I'm not familiar with HDCD beyond knowing that it exists.
As for SACD I'll try to give a quick general overview. It is processed differently than a standard CD and the sample rate is higher (i.e. the digital signal better approximates the analog signal). Standard Redbook CDs have a 44.1 kHz sample rate and, I believe, transfer the data in "packs" of information that are something like 16 bits at a time. SACD is direct stream digital (DSD) and transfers the data a single bit at a time and the sample rate is 2.8 Mhz (2.8 million times per second vs. 44 thousand times per second). In theory, this should allow much better resolution, especially at higher frequencies.
In practice, I have some SACDs that sound better than the majority of my standard CDs, but it's certainly not true in every case. I have several standard CDs that sound better to me than some of my SACDs. The advantage to me is that I have not purchased an SACD that wasn't recorded well so it seems to eliminate, or greatly reduced, the risk of getting a poor recording.
As for SACD I'll try to give a quick general overview. It is processed differently than a standard CD and the sample rate is higher (i.e. the digital signal better approximates the analog signal). Standard Redbook CDs have a 44.1 kHz sample rate and, I believe, transfer the data in "packs" of information that are something like 16 bits at a time. SACD is direct stream digital (DSD) and transfers the data a single bit at a time and the sample rate is 2.8 Mhz (2.8 million times per second vs. 44 thousand times per second). In theory, this should allow much better resolution, especially at higher frequencies.
In practice, I have some SACDs that sound better than the majority of my standard CDs, but it's certainly not true in every case. I have several standard CDs that sound better to me than some of my SACDs. The advantage to me is that I have not purchased an SACD that wasn't recorded well so it seems to eliminate, or greatly reduced, the risk of getting a poor recording.