Why over-sampling and filtering... and why not?

the move to no correction, pretty much has ended.

An anti-aliasing filter is used during recording of the CD so that the inaudible higher frequencies don't run into the 44.1 kHz digital sampling signal. Often 20kHz was chosen as the filter cut-off. It was called a "brick wall" because everything above that frequency was filtered out. The rationale was that we couldn't hear anything above that frequency anyway. Although we can't hear that high, those higher frequencies do interact with frequencies that we can hear. It has been suggested that this is one of the reasons why CD's don't sound as true as analog. Early filters also had a serious phase shift.

With oversampling, the digital sampling circuit of 44.1kHz is increased. If it was 4X oversampling, then 4 X 44.1 equals 176.4kHz. The farther the sampling circuit is from the sound spectrum, the less of a need for a filter with its attendant problems. You could use a filter with a more gentle roll-off, or even no filter at all.

This information is courtesy of people who know more than I do. It's just things I've picked up over the years from Audiogon and other sources.