Any tips on how to play DreamCast?

[Bombardiro Crocodilo]

I'm having difficulties playing DreamCast but I don't know if it's the cores or if I'm downloading the file in the wrong format, I've tried cdi and gdi none worked, the core "sega - naome flycast" also can't read the file or the others "sega - md/cd/gg/ss"
If anyone knows an easy way to play, it can even be through the retro arch of the official website, I only use steam to upload the print and know what I've already played.
I'm more interested in getting achievements in retroachievements

[Fili]

cdi, gdi, cue, chd are all acceptable by the emulator. Can you tell me exactly what happens when you try to launch a game and how you do it? I need a bit more to work with.

Edit: For now, you can try the flycast from the repository and see if it's any diferent. Not sure you can get Retroarch achievements that way though. Or if you have Retroarch Standalone with "Flycast" and "Retro Dream" cores. The latter is discontinued but might still be found with a google search.

[TER]

It says "... invalid cdi image...".

[TheFantaPanda]

A good basic troubleshooting step is to try RetroArch, then compare the behaviour to a standalone emulator like Redream. Helps to narrow down the issue a little at first.

My first thought often goes straight to "it's a bios issue." In your RetroArch/system folder, is there a folder labeled 'dc' with your legal bios placed within?

It is also possible that your .cdi file is just borked. This file format was initially used to compress DC games to the size of a cd-rom, so it isn't the best format to use.

Try a game in a bin/cue or a gdi format - those are the two formats that would work best.

[Fili]

Flycast works without bios except if you want to run Atomiswave games. If the game image is invalid, it's likely borked as stated in the comment above and you have to dump your rom again from your physical CD.