Part II: Determining data-transfer rates, computer backplane buses, and disk-transfer speeds results in a high-speed video-acquisition and archiving system for university use.
By Gary Armstrong and Mongi Abidi
The design of a new high-speed video-acquisition and archiving system has been recently achieved to support the research-and-development activities at the University of Tennessee (Knoxville, TN, USA). System-design specifications call for the acquisition, processing, and storage of 1280 × 1024-pixel color frames at 500 frames/s for 300 s and an upgrade path to faster cameras at 1000 frames/s when these devices become available.
In Part I (Vision Systems Design, July 2003, p. 39), available CMOS-based high-speed camera systems were carefully evaluated. Color cameras are on the drawing board for delivering the 1280 × 1024-pixel and 1000-frame/s design goals, but at system design time, they were not commercially available. Some cameras can provide 200 to 500 frames/s, but most of them are in alpha and beta testing. Based on search results, Maverick Systems identified the high-speed camera, industry-standard computer interface, computer PCI bus, and disk-transfer and storage technologies as critical components of the desired system (see Fig. 1).
Data transfers from camera to computer must occur at a sustained or streaming data flow. Supporting a 1280 × 1024-pixel frame rate of 500 frames/s with 8 bits/pixel requires approximately 655 Mbytes/s of sustained or streaming data flow for 300 s. The options for getting that data to the PC include common data-transfer protocols and their respective speeds (see Table 1).
At first glance, the Camera Link interface seems to be too slow, with a data-transfer rate of 1.5 Gbits/s. But, fortunately, that rate is generated by the Base configuration, and faster modes are available with the Camera Link Full-configuration interface. Using three channels provides a theoretical data rate of 680 Mbytes/s at 85 MHz, which meets the 500-frame/s system requirement but does not meet the 1000-frame/s rate planned for future cameras. However, Camera Link is a scalable technology. More channels can be added in parallel to handle future higher-speed data-transfer requirements.
The personal computer (PC) presents more standards and also limitations to the system-design process. The traditional PCI bus on a typical PC motherboard can handle sustained data rates of 117 Mbytes/s for the 33-MHz PCI bus and 180 Mbytes/s for the 66-MHz PCI bus. New dual PCI data bus backplanes with dual-Pentium CPUs provide 800 Mbytes/s each, for a theoretical maximum of 1.6 Gbytes/s of sustained data flow. But the dual PCI bus motherboards are still in beta testing and are too new to rely on at this time.
Note that PCI speeds vary in the literature and vendor data (see Table 2). The theoretical maximum data flow is obtained by multiplying the clock rate by the byte width of the bus. The burst mode is generally slightly less than the theoretical parameters, and the sustained rate is typically slower than the burst rate.
The sustained data rate for the
PCI 2.1 66-MHz backplane is approximately 360 Mbytes/s. Note that the new PCI-X buses offer higher transfer speeds. However, data rates vary among different manufacturers and operating systems. The dual PCI and PCI-X backplanes are now in beta testing and require special software to achieve high burst rates. However, the dual-high-speed PCI and PCI-X buses show the best promise for providing a PCI-based solution.
Another design problem to be considered is how the data are stored. Most commercially available hard disks can support from 40 to 160 Mbytes/s in burst mode (see Table 3 on p. 41). The Ultra SCSI 160 (also known as SCSI-3) and the Ultra SCSI 320 disk-transfer protocols provide the fastest transfer rates for a disk interface. SCSI also provides the capacity to interface a RAID (redundant array of independent disks) array as opposed to an IDE (independent drive electronics) unit, which is a point-to-point technology that supports only one drive. A RAID array makes several drives act as one large SCSI drive.
However, these protocols fall short of the necessary data rates for the required system. One consideration is a RAM disk to handle the data rates. For example, a 655-Mbyte/s data stream for 300 s translates into 200 Gbytes. However, a solid-state RAM (SSR) device with 300+ Gbytes of storage costs about $650,000. This cost exceeds another important university design constraint—the system budget.
