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Recording movies
================


Audio
=====

If you don't get sound when recording avi movies, double-check the
mixer settings first.  The record source defaults to micro on many
linux sound drivers, you probably have to change this to line-in with
one of the available mixer tools.  Some sound cards have a separate
input gain control which needs to be set to some approximate value.
Also keep in mind that ALSA has all mixer controls at 0 (i.e. muted)
by default.

/me uses kmix (because it doesn't need much space on the screen).
The inputs where the sound cards record from have a red background
color.  With the right mouse botton you'll get a menu where you can
change the settings.  FreeBSD has a aumix version with X11 GUI in
the ports collection which is very nice too (aumix-gtk in debian).

If you want to see something while playing with the mixer settings you
can use the record utility (ncurses terminal application, in the tools
subdirectory), it has a nice input level meter.  motv has one build-in
too (Menu -> Tools -> Record level monitor).

Note on stereo:  xanim seems not be able to playback stereo sound
correctly.


Video
=====

Note that video recording does not work if the Xvideo extension is in
use.  For recording stuff with xawtv you might have to start the
application with the -noxv switch to disable Xvideo.

xawtv/streamer handle video recording with multiple threads:

 - one thread records video (+ displays video on screen).
 - one (or more) thread(s) does color space conversion / compression.
 - one thread records audio (unless you do video only).
 - one thread writes the movie data to the disk.
 - one thread calls sync frequently to make the writeouts more
   smoothly (more smaller chunks instead of few very big ones).


There are buffer fifo's between the recording threads and the
compression / disk writer to avoid recording overruns due to a
temporarily busy hard disk or CPU load peaks.

If you see messages about a full fifo or about v4l(2) waiting for
free buffers it is very likely that your hard disk is too slow,
especially if you try to record uncompressed video.

bttv 0.7.x allows you to use more than just two video buffers, you can
configure the number at insmod time (gbuffers option).  Using more
buffers (say 4-8 instead of just two which is the default) should help
to reduce the number of dropped frames.

If you want to record quicktime movies install libquicktime
(http://libquicktime.sf.net), then (re-)build xawtv.  The configure
script should find the library automatically.


Known problems (and workarounds)
================================

The timestamping for the video frames isn't very exact as it does
_not_ come from the v4l(1) driver but is just a gettimeofday() call
after receiving the video frame.  API design bug, needs fixing.
With v4l2 xawtv uses the frame timestamps provided by the driver.


Troubleshooting syncronisation problems
=======================================

A/V sync should simply work if your box can keep up with the data
rate.  xawtv/streamer displays some debug info (time shift) on stderr.
"a/r" is the difference between audio and real time, "a/v" is the
difference between audio and video.

Ff you see "fifo full" error messages your box likely can't keep up
with the data rate.  Possible fixes: Try using more buffers.  Try
recording compressed video.  Try tuning the hard disk using hdparm.
Buy a faster hard disk.  Buy a faster computer.

If xawtv/streamer says "queueing frame twice" it has put a the same
video frame twice into the output queue to avoid video running out of
sync.  If this happens a lot it indicates that either your computer
can't keep up with compressing the frames or that your v4l device
can't capture frames with the frame rate you are asking for.  A single
message now and then is harmless.


MPEG Encoding
=============

Have a look at the mjpegtools (http://mjpeg.sourceforge.net).  xawtv
can write the yuv4mpeg format accepted by mpeg2enc directly.  mp2enc
accepts xawtv's wav files too.  So you can use xawtv/streamer,
mpeg2enc, mp3enc and mplex to build mpeg movies.

If you don't have enough disk space to store uncompressed yuv video
you can also record compressed quicktime/avi files and then recode
stuff using lav2yuv + mpeg2enc in a pipe.  The streamer help text
(streamer -h) has a few examples.


Large Files
===========

xawtv has LFS support, i.e. it can write files >2GB without problems.
The AVI format has a 2GB limit.  There is a extension to overcome this
[http://www.matrox.com/videoweb/news/press/papers/odmlff2.pdf], but not
all applications can deal with it.  The quicktime format usually works
better as there is the quicktime4linux library everybody uses to
read/write *.mov files, thus there are less compatibility issues.


Read, Convert, Edit + Playback stuff
====================================

Recent xawtv versions come with the "pia" utility.  That is a simple
player which should be able to playback all AVI and QuickTime movies
recorded by xawtv, motv and streamer.

avi format

 * xanim plays everything without problems.
 * QuickTime[tm] (MacOS) plays the uncompressed formats just
   fine and complains about mjpeg/jpeg.
 * Windows Media Player plays the uncompressed formats fine.
   mjpeg/jpeg work too if a codec is installed (/me has a very
   old one from MainConcept).
 * avifile can't deal with the uncompressed video correctly
   [fixed in recent versions].
   mjpeg/jpeg doesn't work either for me as it seems not to
   recognise the MainConcept codec, although I've copied
   stuff to /usr/lib/win32.  Maybe it works with another
   one.
 * MainActor (linux) can read the mjpeg but not the jpeg
   compressed files.


quicktime format

 * xmovie + broadcast2000 can read the files without problems
   (not exactly surprising as they use the quicktime4linux
   library too ...).
 * QuickTime[tm] (MacOS/Windows) plays them without problems.
 * xanim says it can't find any data in the mov file.  It used
   to work with older versions of the quicktime4linux library
   (before 64bit support was added), so I suspect xanim simply
   can't deal with 64bit mov files.


raw data

 * ImageMagick can deal with this, you have to specify the image
   format + size on the command line like this:
	display -size 320x240 rgb:file.raw
   It can handle multiple frames in one big file too.


Have fun,

  Gerd

-- 
Gerd Knorr <kraxel@bytesex.org>

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