[Gllug] running TV and webcam simultaneously

Chris Bell chrisbell at overview.demon.co.uk
Mon Apr 22 22:53:27 UTC 2002

On Mon 22 Apr, Paul Brazier wrote:
> > > So firstly is it possible in theory to have them both running at
> once
> > > and secondly how do I switch between the two video inputs without
> > > rebooting?
> > > 
> > > I'm using Debian with a self-compiled 2.4.18 kernel.
> > > 
> > > --
> > > Paul 
> > > 
> >    You will probably have trouble with any two sources unless you can
> > synchronise them together. A single FIFO frame store requires 
> > rather more
> > memory than that in a TV card; there are some inexpensive 
> > cameras which do
> > have a reference input, but they will not be the cheapest webcams.
> So does this just mean that I can't view both at once?
> I should still be able to switch between one and the other, shouldn't I?
> In xawtv for example there's a setting to select the video input. It
> either says "webcam" or "tv card" depending on whether I've got the
> webcam plugged in, but it doesn't give me the option to switch between
> the two.
> I would have thought that it was something like /dev/video0 was the tv
> card and /dev/video1 was the webcam and /dev/video was a symlink to
> whichever of these was currently being used. But this doesn't seem to
> work.
> Paul.
   I have not seriously played with any of the video cards, but I would
expect any analogue input switch to be a simple selector on the card itself,
followed by a digital converter, and an output digital stream selector if
the card also accepts a digital input. Any switching controls would be
specific to the hardware.
   The analogue to digital converter requires time to lock to the incoming
signal, and is likely to output rubbish until locked.
   If more than one analogue input is to be on screen at the same time, they
must either arrive correctly timed at the mixing point, which implies
complex adjustable locking and analogue signal processing, or each must be
separately converted to digital and buffered in a FIFO memory, (so more than
most cards can do). Anything more than switching between digital sources,
such as mixing and fading, requires very complex digital processing, hence
the cost of dedicated professional equipment.

Chris Bell

Gllug mailing list  -  Gllug at linux.co.uk

More information about the GLLUG mailing list