DSP code

DSP = Digital Signal Processing...

In other words using a computer to calculate the impact of a filter or other transformation on a digital representation of an analogue signal. In analogue electronics a bunch of passive components - capacitors, inductors & resistors - can be arranged in various ways to change a signal. For example to attenuate high frequencies more than lower ones - a low pass filter.

Once a signal is digitised - sampled and turned into numbers - there are various algorithms than can change these numbers in such a was as to replicate the impact of an analogue filter.

Subtle changes in the DSP algorithm or its implementation can be audible, so much effort is put in to getting them right.

Further to Dave's explanation, the actual process of digital to analogue conversion requires the samples to be usually manipulated because of the way the analogue audio signal is reconstructed from a digital signal - the samples are modified - and this is done by DSP - that is the digital signal is mathematically processed.