Re: [Linux-uvc-devel] Connecting multiple cameras

[Laurent P. <lau...@sk...>]

Hi David,

> Hi, I'm interested in connecting more than one camera at once,
> specifically, a pair of Logitech Quickcam Pro 5000s.

I'm currently working on a robotic project which uses 4 cameras, but at 
smaller resolutions.

> I only have one at 
> the moment, but when I run it at full uncompressed resolution
> (640x480x30fps YUV),

Full uncompressed resolution is 1280x960x7.5fps, but that leads to the same 
bandwidth.

> it chooses an ISO endpoint with a packet size of 
> 3060 bytes.  This is more than half of the available ISO bandwidth.
> (cat /proc/bus/usb/devices claims it's 60% -- I expected more than half,
> but not quite that much).  Obviously, this would prevent me from using
> more than one camera.
>
> By my calculation, 640x480x30fps YUV should take 2304 bytes minimum per
> microframe.  So why can't the Quickcam choose its next smaller ISO
> endpoint, which is 2688 bytes per microframe.  This would probably put
> me at less than 50% of the ISO bandwidth?

The driver choose the smallest ISO endpoint compatible with the bitrate 
advertised by the camera (the total bitrate is the payload bitrate plus the 
header bitrate). At 1280x960x7.5fps, the webcam asks for 3024 bytes per 
microframe.

> Do you think this is a hardware limitation of the Quickcam, or is there
> some driver involvement in this decision?

The driver just does what the webcam asks, so that's a hardware limitation. 
I'll contact Logitech to get more information.

Laurent Pinchart