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2 Camera Basics
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While a USB 3.0 standard A plug and a USB 2.0 port can be used together (with the restriction that only USB 2.0
speed is possible), the standard B and micro B plug are no longer compatible with USB 2.0 ports.
2.9.5 Data Transmission and Bandwidth
USB 2.0
The USB 2.0 standard specifies an overall bandwidth of 480 Mbit/s shared between different transmission modes.
DCx Cameras use the USB 2.0 bulk mode for transmitting images. This mode uses error correction to ensure
correct delivery of the image data, but does not guarantee a fixed bandwidth. To ensure error-free communication
with all connected devices at all times, the maximum bandwidth for payload data is limited to 416 Mbit/s.
Theoretically, up to 50 MByte/s of data can be transmitted in this mode, but in practice, this value is hardly ever
reached. A high-performance desktop PC can transmit about 40 MByte/s, most notebooks or embedded PC
systems even less than that.
The overall bandwidth can be increased by the use of USB 2.0 expansion cards. These cards are available for the
PCI and PCIe buses and have their own host controller chip.
Note on hardware selection
To achieve optimum USB bandwidth, it is important to use a powerful mainboard chipset. The mainboard chipsets
from e.g. Intel® or NVIDIA® provide very good results.
Note on image content and bandwidth usage
For USB cameras, you can use a white test image to check the camera's maximum load on the USB bus. Due
to the transmission process, completely white camera images require a somewhat more bandwidth on the USB bus
than completely black images.
USB 3.0
Compared to USB 2.0, USB 3.0 offers a tenfold increased bandwidth of 5 Gbit/s, i.e. images can be transmitted
with a bandwidth of 400 Mbytes/s.
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