Previously I wrote a little about the Blu-Spec CD - a red book CD burnt with a shorter wavelength blue laser. In theory, the digital signal on the Blu-Spec CD should be less prone to errors in playback.
The SMH-CD is a very similar idea: a red book compatible CD created through a new process to reduce errors on playback. In the case of the SMH-CD (Super High Material CD), the base material is a special polycarbonate plastic designed to increase the transparency on the data side of the disc. The increased transparency means less distortion on the signal read by the laser, and a more accurate playback.
I can’t find any SMH-CDs for sale, and there’s very little information about them on the internet. It looks like Prince’s catalog was reissued in Japan at some point in this format, and there’s substantial debate about the sound quality of the manufacturing process. If you know anything else about this format, toss it in the comments!