An ADC (analog-to-digital converter) is the chip that turns the continuous voltage from a microphone or line input into the discrete numbers of digital audio. It does the two defining operations together: sampling, measuring the voltage at a fixed rate such as 48,000 times a second, and quantization, rounding each measurement to the nearest value the bit depth allows. An anti-aliasing filter sits in front of it to remove frequencies above the Nyquist limit, which would otherwise fold into the audio as aliasing. The quality of an ADC — its clock stability, linearity, and noise floor — sets the real ceiling on capture, no matter how high the numbers on the spec sheet.
