[FFmpeg-user] strange video cut

Moritz Barsnick barsnick at gmx.net
Thu Mar 15 18:25:42 EET 2018


On Wed, Mar 14, 2018 at 22:01:01 +0100, Adi Marvillo wrote:
> ffprobe does not show any difference in the file-specs: not with the
> encoder nor in resolution although the small video has bad quality- Does
> anybody know what is the differnce between the 2 files and why ffmpeg
> keeps telling me that everyting is the same in both files?

Is that so? My ffmpeg tells me that things are quite different:

barsnick at sunshine:~/tmp > diff -u <(ffmpeg -i testfile_small.vob 2>&1) <(ffmpeg -i testfile_big.vob 2>&1)
--- /proc/self/fd/63    2018-03-15 17:19:52.992464532 +0100
+++ /proc/self/fd/62    2018-03-15 17:19:52.992464532 +0100
@@ -9,8 +9,8 @@
   libswscale      5.  0.102 /  5.  0.102
   libswresample   3.  0.101 /  3.  0.101
   libpostproc    55.  0.100 / 55.  0.100
-Input #0, mpeg, from 'testfile_small.vob':
-  Duration: 00:00:20.02, start: 0.529978, bitrate: 1012 kb/s
-    Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, progressive), 720x576 [SAR 64:45 DAR 16:9], 25 fps, 25 tbr, 90k tbn, 50 tbc
-    Stream #0:1[0x1c0]: Audio: mp2, 48000 Hz, stereo, s16p, 384 kb/s
+Input #0, mpeg, from 'testfile_big.vob':
+  Duration: 00:00:19.97, start: 0.500000, bitrate: 7258 kb/s
+    Stream #0:0[0x80]: Audio: ac3, 48000 Hz, stereo, fltp, 224 kb/s
+    Stream #0:1[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, top first), 720x576 [SAR 64:45 DAR 16:9], 25 fps, 25 tbr, 90k tbn, 50 tbc
 At least one output file must be specified

> both cutting commands:
> ffmpeg -i Video.vob -ss 00:10:00 -t 00:00:20 ~/Dokumente/test/testfile_small.vob

Here, ffmpeg re-encodes with default parameters. For VOB, that's mpeg2
video (with a very low default bitrate) and mp2 audio. You need to tune
the encoding to get better quality (likely at the cost of larger files,
unless you choose more modern codecs).

> ffmpeg -i Video.vob -ss 00:10:00 -t 00:00:20 -c:v copy -c:a copy ~/Dokumente/test/testfile_big.vob

Here, ffmpeg retains the original streams, with their original codecs
and bandwidths, no quality loss.

You didn't show us your command output. If you had, we could have
pointed out to you that the encoding ffmpeg also shows quite different
things in the console log.

Moritz


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