (however, it decodes so-so, the CW Skimmer program does a much better job).
As you can see, the repeated text DE SVO is transmitted, according to , the station is located in Greece.
Of course, there are few such signals, but they still exist. An example is a long-standing 4331kHz station broadcasting the repetitive “VVV DE E4X4XZ” signals. According to Google, the station belongs to the Israeli Navy. Is there anything else on this frequency? The answer is unknown, those who wish can listen and check for themselves.
The Buzzer (UVB-76)
Our hit parade is completed by the most probably well-known signal - well-known both in Russia and abroad, a signal at a frequency of 4625 kHz.
The signal is used to notify the troops, and is a repeated beep, in between which code phrases are sometimes transmitted from the cipher pad (abstract words like "KROLIST" or "BRAMIRKA"). Some write that they saw such receivers in the military registration and enlistment offices, others say that this is part of the “dead hand” system, in general, the signal is a mecca for lovers of Stalker, conspiracy theories, the Cold War and other things. Those who wish can type in the search "UVB-76", and I'm sure an entertaining reading for the evening is guaranteed (however, you should not take everything written seriously). At the same time, the system is quite interesting, if only because it has been working since the times of the Cold War, although it is difficult to say whether anyone needs it now.
Completion
This list is far from complete. With the help of a radio receiver, you can hear (or rather see) communication signals with submarines, and over-the-horizon radars, and rapidly changing frequency hopping signals, and much more.
For example, here is a picture taken right now at a frequency of 8 MHz, you can count at least 5 signals of various types on it.
What they are is often unknown, at least not all can be found in open sources (although there are sites such as и ). The study of such signals is quite interesting both from the point of view of mathematics, programming and DSP, and simply as a way to learn something new about the world around us.
It is also interesting that despite the development of the Internet and communications, radio not only does not lose ground, but perhaps even vice versa - the ability to transfer data directly from the sender to the recipient, without censorship, traffic control and packet tracking, can become (although let's hope that all the same will not become) relevant again ...
Source: habr.com