[LAU] Introductory seq24 tutorial

Frank Neumann beachnase at web.de
Wed Oct 27 10:33:19 UTC 2010

Hi Leigh,
you wrote:

>> - What toolchain did you use? Was it the Xephyr/jack_capture/ffmpeg
>>   package that others (AutoStatic?) had mentioned here or in other places?
>Yep, I used AutoStatic's Xephyr and ffmpeg setup for capturing the
>video. For the audio, I recorded straight in to Ardour -- that let me

I tinkered around with Xephyr for a while today, and had some problems
with what was described on http://www.rncbc.org/drupal/node/219#comment-3859 -
specificly, starting your X session with simply "/etc/X11/Xsession &" did
not work for me - the harddisk started rattling for a while, but nothing
happened. I'd think that it is trying to "start too much" of the services,
some of which are perhaps already running through my initial login.

What I instead did now was to:
- Enable XDMCP on my PC (via /etc/gdm/custom.conf)
  -> for Ubuntu Karmic, see http://www.peppertop.com/blog/?p=690
- Start Xephyr like this:
  Xephyr :2 -query -once -screen 1280x1024
- Log in as usual, set up applications and continue from there
- Start jack_capture&ffmpeg from outside Xephyr as described by you below.

I also noticed that if I log in as some other user (one with a "default"
Ubuntu desktop, perhaps better for screencasts), I need to allow the "host"
to read the XDisplay of the "guest" by issuing a "xhost localhost" before
I can start ffmpeg.
However, this means that if I run QJackCtl/jackd as that other user, I cannot
get at its audio stream with any JACK client - not even as root. I assume
that is a safety mechanism in JACK. But there are ways to overcome this,
like with sudo.

> I used ffmpeg's default MPEG-4 compression, but with a very high maximum
> bitrate -- that gave me high quality, a manageable (large, but not too
> large) file size, and relatively low CPU usage. Here's the command I
> used:
> ffmpeg -an -f x11grab -s 1280x720 -r 30 -i :2.0 -vb 10000000 tute1.avi
> > - Did you experience any audio/video out-of-sync problems which you had to
> >   compensate for by e.g. time-stretching the video?
> Nope, the audio I exported from Ardour lined up perfectly with the video
> from ffmpeg. To make it easier to line them up in Kdenlive during the
> edit, I started the video by hitting the "send test note" button in
> XSynth; I was then able to line up the button press in the video track
> with the sound of the note being played by XSynth in the audio track.

Ah, ok - a "manual initial sync". As long as video is correctly recorded
thereafter (no framedrops?), this should be fine.

Thanks a lot for these explanations - this is all most useful to me.


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