[doyal gudgel] "Is mp4 a HD file playable on Blue ray players only?"
No, MP4 files are mostly use for HD web video. Blu-ray players use M2TS, M2V, or AVC files.
[doyal gudgel] "Is MP4 as high a quality as AVCHD?"
It can be. AVCHD is a subset of the MPEG4 specification. So AVCHD is MP4 but MP4 can contain formats that are not AVCHD. As for quality, they are the same quality at the same profile, resolution, and bit-rate because they are both MPEG4 video.
[doyal gudgel] "The reason I asked is I thought mp4 was a less quality method for getting stuff on Youtube using less memory."
You are correct that MP4 is a popular file format for Internet video and that AVCHD (.M2TS) is a compatible format for Blu-ray but they are both MPEG4 video and can produce the same quality or different quality videos. Since AVCHD is a subset of MPEG4, it has more limited parameters to choose from.
~jr
http://www.johnrofrano.com
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