Why can ZoneMinder capture images (either at all or just particularly fast) when I can see my camera just fine in xawtv or similar?
With capture cards ZoneMinder will pull images as fast as it possibly can unless limited by configuration. ZoneMinder (and any similar application) uses the frame grabber interface to copy frames from video memory into user memory. This takes some time, plus if you have several inputs sharing one capture chip it has to switch between inputs between captures which further slows things down. On average a card that can capture at 25fps per chip PAL for one input will do maybe 6-10fps for two, 1-4fps for three and 1-2 for four. For a 30fps NTSC chip the figures will be correspondingly higher. However sometimes it is necessary to slow down capture even further as after an input switch it may take a short while for the new image to settle before it can be captured without corruption. When using xawtv etc to view the stream you are not looking at an image captured using the frame grabber but the card’s video memory mapped onto your screen. This requires no capture or processing unless you do
