I am not a programmer but a lurker/project manager. I have programmers who work for me though and for a long time I have thought that the answer is to set the "temporal decimation register" on the capture card itself. Then the frame grabber will throw away the unwanted frames before they ever get to the PCI bus. This should work at least on the BT878 cards we use, but the V4L driver or 878 driver (not sure which) does not react correctly to the card dropping a frame. When you set the card/chipset to decimate and only deliver 5 out of 30 frames for example, when the driver encounters a "skipped" frame it returns the previous frame to the application, not an "empty" buffer. So the video which appears seems to stutter back and forth, V4L unable to "not deliver" a frame to the PCI bus. I have put up to 12 bt878 capture cards in a system with a big backplane and it seems to load and "work" for a bit but garbage starts to appear after about 4 cards and it becomes generally unstable for 4+ cards, and totally unstable beyond 8 cards. This seems to be true for both single port wintv type and 4 port cards (with 4 x BT878) such as the Imaging Source cards. Telling the application layer to decimate does nothing to reduce the demands on the PCI bus which is starting to get saturated at 4 inputs x 320x240 -- and earlier if you try to use 640x480 resolution. or I am completely clueless. but I think that the driver or V4L has to be patched to correctly use the temp. dec. register. If I am wrong, sorry for wasting the bandwidth ;-) Cheers Chris ----- Original Message ----- From: <v4l@xxxxxxx> To: <video4linux-list@xxxxxxxxxx> Sent: Thursday, December 11, 2003 8:20 AM Subject: Re: Frames come in bursty when capturing with low framerate > > > > Grab all frames (i.e. 25 QBUF and DQBUF per second) and copy only the > > ones you want, maybe on the basis of the timestamp or timecode returned. > > Even if your QBUF were *perfectly* rythmed, what counts is the video > > synchro signal, not the system clock. > > > > Hi, > > That's what I used before, and that works ok. The problem is that I'm using > 4 or more grabbers in one PC now, and capturing everything and skipping > frames causes too much data on the PCI bus, resulting in image distortion. > (distorted horizontal lines at random places, etc)... > > -- > :wq > ^X^Cy^K^X^C^C^C > > > -- > video4linux-list mailing list > Unsubscribe mailto:video4linux-list-request@xxxxxxxxxx?subject=unsubscribe > https://www.redhat.com/mailman/listinfo/video4linux-list
