C4FM is a digital modulation for walkie-talkies that hobbyists prefer. It also has a relatively mature history of use in professional digital walkie-talkie applications. The first version of the American digital intercom mechanism Project 25 (P25 for short) adopted C4FM, so do you know what C4FM means? What are the advantages and disadvantages compared with other digital work models?
The C4FM is continuous envelope 4-level frequency modulation. The working principle of C4FM: The RF signal is amplified and transmitted to space through the antenna. The RF signal received by the receiver and passes through the frequency modulation demodulator, which is often referred to as a frequency discriminator circuit in analog mode, and the demodulated result is a C4FM baseband signal whose amplitude varies between four voltage levels. After logical judgment and transformation of the level, a two-digit binary digit sequence is obtained. In order to improve the anti-interference performance, the process of extracting the digital signal from the four-level waveform usually uses a certain algorithm instead of using a simple voltage comparator. It is not substantially different from the well-known traditional analog FM, except that it is not an analog signal of voice that modulates the radio frequency input signal, but a digital signal with only four-level states.
As same as D-STAR, C4FM also uses FDMA. When using FDMA digital modulation mode, the channel interval is 12.5 kHz, which can realize high-speed data communication, reliable voice communication, and strong error correction performance. C4FM FDMA provides a data transfer rate of 9.6 kbps. It is very different from the existing digital radio system and expands the possibility of interesting amateur radio activities in the future.
Compared with walkie-talkies such as DMR, the disadvantages of C4FM walkie-talkies are also very obvious. First of all, the cost of this type of walkie-talkie is relatively expensive, and the link selection is not like DMR walkie-talkies, which can define TalkGroup in each transmission, and can only be determined on the server side like analog walkie-talkies. In addition, DMR walkie-talkies can display detailed personal information such as call signs, addresses, call signs, and C4FM GPS walkie-talkies can only display call signs and distance information. Most importantly, due to the C4FM technology patent problem, only Yaesu walkie-talkies are currently in use, and other brands are basically unavailable.