Audio converters can be Analogue-to-Analogue, Analogue-to-Digital and vice versa. Analogue audio converters can convert between different analog audio formats bi-directionally, for example they can convert audio from unbalanced to balanced and vice versa. Analogue-to-Digital converters convert between different analogue and digital formats, for example AES/EBU, S/PDIF, Toslink etc.
Some machines include SRC-sample rate conversion between the different digital rates used in the industry.