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Movie files are highly sensitive to timing, so some little difference may cause the movie to play back incorrectly. For example, the character might suddenly stop moving, or wander aimlessly, or die. Mario might run continuously into the side of a pipe. Mega Man X might jump into a pit. Or the game might not even leave the starting menu.
Do note that some TASes may have potentially confusing actions that may be mistaken for desyncs.
When the movie you’re trying to view desyncs, there are a few possible causes.
You don’t have the right ROM.
There are generally three versions of games: Japan, Europe and USA version, sometimes marked at the end of the ROM name by (J), (E), or (U) respectively. If you have the wrong one, the movie will likely go out of sync sooner or later.
Sometimes games have multiple revisions, (e.g. REV1, REV2), multiple fixes or hacks. If you have the wrong one, your movie will probably go out of sync.
The author may provide the name of the ROM. It is recommended (but not required) to rename ROMs using GoodTools in order to standardize ROM names.
Do not ask us (the author of this site or the people at the TASvideos forum) to send you ROMs or BIOS images. Any such messages will be deleted.
It is generally best to check the hash of the ROM for an exact match. Hashes ensure, in practice, that you have exactly the same file as the author. Exact hash values using common algorithms such as MD5 and SHA1 are often posted in the discussion threads.
You don’t have the right version of the emulator.
Ensure you used the correct emulator version. Usually, the author will state the version. If not stated, assume it is the most recent version.
Your emulator settings differ too much.
- In Gens, your type of controllers must be the same as the movie expects. Sometimes sound rate must be set to 44100.
- Sometimes, you need to vary the sound settings of SNES9x.
- FCE Ultra may need to be explicitly told if a game is PAL or NTSC.
- VBA only plays movies correctly when the default controller is set to Controller 1. For some movies, it also requires Real Time Clock to be enabled.
- For DeSmuME, check the firmware settings. Due to an oversight they are not loaded from the movie.
If you feel you may need to change the settings, the solution is usually found in the author’s description of the movie.
You have external codes on (ex. Game Genie codes)
Turn them off.
Replay is not quite deterministic
There's not much you can do about this except try and hope. Sometimes runs desync when played back one way and don’t desync when played back another way.
See also: Desync Help TAS