WHITEPAPER. Image processing in the age of 10 Gigabit Ethernet

Transcription

1 WHITEPAPER High resolution, high speed and low cost: CameraLink, CoaXPress or GigE Vision over 10GigE? Authors: Dipl.-Phys. Stephanie Zahner, Technical Product Manager Dr.-Ing. Ronald Müller, Head of Product Marketing

2 Automation, traffic monitoring and research applications are demanding ever increasing accuracy and speed of image processing from industrial cameras, this means that the cameras must offer higher resolution and higher imaging rates. Recent progress in technology from various image sensor manufacturers has provided a solution to this demand. However, both camera manufacturers and users are now confronted with a new problem: How can the resulting data rates be transmitted and processed? In the industrial image processing digital video camera market, gigabit Ethernet plays the most significant role amongst the interfaces for data transmission between cameras and processing units. Here, the nominal transmission speed is only 1 Gbps (gigabit per second). The GigE Vision standard increases the effective data rate for digital video and supports the interaction between cameras and the software products of various manufacturers. For a video at Full HD resolution (1920x1080 pixels), this standard can offer a maximum frame rate of approx. 30 fps (frames per second) and around 7 fps at a 10 MP (megapixel) resolution. Transmission via the new USB 3.0 data interface standard theoretically allows this data rate to increase five-fold to 5 Gbps. In reality, current cameras with a USB 3.0 interface can transfer approx. 4 MP at up to 90 fps. In order to meet the increased requirements described above, faster data interfaces are required, such as CameraLink, CoaXPress and 10 gigabit Ethernet (10GigE). CameraLink is the oldest standard, dating back to the year 2000, and defines a serial communication protocol between the camera and the frame grabber card in the PC. It is structured in three configurations (Base, Medium and Full), which differ in terms of the transmission rate and the number of cables required. In Base mode, the maximum transmission rate is 2 Gbps, whilst, in Full mode, it is 6 Gbps, although two parallel cables are used. The cable length is limited to 10 metres without major technical work being required. On the computer side, a CameraLink frame grabber card and camera-specific files are required for full communication between the PC and the camera. CoaXPress was developed in 2007, in order to implement a greater bandwidth and longer transmission paths than CameraLink. Data transmission of up to 6.25 Gbps is possible using the relatively low-cost 75 Ohm coaxial cables with a maximum transmission distance of 70 metres, although longer distances are possible when the data volume is reduced. According to the standard, this can be scaled to 4 cables and 25 Gbps. As this is an interface used solely in professional image processing, the user also requires a special frame grabber in order to receive the data on the PC side for further processing. GigE Vision over 10GigE is the most recent method in this group for video camera data transmission. It uses the GigE Vision standard, which was established in 2006, but scales transmission from 1 Gbps to 10 Gbps. Data is transmitted via copper or fibre optic cable, in combination with SFP transceivers (Small Formfactor Pluggable), although, depending on the cable quality, lengths of up to 100 m are possible with copper cables and up to 10 km using single-mode fibre optic cables. On the PC side, standard Ethernet network cards for 10 Gbps are used. Standardisation All the interface solutions presented above are standardised, i.e. there is a prescribed communication protocol between the camera and PC. In addition, the standards also specify the hardware components, such as connectors and their pin assignment. The aim of the GigE Vision standard is to make cameras, NICs (Network Interface Cards) and the software of different manufacturers universally compatible. To guarantee quick, flawless data processing, a software filter driver is recommended for the GigE Vision standard, this hands over the image data directly to the appropriate RAM of the software. Due to the 1.

