i don't fully understand the engineering process between various formats, but from a layman's perspective:
regular cd uses one way of encoding and decoding digital information.
HDCD was developed by pacific microsystems to "enhance" regular cd recording/playback. a different type of recording instrument was used, and compatible HDCD players have a different digital filter. many people feel that HDCD discs tend to sound good in all players (could be the remastering though), and the HDCD filter sounds pretty good on non-HDCD discs, but the "magic" happens when an encoded disc is played in a compatible player.
SACD is an entirely different method of encoding/decoding the digital information. the sampling rate is much higher, eg, for every second of music, many more data points are looked at by the player to give you a more accurate portrayle of the music at that moment.
some SACD discs are single layer, meaning they will only play in an SACD player. others are "hybrid" discs, which have both SACD and regular CD layers. these discs will play in any player. and all SACD players will play regular CDs.
i'll let those with more technical expertise explain all that i've missed. i hope that this helped nonetheless.
I thought you pitched that at the right level given the original question. The obvious conclusion with HDCD, given its cross-platform compatibility, is that it can't do that much to improve the sound. The effect is subtle, but sometimes very effective.
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