HDCD VS SACD

I have several HDCDs, they play on my system that usually includes a DAC produced before HDCD was introduced, do they add anything in this case or just play as standard discs? The format came up during my sabbatical from audio and I have never been clear on exactly how it operates.

I can't comment on SACD, but I do own a Rotel CD player with HDCD capability and several HDCD encoded CD's. Let me be blunt, whenever I seriously look for CD's to purchase, HDCD encoding is the closer for me. If you like vinyl, you'll love HDCD discs.

Stanwal,
If your DAC or CDP doesn't have HDCD built in, it cannot decode the HDCD portion of the disc. It would play as a standard redbook CD.

There's a very good explanation of HDCD technology on Wikipedia if you're interested.

I just finished listening to "Life On A String" by Laurie Anderson. One of the best sounding cds in my collection. Probably listened to this cd 25 times but for some reason tonight is the first time I noticed the HDCD light on my cdp.
On well recorded cds it can do wonders.
As for SACDs, I only have a few and the only thing that plays them in my house is the Oppo, DV-980H. The Oppo's audio output section doesn't really compete with my cdp so it is a dedicated dvd player.

"It is a playback only decode system, which means that it's benefits are applied during the playback of standard Redbook CD's and is only compatible with that format."

This is wrong. It has to be recorded HDCD to be played HDCD so it is NOT "a playback only". In HDCD coding the least significant bit is switching dynamic range of the rest. It is known as "in band signaling". Playback on standard player is a little suspicious (if nothing else is done) since the least significant bit becomes garbage (noise). I believe that the sole purpose of introducing schemes like that is to make it very difficult to copy (HDCD) or impossible to copy (SACD).