[linux-dvb] Re: [PATCH] new video event and ioctl

[Marcus Metzler <mocm@metzlerbros.de>]

Andreas Oberritter writes:
 > [1  <text/plain>]
 > On Sat, 2003-12-20 at 01:06, Marcus Metzler wrote:
 > >  > I'd like to add this patch to dvb-kernel's linux/dvb/video.h, so
 > >  > userspace programs can adjust video system settings (PAL/NTSC) depending
 > >  > on the frame rate and video size. I think it won't break binary
 > >  > compatibility. The frame rate values are taken from ITU-T Rec. H.262
 > >  > (2000 E) Table 6-4. May I proceed?
 > >  > 
 > > 
 > > Where is this relevant and wouldn't it make more sense in a
 > > video4linux(1/2) device? 
 > > It makes no sense for playback, because you know what you are
 > > playing. For a demux as source, you can handle it inside the
 > > driver. Any information needed for the display should then be handled
 > > by the display device, i.e. a v4l device. 
 > 
 > While I agree with you that we both know what kind of stream we are
 > playing, John Doe doesn't. He knows the play and stop buttons and can
 > hopefully tell whether his TV supports NTSC and/or PAL.
 > 
 > Then, we have this VIDEO_SET_SYSTEM ioctl which doesn't make much sense

The SET_SYSTEM call was meant to define the output device, i.e. your
TV. This means that the decoder should know that if possible any
signal that is not of the required type should be transformed.


 > unless the driver is able to tell some details about the current stream.
 > It is also nice to have these details displayed on the user interface.
 > 
 > If the mpeg decoder driver switches to NTSC without notifying userspace,
 > then it is not possible to configure the DENC or the Framebuffer
 > accordingly.
 > 
 > Also I don't see the need to create a v4l2 device just for obtaining the
 > frame rate of a stream. Until now you can create quite good DVB
 > applications pretty much without knowing that v4l exists at all.

The av711x driver has a v4l device and the TV norm is switched
automatically. The v4l device can also be very useful, not only for
display matters like scaling, but also for e.g. transcoding to non
MPEG2 formats. It also gives you a lot of available software that
might be helpfull (e.g. webcam or streaming).

Of course, if you don't have a v4l device your solution is easier. 
Maybe we should think about putting some of the decoding
functionalities back into v4l for the next API version. This will help
to unify the DVB decoders with pure hardware decoders and encoders.
Otherwise we will probably have a lot of overlapping functions.

Marcus

-- 
/--------------------------------------------------------------------\
| Dr. Marcus O.C. Metzler        |                                   |
| mocm@metzlerbros.de            | http://www.metzlerbros.de/        |
\--------------------------------------------------------------------/
 |>>>        I like GNU, but I couldn't eat a whole one          <<<|
 


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