Why are some transceivers capable of operating computer-based digital modes without a separate sound card?

Computer-based digital modes, such as FT8 or PSK31, rely on audio tones being passed between the radio and the computer. Traditionally, this required wiring the radio's audio jacks to a separate sound card on the computer to handle the analog-to-digital and digital-to-analog conversions. Modern transceivers often feature a built-in audio codec (coder-decoder). This internal chip performs the necessary audio conversions right inside the radio, allowing audio streams to be sent directly to the computer digitally, usually over a single USB cable. While a USB connector provides the physical path, and CAT (Computer Aided Transceiver) provides the control signals, it is specifically the audio codec that eliminates the need for an external sound card.