The Camera Link Full-configuration interface is capable of maintaining 655-Mbyte/s data transfers to the PCI bus when coupled with enough Ultra SCSI-160/320 boards to get the data to RAID arrays, but it doubles the data-rate requirement on the PCI bus from 655 Mbytes/s to 1.31 Gbytes/s. This higher rate is reachable, though, when using the new specified rates of PCI-X backplanes. The design goal of accomplishing 1280 × 1024-pixel color images at 500 frames/s for 300 s now seems feasible. The Camera Link interface appears to meet the 500-frame/s design goal; moreover, it can be scaled to 1000 frames/s when faster cameras become available.
However, currently available motherboards with single PCI buses cannot handle these high data rates. The dual PCI bus and PCI-X backplanes could be considered if design time permits beta testing of the backplane and alpha testing the accompanying software. Integration of such state-of-the-art technologies would inevitably result in delays caused by fixing unexplained faults.
Final system design
The final system design has been chosen based on the expertise of systems integrators who provided turnkey solutions to the high-speed video-acquisition and storage system requirements: Spica Technology Corp. (Kihel, Maui, HI, USA), IO Industries (London, ON, Canada), Photron USA Inc. (San Diego, CA, USA), and NAC Image Technology (Simi Valley, CA, USA). Spica Technology has solved the data-flow problem by dividing the Camera Link information into multiple streams that are then processed in parallel by networked processors. The company's IDAS high-bandwidth, digital-image data-acquisition system provides real-time image processing, display, and 3-D tracking. This multiprocessor approach allows timing, INS (inertial navigation system), and GPS (global positioning system) data to be stamped onto every frame. The method is accomplished without the need for custom hardware but does require custom software to integrate an array of parallel computers. Upward scalability is achieved by adding more computers in parallel.
For high-speed video acquisition and storage, IO Industries splits the Camera Link channels and bypasses the PCI bus by transferring the data directly to a RAID array through a SCSI Ultra-160 interface. The company's DVR (digital video recorder) Express product accomplishes this task with a single PC. It uses a custom I/O card that inputs one channel of the Camera Link interface and streams the data on a 160-Mbyte/s Ultra SCSI 160 bus directly into a RAID array. Multiple cards and multiple RAID arrays are needed to meet the system data-rate and data-storage requirements. This approach frees the CPU to process other activities and keeps the PCI bus open for stamping the frames with the IRIG (inter-range instrumentation group) timing, INS, and GPS data. In addition, the DVR Express card subsamples the video stream, which allows the video to be displayed and ensures that the camera is focused and functioning properly. Scalability is achieved by adding more I/O cards and RAID arrays.
The Photron FastCAM digital video recorder comes with a 24-bit, color Photron CMOS camera in a system that can sustain 640 × 480 pixels at 500 frames/s to a 128-Gbyte IDE hard drive. The company uses 10:1 JPEG data compression to reduce the data-transfer rate. This is an acceptable platform if the application can accept losing some data during compression.
The integrated NAC Image Technology HSV- 500C3 high-speed, color video cassette recorder, color video camera, and lens system delivers 510 × 105 pixels at 500 frames/s for 43 minutes of video data recording to S-VHS cassette tapes (see Fig. 2). Its three-chip, 1/3-in. image sensor captures true color but loses up to half the resolution during storage to tape. This use of a tape drive is advantageous for airborne and mobile applications that are concerned with the effects of shock and vibration on the hard disks.
Vision Systems Design, Camera Link Special Report, May 2002, p. S4.
A. Wilson, Vision Systems Design, April 2002, p. 29.
Vision Systems Design, Camera Link Special Report, May 2002, p. S10.
S. Arramreddy [SeverWorks] and D. Riley [Compaq], PCI-X 2.0 White Paper, April 2002.
The SCSI and IDE Interfaces, 2nd ed., Friedhelm Schmidt, Addison Wesley (1997).
GARY ARMSTRONG is president, Maverick Systems Inc., Kingston, TN, USA; www.MavSysInc.us; MONGI ABIDI is professor and director of the Imaging, Robotics, and Intelligent Systems Laboratory, University of Tennessee, Knoxville, TN, USA; imaging.utk.edu.
Imaging, Robotics, and Intelligent Systems Laboratory University of Tennessee imaging.utk.edu
IO Industries www.ioindustries.com
Maverick Systems Inc. www.MavSysInc.us
NAC Image Technology www.nacinc.com
Photron USA Inc. www.photron.com
Spica Technology Corp. www.spicatek.com