Why are AVCHD files .m2ts format?

LSHorwitz wrote on 6/25/2008, 7:15 AM

I have been authoring AVCHD disks with several NLE programs and notice that most of them including Vegas produce streams encoded as .m2ts.

It doesn't make much sense to me why h.264/mpeg4 files should be encoded as MPEG2 transport streams. The transcoding to mpeg4 from the original AVCHD camcorder footage into MPEG2 for playback makes no sense, both from the viewpoint that the transcoding will degrade the image, take additional rendering time, and serves no purpose since the BluRay player has AVC codec playback built-into the BluRay spec.

For whatever it is worth, Sony, Ulead, and Pinnacle all do it this way. Nero, on the other hand, makes its AVCHD disks using .mts streams, the very same forfmat which my Canon AVCHD format produces as raw output. These play fine on my BluRay player, as do all the rest.

Anybody able to explain why there are .m2ts files which seemingly contain h.264/mpeg4 content when Sony, Ulead, and Pinnacle author their AVCHD disks?

Thanks in advance,

Larry

Comments

LSHorwitz wrote on 6/27/2008, 7:33 AM

Looks like I am not the only one who is puzzled by this.....

Another surprise is that GSpot reports that the file format for AVCHD streams is "mpeg2 transport stream". How can this be?

Larry

Xander wrote on 6/27/2008, 7:57 AM

You need to remember that the MPEG-2 Transport Stream is just a wrapper. In days gone by, it was exclusively for the use of MPEG-2 video. With MPEG-4, the standard was modified to allow the inclusion of MPEG-4 video within the same wrapper. There was no need to re-invent the wheel.

The above stems from professional DVB broadcast systems. There is a lot of MPEG-2/DVB based equipment out there. By modifying the Transport Stream standard, older systems are still able to pass MPEG-4 content, albeit, they may not understand it.

Just remember that a Transport Stream is not the same as a Program Stream. File extensions are usually semantic.

Also, remember that MPEG-2 Transport Streams also include support for VC-1 as well as for the various audio formats. Understanding the content within the wrapper is not the same as understanding the wrapper itself. Most compatibility issues occur with the understanding of the content within the wrapper.

LSHorwitz wrote on 6/27/2008, 12:26 PM

Thanks very much. I was not aware that h.264/mpeg4 content was being wrapped in mpeg2 containers, and assumed that the .mp4 file name more commonly seen with mpeg4/h.264 content or .mts was used in some consistent way. It is suprising for VC1 for similar reasons.

Your explanation makes sense but does add an element of confusion. Up until now, I had only seen mpeg2 content wrapped in .m2t wrappers, and never expected that the definition had been revised to allow for other compression methods to be used on files contained within.

Thanks very much for clarifying this!

Larry