3 high data volume per second, such a filter driver is essential for 10GigE. The CameraLink standard also specifies the shape of the connectors, their pin assignment and the communication protocol, allowing combination of cameras and frame grabbers from different manufacturers. However, additional camera and frame grabber-specific configuration files are required, as described by the individual communication between the components. It is possible to optimise transmission performance, depending on the requirements of the application in question, although handling this standard requires some knowledge and experience. In addition to the hardware (75 Ohm coaxial cable) and the transmission protocol, the CoaX- Press standard also defines the electrical signals to be transmitted. A disadvantage of this standard is that it does not provide any mechanism for resending data. If data is lost during transmission, then these data packets cannot be resent. This is a particular problem in areas with strong electromagnetic interference. Maximum data volume A key criteria in the selection of the (high-speed) interface is the maximum data volume that can be transmitted over time. Both CameraLink Full and CoaXPress offer transmission speeds up to 6.25 Gbps, although for CoaXPress one cable is required for data transmission and CameraLink Full requires two cables. 10 GigE can surpass this data volume using only one cable. In the CoaXPress standard, it is possible to increase the maximum data rate with 4 cables, a data rate of 25 Gbps is possible - however, appropriate powerful frame grabbers must also be used. In a comparison of these three interface standards, GigE Vision over 10 GigE can achieve the greatest throughput, even if up to 20% overhead must be deducted from the maximum data rate, due to the TCP/IP protocol used. The use of a single cable not only saves costs but also increases flexibility in the installation of the camera system. Overview of the different features 2.

4 Hardware comparison A special frame grabber is required on the PC side, for both CameraLink and CoaXPress, however, standardisation means that it is possible to combine cameras and frame grabbers from different manufacturers. A benefit of this system architecture is that some image processing steps, such as debayering (calculation of the colour values per pixel using the red, green and blue values of the neighbouring pixels), can already be calculated efficiently on the frame grabber, reducing the PC performance requirements. It should be noted that the frame grabbers require software libraries compatible with the device and the data volume, which may vary between manufacturers. No special frame grabber card is required for GigE Vision over 10GigE, where only a 10 gigabit Ethernet connection is required. However, as this connection is not yet available as standard on every computer, a 10GigE network card must be available. Assuming that the 10 Gbps can be fully exploited, normal software filter drivers, such as the Universal Pro from Intel, and a standard dual core CPU are insufficient. The network card manufacturer Myricom has specially developed a Machine Vision Accelerator (MVA), which reduces the overhead of the host processor, thus increasing the maximum data rate. The data of the GigE Vision standard protocol is written directly to the user s RAM, without intermediate storage and copying of the data, which reduces the CPU load considerably. Maximum cable length and flexibility Cabling has a significant role to play during system design. The size of the connectors, the transmission length, the minimum bend radius and lifespan in the case of continuous mechanical bending and torsion all have to be taken into account. The bending properties of the different cables and the size of the connectors are comparable, although the CameraLink connectors are larger than the BNC connectors (CoaXPress ) and the SFP Transceiver for 10 GigE, which may make routing in cable ducts more difficult. In addition, with multiple lines, CameraLink Full and CoaXPress use two or more cables. This increases space requirements in the cable duct and also has a negative effect on the overall bending properties. As there are various cabling options for GigE Vision over 10 GigE, e.g. CAT7 copper cables, Mulitmode fibre optic cables and Single mode fibre optic cables, the bend radii and the cable diameters are also slightly different. Cable manufacturers are currently developing fibre optic cables for industrial image processing applications. However, these cables still have the largest minimum bend radii and are thus not yet suitable for challenging robotics or drag chain applications. The same applies to a great extent to CAT7 cables. The greatest difference between the three interfaces is in the maximum cable length, which is a key parameter for the majority of applications in transport monitoring and safety systems. CameraLink has the shortest range here as only 10 m are possible without major technical effort. Special, cost-intensive cables and frame grabbers allow transmission distances of up to 20 m. CoaXPress can achieve three times the range with a maximum cable length of 68 m. Up to 130 m can be achieved, albeit at the expense of the data rate (the standard only specifies 1.25 Gbps). However, these distances cannot be compared to the enormous range of 10 GigE, as an individual Single mode fibre optic cable can run for several kilometres or an existing fibre optic network can be used. In addition, the costs and performance can be optimised for classic IT installations, on account of the range of different cables. 3.

