When purchasing a medium-high-end walkie-talkie or setting up an amateur relay station, it is inevitable to mention the subsonic squelch function. Many radio communication enthusiasts, especially the newly-started "rookie" people, have always seemed to understand and understand what they can do with CTCSS. To solve this problem, today's system will talk in detail and help. Everyone completely eliminates confusion.
The so-called CTCSS, is the English "Continuous Tone ControlLEDSquelch System" abbreviation, translated into Chinese is the continuous voice control squelch system, because the control of the voice signal frequency is lower than the standard audio frequency, so commonly known as subsonic, such as used in the walkie-talkie The 38 sets of standard subtones are all within the frequency range of 67.0Hz to 250.3Hz. The more commonly used ones are 74.4Hz and 88.5Hz. Since the ear is not sensitive to these lower frequencies, the normal voice calls between each other are hardly affected. It is used to control the squelch function of turning on and off the intercom to increase the sensitivity of the equipment and avoid interference and noise. When using the CTCSS device, the transmitter must have a subtone coding function and the receiver must have a corresponding subsonic decoding function. When the transmitter presses the PIT key, the device will automatically transmit continuous subsonic coded modulated signals first. After the receiving party receives and demodulates the subsonics, it is filtered and shaped and finally input into the CPU for comparison. If the subtone frequency is consistent with the local setting, the CPU will issue instructions to control the corresponding actions of the transfer switch. When squelch is enabled, the speaker at the end of the audio amplifier circuit starts to make a sound. If the other party speaks, the sound can be heard. If no subtone is detected, or if the subtone frequency is inconsistent with the local setting, the receiving side squelch. Is turned off, the device is always in a mute state, which can effectively avoid the interference of incoherent calls or industrial radio noise. This is the basic principle of the subsonic squelch function.
The subsonic squelch function is mainly used to solve the problem that multiple walkie-talkie users use the same frequency to interfere with each other. Since the frequency occupied by the walkie-talkie is not always continuous, as long as the gap is staggered, the same frequency is introduced after the subsonic mute function is introduced. But between different subsonic users. Basically it can be done peacefully. Therefore, the V/U segment of amateur equipment produced in the last ten years either has a subsonic mute function, either as standard or as an option. Unfortunately, most devices only support encoding and do not support decoding, ie, only Subsonics are emitted and subsonics are not received. In order to ensure the reliability of the squelch function, these low-frequency subsonic signals use a completely sine wave, and the frequency difference requirement is very strict, usually ±0.5Hz, which is mixed with the speech audio signal and sent to the modulation during transmission. The circuit, its frequency modulation frequency deviation of the high-frequency carrier is about ± 0.4kHz ~ O.8kHz, far smaller than the speech frequency of the speech frequency ± 5kHz FM frequency deviation. The demodulation circuit at the time of reception will restore the subsonic signal of the transmitting end, and compares the receiving subsound set by this unit to control the switching of the squelch. Commercial radio stations usually use special circuits to filter out these subsonic frequencies before audio processing circuits, but amateur devices do not filter, so sometimes we can hear these low frequency frequencies, and many people mistakenly recognize it as a deaf power source. AC noise.
Through the above principle analysis, CTCSS can not completely eliminate the same frequency interference of radio frequency signal. Once two frequency modulation signals of the same frequency appear at the same time, they will still suppress each other unless the strength of one signal is higher than the other 6dB. However, after the introduction of CTCSS, multiple user groups with different sub-tones are set up, which can be connected via a repeater without hearing each other. Of course, there will still be normal reception signal indications on the intercom, if you want to monitor other users. For group talk, you can press the MONI “monitor†button near the PPT button to manually turn on squelch. A walkie-talkie that does not have a reception tone can listen to all signals in the frequency coverage without interruption, naturally including the radio noise in the surrounding environment. The use of CTCSS in a large number of radio communication relay systems aims to increase an invisible “code lockâ€, reduce the possibility of false triggering, and limit the unauthorized use of unauthorised personnel. It should be pointed out that for the amateur relay station used for emergency rescue and disaster relief, in order to ensure the openness of communication, the subsonic function is generally not to be set.
The implementation of the subsonic squelch function is, of course, inseparable from the support of the relevant hardware. Foreignly-adopted walkie-talkies mostly use special circuit chips and have been integrated into the whole machine. There are also some brands of machines that use an independent pluggable circuit board. Commonly known as sub-notes, they are usually sold separately as options (the circuit works much the same). The domestically-made or cheaper walkie-talkie does not have a subsonic mute function, but it can also be implemented by adding a home-made “subsonic signal generatorâ€. "Subsonic signal generator" is actually a simplified version of the sub-notes. It is only a single function than the finished product. It does not have the decoding function, and it discards the interface and uses several wires to connect directly to the host circuit. The one dollar coin is even smaller and can easily be added to various radio transceivers.
Sub-notes have a single frequency, but also a number of frequencies, either using a single-chip microcomputer, can also use DTMF telephone dual-tone multi-frequency integrated circuits. If the microcontroller is used to control the digital frequency synthesizer, for example, Taiwan's ELAN's EM78P156E microcontroller with quartz crystal. It can provide a high frequency stability and low clutter signal. And the design is simple, the price is low, the frequency is adjustable, and the application is convenient. However, it is required that the creator can understand the software programming at the same time. This is awkward for the average amateur. The use of TCM5087, CSC5087 and other dual-tone multi-frequency dial-up integrated circuits, it is very easy, but the use of less effective, the specific production method will be introduced in another article. If you need two transmit subsonic frequencies, you can also use two different frequencies in combination. The sub-notes with decoding function are relatively difficult to make under amateur conditions and generally do not have to be produced.
In addition, there is a CDCSS (Continuous Digitally Controlled Squelch System), which is a continuous digital control squelch system. The principle is the same as the CTCSS. The only difference is that it uses digital coding to control whether squelch is enabled or not. It is also called digital subsonic.
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