Analogue to digital converters take video in one form, composite, Y/C or component, and convert it to another format - like DV, HDV or SDI.
Built-In Signal Processing
Many converters are bi-directional, so they can convert from analogue video to digital, or from digital to analogue. Some analogue to digital converters have built in signal processing and time base correctors to help them convert a signal from poor video sources.
When an analogue to digital converter has SDI capability, some are able to use embedded audio.