Intercom CTCSS is a continuous voice control squelch system, commonly known as sub-audio, which is a technology that attaches frequencies lower than audio (67Hz-250.3Hz) to audio signals and transmits them together. Because its frequency range is lower than standard audio, it is called infrasound.

After the interphone demodulates the received intermediate frequency signal, it filters and shapes the secondary audio signal, and then inputs it into the CPU, and compares it with the CTCSS frequency set by this machine, so as to decide whether to turn on the mute.

If your receiver is equipped with sub-audio, and the transmitter you are talking to is also equipped with corresponding sub-audio, then when you receive the signal from the other party, the sub-audio signal will be separated from the special chip, filtered and shaped and input to the CPU, and the CPU will compare the foreign sub-audio with the sub-audio set by the machine. If it corresponds, the CPU will issue an instruction to turn on the mute, so that you can hear the voice of the other party.