I have a player that plays both. Explain these to me. Are they worthwhile
???? There is no such thing as "HD SACDs."

Wikipedia has an overview of SACD.
http://www.sa-cd.net has a list of available ones and a forum.

What other are you asking about? What player do you have?

Probably means HDCD.
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.
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.
if i glean correctly, your central question is whether hdcd/sacd are superior than redbook cd. hdcd does contain a little more musical information and has more dynamic range; all things being equal an hdcd should sound richer and better than a redbook cd, though not night-and-day better--hdcd is still just an enhanced redbook cd at core. presumably because the difference between hdcd/redbook is comparatively small, the hdcd format doesn't get a great deal of attention. sacd, on the other hand, is an entirely different beast--it utilizes much higher sampling frequency and provides much greater resolution and dynamic range, as well as much greater disk capacity, multi-channel capability, etc. all things being equal, sacd ought to sound vastly better than hdcd or redbook. however (and there's always a however) there have been a number of blind listening tests in which subjects (including recording engineers) could not discern an audible difference between sacd and regular cds, conceivably because most people can't distinguish audio over a certain frequency threshold. interesting stuff.
Vernneal, I suspect that the earlier posters are correct that you are really asking about HDCDs. Only the Crystal chip set, I believe, plays HDCDs, which have been somewhat lost with Micro Soft buy this format. The original Oppo uses this chip set and does play both HDCDs, CDs, and SACDs. I have never figured out how to force the Oppo to play HDCDs as CDs, but the HDCDs do sound excellent. Many performers want their recording done on HDCDs.