The D/A converters developed for DVD application are 24 bit devices that can run at 192 KHz. When these readily available and relatively cheap devices are used to read a 16bit CD disc and are run faster than the 44.1 KHz data rate of the CD disc, the result is a higher bandwidth analog signal, where the signal in between the points corresponding to the original 16 bit data on the disc has been smoothly filled in. The so called "Brick wall" filter at 20 KHz used in older CD players can be moved up to a higher frequency well removed from the audio range. This ought to improve sonic performance, and most people agree that it does, although other aspects of a particular CD player design may be as much or more responsible.
By the way, in another post there was some confusion (on my part) regarding the sampling rate for DVD-A discs. The 192 KHz sampling rate is for a Stereo DVD-A disc. Multichannel DVD-A are 96KHz/24bits. But I have never seen a stereo DVD-A, and I don't know if such an animal actually exists.
