So, I was testing out some exports with Pro CS4, finally working with some AVCHD from an SR12 a friend sent me, and exported it using CS4's media encoder. H.264 MP4. Anyway, plays back perfectly on my machine, so no problems there.

I sent the edited, exported files back to my friend. He's using an Apple (Final Cut) and the playback of the MP4 ran very choppy. Also identified the files as AVCHD codec, and not the H.264 MP4 that I'd exported them to.

These were his m2ts files, and worked fine on his machine when he exported them. Any ideas as to why it would still be identified as AVCHD on his Mac even through it went through my encoding process as H.264 MP4?

I would like to know this answer too. The 1080 30p files I shoot play fine on my Mac. But some edited 720p files from the same camera on Vimeo are choppy. In fact it seems quite a few of the posted 5DII files are damaged in IQ or choppy. Clearly editing H264 still has issues.

Well, avchd uses H.264 and mp4. Info from Wikipedia that might be useful:

"AVCHD (AVC-HD, AVC HD) video is recorded using the MPEG-4 AVC/H.264 video compression codec. Audio is stored in either compressed form (Dolby AC-3), or uncompressed form (multichannel PCM). Aside from recorded audio and video, AVCHD includes features to improve media presentation: menu navigation, slide shows and subtitles. The menu navigation system is similar to DVD-video, allowing access to individual videos from a common intro screen. Slide shows are prepared from a sequence of AVC still frames, and can be accompanied by a background audio track. Subtitles are used in some camcorders to timestamp the recordings."

Battle Vaughan/miamiherald.com video team