extra-dependencies/debian/transcode/transcode-1.1.7/docs/import_v4l2.txt

--- README ---

What

This module captures from v4l2-compatible cards. I made it because the
existing v4l import module uses the v4l-compatibility-layer of most
cards nowadays, which is not what you want. Also the v4l module does not
keep track of audio-to-video synchronisation in any way. This may be
sufficient for some capturing cards, but my own SAA7134-based looses a
frame every now and then so audio and video will become unsynchronised
after a while.

Who

Anyone using a capturing card compatible to v4l2 should be able to use
the module, there are some warnings applicable though. First, I can only
test on a SAA7134-based card. I do follow general v4l2 programming
directives, so in theory it should work on any v4l2 compatible card. The
SAA7134 driver does have some quirks though. The module detects SAA7134
based cards and enables some workarounds. I can imagine that other
drivers that also need workarounds, these are of course not present in
the module. Second, the v4l2 api changes frequently. If you'd like to
compile the module yourself, you may run into all sorts of problems,
including compiling errors and errors during runtime of transcode.
Sometimes these problems can be solved by adding another include file to
the source, but you need to have linux kernel version > 2.4.20 anyway
and also with the latest v4l2 patches applied (see
http://www.bytesex.org). Linux version >= 2.6.0 should work in theory,
but I didn't test on it.

How

1) Set v4l2 parameters (station, bright, volume, etc.) using your
favourite tv viewing program or something like v4lctl. I deliberately
did not include any setting of parameters in the module as these
external programs are much better at it anyway ;-)

2) Try!!! You should be able to record from v4l2 now with transcode
using "-x v4l2". You may have to use "-D" to correct any static (!)
difference between audio and video, this depends on your card etc. You
should also use -V, because the SAA7134 driver seems to have a problem
with RGB capturing, which crashed my computer, so I disabled the
capture-in-RGB mode for the moment. Most encoders work much better and
faster with YUV anyway.

For SAA7134 cards, you may choose to record audio from the card directly
(using direct PCI transfers, not using sound cards etc.) To enable this
mode, add "oss=1" to the saa7134 driver module parameters. You now
should have another dsp device (mostly /dev/dsp1, sometimes /dev/dsp2).
To use this  device for capturing, pass it to transcode with the -p
flag. Keep in mind that this device can only capture at 32 Khz, although
it will happily accept any other rate. You may have to use the resample
filter.

3) If your cpu can hardly keep up it is advisable to boost up the number
of internal transcode buffers to e.g. 128 (-u 128). You may run into
this situation if you're encoding to mpeg2 in real time. If that's not
enough, you cpu is simply too slow for what you want :-( You'll have to
tweak. Sometimes, if you're also using filters, it helps to change the
number of threads.

4) If your cpu can cope and also the number of v4l2 buffers is
sufficient  (full screen capture should report six buffers) and the
transcode buffers are not overrun (watch the number between
parenthesises), and your audio and video become out-of-sync, then you
should try the force resync mode. Else happy capturing! ;-)

5) You can enable and tune the resync mode using parameters to
the v4l2 module (-x v4l2="<param>:<param>:etc").

Possible parameters:

resync_margin=x     this many frames the audio and video may drift
    apart before frame dropping or cloning starts.
resync_interval=x   this many frames between each resynchronisation
    check
overrun_guard=x     set this to 1 if you have a saa7134 and a very old
    driver (pre-2004)
crop=wxh+txl        use hardware cropping width * height at top / left;
    only works on recent saa7134 driver
convert=x           specifiy conversion scheme manually.
    x is a number, obtained with convert=-1, -2 = auto (default)
format=x            specify colour format manually, use format=list
    to obtain a list.

Please note: if you do not need the resync mode, do not enable it! If
your card produces good a/v sync ON AVERAGE (although short variations
may exist) the resync code will do more harm than good. Resyncing works
by cloning and dropping frames and yes, this is visible! On the other
hand, a hickup every now and then may be more acceptable than audio
running before or behind. Your milage may vary ;-) If you determined
that you definitely need the resync mode, then you should tune the
resync parameters. A good start is resync_margin=2,resync_interval=15.
Whenever "info output" is "on" (-q 1) and a frame is cloned or dropped,
transcode will show a line with some parameters, like whether the frame
is dropped or cloned and the sequence numbers of audio and video. When
finished encoding, a summary is shown.