PS1:Optical Drive Emulators

These are various hardware solutions that allow the PS1 to run disc image files - there is currently no software only method for doing this, and given how close PS1 games run to the hardware it's likely there never will be.If you want to be really pedantic then none of these are strictly "optical drive emulators", because they all work at a higher level than the actual drive, but it's still a useful name for describing what function they perform. All of them do basically the same thing, so it just comes down to what set of features you are interested in. They are listed in order based on original release (I.E. earlier devices first).

PSIO
This was the original optical media emulation solution for the PSX - it consists of a cartridge that plugs into the parallel expansion port and a small (postage stamp sized) PCB that needs to be installed inside the console and which reroutes the CD interface chip CS/ and INT/ lines to the expansion port, with a switch function that will reconnect the original signals when the cartridge is unplugged. This is the only currently available solution that allows you to retain the use of the original optical drive, and even provides a function in the menu for booting from CD with the cart installed. PSIO has a different design approach from the other products here in that it replaces the entire CD subsystem including the CD interface chip and the mechacon CPU with it's own logic.In principle, this makes it a lot more flexible, although the current implementation doesn't make use of this.

PSIO - Pros
Allows continued use of physical media. Can be installed on most consoles (officially supported up to SCPH-750x/PU22, Installation on SCPH-900x/PSone possible if you are a masochist). Can simply be unplugged to revert the console to stock functionality.

PSIO - Cons
Still has some compatibility issues with certain titles like Vib Ribbon. Since it occupies the expansion port, not usable with cheat carts (can still be used with cheat CDs). DRM on the firmware is annoying and makes swapping cards between carts needlessly complex. Constantly out of stock and difficult to buy.

Xstation
This is a later development, and uses a significantly different approach from the PSIO. The PSX CD subsystem effectively consists of a regular audio type CD player feeding through a CD interface section that carries out the CD-ROM decoding functions, and (at least on the earlier models) these two functions are contained in different physical chips.What Xstation does is disconnect the "CD player" part of the system (the CD DSP) and create its own transport stream that gets fed into the CD interface section along with the subcode while monitoring the commands sent to the CD DSP and various status signals. This has both advantages and disadvantages - the biggest advantage is that since it's using the original CD interface chip and mechacon CPU it gets a whole lot of definitely correct behavior for free, the flip side of this is that it can't do things the original hardware doesn't support even if it wants to.

Xstation - Pros
Good compatibility with the PS1 library. Generally good availability. Reasonably easy to install using the QSB.

Xstation - Cons
Installation disables the CD drive Only works on certain specific models It's not THAT easy to install - in particular lifting the DSP pins might cause stress to beginners.

MODE
This is listed last because its PSX compatibility came after the Xstation - it uses pretty much the same basic operating approach as the Xstation (injecting signals at the CD DSP level) and I would expect it to have about the same level of compatibility (maybe someone that's used it fairly extensively could confirm?).The install is very similar to Xstation and even uses a lot of the same soldering points.

MODE - Pros
Supports multiple media types including SATA HDDs. Installation is about the same as Xstation. In theory the same unit can be used for PSX, Saturn and Dreamcast.

MODE - Cons
No physical CD support. Only supported on same machines as Xstation. Expensive. Stupid DRM nonsense on firmware like PSIO.