What kind of video capture are you trying to perform? Analog or digital?
If this is a capture of a digital camera (with miniDV tape), you need to connect the camera via FireWire/IEEE1394. This is the only really supported option. If you have a DV camera with a USB interface and you can get it to work in the Magix program at all, the results will most likely be less than perfect, to say the least. So, once again: DV capture always means using FireWire.
If we're actually talking about some kind of analog camera here and you mean that you're using some kind of USB unit for the analog capturing process, I suggest that you make sure that the drivers for the USB capture hardware are up-to-date. I was also going to suggest that you try the video connection from the source to the USB video grabber via Composite, instead of S-Video, but this should only be relevant for VCRs with Scart output, not analog video cameras.