What is oversampling?
To take distortion-less samples at 44.1kHz requires that the analogue signal be band limited to 22.05kHz. Since the audio band is reckoned to extend to 20kHz we require an analogue filter that cuts off very sharply between 20kHz and 22kHz to accomplish this. This is expensive, and suffers from all the ailments associated with analogue electronics. Oversampling is a technique whereby some of this filtering may be done (relatively cheaply and easily) in the digital domain. By sampling at a high rate (for example 4 times 44.1kHz, or 176.4kHz) the analogue filter can have a much lower slope since its transition band is now 20kHz to 88kHz (i.e. half of 176kHz). The samples are then passed through a digital filter with a sharp cutoff at 20kHz, after which three of every four are discarded, resulting in the sample stream at 44.1kHz that we require.
