The Universal Serial Bus (USB) standards provide a high bandwidth (480 Mb/s for USB 2.0, 4.8 Gbps for USB 3.0) interface that is well-suited for digital imaging, and is one of the most well-established and popular serial interfaces in use. USB 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 PixeLINK IEEE-1394 cameras are also available on USB cameras, including:
A "plug and play" camera system supported by the same PixeLINK API and SDK.
Nevertheless, when evaluating a USB camera for purchase, it is important to be aware of the limitations of USB compared to cameras with a FireWire interface.
Wherever possible, PixeLINK FireWire cameras should be used for applications that require multiple cameras running simultaneously on the same computer.
Other considerations of USB compared to FireWire include the following:
Cable Length: The standard cable lengths are 4.5 meters for IEEE-1394 devices, 5.0 meters for USB 2.0 devices, and a recommended length of 3 meters for USB 3.0 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.
Power Requirements: USB 2.0 and 3.0 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 2.0 is 480 Mb/s, and is configured as a bulk transfer. The speed of USB 3.0 is 4.8 Gbps, and USB 3.0 supports bulk transfer and bulk stream modes.
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.