Ok thank you Dmitri, I think I see what is happening. Are there any cameras that do not take the full bandwidth? The time when both cameras showed images in xawtv was at default resolution 320x240(?) for two IBM Zirlink cameras (the ibmcam.o module). Ted Dmitri wrote: > Quoting Ted Huntington <thunting@xxxxxxx>: > > > I do not understand why I can not even get 1 image from both cameras at two > > different times. That indicates to me that v4l or the usb code in the kernel > > is not allowing the bus to be shared. > > When one camera app opens the device, the device grabs all available > bandwidth, and so another camera will not get enough data through to > recover a frame. Isoc transmissions are not guaranteed, meaning that any > Isoc transfer can be dropped by the bus (HC or device) at any time. But > most cameras use Isoc transfers because they are somewhat faster and > straightforward. > > So if you think the USB is shared - that is not exactly so. A restaurant > is shared by every visitor, but it does not guarantee that you will be > served any time soon :-) > > > I am sure that I was able to see images from two usb cameras going at one time > > before, but have not been able to reproduce this. One thing that I changed (I > > changed back but still the same problem) was XFree86 from 4.0.3 to 4.1.0. > > Possibly, cameras were running at lower resolution? > > > Are there any changes for v4l (or linux USB) in the future to change this? > > This is just the USB 1.1 standard, and badly programmed USB cameras. There > is nothing to fix in Linux. If a device demands full bandwidth - it gets > it; if another device wants to compete for the bus frames... they will > hurt each other. That's how USB is supposed to work. If you want > guaranteed transfers, bulk and control transfers are provided for that - > but they are not guaranteed in delivery time... you can't have both! > > Dmitri > > -- > http://www.linux.org => Where do you want to go tomorrow? > > ------------------------------------------------------------------------ > Part 1.2Type: application/pgp-signature -- Ted Huntington Programmer Analyst I Main Library University of California, Irvine PO Box 19557 Irvine, CA 92623-9557 Phone Bus Off 949 824 8926 Phone MRC 949 824 1674 emesg: thunting@xxxxxxx webpage: http://sun3.lib.uci.edu/~busoff/webpages/ted.htm
