what is the relation between mp4 files and mov files?

[donaldxdog]

When I open a abc.mp4 movie in quick time and then save it, it saves as a abc.mov file. The mov file is the same size as the mp4 file and appears to be the same quality. Is it truly the same quality? When I try to use quick time to convert the mov file back to a mp4 file it takes much longer than the mp4 to mov conversion, gives a larger file size and the quality is much poorer. What is happening here?
Thank You!

[Case]

When I open a abc.mp4 movie in quick time and then save it, it saves as a abc.mov file. The mov file is the same size as the mp4 file and appears to be the same quality. Is it truly the same quality?

This procedure only saves the file in a new container format (.MOV) without changing the video or audio. QuickTime Pro can do that with many source formats.

When I try to use quick time to convert the mov file back to a mp4 file it takes much longer than the mp4 to mov conversion, gives a larger file size and the quality is much poorer.

This procedure re-encodes everything to the specified codecs, regardless of the codecs used in the source file. It won't look if the original video or audio is usable, but converts everything. The upside is that anyone can do it, without knowing much about video conversions. The downside is that sometimes you'd wish for pass-through, as it saves time and quality.
MOV to MP4 is possible for matching codecs, but you'd need a different tool.

[terryj]

Fisrt I t seems like you are just doing File-->Save As in Quicktime, and selecting reference movie ?
This would just take the .mp4 and put it
inside of a QT .mov container.
When you then wanted to convert that .mov into something else,
yes it would take quite a while because it is actually decoding an .mp4,
NOT a .mov file.
....

Best to do is to use File--> Export, and then choose QT .mov as the format,
and select a codec to encode the file properly into a true QT .mov file,
like Apple Motion Jpeg-A for the video, uncompressed 16bit aiff for the audio.

[AeroQ]

Fully agree with the answers above.

A tool to save an MP4 file into MP4 container without re-encoding (ie fast and without loss of quality) is SimpleMovieX.
It does passthrough saving, just do File>Save and you have an mp4 file.

Regards, BJ