Later the frequency modulation (FM) has developed, and it significantly decreased the sensitivity of the received signal to interferences. This method was suitable for high fidelity (Hi-Fi) in wide FM, which is still being used in the VHF band broadcasts. Amateurs and professionals were using narrow-band FM, primarily because only human speech is transmitted, which only requires telephone quality. The main aspect is readability, and recognition is less important. The TELEX, which was commonly used in wire telecommunications, has become popular amongst radio amateurs as well. In the beginning, interface was connected between the radio and the mechanical telex machine.
With the introduction of the computers, especially when the prices of the ZX Spectrum and the Commodore C64 dropped to an acceptable level (many of us still remember the "shopping tours" on the Mariahilfer Strasse in Vienna), many radio amateurs started using the radio teletype (RTTY) mode. This was really the beginning of the digital telecommunication! Almost without a transition, we found ourselves in the PACKET radio era, thanks to the low-cost ZX and C64 machines. Then we already used Bulletin Board Systems (BBS), and could leave messages for fellow amateurs in these systems. These operated in AFSK mode, at a relatively low, 1200 bps (baud) speeed. Then all kinds of new digital modes were introduced, such as the Pactor, Amtor, Sitor, and lately the JT6 modes for weak-signal communication, developed by the Nobel Prize winner K1JT.
Meanwhile, AFSK was replaced by FSK, and its numerous variants, which could easily reach the speed of 9600 bps on FM radios. Compared to the bandwidth we are using on the Internet today, this is considered very slow, but at the time, the transmission of large files and videos was not necessary.
It is interesting, that the digital transmission of voice came rather late. Overall, the real breakthrough came in the second half of the 1990's, by the penetration of the modulation modes and the special IC's for them. First the NMT450 mobile phones (which were partially digital, partially analog units), then the GSM phones started using the modes. The industrial transceivers started using digital modes rather late, in the 2000's. In Europe the best known system is TETRA.
For the digital voice communication, the voice first has to be digitalized and compressed. This is what VOCODER is good for. The analog signal is first compressed by a dynamic compressor, then digitalized by an A/D converter, and finally compacted. The nature of the compaction is that the less important content (like silence) is cut, and the rest is compressed by using algorythms, where every bit is carrying important information. This makes the voice a little "digital", but you get soon used to it... In the same time the error-correcting signals are produced, and packed with the useful signal. Generally this is done by the modulator IC. One of the most popular, and therefore commonly used error-correcting method is Forward Error Correction (FEC). Then the FM signal is transmitted by some kind of a modulation.
On the receiving side, the things turn around. The received and demodulated signal appears as a digital signal on the output of the demodulator IC, after decompaction and error-correction the VODECODER restores the analog signal (speech), which is then heard in the loudspeaker. The block diagram of the DMR digital modulation is shown on the following picture:
In digital VHF/UHF radio communication, three goups can be differentiated:
- FDMA, that is Frequency Division Multiple Acces. It is named after the frequency division system it is using. In professional radios Icom and Kenwood are using the FDMA technology, while Motorola, Vertex and Hytera are using the DMR technology. In amateur radio, the FDMA technology is popular. This is used by all three large manufacturers Icom, Kenwood and Yaesu. While the DMR radios communicate with each other, regardless of the manufacturer, the amateur equipment manufacturers are using different coding (modulation). No standardization was made.
Earlier both the professional and amateur FM VHF and UHF channels were spaced 25 kHz from each other, today they are mostly spaced 12.5 kHz. Further narrowing could only be realized by seriously sacrificing redability and recognition, not to mention the significant deterioration of the signal-to-noise ratio. In 12.5 kHz spaced channels the maximum permitted deviation is 2.5 kHz, while in 6.25 kHz spaced channels this would obviously be halved. The FDMA systems, using the possibilities of the digital modulation and its compaction, split the existing 12.5 kHz wide channels to two 6.25 kHz wide channels. This makes two simultaneous digital communications possible on a single 12.5 kHz analog channel. This technology has been used in space technology. You can find plenty of further information on the Internet. e.g. https://en.wikipedia.org/wiki/Frequency-division_multiple_access
- TDMA, that is Time Division Multiple Acces. It is named after the time division system it is using. In this system several time windows are dedicated on the FM channel: the GSM is using a maximum of 8 time windows at 50 kHz, the TETRA is using 4 time windows at 25 kHz, and the DMR is using 2 time windows at 12.5 kHz. Professional and amateur systems are mostly using DMR, while government communication is done with TETRA.
In TETRA the high-speed data communication was important, thus the timing is set up so that the system can support this. The data transfer speed can be increased by linking the 4 SLOTs (time windows). For example, policemen download data from their central servers using their TETRA equipment, like license plate number, personal identity number, personal data, etc. The demand for higher speed made the infrastructure much bigger. In the same service area, twice as much repeaters have to be installed, then with the DMR system. DMR is mostly focused on voice communication, by using less data transfers (e.g. GPS data). Because of its low cost and good parameters, DMR is more and more popular, and replaces the existing analog FM systems. The DMR equipment are suitable for analog FM as well, so a transfer period can be provided for the transition. The DMR technology is being also used by amateurs, although the standard is not really developed for this usage, and the professional equipment has to be pre-programmed. The association of DMR manufacturers and developers standardize the technology, providing the interoperability of the different manufacturers. Their web page: http://dmrassociation.org/
- CDMA, that is Code Division Multiple Access. This code division technology is primarily used in military telecommunication, but also spread in the US in mobile phone systems. This is a kind of spread spectrum transmission. In this article I will not go into more details, but those interested can find tons of literature on the Internet. e.g. https://en.wikipedia.org/wiki/Code_division_multiple_access
The above technologies are compared visually on the following diagrams:
It is an important question, what coverage can be provided, and how does the digitally received signal sound in your speaker?
In analog FM receivers, the other party is heard with more and more noise and sizzle, as we go farther and farther away from the transmitter (repeater), when we finally reach a limit where the signal is breaking up, and we only hear parts of the transmission, but the information can be inferred.
In a digital radio, the aother party is heard crystal clear until the same limit, where the analog signal started to break up. Here the digital reception ceases. That is, until we hear it, it is crastal clear, and then we hear nothing. The error-correcting code systems, which are transmitted together with the useful signal, play an important role in the digital technology. To put is simple: the receiver can correct the smaller errors, but when there are too much errors, it shuts down the speaker.
In my next article the digital systems of a major amateur equipment manufacturer, and the C4FM mode, will be covered.