When the relay station communicates with the walkie-talkie, the function of the sub-audio setting is the same as CTCSS, but the difference is that it uses digital coding as a condition for whether the mute is turned on or not. If the receiver is equipped with auxiliary audio, and the sender of the call is also equipped with corresponding auxiliary audio, when receiving the signal sent by the other party, the auxiliary audio signal will be separated from the special chip, filtered and shaped and input to the CPU, and the CPU will compare the foreign auxiliary audio with the auxiliary audio set by the local computer. If it corresponds, the CPU will issue an instruction to turn on the mute, so that the voice of the other party can be heard.

CTCSS (continuous tone controlled squelching system) and continuous voice controlled squelching system, commonly known as sub-audio, is a technology of adding frequencies lower than audio (67Hz-250.3Hz) to audio signals and transmitting 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.