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The principle and application of two-way radio sub-tone squelch
When buying a high-end walkie-talkie or setting up an amateur repeater, it is inevitable to mention the subwoofer squelch function. Many radio communication enthusiasts, especially the "beginners" who have just started, seem to understand this. They are still confused about what CTCSS can do. In response to this problem, I will talk about it in detail and help today. Everyone eliminates confusion.
The so-called CTCSS is an abbreviation of "Continuous Tone ControlLEDSquelch System" in English. Translated into Chinese is a continuous voice control squelch system. Because the frequency of the voice signal controlling the frequency is lower than the standard audio frequency, it is commonly known as subtone. The 38 sets of standard subtones are in the frequency range of 67.0Hz to 250.3Hz. The more commonly used are 74.4Hz and 88.5Hz. Since the ears are not sensitive to these lower frequencies, normal voice calls between them are almost not affected. It is used to control the mute function of the radio to turn it on and off to improve the sensitivity of the device and avoid interference and noise. When using CTCSS equipment, the transmitter must have a subtone encoding function, and the receiver must have a corresponding subtone decoding function. When the transmitter presses the PIT button, the device will automatically transmit a continuous subtone encoded modulated signal first. After receiving and demodulating the subtone, the receiver will filter and shape it, and then input it to the CPU for comparison. If it matches the subtone frequency set by the machine, the CPU will issue a command to control the corresponding action of the switch. Turn on the squelch, and the speaker at the end of the audio amplifier circuit starts to make a sound. If the other party speaks at this time, you can hear the sound; if no sub-tone is detected, or the sub-tone frequency is not consistent with the setting of the local unit, the squelch of the receiver It is turned off, and the device is always in the mute state, which can effectively avoid the interference of irrelevant calls or industrial radio noise. This is the basic principle of the subtone squelch function.
The subwoofer squelch function is mainly used to solve the problem of multiple radio users using the same frequency to interfere with each other. Since the frequency occupied by the radio is not always continuous, as long as the use gap is staggered, the same frequency is introduced after the subwoofer squelch function is introduced. But between different tone users. Basically, we can do everything together. Therefore, V / U amateur equipment produced in the past ten years, whether as standard or optional, already has a subsonic squelch function, but unfortunately, most devices only support encoding but not decoding, that is, only There is a function of transmitting subtones without receiving subtones. In order to ensure the reliability of the squelch function, these low-frequency sub-tone signals use a complete sine wave, and have strict requirements on the frequency difference, usually ± 0.5Hz, which is mixed with the speech audio signal during transmission and sent to the modulation Circuit, its FM frequency deviation for high-frequency carrier is about ± 0.4kHz to 0.8kHz, which is far less than the FM frequency deviation of the speech frequency of ± 5kHz. When receiving, the demodulation circuit will restore the sub-tone signal at the transmitting end, and then control the mute switch by comparing with the received sub-tone set by this unit. Commercial radio stations usually use special circuits to filter out these sub-tone frequencies before the audio processing circuit, but amateur equipment does not filter them, so sometimes we can hear these low-frequency frequencies, and many people mistakenly think of them as power sources for people. AC noise.
Based on the analysis of the above principles, CTCSS cannot completely eliminate the same frequency interference of radio frequency signals. Once two FM signals of the same frequency appear at the same time, they will still suppress each other, unless the strength of one signal is more than 6dB higher than the other. However, after the introduction of CTCSS, multiple user groups with different subtones can be set up to communicate through a repeater without hearing each other's speech. Of course, there will still be normal receiving signal instructions on the intercom. If you want to monitor other users, In the group's speech, you can press the MONI "monitor" key near the PPT button to manually turn on the squelch. There is no intercom for receiving subtones, and you can listen to all signals in the frequency coverage unobstructed, naturally including the radio noise of the surrounding environment. Radio communication relay systems use CTCSS extensively, the purpose is to add an invisible "password lock", reduce the possibility of being triggered by mistake, and restrict unauthorized use of unauthorized personnel. It should be pointed out that in order to ensure the openness of communication, amateur repeaters used for emergency rescue and disaster relief do not generally have a subtone function.
The realization of the subwoofer squelch function is of course dependent on the support of related hardware. Most of the walkie-talkies imported from abroad use dedicated circuit chips and have been integrated into the whole machine. Some brands of machines use an independent pluggable circuit board. , Commonly known as subtones, are usually sold separately as options (their circuit works in much the same way). Generally speaking, domestic or less expensive walkie-talkies do not have a subtone squelch function, but they can also be implemented by adding a homemade "subtone signal generator". The "subsonic signal generator" is actually a simplified version of the subsonic film, but it has a single function than the finished film, does not have a decoding function, and discards the plug-in interface and uses a few wires to directly connect to the host circuit. The volume ratio One yuan coin is even smaller and can be easily added to various radio transceivers.
There are single-frequency and multiple-frequency subtones, which can use either a single-chip computer or a DTMF telephone dual-tone multi-frequency integrated circuit. If a single-chip microcomputer is used to control the digital frequency synthesizer, for example, Taiwan's Elan Electronics EM78P156E single-chip microcomputer is used with a quartz crystal. It can provide sub-tone signals with high frequency stability and low clutter. And the design is simple, the price is low, the frequency is adjustable, and the application is convenient, but it requires the producer to understand software programming at the same time, which is a bit difficult for ordinary amateurs. And using TCM5087, CSC5087 and other dual-tone multi-frequency dial-up integrated circuits, it is very easy, but the use effect is slightly worse, the specific production method will be described in another article. If two transmit sub-tone frequencies are required, two chips with different frequencies can also be used together. Subtones with decoding function are relatively difficult to produce under amateur conditions, and generally do not need to be produced.
In addition, there is a CDCSS (Continuous Digital Controlled Squelch System), which is the same principle as CTCSS. The only difference is that it uses digital coding to control whether the squelch is turned on, which is also called digital subtone.