In my understanding, DTMF, MDC/HDC, and APRS all fall under the category of digital signals modulated using frequency-shift keying (FSK). The main difference lies in how the latter two use dedicated protocols for encoding and decoding. DTMF is simpler, relying on combinations of two tones to represent numbers or letters. Theoretically, any device capable of producing these two tones, such as a mobile phone keypad, could be used. However, when transmitting longer messages with DTMF, a long sequence of tones would be required.
In practice, MDC/HDC signaling also serves to control communication on dedicated devices, such as group calls and individual calls. However, in emulated devices like Sennheiser's GT12 or the K5 firmware, it can only perform simple signaling decoding, with only the "signal" but not the "command." DTMF can also provide similar functionality on entry-level devices, although the implementation differs slightly. Based on my testing on the K5, when the DTMF decoding function is enabled, it essentially forces the handset to mute. The specific method used could be either direct muting or adding a low-frequency signal. I haven't further tested this (note: this is not noise cancellation; at this point, the device still receives and processes signals, with the field strength meter and signal indicator lights responding normally. When a call is received and the caller ID matches your pre-set ID, the communication mode is activated, muting is canceled, and the communication mode is entered.
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