It turned out that my kernel had an old uvcvideo module which didn't
have the right debug log messages, and so I'm in the process of
upgrading to a current kernel version. So far, I've managed to render my
machine unbootable [it is a little embedded linux box].
Philip
On 3/6/2011 6:14 AM, Laurent Pinchart wrote:
Hi Philip,
On Sunday 20 February 2011 21:57:24 Philip Gladstone wrote:
I attached two C910s to a small linux box, and ran into the 'Failed to
submit URB' problem.
When I look at the descriptors for this camera, I think I understand the
problem. I'm capturing at 5 Mpixels and I'm trying two cameras.
VideoStreaming Interface Descriptor:
bLength 38
bDescriptorType 36
bDescriptorSubtype 7 (FRAME_MJPEG)
bFrameIndex 28
bmCapabilities 0x01
Still image supported
wWidth 2592
wHeight 1944
dwMinBitRate 403107840
dwMaxBitRate 806215680
dwMaxVideoFrameBufferSize 10077696
dwDefaultFrameInterval 1000000
bFrameIntervalType 3
dwFrameInterval( 0) 1000000
dwFrameInterval( 1) 1333333
dwFrameInterval( 2) 2000000
The video frame size is set to 10Mb. This is surprisingly large as
actual frames captured with MJPEG on this camera are typically 500kb or
less. When I checked the descriptor for the uncompressed version of the
same frame, it came back with the same value of dwMaxVideoFrameBufferSize
(effectively 16 bits per pixel). The values for min/max bit rate are
(correctly) calculated from the frame intervals and the buffer size.
The uvcvideo driver doesn't use the dwMaxVideoFrameBufferSize field to compute
the required bandwidth but queries the device at runtime instead.
You can enable the UVC_TRACE_VIDEO trace flag to get the driver to print the
bandwidth requested by the device to the kernel log.
Typical JPEG compression gets down to 1 bit per pixel, and 2 bits is
very unusual.
What I want to know is what the impact would be of defining a new QUIRK
that overrode the frame buffer size for compressed frames and calculated
them at (say) 2 bits per pixel?
Do other webcams get this right?
You would be surprised by how many webcams get things wrong.
--
Philip Gladstone
Ham: N1DQ
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