This article presents the main differences and limitations of USB 2.0 functionality compared to 1394-based cameras.
The Universal Serial Bus (USB) 2.0 standard provides a high bandwidth 480 Mb/s interface that is well-suited for digital imaging, and is one of the most well-established and popular serial interfaces in use. USB 2.0 ports are widely available on a variety of hardware platforms, including Macintosh and PC systems, and a variety of laptop, desktop, and small form factor (embedded) systems. This availability maximizes system compatibility and minimizes the need for add-in PCI or PCI Express interface cards.
Most of the features familiar to users of Point Grey Research IEEE-1394 cameras are also available on USB cameras, including:
- A "plug and play" camera system supported by the same FlyCapture API and SDK.
- Updatable firmware designed to the same IIDC-1394-based Digital Camera Specification.
Nevertheless, when evaluating a USB 2.0 camera for purchase, it is important to be aware of the limitations of USB compared to cameras with a FireWire interface.
Multiple cameras on the same bus: The USB standard allows for multiple USB devices on a single bus. However, Point Grey does not currently support these configurations. We have not performed any rigorous qualification of the ability of various hardware platforms, operating systems, software, and drivers to handle multiple USB 2.0 image streams, and we are therefore not able to address any questions directly or troubleshoot issues related to doing so. For more information, see Knowledge Base Article 336, below.
Wherever possible, Point Grey FireWire cameras should be used for applications that require multiple cameras running simultaneously on the same computer.
If you do wish to operate multiple USB cameras on one bus, keep in mind the following:
- Multiple Camera Synchronization: For reasons similar to the above, Point Grey does not support synchronizing multiple cameras on the same bus.
- Broadcast Properties: There is no support for broadcasting camera settings to other cameras on the same bus, including trigger and strobe broadcasting.
Other considerations of USB compared to FireWire include the following:
- Cable Length: The standard cable lengths are 4.5 meters for IEEE-1394 devices, and 5.0 meters for USB devices. Options for extending the distances of 1394 devices using hubs and/or other types of cables range from 10 meters to 100+ meters. For USB devices, distances can be extended 25+ meters. See the link below for more information.
- Power Requirements: USB cameras require a 4.75 V to 5.25 V power source, while 1394 cameras require 8 V to 32 V of power. In the case of USB, the maximum power restriction limits devices to sensors that consume less power and may output a narrower spectral response than higher voltage sensors.
- Setting bus speed: FireWire cameras allow the option of configuring bus speed of isochronous and asynchronous image transmission, up to the maximum of 1394a (S400) or 1394b (S800). The speed of USB is 480 Mb/s, and is not configurable.
- Less precise timestamp: Timestamps derived from FireWire cameras are based on the 1394 cycle timer, but USB has no such mechanism. As a result, timestamps originating from USB are less accurate. USB timestamps can range in accuracy from within 0 to 125 us of the end of shutter integration. This range can increase if there are higher priority threads running on the system.
- PHY register and Network Topology: USB does not provide an interface for viewing the PHY register information or network topology of cameras on the bus.
- Other advanced imaging capabilities: The Point Grey driver for USB cameras does not support the following additional features that are available on IEEE-1394 cameras:
- Access to image packet information: including packet information from custom images
- Receiving bus error messages
- Partial image event notification: for accessing image data in memory, before display.
1.) Extending the distance between a PGR camera and the controlling host system.
2.) Providing power to Point Grey Research cameras
3.) Using USB 2.0 PCI host adapter cards with USB cameras
4.) Customizing applications to control USB cameras
5.) Troubleshooting problems when running multiple USB cameras on the same bus
|2/26/2009 11:36:14 AM
|4/26/2012 10:32:59 AM
|USB, FireWire, Comparison, 1394