Originally Posted by Retro-Nerd
They aren't different consoles. It's just a Genesis CD-Rom add-on with some improvements.
The Sega CD-"addon" has an own CPU clocked at 12.5MHz which is faster than the Genesis' CPU (runs at 7.something MHz).
It also added 8 PCM-channels to the audio capabilities and some gfx stuff.
Most games for Sega CD were really just Genesis-titles with added CD music - they didn't make use of the faster CPU and digital audio channels.
Very few titles were created specifically for the Sega CD and used its new CPU - but they do exist...
Thus it's more than just a "CD-Rom add-on with some improvements". It's an own system (although it needs to be added on to the Genesis).
The PC Engine CD on the other hand is indeed more or less just a CD-drive and adds more RAM. So maybe we could allow the CD games, after all. Or not - now it's too late...