PC interface speeds image capture and display
Gary T. Forrest
Hardware modifications allow high-resolution,fast imaging on a laptop platform.
In virtually all personal-computer (PC)-based instrumentation applications, one of the fundamental tradeoffs is speed versus resolution. In general, higher data rates not only carry higher cost burdens but can rapidly conflict with bus speed limitations of the computer. This limitation is most evident in video-based instrumentation in which eight-bit two-dimensional arrays represent larger data sets. And this volume of information must yield to manipulation by an operating system, such as Windows, for effective presentation of the visual and quantitative data.
Resolution can be a confusing concept. For video instrumentation, the camera is the primary factor governing resolution. Camera resolution is expressed in pixels, with pixel sizes commonly 10 µm . But users may not realize this resolution if the frame grabber samples the analog camera signal at a lower resolution. This caveat is easily illustrated by looking at the file size of two typical captured images: for 640 ¥ 480-pixel images, the file size is around 300,000 bytes, but for a 320 ¥ 240 image the file is only about 77,000 bytes. The lower-resolution image can be captured four times as quickly because it has only one quarter the data of the higher-resolution image.
The Windows display of a captured-image file can be adjusted to screen resolutions ranging from 640 ¥ 480 to 1280 ¥ 1064 pixels. Even though the captured-image resolution is fixed, adjustment of the Windows resolution allows users to optimize the relative size of the image and text on the computer screen.
The need for speed
Most video-based instruments require a live image display during set-up. This live rate has to be fast enough so changes in the system are easily observed. The ideal goal is 30 Hz, but in practice rates around 20 Hz are acceptable. The main limitation on live update rates is the industry standard architecture (ISA) bus speed. Despite more than tenfold advances in processor speeds to greater than 100 MHz, the ISA bus speed remains at the 8 MHz rate found in the original IBM PC-AT of the mid-1980s. The true live data rate is actually on the order of a few megabytes per second because the data must be written to memory and then back to the display. Combined with various wait states and housekeeping delays, a 640 ¥ 480-pixel eight-bit resolution image can at best be displayed at rates of about 0.5 Hz.
To improve the live data display rate, video signals can be shunted directly to the VGA card via a feature connector. However, a direct shunt requires the VGA card to have a matching connector and thus is video-hardware dependent. Furthermore, the image must still go through the slow PC bus to be saved in memory, limiting data-acquisition rates to less than 1 Hz. Of course the data resolution can also be reduced, but this forces a trade-off of resolution for speed.
Alternative hardware approaches
Over the past five years a variety of bus architectures have been proposed to increase video display rates, the latest being the PowerPC. At present local buses are the only generic approaches that utilize standard, low-cost PC hardware and software. The Video Electronics Standards Association (VESA) local bus and the peripheral component interconnect (PCI) local bus offer 640 ¥ 480-pixel display rates of about 12 Hz and 10-100 Hz, respectively. These local buses require special connectors on the PC motherboard and, as indicated by the range in PCI video rates, can be PC-system dependent. This dependency gives a PCI bus twice the bandwidth of a VESA bus, which in turn is quicker than an ISA bus fixed on the motherboard.
In another example, the installation of PCI cards can become an exercise in computer configuration that tests the user`s knowledge of the interrupts and addresses of other peripherals. With the advent of Windows 95 "Plug and Play" technology, it is anticipated that newer computers will be able to recognize and automatically adjust the installation of new plug-and-play cards.
In an alternate approach, SensorPhysics (Oldsmar, FL) has employed local processing on a standard VGA card in its video-based laser beam profiler. During installation, the computer recognizes the card as the VGA controller by simply looking for the industry-standard VGA address code in the card`s basic input/output system (BIOS) software. Because the incoming video signals arrive directly on the VGA card, the live display can proceed at the desired 30 Hz.
In addition, by incorporating memory on the same card, 640 ¥ 480-pixel images can be captured at 30 Hz and then relayed to the PC memory at a slower rate (see Fig. 1). This was achieved with a modification of an off-the-shelf industrial vision card, giving higher performance at only a modest increase in cost. As in most cases of modified hardware, DOS and Windows drivers were written to capture data at 30 Hz.
Gathering data with laptops
Laptop, or notebook, computers present a different set of problems for computer-based instrumentation. Until last year, screen resolutions were limited to 320 ¥ 240 pixels for 256 color modes. Now that 640 ¥ 480 ¥ 256 and 800 ¥ 600 ¥ 256 color screens are available there is sufficient real estate on the screen to display both a high-resolution image and text.
Another significant advance in notebook computers is the availability of PCMCIA "credit cards" incorporating software and data-storage memory. In particular, SensorPhysics has used such a card with Application Specific Integrated Circuit (ASIC ) hardware video compression to display monochrome and RGB color images on a Toshiba T4700CT notebook computer at 30 Hz (see Fig. 2). This ASIC approach allows video images to be rapidly compressed for entry into the VESA display local bus. Software image-data compression would be too slow and result in a single-frame-capture beam profiler with limited usefulness for live alignment.
During implementation of the PCMCIA card architecture it was discovered that some notebook computers offer up to twice the live-video display rate of other models, even those that use the same generic VESA local bus. The specific hard-wired chip sets and software in the computer govern the display rate. Also, the installation of the PCMCIA card can be dependent on both the computer BIOS and socket-card services software, which looks for the PCMCIA interface and configures the PC to communicate with it. Because of these system variables, the SensorPhysics notebook beam profiler is offered only as a turnkey system to ensure consistency. However, the companion desktop beam profiler is easily installed on any generic PC.
Finally current PCMCIA video interfaces are optimized for RGB video input, but many instrumentation applications such as beam profiling and thermography use monochrome cameras and false color to indicate intensity. This leads to unusual color palette requirements, which may not be easily achieved on PCMCIA cards designed for color cameras. The SensorPhysics notebook profiler is optimized for 320 ¥ 240-pixel display and acquisition speeds of about 10 Hz. If portability is of key importance, this would be an acceptable tradeoff for most users. With improvements in PCMCIA cards and 32-bit operating systems, these speeds are expected to approach the 30-Hz benchmark soon. n
FIGURE 2. Hardware image-data compression via PCMCIA card allows display of RGB color images acquired at 30 H¥on a laptop-based beam-profiler system. For monochrome images that have to be processed in false color, the capture rate is 10 Hz.
GARY T. FORREST is president of SensorPhysics, 105 Kelleys Trail, Oldsmar, FL 34677.
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