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Here's the info from Simple MovieX's help file:
Saving and Converting
Once you're done with editing, it's time to save your work.
The important thing to remember is: You can't always save your work in the original format.
What ? Wasn't this supposed to be a nice movie editor for the rest of us ?
OK, we're not perfect, but SimpleMovieX is still probably one of the best product to this regard: It supports native editing of QuickTime, AVI, MPEG-1, MPEG-2 and MPEG-4.
Saving has a few limitations coming from the underlying multimedia technology. First of all, some codecs are one-way, they just open and decode but cannot encode or save. This is the case for example of mp3. In this case, the only possible saving options are QuickTime format, that always works, or re-encoding if the codec allows to do it.
Re-encoding means loss of quality and a very lengthy process.
QuickTime format is great as long as the "final customer" can read it. For example, set-top boxes can usually read DivX AVI and MPEGs, but not QuickTime format.
The second limitation is related to container format capabilities. While QuickTime can do almost anything, MPEG and AVI cannot handle chapter, resizing or mixing media from several sources. Therefore, for AVI, MPEG, the following actions will be discarded when saving:
Resize or rotate movie
and finally mixing media from incompatible sources will probably force you to save in QuickTime format.
A lot of ways to save your work
File>Save tries to keep same file location and format.
File>Save As... gives you the choice of location and of format.
File>Export or Convert is a different way to save a movie. It involves re-encoding the media to the same or to a different codec and format, thus is very slow and involves some quality loss.
Save RefMovie... is like regular QuickTime format saving, but without "flattening". It means that SimpleMovieX doesn't copy the media data, just the recipe to describe the movie. You can think of it as a kind of playlist where you can specify which chunks of movies are played in sequence. The main advantage is speed and low file size, the main drawback is fragility as if you move or delete any of the referenced media data, the RefMovie can be lost.
We call Native the formats that SimpleMovieX can read, edit and save.
SimpleMovieX can also read and edit more formats, actually all the formats supported by QuickTime and codecs installed in the system. However, saving is restricted to the following ones:
MPEG-1 and MPEG-2 (more info)
Native vs Transcoding
SimpleMovieX also leverages the exporter components compatible with QuickTime to make transcoding of format possible. For example, a DV clip can be converted to MPEG-4 through the Export menu.
Transcoding is not as good as Native editing, because it is about 10 times slower to perform and some quality is lost. If you have a 5 minutes DivX .avi clip, you can either save it natively in avi format, which will take a dozen of seconds, or transcode it with DivX avi exporter, which will take 5 minutes and erode the image quality.
No restriction. Any file that SimpleMovieX is able to open, can be edited and saved in .mov format. Movie is saved as it is in the editor window.
MPEG-1 and MPEG-2
Editing is possible as long as files are homogenous, i.e. they share the same audio and video encoding, bitrate and pixel size. It means that cutting and pasting is not only possible within a document, but also between separate files.
The main restriction is about editing accuracy : If the document is saved in .mov format, frame accurate editing is permitted. However, if you save in MPEG format, editing points are rounded to the nearest keyframe (Group of Picture), which means that accuracy is around 15 frames. SimpleMovieX v3.0 features a keyframe indicator and a sticky mode to help you.
The last restriction is relative to MPEG-2, and comes from Apple's component : Only mp2 audio codec is supported, not AC3 (a52, Dolby not supported). See also MPEG support.
Main advantages with respect to QuickTime Pro are, besides editing and native saving capabilities :
Audio transcoding. You can convert MPEG files without loosing audio.
Support for files beyond 4GB.
No restriction as long as the movies involved have the same pixel size. Movie is saved as it is in the editor window, but chapters are lost.
Any movie (any format or container) can be saved as MPEG-4 if its video codec is H264/AVC1 or MPEG-4 Video and its audio is AAC.
AVI files contain in general one DivX or 3vix video track and one mp3 audio track. SimpleMovieX supports any video codec that QuickTime understands (this includes DivX and 3vix if you install the respective codecs) and the following audio codecs:
raw, integer, a-law, mu-law, AAC, AC-3 (A52 Dobly, 5 channels), MP2, MP3, DVI ADPCM.
Other audio formats are not supported at this stage. This is why an .avi file containing an Ogg Vorbis sound track cannot be saved as .avi, even untouched.
Several files can be edited and mixed if they share the same homogenous video and audio codecs, bitrates and pixel size. If you save in native AVI format, editing points are rounded to the nearest keyframe. SimpleMovieX features a keyframe indicator and a sticky mode to help you.
Main advantages with respect to QuickTime Pro, besides native saving capability, are support for OpenDML and for big files beyond 1, 2, and even 4 GB. (QT Pro would truncate them).