Digital Video Protocol – The New Age

Technology advancement has triggered more choices for consumers as well as vision design engineer. Analog video output may soon become history. The new age digital video outputs are here to stay. However - How to choose? What is the difference? The decision is application specific. Knowing the advantages and limitations of each protocol will helps in your vision system design.

Camera Link
Camera Link, designed for high-performance vision applications, streams data reliably at very high rates—up to 7.14 Gbits/s—over dedicated point-to-point copper links of 10 meters or less. This short reach limits its usefulness in many applications because PCs are essentially tethered to cameras. Fiberoptic extenders stretch the reach to 500 m, but at significant expense.

Camera Link is also limited on the networking front, with no flexibility for interconnecting multiple cameras or centralizing control and maintenance. It also runs over specialized cable and terminates on PCI frame grabbers, both of which enjoy few economies of scale. Despite its limitations, Camera Link delivers unmatched data rates, and is supported by a range of high-end camera manufacturers.

FireWire
IEEE 1394b (FireWire) is a consumer standard developed for linking digital camcorders to PCs. It offers "plug-and-play" usability, and uses a readily available, low-cost PC interface. FireWire is based on a bus topology, where 800 Mbits/s is shared by up to 63 devices in a "daisy-chain" network. Devices can be separated by 4.5 m, to a maximum length of 72 m, over twisted pair copper cable.

FireWire sends data over both asynchronous and isochronous channels. Asynchronous links are typically used for latency (delay)-tolerant data, such as control signals, and isochronous channels for latency-sensitive data-like video. Up to 80% of the bandwidth, or 640 Mbits/s, can be allocated to a single camera over an isochronous channel. With the shared bus, however, only one camera can access this bandwidth at a time, which means high-priority data can be delayed and reliability compromised. Moreover, FireWire does not include error-checking for isochronous transfers, so data delivery over these links is not guaranteed.

Since one PC can remotely control multiple cameras, the scalability and networking flexibility of IEEE 1394b is superior to that of Camera Link. However, even at the maximum rate of 640 Mbits/s, IEEE 1394b data transfers are too slow to support higher-end digital cameras. Many high-speed applications also require real-time PC processing, which is difficult with IEEE 1394b's Windows-based driver, which "hogs" the PC's CPU during data transfers. Some companies have addressed this limitation by developing their own drivers.

Another drawback of IEEE 1394b is the price of its copper cable. Category-5 local-area network (LAN) cable, which costs up to 10 times less, can be used instead, but this limits total available bandwidth to 100 Mbits/s.

Numerous companies support IEEE 1394b, and their cameras are gaining ground in applications where performance requirements are not overly rigorous, such as microscopy, scientific imaging, and process triggering.

Universal Serial Bus
USB 2.0, a consumer standard for connecting peripherals to PCs, has much in common with IEEE 1394b. It leverages a built-in PC interface, uses a shared bus, and supports asynchronous and isochronous transfers. USB 2.0 delivers up to 480 Mbits/s of bandwidth, shared by up to 127 hub-connected devices in a master/slave relationship. Direct PC connections extend up to 5 m. Hubs extend the reach to 30 m, with maximum spans of 5 m between devices.

Like IEEE 1394b, USB 2.0 is best suited for less-demanding applications.

Gigabit Ethernet
The fourth standard, Ethernet, was launched about 25 years ago and has evolved into the dominant global local-area-network technology, covering 97% of installed network connections. Ethernet is flexible, easy to implement and manage, and highly scalable.

On one network, over low-cost Category-5 copper, Ethernet connections operate at 10 Mbits/s, 100 Mbits/s, or 1000 Mbits/s (1 Gbit/s). The top data rate—1 Gbit/s or Gigabit Ethernet (GigE) —is fast enough to support 90% of today's vision-system applications. The next generation of Ethernet, 10GigE, which delivers 10 Gbits/s, is available today over fiber and is expected to run over copper in 2004. Links at all rates interwork seamlessly, allowing users to allocate bandwidth as needed in a multi-pronged network.

Ethernet uses dedicated links, so bandwidth is not shared between cameras, as with IEEE 1394b and USB 2.0. Ethernet supports many connection options, including one camera to one PC, multiple cameras to one PC, one camera to multiple PCs, and multiple cameras to multiple PCs. In configurations with multiple cameras or PCs, interconnections are through full-duplex, inexpensive Ethernet switches. PCs links are through RJ-45 plugs, which are either already on the PC or added via low-cost network interface cards.

Ethernet also goes the distance, supporting individual links of 100 m over Category-5 copper. With switches, the reach is unlimited. This means PCs can migrate out of operations areas, and control and maintenance functions can be centralized in one room. Ethernet's networking flexibility also allows image data to be "multicast," or simultaneously distributed, to multiple PCs. This permits, for example, one PC to display the image, one or more PCs to process it, and another PC to archive it.

Furthermore, Ethernet is robust. Most of today's commercial Ethernet equipment supports sophisticated Quality of Service rules, making it suitable for carrying latency-sensitive traffic like video.

Reference:
Vision Systems Design
(Author: George Chamberlain)

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