Digital cameras are available with a variety of interface options to suit the particular needs of any given application. Before selecting a digital interface, system integrators need to consider a number of factors such as data transfer rate and maximum cable length. At present, the machine vision community has settled on four preferred standards for transferring digital video signals from the camera to the image-processing computer or workstation. Each of the four standards we describe here— FireWire (IEEE 1394), USB 2.0, Gigabit Ethernet, and Camera Link— has its unique advantages.
FireWire (IEEE 1394) is a high-speed serial bus originally developed to let users plug bandwidth-intensive multimedia peripherals into a computer. FireWire has the advantage of being built into most general-purpose computers currently available. This ubiquity lowers cost through volume manufacturing savings. Because of the high degree of standardization in the hardware, software, and cabling, FireWire has become a popular interface in machine vision. A single low-cost cable provides camera control, power, and data.
With FireWire, no frame grabber is required and many computers now include a built-in FireWire port. Even if the computer does not have that port, FireWire PCI or PCMCIA cards are relatively inexpensive and generally available through local consumer electronics stores. Furthermore, most interface cards enable users to plug multiple cameras into a single card.
Implementing a machine vision system with FireWire cameras has benefited from a standardized software interface known as DCAM/IIDC. This means that any DCAM/ IIDC-compliant driver can operate any DCAM/IIDC-compliant camera.
With the introduction of IEEE 1394.b, data rates as high as 800 Mbps are possible, up from 400 Mbps with IEEE 1394.a. The maximum cable length of an IEEE 1394.b connection is 100 m (on glass optical fiber), significantly longer than the 4.5 m max. cable length of IEEE 1394.a. Another advantage of FireWire is its very low CPU overhead that leaves plenty of processing power for image processing applications.
Whereas FireWire was originally developed for multimedia peripherals, USB (Universal Serial Bus) was intended for less bandwidth-intensive peripherals such as keyboards, mice, printers, etc. With the advent of USB 2.0, however, devices with data transfer rates of 480 Mbps that can support a maximum cable length of 5 m were introduced. USB ports are truly universal in that virtually every PC manufactured during the past 10 years has at least two USB ports built in. This plug-and-play feature makes it extremely easy to use.
USB relies on the CPU to achieve its full bandwidth, which means the camera frame rate can be negatively affected if the host computer is concurrently used for image analysis or other bandwidth-intensive tasks. For applications requiring documenting and archiving of high-resolution images at low frame rates, USB is more than adequate, but it is generally not recommended for high-performance machine vision applications.
Gigabit Ethernet (or GigE for short) is a high-speed, 1-Gbps version of Ethernet, the world's dominant LAN connection protocol. It can carry data at up to 1000 Mbps. Being an Ethernet standard, it is designed for long-range applications (up to 100 m without repeaters) and connectivity to an unlimited number of other devices. It is increasingly being adapted to high-performance cameras for industrial applications.
On the down side, GigE suffers from packet-switching indeterminacy. Depending on network traffic, information can be delayed or not delivered at all. Still, its very high bandwidth and long cable lengths give GigE a great advantage in applications such as traffic monitoring, factory automation, and surveillance. GigE is therefore poised to become a dominant machine vision interface standard.
The Automated Imaging Association (AIA) is working on the GigE Vision standard for high-performance machine vision cameras. Analogous to FireWire's DCAM/IIDC interface standard, GigE Vision has great value for reducing camera system integration costs and for improving ease of use. The common software interface will enable third-party software to communicate with cameras from various manufacturers—without customization.
In contrast to the other three standards, Camera Link is not intended for general-purpose peripherals. It was designed specifically to expedite digital-video signals from camera to computer. Using Camera Link requires installing a Camera Link board into an expansion slot, just as is the case with a frame grabber. The difference is that, while the expansion-slot board superficially resembles a frame grabber, and can perform many of the same functions (such as isolating a particular frame from the video data stream), it works with digital video information that is organized as a stream of still pictures, as is a film-based motion picture.
Figure 1. Video interface standards
Because it has been a machine-vision-oriented standard right from the start, Camera Link has a host of features that would be of no use to someone downloading prerecorded video or music. Moreover, it offers none of the compromises consumer-computer standards live with. For example, Camera Link is a point-to-point standard. That is, one camera links to one frame grabber board at a time. To use two cameras simultaneously, it's necessary to install two frame grabbers.
Perhaps Camera Link's biggest advantage is blazing speed—up to 3.6 Gbps. Another is deterministic communication back to the camera electronics. This allows exquisite on-the-fly control of camera functions, such as frame rate, zoom, etc. Camera Link's biggest disadvantage is that it's fairly esoteric compared to the other standards. It will be much harder to find engineers and technicians within your organization who have Camera Link knowledge. Much lower manufacturing volumes also make costs higher.
Selecting a Standard
Figure 1 lists the major characteristics of all four standards. No one standard, of course, will serve best in all applications. When resolution/speed requirements are low, and cost is an important constraint, USB is a likely choice. On the other end of the scale, when high performance is the goal and cost is secondary, Camera Link is a strong candidate. When the overall system architecture is that of a large network to begin with, GigE will likely fit the bill best.
As usual, making the best choice ultimately hinges on understanding your application's requirements as well as your organization's strengths, constraints, and resources; knowing each standard's characteristics; and taking all of these into account during the selection process.