Have you gotten an answer yet?
I'm having the same issue.
I have narrowed it down to the fact that the GV-800 cards share the /dev/videoX streams.
If you remove monitors 5 through 16, I bet the first 4 will work. You can also change the setting in ZM under Options->Config and change the "V4L_MULTI_BUFFER" setting to a larger number. This will compensate for the image interlacing.
I got my images to be clear when I have them all set on NTSC, and YUV420 with the V4L_MULTI_BUFFER value greater than 4, however, I can only get channels 1-4. So I'm hoping that maybe you have some type of an answer, or someone else does.
