The capabilities of the different digital communication systems are different, which depend on the type of modulation, the method of multiplication and the protocol. While the binary signals have two stabile states, 0 and 1, the 4-level signal has four stable states, namely 00, 01, 11 and 10. The technology can be best understood by the following small illustration. The block diagram of the modulator is shown on Figure 3. This modulation system is getting more and more popular among amateurs as well. This is used by the DMR system, which is also known in Hungary.
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The C4FM modulation system is very popular among professional radios. One of the best known systems is the APCO P-25, which is mainly used for governmental communication (police, disaster prevention, etc.) in the USAand some other countries. Because of these benefits, this one soon superseded the GMSK modulation in the development of the professional radios. If you want to take a deep dive in the world of the DQPSK modulation, a detailed description can be found at: Ĭompared to the previous two modulations, the circuitry of the C4FM is simpler, and the bit error rate ( BER) is also more favourable. The block diagram of this modulation is shown on Figure 2. One of them is the European TETRA system (in Hungary it is known under the name EDR), another one is the TDMA-based mobile phones ( USA). Therefore this mode is not quite widespread, except for a few applications. This is also accompanied with the lower efficiency of the transmitter stage, which has a negative effect on the service life of the battery, too. One of the biggest advantages it has is that the distortion of the signals is very low, although this mode requires linear RF stages (Class A stages, just like SSB), and in addition, the circuitry is rather complicated. You can find detailed information about the MSK and GMSK modulation on the following webpage: The block diagram of the modulator is shown on Figure 1.
The GMSK modulation is still being used by D-STAR. Nowadays it is considered to be obsolete. This modulation mode has been used in the mobile phones and the TETRAPOL system between 19. The GMSK modulation circuit is relatively simple. Let's have a look at the benefits and disadvantages of modulation modes! While for an analog radio, only the type of modulation is important, for the digital one, the type of communication and the protocol is also important for the contact. Without the latter one, the voice in the loudspeaker of the receiver would be broken up.
One of its most important elements is the method of compression, in order to reduce the amount of data, not containing useful information, and to detect and correct the faulty bits. The protocol is probably the most important factor in the design of digital radios, even if they are using the same modulation and communication system. The third one is the protocol: this is the procedure how the signal is rendered from the transmitter to the loudspeaker of the receiver. Like I mentioned in the previous part, FDMA (Frequency Domain Multiple Access), TDMA (Time Division Multiple Access), CDMA (Code division Multiple Access) are the widespread ones. The second one is the type of communication: this is the method of multiplication and transmission of the modulated signals. Each of these have their benefits and disadvantages, which will be discussed later. The most widespread modulation modes are GMSK(MSK), 16QAM, π/4 DPQSK and C4FM (4-level FSK). The first one is the type of modulation: this is the modulation scheme itself, which is used to convert the analog speech to digital signals (also called digitizing).
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