mkvmerge: MPG -> MKV causing double the framerate?

[KeyMs92]

Hi,

I was trying to convert a couple of dvds to mkv files. I've already extracted a number of mpg files (each with one episode) using DVD Decrypter. As mpg doesn't handle chapter points, I decided to change the container to mkv. I found out this could be easily done using mkvmerge. However, I noticed looking at the output file that the original framerate was strangely doubled (25 to 50 fps). I'm wondering what could be the reason for this. The video itself seems perfectly fine. Also, if I specify a framerate of 25 in mkvmerge the resulting file is almost identical to the 50fps version, only being 400kb smaller.

Original (mpg):

Video
ID                               : 224 (0xE0)
Format                           : MPEG Video
Format version                   : Version 2
Format profile                   : Main@Main
Format settings, BVOP            : Yes
Format settings, Matrix          : Custom
Format settings, GOP             : M=3, N=15
Duration                         : 28mn 48s
Bit rate mode                    : Variable
Bit rate                         : 5 033 Kbps
Maximum bit rate                 : 7 500 Kbps
Width                            : 720 pixels
Height                           : 576 pixels
Display aspect ratio             : 4:3
Frame rate                       : 25.000 fps
Standard                         : PAL
Color space                      : YUV
Chroma subsampling               : 4:2:0
Bit depth                        : 8 bits
Scan type                        : Interlaced
Scan order                       : Top Field First
Compression mode                 : Lossy
Bits/(Pixel*Frame)               : 0.485
Stream size                      : 1.01 GiB (91%)

mkvmerge output (mkv):

Video
ID                               : 1
Format                           : MPEG Video
Format version                   : Version 2
Format profile                   : Main@Main
Format settings, BVOP            : No
Format settings, Matrix          : Custom
Codec ID                         : V_MPEG2
Codec ID/Info                    : MPEG 1 or 2 Video
Duration                         : 28mn 48s
Bit rate mode                    : Variable
Bit rate                         : 4 886 Kbps
Maximum bit rate                 : 7 500 Kbps
Width                            : 720 pixels
Height                           : 576 pixels
Display aspect ratio             : 4:3
Frame rate                       : 50.000 fps
Original frame rate              : 25.000 fps
Standard                         : PAL
Color space                      : YUV
Chroma subsampling               : 4:2:0
Bit depth                        : 8 bits
Scan type                        : Interlaced
Scan order                       : Top Field First
Compression mode                 : Lossy
Bits/(Pixel*Frame)               : 0.236
Stream size                      : 1 007 MiB (91%)

[Baldrick]

Probably because of Scan type : Interlaced. So its 50fps interlaced=25fps for the mkv container.

[KeyMs92]

That's what I was thinking, so just leave it like that?

[jagabo]

Does it play properly?

[KeyMs92]

Yes, it plays perfectly fine.

[jagabo]

Then don't worry about it.

[TSJNachos]

Just to be on the safe side, I would force MKV Merge to acknowledge the framerate of 25, Especially if you plan to convert to XviD or something.

[KeyMs92]

Just to be on the safe side, I would force MKV Merge to acknowledge the framerate of 25, Especially if you plan to convert to XviD or something.

Right, can you explain why? I'm happy to leave it at 50 or change it to 25 but it still feels a bit uncomfortable when I don't really know what the difference is. Or is it just, what Baldrick said, that 50 fps interlaced can be seen as either 50fps or 25fps?

[TSJNachos]

When you play the file back, the timecodes will dictate the when each frame will be played, so most video players will still play the file at 25 FPS. BUT... some converters will get confused, and will try to double the frame rate, causing jerky video, as well as increased storage space.

IDK whether interlacing has anything to do with it. I could be that MKV Merge does not read vob files very well, or it could be interlacing. IDK.

[KeyMs92]

I see, thanks. I think I'll just leave it all at 50 fps and than convert it when I experience problems.