Mceljo couldn't have stated it better and that is an excellent overview of the SACD format. The sampling rate is very well explained. If I could add to it: SACDs come in both multi-channel and stereo 2 channel. IMHO, those that are mastered in multi-channel sound a great deal better than in stereo. I have both (mostly multi-channel) and the comparison is like night and day. SACD technology shines when taking an older recording and remastering it from the original tapes. Some turn out great while others not so good. Many variables.
EXPLAIN HD SACDs to me.
I have a player that plays both. Explain these to me. Are they worthwhile