Why A soundcard-style digital mode setup needs three things to pass between the computer and the transceiver: audio out of the radio's receiver into the computer so the software can decode it, audio from the computer into the radio's mic or accessory input to be transmitted, and a keying line (PTT, often via a serial line or a CAT command) so the software can switch the radio to transmit at the right moment. Modern interfaces and built-in USB codecs bundle all three into one cable, but those are still the signals involved. Without the keying signal the computer could generate tones but never get the radio to transmit them.