bt878 performance

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Hello,
I've got an Hauppauge BT878 based card, i was trying to write a simple
app to display captured TV frames. 
The app goes like this : I call ioctl VIDIOCMCAPTURE, to start the
capture and then ioctl VIDIOCSYNC. The problem comes from the fact that
the call takes 100ms to return, time that is far too slow to be
acceptable.
I've tried then to use several buffers (four). I then call four times
VIDIOCMCAPTURE and then call VIDIOCSYNC four times. The first call to
VIDIOCSYNC still takes about 100ms, and the subsequent calls take a bit
less than 40ms. This is too slow for a TV app too.
My understanding is that when you call VIDIOCMCAPTURE, it starts the
capture on the device, so why do the 2nd, 3rd and 4th frames take so
long to grab?
Any hints? Do you have links that describe the hardware and could give
an explanation?

Thanks in advance,

Frederik Deweerdt




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