Sub-audio 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 the standard audio, it is called secondary audio. 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 the intercom is not equipped with sub-audio, you can't set it; In addition, relay stations are generally set with sub-audio. If your walkie-talkie is not set with sub-audio or is set incorrectly, the relay station cannot be turned on.