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?
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