CHD: compression, preservation, and convenience CHD (Compressed Hunks of Data) originated with MAME to store disc and hard-drive images more efficiently while preserving sector-level details like subchannels and copy-protection metadata. For optical-media-based systems like the PSP (UMD) or older consoles, CHD offers a pragmatic middle ground: lossless or near-lossless preservation with substantial space savings compared with raw ISO or BIN/CUE images.