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AVCHD is a format for the recording and playback of high definition
video. AVCHD (AVC-HD, AVC HD) utilizes MPEG-4 AVC/H.264 (AVC) video
compression codec and Dolby AC-3 (Dolby Digital) audio compression
codec. Uncompressed linear PCM audio is not supported in any consumer
devices. iMovie is created for this purpose, just several steps, you
can add movies to your website, publish them on YouTube, create
versions for iPod, iPhone, and Apple TV, or play on iTunes, QuickTime.

Most of high definition camera/camcorder videos(AVCHD\MTS\M2TS, MOD\MPG
\MPEG\MOI, TOD\M2T, TR, TRP, TS) can be imported into iMovie, but the
formats are still very limited as following:
Tapeless Camcorder(MPEG-2, MPEG-4 and AVCHD) ;
Tape based DV-Standard and HDV (High Definition Video) ;
Part Camera\Mobile Phone video\other video recorder devices(MPEG-1,
MEPG-2, MPEG-4, H.264, 3GP, MP4);
Part QuickTime compatible MOV videos;
MPEG-4 video.
Since imovie can't directly play avchd video files, so it is necessary
to convert the video files to imovie suitable formats like mov or mp4.
AVCHD to iMovie Converter can transfer avchd video files into imovie
on mac os x. With this AVCHD to iMovie Converter, you can import avchd
into imovie.

How to import avchd video files into imovie on mac os x?

http://www.4yousoft.com/imovie/avchd-to-imovie.htm

Step 1: Download, install and run AVCHD to iMovie Converter
It's simple and quickly.

Step 2: Add AVCHD video files
Click the "Add files" button to import your AVCHD files. This AVCHD
into imovie converter support batch conversion, so you can import
multiple files at a time.

Step 3: Select output format: "MPEG-4 Movie (*mp4)"

Step 4: Convert
Click "Convert" button, it begins to convert your AVCHD files to
imovie mp4 format.

Step 5: Importing convertion mp4 files to imovie
Launch iMovie, go to "file import movies". Select the converted MP4
video then click "import".

http://www.4yousoft.com/imovie/avchd-to-imovie.htm