great confusion in video capture please help

Hai all!
           I am working as research assistant at
university o kansas . I have downloaded latest drivers
for video for linux -2 and installed them all along
with required packages. Got source from
bttv-v4l2.sourceforge,net. I tested some sample
applications and they work. I have set capture frame
rate to 30 frames per second and this is set as
default frame rate if capture format is  NTSC. Now i
calculated number of frames captured per second in
another way by keeping a loop count as:

startTime = time(NULL);
i=0;
while(i <= 1000)
{
  
read(fd,imagesizeinbytes(for 320x240x24bpp),&buffer);	
i++
}

endTime = time(NULL);

framerate = 1000/(endTime-startTime);
(start , end times have been declared long ints)

But this frame rate comes out to be 10
framespersecond.
Why is there such discrepancy. WHats flaw in this??
(By making ioctl call i find framerate of 30fps.)
Help will be greatly appreciated.

		THanks for one and all;
			bye,
				SUNIL.














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