EXPLAIN HD SACDs to me.

Mceljo - SACD is also created without anti-alias filters that tend to screw-up transient response. SACD is basically the same as output of Delta-Sigma converter before filtering (PWM). Many audiophiles dislike Delta-Sigma converters but the same time enjoy SACD (strange).

HDCD is basically regular CD where the least significant of 16-bits is used to switch dynamic range (technique known as "in-band signaling") of the remaining 15 bits. Theoretically it is playable on standard CDP but it will play 15 bits with strange dynamics plus noise on bit #16.

I don't know how much better it can sound, but often recordings selected for such formats are excellent to start with making impression of new format superiority - I don't have opinion since I've never heard HDCD.

HDCD may or may not be manipulating 16th bit, but it sounds just fine on a non-HDCD decoding player. From what I heard it's using special noise shaping algorithm to extract close to 20 bits in certain audio spectrum. Probably similar to what Meridian 518 does as described here.

Meridian 518

It's quite amazing how the noise can be "shaped" out and away from the critical midrange in order to provide higher resolution.

Someone aske which player. I have gone from a Meridian 506(non 24) to a NOS McIntosh MVP 861. There are SACDs and there are HD CDs.

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.

Bear in mind that not all players that purportedly play both SACD and RBCD do a "proper" job with SACD. Many will convert the SACD datastream into PCM, and then process it through a standard 44.1kHz DAC. You will lose the advantage of SACD and maybe even get sound which is worse than RBCD. To play SACD properly, check if your player has a DSD DAC.