Windows or Mac? Is there a particular editing program you’re using? Is the program not recognizing the .cpi? What program?
A .cpi file is a kind of .mts file, basically MPEG2, but Sony and Panasonic use .cpi because it can include other settings info in the video file. Note that a .cpi can also include this metadata but *not* include the video data, so be sure you’ve looked at the sizes of all the files on the cards. The big ones contain video data.
For conversion, the first place to look would be in the documentation that came with your cousin’s camera. Is there a disk, or a link for conversion software?
There are freeware solutions out there, that include some batch abilities. Handbrake is #1 on my list, and VLC Media Player is #2, but I’ve not tried .cpi, and the different versions from different camcorders are... ... different!
But do try sony resources first!
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Photos, Video & Pano r us.
College and workshop instructor in video and audio.
70D, Sigma 8mm, Tokina f2.8 11-16, Canon EF-S f2.8 17-55, Sigma f2.8 50-150 EX OS, Tamron 150-600VC. Gigapan Epic Pro, Nodal Ninja 5 & R10.