Subaudio, CTCSS (Continuous Tone Control Silencing System) and Continuous Voice Control Silencing System, commonly known as subaudio, is a technology that attaches frequencies lower than audio (67Hz-254. 1Hz) to audio signals and transmits them together. Because its frequency range is below 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, inputs it to the CPU, and compares it with the CTCSS frequency set by this machine, so as to decide whether to turn on the mute.

The English name of digital subaudio (Coutinus digitally controlled squelech system) means continuous digital coding squelch system in Chinese, or DCS or DTCS for short. DQT also stands for digital sub-audio, and Motorola calls it DPL (Digital Private Line).

Digital sub-audio has the same function as sub-audio, except that it is transmitted with audio signal in digital coding mode. CTCSS and DCS are both international signaling standards.