Psx Games Highly Compressed Access

Often used for PSP games but sometimes applied to PS1 ISOs. It uses lossy or lossless compression to shave off MBs, though it is less common for PS1 than CHD. ECM (Error Code Modeler):

10 Highly Compressed PSX Games That Still Look & Play GREAT (Under 100MB) psx games highly compressed

Even with CHD, sometimes the emulator needs a .CUE sheet for audio tracks. If you have a CHD of Wipeout XL and there is no music, you need a .CUE file next to the CHD. You can generate a blank one using "CUE Maker" software. Often used for PSP games but sometimes applied to PS1 ISOs