What 3GP is, and why nothing opens it anymore
3GP (.3gp and the newer .3gpp) was the video format of pre-smartphone and early-smartphone handsets — built to squeeze video down a slow 3G network and onto a phone with almost no storage. Inside the container is usually H.263 or MPEG-4 Part 2 video with AMR or AAC audio, at small frame sizes and low bitrate. Those codecs are retired now: modern phones, browsers, and editors frequently refuse to open them. Re-encoding to H.264 + AAC inside an MP4 wrapper makes the clip playable again.
What this tool does, exactly
Native server-side ffmpeg decodes the old phone video stream and re-encodes it to H.264 at CRF 23, re-encodes the audio to AAC at 160 kbps, and writes the result as an MP4 with the +faststart flag so it begins playing immediately when streamed. Pixel format is forced to yuv420p so picky hardware and browser decoders accept it. One file in, one MP4 out — no options to fiddle with. The source is low-resolution by design, so the conversion makes it playable; it does not add detail that was never captured.
Where your old 3GP clips came from
Most .3gp files still kicking around were shot on a feature phone or an early Android or iPhone, or arrived as an MMS video clip back when carriers capped attachments at a fraction of a megabyte. The fix is always the same: lift the old H.263 / MPEG-4 Part 2 video and AMR / AAC audio out of the 3GP container and re-encode into a standard H.264 + AAC MP4 that today's devices and editors actually accept.
Sister tools
For other legacy and broadcast containers: MPG to MP4 converter handles old MPEG-1 / MPEG-2 files, TS to MP4 converter and MTS to MP4 converter handle camcorder and broadcast streams, and AVI to MP4 converter handles old DivX and Xvid files. Shrinking a finished MP4 further? Video compressor.
Skipping the conversion entirely
3GP exists because old phones recorded it. If you are still capturing new video and want it to play everywhere from the start, record straight to a shareable link instead. Clipy records your screen to a modern H.264 MP4 behind a link — no local file, no format conversion, no watermark, no install.