5 Power supply and energy consumption When considering the cabling, the power supply cable must also be taken into account. With both the CameraLink and CoaXPress, the power is supplied via the data cable (PoCL, PoC), meaning that no other cable is required. Apart from some cable products which have two fibre optic cables for data transmission and two copper cables for the power supply, this is not possible for 10GigE. However, power can often be supplied locally, whilst only the data cable only needs to be run to the computer (which may some distance away). Greater data rates require increased power consumption and can cause a higher level of heat development in the camera. All three interfaces have this disadvantage. In the case of 10GigE, the image data must be specially packaged for shipment, appropriate calculation steps must be performed on the camera side regarding power consumption and heat. In the context of these calculation steps, image optimisation elements can also be performed directly in the camera. Irrespective of the data interface, an ingenious camera design will permit efficient heat dissipation from the image sensor and the communication interface. However, 10GigE suffers from certain thermal limits in this field, which in turn affect the size of the camera housing. Distribution of the video data to different recipients Although only point-to-point connections are possible for both CameraLink and also CoaX- Press, GigE Vision can perform a multicast via 10 GigE, i.e. there is the option of distributing images from a single camera to multiple destinations simultaneously. The multiple shipment function allows the development of extended analytical systems, in which multiple computers receive images from a camera simultaneously and analyse or save them. In addition, it is also possible to create a network of multiple cameras and a computer. It is possible to contact multiple cameras with a single piece of software, although GigE and 10GigE cameras can be combined, allowing the expansion of existing systems to include high-speed cameras. Costs Of course, when choosing the system design, the overall costs for the hardware and development costs are of interest. Table 2 offers an overview of the hardware costs of the three solutions discussed. When calculating the overall costs, the use of two cables was assumed for both CameraLink Full and CoaXPress. The most expensive, yet fastest, standard is CoaXPress, as the frame grabber is particularly expensive, although cheaper cables can be used. However, the costs also increase when specially-shielded cables have to be used. GigE Vision over 10GigE offers lower overall cost, low-cost NICs can be used in comparison to special, and thus more expensive, frame grabbers (costs for the necessary software licenses are taken into account in the price of the NIC). In this example for a 10-metre cable, the price difference between copper and fibre optic cables is very small as copper cables can be very economic for shorter transmission lengths. However, for long distances the characteristic feature of GigE Vision over 10 GigE, can only be implemented with fibre optic cables. CameraLink has the lowest hardware cost, compared to CoaXPress and GigE Vision over 10GigE. However, as any serious image processing library will support the GigE Vision 4.

6 standard, software development systems using cameras based on 10GigE offer the smallest risk and greatest flexibility. The comparison of the interfaces shows that the peripheral costs of 10GigE, just 510, are less than half those of CameraLink and CoaXPress. In particular, it is the frame grabber which determines the overall costs of the system. Currently, no manufacturer offers cameras with all three interfaces, meaning that a pure price comparison will always rely on the currently available models of various brands. At these high frame rates and resolutions the price of the image sensor has the greatest influence on the camera price. It can be expected that the price difference between the cameras with the appropriate data interfaces will reduce in larger volumes. Overview of costs Summary and outlook To a certain extent, all three interface standards meet the various requirements of modern high-speed or high-resolution image processing methods. GigE Vision over 10 GigE shows clear advantages over the other interfaces for cable length and the maximum data rate when using just a single cable, the ability to multicast, the peripheral costs and the native support in all available software libraries. These advantages will continue to increase in future, as soon as 10GigE is commonly used in consumer PCs and networks. By comparison, CameraLink and CoaXPress are solely limited to the users in the image processing industry. This means that the GigE Vision standard over 10 GigE has the potential to become THE transmission medium for high data rates. Its technical performance and financial aspects are already winning business. 5.

7 About FRAMOS Founded in 1981, FRAMOS is a leading technology provider in the industrial, scientific and medical imaging sector. Headquartered in Munich, Germany, and with 5 subsidiaries worldwide we enable manufacturers, system integrators and researchers to benefit from imaging technologies. Our team of 60 employees provides a fully comprehensive portfolio of imaging components (image sensors, cameras, optics, illuminators, frame grabbers, etc.) with individual logistic services, technical consulting and after-sales support. Based on our deep industry knowledge we offer engineering services for tailored camera development as well as complete turn-key-solutions. FRAMOS GmbH Mehlbeerenstraße Taufkirchen, GER Phone: +49 (0) Fax: +49 (0) info@framos.com 7.