3GP to MP4
Repackage an old 3GP mobile video into MP4. 3GP is already an ISOBMFF variant carrying H.264/AAC or the older H.263/AMR, so a modern player that refuses the 3GP extension plays the identical packets once they sit in an MP4 box.
3GP to MP4 moves an old mobile video file into an MP4 wrapper, copying its packets with no decode at all. The 3GP container already follows the ISOBMFF layout, typically wrapping H.264 and AAC, or on older phones H.263 and AMR. A player that turns away the .3gp extension opens the identical bytes once boxed as MP4.
Questions
Why would a modern player refuse a 3GP but accept the same content as MP4?
Because some players key off the file extension rather than reading the container, and 3GP is already an ISOBMFF variant carrying H.264 and AAC or the older H.263 and AMR. Wrapping the identical packets in an MP4 box is often enough to make a player that refused the 3GP file accept the result.
Does the conversion change the picture or sound?
No. Packets are copied with no decode, so whatever codec the 3GP used comes across unchanged. A codec MP4 cannot legally hold is refused by name instead.
Which engine handles this, and where can it run?
A packet-copy remux tool moves data through mediabunny with no decode step, running in Node, on the command line and through MCP as well as the browser.
What size should I expect?
Close to the source 3GP size, since the compressed data does not change; only the container structure differs.
What is the reverse page?
There is none. Nothing on this site writes 3GP.