Q: My application uses multiple IIDC cameras to capture video with a frame size of 640x480 @ 30 fps, but I've found I can only capture with three cameras. Is this a known limitation?A: The limitation you're encountering comes from the bandwidth each camera is using on the FireWire bus. In general, the FireWire bus that IIDC cameras connect to can run at a maximum speed of 400Mb/sec. The FireWire spec allows up to 80% of the total bandwidth to be used for isochronous traffic, like video. IIDC cameras can operate at different frame rates, different frame sizes and can also produce different flavors of YUV data -- YUV 4:1:1 and YUV 4:2:2. Depending on these factors the amount of bandwidth consumed will vary. Note: You can also connect IIDC cameras to an 800Mb/sec FireWire bus but they will operate at slower speeds. Multiple instances of the QuickTime IIDC Video Digitizer can be opened allowing multiple IIDC streams to be active simultaneously. The number of active streams is only limited by the number of cameras present and the amount of FireWire bus bandwidth available. If there is insufficient bandwidth for a given setting (pixel format / frame size / frame rate), the QuickTime IIDC Video Digitizer will attempt to use a smaller frame size. If that fails, it will try a slower frame rate. The solution is to work with smaller frame sizes, lower frame rates or to add an additional FireWire bus using a 3rd party PCI card giving you more bandwidth to play with. Note: For developers thinking about DV, at the time of this writing simultaneous recording from multiple DV cameras is not supported. The current implementation of the DV Video Digitizer (QuickTime 6.5.1) only supports a single connection. This is a known limitation. Document Revision History| Date | Notes |
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| 2004-08-25 | First Version |
Posted: 2004-08-25
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