Dear fellow Matterhorn users,
as we start to use our Matterhorn 1.3 in these days, I wanted to improve
our workflow definitions. I decided that I wanted to use H.264 only, as
we luckly updated to a Wowza Media Server [1] that allows us to stream
H.264 [2].
So I created some MP4 encoding profiles, updated the multi-quality
workflow definition and created a HD-workflow definition based on
multi-quality. These updates are documented here [3].
In creating the MP4 encodings I recognized that MP4 behaves quite
different from FLV. I don't know how much I like these changes and I
must say that FLV had some great features that I currently miss. So what
has to be considered when encoding MP4:
1. If you want synchronous playback of two streams the two videos for
presenter and presentation need to have the same framerate and GOP
structure. If this is not given the videos can be out of sync for
several seconds.
2. The Engage Player can only jump to key-frames. These are usually set
every 300 frames. That means every 10s on NTSC, 12s on PAL. If you
reduce the framerate to save bandwith the intervall may become even
greater. You can try to set the key frame interval manually, but the
presets may overwrite these changes. The consequence is that when you
jump to a chapter in the Matterhorn engage player you will not jump to
the exact second but to the last keyframe, that may be several seconds
earlier. FLV did not have this problem as the format somehow stores the
reference to the keyframes in the metadata.
3. At least in the engage player there seems to be a problem that the
h.264 videos are not always lip sync, which means that the delay of
audio and video is more than 80ms.
4. There are several "profiles" for H.264. I have not understood the
differences between these profiles in depth. The advantage of baseline
profile is that it should run on the most devices and is probably
hardware accelerated. So at least one of you profiles should be a
basline profile. From my subjective tests the lip-sync issues is not
that recognizeable on main and high profiles somehow.
5. There are several encoding presets (from ultrafast to veryslow) for
x264. There is a dublication of presets ffmpeg has its own which are
called with the "-vpre" option and the presets from the x264 codec
itself which can be called with the "-preset" option. The vpre-settings
create a MP4 that is not playable in Flash, so I recommend to use the
x264 presets that all seem to work fine. In my tests ultrafast was more
than 10x faster than veryslow. From my test-encodings I decided to use
the medium setting (2-3x the encoding time of ultrafast), as the
video-quality seemed okay for me than, but I might adjust this with more
test videos around. (In general the baseline profile encoded 30% in my
examples than the other profiles).
I hope my findings are helpful to others, as it seemed to me that there
was quite a lot chatting on IRC about using H.264.
Rüdiger
[1] A guide on how to setup Wowza with Matterhorn can be found here:
http://opencast.jira.com/wiki/display/MHDOC/Wowza+Media+Server+3+v1.3
[2] Red5 does not allow seeking in MP4 files somehow.
[3]
http://opencast.jira.com/wiki/display/MH/HD-Video+%28720p%29+with+H.264+encoding+only
--
________________________________________________
Rüdiger Rolf, M.A.
Universität Osnabrück - Zentrum virtUOS
Heger-Tor-Wall 12, 49069 Osnabrück
Telefon: (0541) 969-6511 - Fax: (0541) 969-16511
E-Mail: [email protected]
Internet: www.virtuos.uni-osnabrueck.de
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